Which hardware specification should one be looking for to optimize the visualization of a tendinopathy through ultrasound?

Which hardware specification should one be looking for to optimize the visualization of a tendinopathy through ultrasound?

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The 2019 study {1} reports that hip abductor tendon pathology is better visualized by 3.0 Tesla MRIs than by 1.5 Tesla MRIs.

The 2014 study {2} also mentions the usefulness of high teslas for tendon analysis:

MRI of tendons and ligaments benefits from high spatial resolution. Stronger magnetic fields lead to higher signal-to-noise ratios and improvements in image resolution; for this reason, 3-T MRI may be more sensitive than 1.5 T for detection of partial thickness tears [26]. Alternatively, higher resolution may be achieved by using a local surface coil [27]. Imaging with shorter echo times improves sensitivity to tendon changes, although this may come at the expense of specificity [28,29]. T2 weighted images are helpful for identifying fluid signal in tendon or ligament tears (Figure 5) as well as for demonstrating changes in the surrounding tissues [30]. If the orientation of a tendon changes over its course, magic angle effects may be problematic; it may therefore be helpful to acquire images with a sufficiently long echo time to avoid these artefacts.

Which hardware specification should one be looking for to optimize the visualization of a tendinopathy through ultrasound?


  • {1} Oehler, Nicola, Julia Kristin Ruby, André Strahl, Rainer Maas, Wolfgang Ruether, and Andreas Niemeier. "Hip abductor tendon pathology visualized by 1.5 versus 3. 0 Tesla MRIs." Archives of Orthopaedic and Trauma Surgery (2019): 1-9.;
  • {2} Hodgson, R. J., P. J. O'Connor, and A. J. Grainger. "Tendon and ligament imaging." The British journal of radiology (2014). Harvard ;,22 ;

First-arrival traveltime sound speed inversion with a priori information

A first-arrival travel-time sound speed algorithm presented by Tarantola [ Inverse Problem Theory and Methods for Model Parameter Estimation (SIAM, Philadelphia, PA, 2005) [Google Scholar] ] is adapted to the medical ultrasonics setting. Through specification of a covariance matrix for the object model, the algorithm allows for natural inclusion of physical a priori information of the object. The algorithm's ability to accurately and robustly reconstruct a complex sound speed distribution is demonstrated on simulation and experimental data using a limited aperture.


The algorithm is first demonstrated generally in simulation with a numerical breast phantom imaged in different geometries. As this work is motivated by the authors' limited aperture dual sided ultrasound breast imaging system, experimental data are acquired with a Verasonics system with dual, 128 element, linear L7-4 arrays. The transducers are automatically calibrated for usage in the eikonal forward model.A priori information such as knowledge of correlated regions within the object is obtained via segmentation of B-mode images generated from synthetic aperture imaging.


As one illustration of the algorithm's facility for inclusion ofa priori information, physically grounded regularization is demonstrated in simulation. The algorithm's practicality is then demonstrated through experimental realization in limited aperture cases. Reconstructions of sound speed distributions of various complexity are improved through inclusion of a priori information. The sound speed maps are generally reconstructed with accuracy within a few m/s.


This paper demonstrates the ability to form sound speed images using two opposed commercial linear arrays to mimic ultrasound image acquisition in the compressed mammographic geometry. The ability to create reasonably good speed of sound images in the compressed mammographic geometry allows images to be readily coregistered to tomosynthesis image volumes for breast cancer detection and characterization studies.

Shoulder pain is one of the most common musculoskeletal symptoms that prompt medical evaluation and can result in disability, lost wages, and substantial health care costs. It is the third most common reason for musculoskeletal consultations in the primary care setting, affecting up to one-third of the general population—particularly older individuals (1–3). Ultrasonography (US), magnetic resonance (MR) imaging, and MR arthrography are advanced imaging modalities that have been used to examine patients with shoulder pain, and each has unique advantages and disadvantages (3). Advances in technology, training, and research have improved clinicians’ ability to diagnose common shoulder diseases with high accuracy using US and MR imaging.

However, several barriers preclude the widespread use of US. These barriers include the steep learning curve in becoming proficient in performing high-quality evaluation, the lack of dedicated musculoskeletal imaging training for sonographers and technologists in U.S. training programs, and the limited time that radiologists have to perform these examinations in the absence of musculoskeletal imaging–trained sonographers. Using a standardized approach to the shoulder US examination can help reduce these barriers and provide a framework to facilitate high-quality diagnostic imaging(11).

The purpose of this review is to describe the components of a standardized shoulder US examination, review the basic technique of shoulder US and the normal shoulder anatomy, identify common indications for shoulder US in clinical practice, and illustrate characteristic US appearances of common shoulder diseases, with MR imaging and arthroscopic correlation.


The primary role of any UPS is to provide short-term power when the input power source fails. However, most UPS units are also capable in varying degrees of correcting common utility power problems:

    or sustained overvoltage
  1. Momentary or sustained reduction in input voltage
  2. Noise, defined as a high frequency transient or oscillation, usually injected into the line by nearby equipment
  3. Instability of the mains frequency , defined as a departure from the ideal sinusoidalwaveform expected on the line

Some manufacturers of UPS units categorize their products in accordance with the number of power-related problems they address. [2]

The three general categories of modern UPS systems are on-line, line-interactive and standby: [3] [4]

  • An on-line UPS uses a "double conversion" method of accepting AC input, rectifying to DC for passing through the rechargeable battery (or battery strings), then inverting back to 120 V/230 V AC for powering the protected equipment.
  • A line-interactive UPS maintains the inverter in line and redirects the battery's DC current path from the normal charging mode to supplying current when power is lost.
  • In a standby ("off-line") system the load is powered directly by the input power and the backup power circuitry is only invoked when the utility power fails.

Most UPS below one kilovolt-ampere (1 kVA) are of the line-interactive or standby variety which are usually less expensive.

For large power units, dynamic uninterruptible power supplies (DUPS) are sometimes used. A synchronous motor/alternator is connected on the mains via a choke. Energy is stored in a flywheel. When the mains power fails, an eddy-current regulation maintains the power on the load as long as the flywheel's energy is not exhausted. DUPS are sometimes combined or integrated with a diesel generator that is turned on after a brief delay, forming a diesel rotary uninterruptible power supply (DRUPS).

A fuel cell UPS was developed by the company Hydrogenics using hydrogen and a fuel cell as a power source, potentially providing long run times in a small space. [5]

Offline/standby Edit

The offline/standby UPS offers only the most basic features, providing surge protection and battery backup. The protected equipment is normally connected directly to incoming utility power. When the incoming voltage falls below or rises above a predetermined level the UPS turns on its internal DC-AC inverter circuitry, which is powered from an internal storage battery. The UPS then mechanically switches the connected equipment on to its DC-AC inverter output. The switch-over time can be as long as 25 milliseconds depending on the amount of time it takes the standby UPS to detect the lost utility voltage. The UPS will be designed to power certain equipment, such as a personal computer, without any objectionable dip or brownout to that device.

Line-interactive Edit

The line-interactive UPS is similar in operation to a standby UPS, but with the addition of a multi-tap variable-voltage autotransformer. This is a special type of transformer that can add or subtract powered coils of wire, thereby increasing or decreasing the magnetic field and the output voltage of the transformer. This may also be performed by a buck–boost transformer which is distinct from an autotransformer, since the former may be wired to provide galvanic isolation.

This type of UPS is able to tolerate continuous undervoltage brownouts and overvoltage surges without consuming the limited reserve battery power. It instead compensates by automatically selecting different power taps on the autotransformer. Depending on the design, changing the autotransformer tap can cause a very brief output power disruption, [6] which may cause UPSs equipped with a power-loss alarm to "chirp" for a moment.

This has become popular even in the cheapest UPSes because it takes advantage of components already included. The main 50/60 Hz transformer used to convert between line voltage and battery voltage needs to provide two slightly different turns ratios: One to convert the battery output voltage (typically a multiple of 12 V) to line voltage, and a second one to convert the line voltage to a slightly higher battery charging voltage (such as a multiple of 14 V). The difference between the two voltages is because charging a battery requires a delta voltage (up to 13–14 V for charging a 12 V battery). Furthermore, it is easier to do the switching on the line-voltage side of the transformer because of the lower currents on that side.

To gain the buck/boost feature, all that is required is two separate switches so that the AC input can be connected to one of the two primary taps, while the load is connected to the other, thus using the main transformer's primary windings as an autotransformer. The battery can still be charged while "bucking" an overvoltage, but while "boosting" an undervoltage, the transformer output is too low to charge the batteries.

Autotransformers can be engineered to cover a wide range of varying input voltages, but this requires more taps and increases complexity, as well as the expense of the UPS. It is common for the autotransformer to cover a range only from about 90 V to 140 V for 120 V power, and then switch to battery if the voltage goes much higher or lower than that range.

In low-voltage conditions the UPS will use more current than normal, so it may need a higher current circuit than a normal device. For example, to power a 1000-W device at 120 V, the UPS will draw 8.33 A. If a brownout occurs and the voltage drops to 100 V, the UPS will draw 10 A to compensate. This also works in reverse, so that in an overvoltage condition, the UPS will need less current.

Online/double-conversion Edit

In an online UPS, the batteries are always connected to the inverter, so that no power transfer switches are necessary. When power loss occurs, the rectifier simply drops out of the circuit and the batteries keep the power steady and unchanged. When power is restored, the rectifier resumes carrying most of the load and begins charging the batteries, though the charging current may be limited to prevent the high-power rectifier from damaging the batteries. The main advantage of an online UPS is its ability to provide an "electrical firewall" between the incoming utility power and sensitive electronic equipment.

The online UPS is ideal for environments where electrical isolation is necessary or for equipment that is very sensitive to power fluctuations. [7] Although it was at one time reserved for very large installations of 10 kW or more, advances in technology have now permitted it to be available as a common consumer device, supplying 500 W or less. The online UPS may be necessary when the power environment is "noisy", when utility power sags, outages and other anomalies are frequent, when protection of sensitive IT equipment loads is required, or when operation from an extended-run backup generator is necessary.

The basic technology of the online UPS is the same as in a standby or line-interactive UPS. However it typically costs much more, due to it having a much greater current AC-to-DC battery-charger/rectifier, and with the rectifier and inverter designed to run continuously with improved cooling systems. It is called a double-conversion UPS due to the rectifier directly driving the inverter, even when powered from normal AC current.

Online UPS typically has a static transfer switch (STS) for increasing reliability.

Hybrid topology/double conversion on demand Edit

These hybrid rotary UPS [8] designs do not have official designations, although one name used by UTL is "double conversion on demand". [9] This style of UPS is targeted towards high-efficiency applications while still maintaining the features and protection level offered by double conversion.

A hybrid (double conversion on demand) UPS operates as an off-line/standby UPS when power conditions are within a certain preset window. This allows the UPS to achieve very high efficiency ratings. When the power conditions fluctuate outside of the predefined windows, the UPS switches to online/double-conversion operation. [9] In double-conversion mode the UPS can adjust for voltage variations without having to use battery power, can filter out line noise and control frequency.

Ferroresonant Edit

Ferroresonant units operate in the same way as a standby UPS unit however, they are online with the exception that a ferroresonant transformer, is used to filter the output. This transformer is designed to hold energy long enough to cover the time between switching from line power to battery power and effectively eliminates the transfer time. Many ferroresonant UPSs are 82–88% efficient (AC/DC-AC) and offer excellent isolation.

The transformer has three windings, one for ordinary mains power, the second for rectified battery power, and the third for output AC power to the load.

This once was the dominant type of UPS and is limited to around the 150 kVA range. These units are still mainly used in some industrial settings (oil and gas, petrochemical, chemical, utility, and heavy industry markets) due to the robust nature of the UPS. Many ferroresonant UPSs utilizing controlled ferro technology may interact with power-factor-correcting equipment. This will result in fluctuating output voltage of the UPS, but may be corrected by reducing the load levels, or adding other linear type loads. [ further explanation needed ]

DC power Edit

A UPS designed for powering DC equipment is very similar to an online UPS, except that it does not need an output inverter. Also, if the UPS's battery voltage is matched with the voltage the device needs, the device's power supply will not be needed either. Since one or more power conversion steps are eliminated, this increases efficiency and run time.

Many systems used in telecommunications use an extra-low voltage "common battery" 48 V DC power, because it has less restrictive safety regulations, such as being installed in conduit and junction boxes. DC has typically been the dominant power source for telecommunications, and AC has typically been the dominant source for computers and servers.

There has been much experimentation with 48 V DC power for computer servers, in the hope of reducing the likelihood of failure and the cost of equipment. However, to supply the same amount of power, the current would be higher than an equivalent 115 V or 230 V circuit greater current requires larger conductors, or more energy lost as heat.

High voltage DC (380 V) is finding use in some data center applications, and allows for small power conductors, but is subject to the more complex electrical code rules for safe containment of high voltages. [10]

Rotary Edit

A rotary UPS uses the inertia of a high-mass spinning flywheel (flywheel energy storage) to provide short-term ride-through in the event of power loss. The flywheel also acts as a buffer against power spikes and sags, since such short-term power events are not able to appreciably affect the rotational speed of the high-mass flywheel. It is also one of the oldest designs, predating vacuum tubes and integrated circuits.

It can be considered to be on line since it spins continuously under normal conditions. However, unlike a battery-based UPS, flywheel-based UPS systems typically provide 10 to 20 seconds of protection before the flywheel has slowed and power output stops. [11] It is traditionally used in conjunction with standby generators, providing backup power only for the brief period of time the engine needs to start running and stabilize its output.

The rotary UPS is generally reserved for applications needing more than 10,000 W of protection, to justify the expense and benefit from the advantages rotary UPS systems bring. A larger flywheel or multiple flywheels operating in parallel will increase the reserve running time or capacity.

Because the flywheels are a mechanical power source, it is not necessary to use an electric motor or generator as an intermediary between it and a diesel engine designed to provide emergency power. By using a transmission gearbox, the rotational inertia of the flywheel can be used to directly start up a diesel engine, and once running, the diesel engine can be used to directly spin the flywheel. Multiple flywheels can likewise be connected in parallel through mechanical countershafts, without the need for separate motors and generators for each flywheel.

They are normally designed to provide very high current output compared to a purely electronic UPS, and are better able to provide inrush current for inductive loads such as motor startup or compressor loads, as well as medical MRI and cath lab equipment. It is also able to tolerate short-circuit conditions up to 17 times larger than an electronic UPS, permitting one device to blow a fuse and fail while other devices still continue to be powered from the rotary UPS.

Its life cycle is usually far greater than a purely electronic UPS, up to 30 years or more. But they do require periodic downtime for mechanical maintenance, such as ball bearing replacement. In larger systems redundancy of the system ensures the availability of processes during this maintenance. Battery-based designs do not require downtime if the batteries can be hot-swapped, which is usually the case for larger units. Newer rotary units use technologies such as magnetic bearings and air-evacuated enclosures to increase standby efficiency and reduce maintenance to very low levels.

Typically, the high-mass flywheel is used in conjunction with a motor-generator system. These units can be configured as:

  1. A motor driving a mechanically connected generator, [8]
  2. A combined synchronous motor and generator wound in alternating slots of a single rotor and stator,
  3. A hybrid rotary UPS, designed similar to an online UPS, except that it uses the flywheel in place of batteries. The rectifier drives a motor to spin the flywheel, while a generator uses the flywheel to power the inverter.

In case No. 3 the motor generator can be synchronous/synchronous or induction/synchronous. The motor side of the unit in case Nos. 2 and 3 can be driven directly by an AC power source (typically when in inverter bypass), a 6-step double-conversion motor drive, or a 6-pulse inverter. Case No. 1 uses an integrated flywheel as a short-term energy source instead of batteries to allow time for external, electrically coupled gensets to start and be brought online. Case Nos. 2 and 3 can use batteries or a free-standing electrically coupled flywheel as the short-term energy source.

Smaller UPS systems come in several different forms and sizes. However, the two most common forms are tower and rack-mount. [12]

Tower models stand upright on the ground or on a desk or shelf, and are typically used in network workstations or desktop computer applications. Rack-mount models can be mounted in standard 19-inch rack enclosures and can require anywhere from 1U to 12U (rack units). They are typically used in server and networking applications. Some devices feature user interfaces that rotate 90°, allowing the devices to be mounted vertically on the ground or horizontally as would be found in a rack.

N + 1 Edit

In large business environments where reliability is of great importance, a single huge UPS can also be a single point of failure that can disrupt many other systems. To provide greater reliability, multiple smaller UPS modules and batteries can be integrated together to provide redundant power protection equivalent to one very large UPS. "N + 1" means that if the load can be supplied by N modules, the installation will contain N + 1 modules. In this way, failure of one module will not impact system operation. [13]

Multiple redundancy Edit

Many computer servers offer the option of redundant power supplies, so that in the event of one power supply failing, one or more other power supplies are able to power the load. This is a critical point – each power supply must be able to power the entire server by itself.

Redundancy is further enhanced by plugging each power supply into a different circuit (i.e. to a different circuit breaker).

Redundant protection can be extended further yet by connecting each power supply to its own UPS. This provides double protection from both a power supply failure and a UPS failure, so that continued operation is assured. This configuration is also referred to as 1 + 1 or 2N redundancy. If the budget does not allow for two identical UPS units then it is common practice to plug one power supply into mains power and the other into the UPS. [14] [15]

Outdoor use Edit

When a UPS system is placed outdoors, it should have some specific features that guarantee that it can tolerate weather without any effects on performance. Factors such as temperature, humidity, rain, and snow among others should be considered by the manufacturer when designing an outdoor UPS system. Operating temperature ranges for outdoor UPS systems could be around −40 °C to +55 °C. [16]

Outdoor UPS systems can either be pole, ground (pedestal), or host mounted. Outdoor environment could mean extreme cold, in which case the outdoor UPS system should include a battery heater mat, or extreme heat, in which case the outdoor UPS system should include a fan system or an air conditioning system.

A solar inverter, or PV inverter, or solar converter, converts the variable direct current (DC) output of a photovoltaic (PV) solar panel into a utility frequency alternating current (AC) that can be fed into a commercial electrical grid or used by a local, off-grid electrical network. It is a critical BOS–component in a photovoltaic system, allowing the use of ordinary AC-powered equipment. Solar inverters have special functions adapted for use with photovoltaic arrays, including maximum power point tracking and anti-islanding protection.

The output of some electronic UPSes can have a significant departure from an ideal sinusoidal waveform. This is especially true of inexpensive consumer-grade single-phase units designed for home and office use. These often utilize simple switching AC power supplies and the output resembles a square wave rich in harmonics. These harmonics can cause interference with other electronic devices including radio communication and some devices (e.g. inductive loads such as AC motors) may perform with reduced efficiency or not at all. More sophisticated (and expensive) UPS units can produce nearly pure sinusoidal AC power.

A problem in the combination of a double-conversion UPS and a generator is the voltage distortion created by the UPS. The input of a double-conversion UPS is essentially a big rectifier. The current drawn by the UPS is non-sinusoidal. This can cause the voltage from the AC mains or a generator to also become non-sinusoidal. The voltage distortion then can cause problems in all electrical equipment connected to that power source, including the UPS itself. It will also cause more power to be lost in the wiring supplying power to the UPS due to the spikes in current flow. This level of "noise" is measured as a percentage of "total harmonic distortion of the current" (THDI). Classic UPS rectifiers have a THDI level of around 25%–30%. To reduce voltage distortion, this requires heavier mains wiring or generators more than twice as large as the UPS.

There are several solutions to reduce the THDI in a double-conversion UPS:

Classic solutions such as passive filters reduce THDI to 5%–10% at full load. They are reliable, but big and only work at full load, and present their own problems when used in tandem with generators.

An alternative solution is an active filter. Through the use of such a device, THDI can drop to 5% over the full power range. The newest technology in double-conversion UPS units is a rectifier that does not use classic rectifier components (thyristors and diodes) but uses high-frequency components instead. A double-conversion UPS with an insulated-gate bipolar transistor rectifier and inductor can have a THDI as small as 2%. This completely eliminates the need to oversize the generator (and transformers), without additional filters, investment cost, losses, or space.

  1. The UPS to report its status to the computer it powers via a communications link such as a serial port, Ethernet and Simple Network Management Protocol, GSM/GPRS or USB
  2. A subsystem in the OS that processes the reports and generates notifications, PM events, or commands an ordered shut down. [17] Some UPS manufacturers publish their communication protocols, but other manufacturers (such as APC) use proprietary protocols.

The basic computer-to-UPS control methods are intended for one-to-one signaling from a single source to a single target. For example, a single UPS may connect to a single computer to provide status information about the UPS, and allow the computer to control the UPS. Similarly, the USB protocol is also intended to connect a single computer to multiple peripheral devices.

In some situations it is useful for a single large UPS to be able to communicate with several protected devices. For traditional serial or USB control, a signal replication device may be used, which for example allows one UPS to connect to five computers using serial or USB connections. [18] However, the splitting is typically only one direction from UPS to the devices to provide status information. Return control signals may only be permitted from one of the protected systems to the UPS. [19]

As Ethernet has increased in common use since the 1990s, control signals are now commonly sent between a single UPS and multiple computers using standard Ethernet data communication methods such as TCP/IP. [20] The status and control information is typically encrypted so that for example an outside hacker can not gain control of the UPS and command it to shut down. [21]

Distribution of UPS status and control data requires that all intermediary devices such as Ethernet switches or serial multiplexers be powered by one or more UPS systems, in order for the UPS alerts to reach the target systems during a power outage. To avoid the dependency on Ethernet infrastructure, the UPSs can be connected directly to main control server by using GSM/GPRS channel also. The SMS or GPRS data packets sent from UPSs trigger software to shut down the PCs to reduce the load.

There are three main types of UPS batteries: Valve Regulated Lead Acid (VRLA), Flooded Cell or VLA batteries,Lithium Ion batteries. The run-time for a battery-operated UPS depends on the type and size of batteries and rate of discharge, and the efficiency of the inverter. The total capacity of a lead–acid battery is a function of the rate at which it is discharged, which is described as Peukert's law.

Manufacturers supply run-time rating in minutes for packaged UPS systems. Larger systems (such as for data centers) require detailed calculation of the load, inverter efficiency, and battery characteristics to ensure the required endurance is attained. [22]

Common battery characteristics and load testing Edit

When a lead–acid battery is charged or discharged, this initially affects only the reacting chemicals, which are at the interface between the electrodes and the electrolyte. With time, the charge stored in the chemicals at the interface, often called "interface charge", spreads by diffusion of these chemicals throughout the volume of the active material.

If a battery has been completely discharged (e.g. the car lights were left on overnight) and next is given a fast charge for only a few minutes, then during the short charging time it develops only a charge near the interface. The battery voltage may rise to be close to the charger voltage so that the charging current decreases significantly. After a few hours this interface charge will not spread to the volume of the electrode and electrolyte, leading to an interface charge so low that it may be insufficient to start a car. [23]

Due to the interface charge, brief UPS self-test functions lasting only a few seconds may not accurately reflect the true runtime capacity of a UPS, and instead an extended recalibration or rundown test that deeply discharges the battery is needed. [24]

The deep discharge testing is itself damaging to batteries due to the chemicals in the discharged battery starting to crystallize into highly stable molecular shapes that will not re-dissolve when the battery is recharged, permanently reducing charge capacity. In lead acid batteries this is known as sulfation but also affects other types such as nickel cadmium batteries and lithium batteries. [25] Therefore, it is commonly recommended that rundown tests be performed infrequently, such as every six months to a year. [26] [27]

Testing of strings of batteries/cells Edit

Multi-kilowatt commercial UPS systems with large and easily accessible battery banks are capable of isolating and testing individual cells within a battery string, which consists of either combined-cell battery units (such as 12-V lead acid batteries) or individual chemical cells wired in series. Isolating a single cell and installing a jumper in place of it allows the one battery to be discharge-tested, while the rest of the battery string remains charged and available to provide protection. [28]

It is also possible to measure the electrical characteristics of individual cells in a battery string, using intermediate sensor wires that are installed at every cell-to-cell junction, and monitored both individually and collectively. Battery strings may also be wired as series-parallel, for example two sets of 20 cells. In such a situation it is also necessary to monitor current flow between parallel strings, as current may circulate between the strings to balance out the effects of weak cells, dead cells with high resistance, or shorted cells. For example, stronger strings can discharge through weaker strings until voltage imbalances are equalized, and this must be factored into the individual inter-cell measurements within each string. [29]

Series-parallel battery interactions Edit

Battery strings wired in series-parallel can develop unusual failure modes due to interactions between the multiple parallel strings. Defective batteries in one string can adversely affect the operation and lifespan of good or new batteries in other strings. These issues also apply to other situations where series-parallel strings are used, not just in UPS systems but also in electric vehicle applications. [30]

Consider a series-parallel battery arrangement with all good cells, and one becomes shorted or dead:

  • The failed cell will reduce the maximum developed voltage for the entire series string it is within.
  • Other series strings wired in parallel with the degraded string will now discharge through the degraded string until their voltage matches the voltage of the degraded string, potentially overcharging and leading to electrolyte boiling and outgassing from the remaining good cells in the degraded string. These parallel strings can now never be fully recharged, as the increased voltage will bleed off through the string containing the failed battery.
  • Charging systems may attempt to gauge battery string capacity by measuring overall voltage. Due to the overall string voltage depletion due to the dead cells, the charging system may detect this as a state of discharge, and will continuously attempt to charge the series-parallel strings, which leads to continuous overcharging and damage to all the cells in the degraded series string containing the damaged battery.
  • If lead-acid batteries are used, all cells in the formerly good parallel strings will begin to sulfate due to the inability for them to be fully recharged, resulting in the storage capacity of these cells being permanently damaged, even if the damaged cell in the one degraded string is eventually discovered and replaced with a new one.

The only way to prevent these subtle series-parallel string interactions is by not using parallel strings at all and using separate charge controllers and inverters for individual series strings.

Series new/old battery interactions Edit

Even just a single string of batteries wired in series can have adverse interactions if new batteries are mixed with old batteries. Older batteries tend to have reduced storage capacity, and so will both discharge faster than new batteries and also charge to their maximum capacity more rapidly than new batteries.

As a mixed string of new and old batteries is depleted, the string voltage will drop, and when the old batteries are exhausted the new batteries still have charge available. The newer cells may continue to discharge through the rest of the string, but due to the low voltage this energy flow may not be useful, and may be wasted in the old cells as resistance heating.

For cells that are supposed to operate within a specific discharge window, new cells with more capacity may cause the old cells in the series string to continue to discharge beyond the safe bottom limit of the discharge window, damaging the old cells.

When recharged, the old cells recharge more rapidly, leading to a rapid rise of voltage to near the fully charged state, but before the new cells with more capacity have fully recharged. The charge controller detects the high voltage of a nearly fully charged string and reduces current flow. The new cells with more capacity now charge very slowly, so slowly that the chemicals may begin to crystallize before reaching the fully charged state, reducing new cell capacity over several charge/discharge cycles until their capacity more closely matches the old cells in the series string.

For such reasons, some industrial UPS management systems recommend periodic replacement of entire battery arrays potentially using hundreds of expensive batteries, due to these damaging interactions between new batteries and old batteries, within and across series and parallel strings. [31]

Most UPS devices are rated in volt-amperes giving the peak load power that they can support. This however gives no direct information as to what duration of support is possible, which would require an indication of total power stored in for example Joules or kilowatt-hours. [ citation needed ]


Ultrasound elastography (UE) and ultrasound tissue characterisation (UTC) are two newer modes of ultrasound (US) which have begun to attract scientific interests as ways to improve tendon characterisation. These modes of US show early promise in improved diagnostic accuracy, prediction of at-risk tendons and prognostication capability beyond conventional grey-scale US. Here, we provide a review of the literature on UE and UTC for Achilles, patellar and rotator cuff tendons.

The translational potential of this article: The present literature indicates that UE and UTC could potentially increase the clinician's ability to accurately diagnose the extent of tendon pathology, including preclinical injury.

Which hardware specification should one be looking for to optimize the visualization of a tendinopathy through ultrasound? - Biology

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Musculoskeletal ultrasound involves the use of high-frequency sound waves to image soft tissues and bony structures in the body for the purposes of diagnosing pathology or guiding real-time interventional procedures. Recently, an increasing number of physicians have integrated musculoskeletal ultrasound into their practices to facilitate patient care. Technological advancements, improved portability, and reduced costs continue to drive the proliferation of ultrasound in clinical medicine. This increased interest creates a need for education pertaining to all aspects of musculoskeletal ultrasound. The primary purpose of this article is to review diagnostic ultrasound technology and its potential clinical applications in the evaluation and treatment of patients with neurologic and musculoskeletal disorders. After reviewing this article, physicians should be able to (1) list the advantages and disadvantages of ultrasound compared with other available imaging modalities, (2) describe how ultrasound machines produce images using sound waves, (3) discuss the steps necessary to acquire and optimize an ultrasound image, (4) understand the different ultrasound appearances of tendons, nerves, muscles, ligaments, blood vessels, and bones, and (5) identify multiple applications for diagnostic and interventional musculoskeletal ultrasound in musculoskeletal practice. Part 1 of this 2-part article reviews the fundamentals of clinical ultrasonographic imaging, including relevant physics, equipment, training, image optimization, and scanning principles for diagnostic and interventional purposes.

Autonomous Systems

Autonomous systems in the field of robotic ultrasound may be considered to be systems facilitating independent task plan generation and consequent control and movement of the robot to acquire ultrasound for diagnostics or interventional tasks. First, autonomous image acquisition systems and, afterwards, systems for autonomous therapy guidance with respect to the medical fields of minimally invasive procedures, high-intensity focused ultrasound (HIFU), and radiation therapy are reviewed in this section. The systems described in this section may have a LORA between five and nine. However, the highest LORA obtained in this review is seven. The systems are presented in Table 2.

Autonomous Image Acquisition

Autonomous image acquisition systems are categorized into the following three main objectives: (1) using robotic ultrasound systems to create a volumetric image by combining several images and spatial information, (2) autonomous trajectory planning and probe positioning, and (3) optimizing image quality by probe position adjustment.

3D Image Reconstruction

A robotic ultrasound system to reconstruct peripheral arteries within the leg using 2D ultrasound images and an automatic vessel tracking algorithm was developed in [41]. The physician initially places the probe on the leg such that a cross-section of the vessel is visible. Thereafter, the vessel center is detected, and the robotic arm moves autonomously such that the vessel center is in the horizontal center of the image. A force-torque sensor is placed between probe holder and end-effector that allows keeping a constant pressure during the scan. The 3D reconstruction was performed online during the acquisition. Huang et al. [42] presented a more autonomous system that encompasses a depth camera in order to identify the patient and independently plan the scan path of the ultrasound robot. After spatial calibration, the system could autonomously identify the skin within the image and scan along the coronal plane using a normal vector-based approach for probe positioning (Fig. 3a). Two force sensors placed at the bottom of the probe ensured proper acoustic coupling during image acquisition.

Overview of different robotic ultrasound systems for autonomous image acquisition. a A robotic ultrasound system autonomously scanning along a lumbar phantom (left) and the reconstructed ultrasound volume from 2D images (right) (copyright © [2019] IEEE. Reprinted with permission from [42]). b System setup including transformations (arrows) between robot, camera, ultrasound probe, and patient (left). MRI atlas displaying the generic trajectory (dotted red line) to image the aorta (right) (copyright © [2016] IEEE. Reprinted with permission from [44•]). c Robotic ultrasound system and phantom (left) with the target (red) in the ultrasound image (top right). A confidence map is calculated, and the current and desired configuration (red and green line, respectively) are calculated (bottom right) (copyright © [2016] IEEE. Reprinted with permission from [49])

Trajectory Planning and Probe Positioning

Hennersperger et al. [43] developed a robotic ultrasound system using an LBR iiwa robot that can autonomously conduct trajectories based on selected start and end points selected by a physician in preinterventional images such as MRI or CT. Given the start and end points within the MRI data, the trajectory was calculated by computing the closest surface point and combining it with the corresponding surface normal direction. Drawbacks of this method are the need for patients to hold their breath and the necessity of preinterventional image acquisition prior to selecting start and end points. The same research group overcame this drawback and used the system for quantitative assessment of the diameter of the abdominal aorta [44•]. Based on an MRI atlas and the registration to the current patient, the robot follows a generic trajectory to cover the abdominal aorta (Fig. 3b). An online force adaptation approach allowed measuring the aortic diameter even while the patient was breathing during acquisition. The system setup proposed by Graumann et al. [45] was similar but with the main objective to autonomously compute a trajectory in order to cover a volume of interest within previously obtained images such as CT, MRI, or even ultrasound. The robotic ultrasound system could cover the volume by single or multiple parallel scan trajectories. Kojcev et al. [46] evaluated the system regarding the reproducibility of measurements performed by the system producing ultrasound volumes compared to an expert-operated 2D ultrasound acquisition.

Von Haxthausen et al. [47•] developed a system that, after a manual initial placement of the probe, can control the robot in order to follow peripheral arteries, whereas the vessel detection is realized using convolutional neural networks (CNNs).

A system that provides an automatic probe position adjustment with respect to an object of interest was proposed in [48]. Their approach is based on visual servoing using image features (image moments). The authors used a 3D ultrasound probe and extracted features from the three orthogonal planes to servo in- and out-of-plane motions.

Image Quality Improvement

Since ultrasound imaging suffers from high user dependency, there is a strong interest in autonomously improving the image quality by means of probe positioning of the robot. Chatelain et al. dedicated several publications to this topic. The authors proposed a system that can automatically adjust the in-plane rotation for image quality improvement while using a tracking algorithm for a specific anatomical target [49]. The main objective was to keep the object horizontally centered within the ultrasound image while scanning the best acoustic window for the object (Fig. 3c). However, out-of-plane control is not considered. Their following work [50•] utilized the same approaches but for an ultrasound volume instead of a 2D image that in turn could provide tracking and image quality improvement for all six DOF.


Several systems and approaches have been proposed to provide autonomous image acquisition with respect to 3D image reconstruction, trajectory planning, probe positioning, and image quality improvement. A key component for initial autonomous probe placement is a depth camera to capture relative positions of robot and patient. Mostly, preinterventional images such as CT or MRI were used to calculate the trajectory needed to image the desired volume of interest. To improve image quality during acquisition, the systems rely on ultrasound image processing and force information. Even though some studies provide in vivo results, safety aspects with respect to the workflow are rarely considered within the reviewed articles.

Autonomous Therapy Guidance

This subsection presents systems that eliminate the need of human intervention for imaging during therapy. Using an autonomous system has the benefit that the physician can concentrate on the interventional task while a robot performs ultrasound imaging. To realize this, ultrasound images need to be interpreted automatically to be able to continuously track and visualize the ROI for guidance.

Minimally Invasive Procedures/Needle Guidance

In [51•], the authors proposed an autonomous catheter tracking system for endovascular aneurysm repair (EVAR). As illustrated in Fig. 4a, an LBR iiwa robot with a 2D ultrasound probe is used to acquire ultrasound images. In a preinterventional CT, the vessel structure of interest is segmented and subsequently registered to the intrainterventional ultrasound images. During the intervention, a catheter is inserted into the abdominal aorta by a physician, and the endovascular tool is guided to the ROI. The robot follows the catheter using a tracking algorithm and force control law so that the catheter tip is continuously visible in the ultrasound images. For needle placement tasks such as biopsies, Kojcev et al. [52] proposed an autonomous dual-robot system (Fig. 4b). The system can perform both ultrasound imaging and needle insertion. In this phantom study, two LBR iiwa robots are used, one holding the needle and the other one holding the ultrasound probe. Preinterventional planning data is registered to the robot coordinate system in the initialization phase using image registration. The physician selects the ROI on the patients’ surface images acquired by RGB-D (depth) cameras mounted on the robots. The robots move the ultrasound probe and the needle to the ROI and start target tracking based on a predefined target and also needle tracking to perform needle insertion as planned. A dual-robot system provides higher flexibility than a one-robot system as used in [39•, 53], but its setup is more complicated to implement.

Examples of autonomous therapy guidance systems. a Autonomous robotized catheter tracking for EVAR with an LBR iiwa robot. Robot ultrasound setup (top), ultrasound image (bottom left), and 3D vessel model (bottom right) (copyright © [2019] IEEE. Reprinted with permission from [51•]). b Dual-robot system with two LBR iiwa robots performing both target tracking and needle insertion in a water bath phantom (reproduced from [52] with permission from Springer Nature)

High-Intensity Focused Ultrasound

Another application field is tumor treatment with HIFU. In [54], one 2D ultrasound probe and the HIFU transducer are mounted to a six DOF robotic arm. The HIFU focus is adapted by using speckle tracking to determine the difference between target and HIFU focus. While this phantom study only considered one-dimensional (1D) motion, the authors plan to extend the system to 2D motion. In the system developed by An et al. [55], an optically tracked 2D ultrasound probe is handheld, and a YK400XG robot (YAMAHA) holds the HIFU transducer. The robot adapts the HIFU focus to the target position that is identified in the ultrasound images. In contrast to other systems, the treatment transducer, but not the ultrasound probe, is robot controlled. Another approach is proposed in [56] where a tracking accuracy study is performed. Here, two 2D ultrasound probes mounted on the HIFU transducer are used to track the target position using image registration with preinterventional image data. So far, the ultrasound probes and the transducer are static, but the authors plan to use a dual-robot system to reach higher flexibility in the future.

Radiation Therapy

In radiation therapy, tumors are treated by using ionizing radiation. Especially treatment of soft-tissue tumors is a challenging task due to organ motion [6]. For example, various approaches have been proposed to track tumor motion and adapt the radiation beam using ultrasound guidance [57, 58•]. However, in the treatment room, the probe needs to be placed on the patient for image acquisition. To help the operator with this task, Şen et al. [59] proposed an autonomous robotic ultrasound-guided patient alignment. Kuhlemann et al. [60] proposed a robotic camera-based patient localization approach where a depth camera is used to localize the patient within the treatment room and register the body surfaces from the preinterventional CT and the depth camera. In addition, optimal ultrasound view ports were calculated from the preinterventional CT. For treatment delivery, Schlüter et al. [61] proposed the usage of a kinematically redundant robot (LBR iiwa) to be able to avoid beam interferences caused by the robotic system and developed strategies for automatic ultrasound probe placement [62••]. In addition, safety aspects need to be considered [63] to prevent collisions and ensure that robot forces do not exceed acceptable values.


Autonomous therapy guidance systems are highly application-specific and depend on the ultrasound image analysis capability. While robotic motion compensation can already be performed using force sensitive robots, the automatic detection of target motion in 2D and 3D ultrasound images is still under active research. Furthermore, most evaluations were limited to phantom experiments, highlighting the need for more realistic in vivo studies.

Which hardware specification should one be looking for to optimize the visualization of a tendinopathy through ultrasound? - Biology

OBJECTIVE. Dual-energy CT (DECT) characterizes the chemical composition of material according to its differential x-ray attenuation at two different energy levels. Applications of DECT in musculoskeletal imaging include imaging of bone marrow edema, tendons, and ligaments and the use of monoenergetic techniques to minimize metal prosthesis beam-attenuating artifacts.

CONCLUSION. The most validated application of DECT is undoubtedly its noninvasive and highly specific ability for confirming the presence of monosodium urate deposits in the assessment of gout.

Dual-energy CT (DECT), also known as spectral imaging, was initially designed to identify uric acid deposition within kidneys (i.e., kidney stones) and has been validated to do so in both in vitro and in vivo studies [1, 2]. However, DECT now has been successfully modified and applied to the realm of musculoskeletal imaging with unique applications.

Dual-source CT scanners are equipped with two x-ray tubes allowing simultaneous acquisition at two different energy levels (e.g., 80 or 100 and 140 kVp) thus, it is superior to the present single-energy (single-source) CT because of its ability to extract information and characterize the chemical composition of material according to the differential x-ray photon energy–dependent attenuation of the compounds being examined at the two different energy levels [1, 2]. The material-specific differences depend on the absorption of the x-ray beam by different degrees, which is directly related to the atomic weight number and electron density of the material being examined [3].

Utilizing this ability, DECT has been successfully shown in the literature to confirm the presence of monosodium urate (MSU) crystals in and around joints in gout arthropathy [4–6], identify bone marrow edema [7], visualize tendons and ligaments [1, 8–11], and minimize beam-hardening artifacts from bone prostheses [12–14]. However, to date, the most successful application of DECT in musculoskeletal imaging is the visualization of MSU crystals in confirming the diagnosis of gout.

Although there are some slight differences with each musculoskeletal-specific application of DECT, at our institution, the general protocol we use includes the use of a CT scanner (SOMATOM Definition Flash, Siemens Healthcare) with two x-ray tubes and corresponding detectors offset by 95°. Scan parameters are currently as follows: tube A, 140 kV and 55 mA with a tin filter tube B, 80 kV and 243 mA and collimation of 0.6 mm reconstructed to 0.75-mm-thick slice. Postprocessing occurs on a multimodality workplace (Leonardo, Siemens Healthcare). The two datasets are loaded into organ-specific dual-energy programs utilizing material decomposition algorithms that yield color-coded multiplanar reformatted cross-sectional and volumetric-rendered images.

Gout is a disease that results from the deposition of uric acid crystals that can accumulate both intra- and extraarticularly, as well as in soft tissues, tendons, and intraosseous regions. It is the most common type of crystalline arthropathy, as well as the most common inflammatory arthropathy in male patients, with incidence estimated to affect 6.1 million people in the Unites States [15–17]. In addition, the incidence has been increasing as a result of changing dietary habits, such as the higher intake of fructose products. Choi and Curhan [18] have reported a 1.85 relative risk factor for developing gout in male patients who consume more than two sugar-sweetened soft drinks daily, because there is increased uric acid produced by the liver and decreased renal clearance secondary to excessive fructose consumption [3]. Peak incidence occurs between age 30 and 50 years, and the condition is five times more common among men than among women [15–17]. Acute gout most commonly affects the first metatarsal joint of the foot, also known as podagra however, almost any joint can be involved. The classic clinical picture of gout is that of acute excruciating burning joint pain. If left untreated, gout can lead to recurrent episodes of joint inflammation and eventual joint destruction [15–17].

The greater the degree and duration of high urate levels, the greater the severity of disease. Thus, confirming the presence of gout with high accuracy would be highly desirable because it is crucial to start lowering the urate levels early on in the disease process to best prevent the development of complications and future exacerbations of gout. Tophaceous gout refers to the deposition of urate and inflammatory cells in the tissues and can result from long-standing disease or after one episode of an acute gout attack. This deposition may be in joints, synovium, tendons, ligaments, cartilage, bone, and other soft tissues that can result in joint destruction or weaken tendons or ligaments and predispose them to rupture.

Asymptomatic hyperuricemia, as its name implies, is characterized by elevated uric acid without symptoms of the disease and might offer the potential of unexplored research avenues. With DECT as a noninvasive technique to identify subclinical disease, this would allow the institution of urate-lowering treatment at the onset of disease and avert joint destruction and patient immobility, leading to improvement in a patient’s quality of life [3]. Figures 1A and 1B shows an example of the application of DECT for the detection of MSU crystals in an asymptomatic hyperuricemic patient.

Diagnosis of gout—Although certain clinical features can aid with establishing the diagnosis, there are several other diseases that can mimic or coexist with gout [15–17], including septic arthritis, rheumatoid arthritis, osteoarthritis, erosive osteoarthritis, psoriasis, calcium pyrophosphate dihydrate crystal deposition, xanthomatosis, amyloidosis, pigmented villonodular synovitis, amyloid arthropathy, sarcoidosis, and malignant intra- and paraarticular masses, such as synovial sarcoma and giant cell tumor. The definitive diagnosis of gout requires microscopic analysis of fluid aspirated from the joint with the finding of negatively birefringent MSU crystals [16]. Joint aspiration may appear to the novice as a relatively simple technique however, it can often be technically challenging to perform because of an inadequate amount of fluid collection, particularly when the joints are acutely inflamed, or as a result of challenging anatomic sites clinically, it is only performed in 17% of cases [19, 20]. In the acute setting of acute gout, Swan and colleagues [21] found that aspiration can be negative 25% of the time. Thus, a sensitive, specific, and noninvasive means of diagnosing gout, such as DECT, is highly desirable clinically.

A, Two-material decomposition coronal multiplanar reformatted color-coded (A) and volume-rendered (B) images of feet show mild degree of monosodium urate deposition (arrows) surrounding right first metatarsophalangeal joint and along left midfoot, confirming presence of subclinical gout.

B, Two-material decomposition coronal multiplanar reformatted color-coded (A) and volume-rendered (B) images of feet show mild degree of monosodium urate deposition (arrows) surrounding right first metatarsophalangeal joint and along left midfoot, confirming presence of subclinical gout.

Fig. 2 Dual-energy gout application class for multimodality workplace (Leonardo, Siemens Healthcare). Screen shot of gout algorithm (left) displays differences in attenuation between trabecular bone, uric acid, and cortical bone: y-axis represents attenuation values of lower kilovoltage tube (80 kV), and x-axis represents attenuation values of higher kilovoltage tube (tin-filtered 140 kV). Compounds above line represent calcium (high atomic weight number, color-coded in blue), and compounds below line represent uric acid (low atomic weight number, color-coded in green). Color-coded two-material decomposition cross-sectional and volume-rendered images (right) show deposition of uric acid color-coded in green.

DECT for gout—Imaging methods, including radiography, sonography, single-energy CT, and MRI, have been previously suggested as potentially useful noninvasive imaging modalities for diagnosing gout [16, 22–26]. However, none of these methods has been found to be sensitive or specific enough to confirm the presence of MSU crystals [16, 23–26]. There has also been criticism regarding the interobserver correlation for ultrasound and MRI in the assessment of tophus size [23, 27].

Dual-energy imaging easily allows the separation and characterization of calcium, a high-molecular-weight compound, from uric acid, a low-molecular-weight compound, making DECT an important noninvasive tool in diagnosing gout. The two datasets—80 and 100 kVp and tin-filtered 140 kVp—are loaded in the multimodality workplace. Then a two-material decomposition calculation occurs ( Fig. 2 ). The material-specific differences in attenuation of the two datasets (80 and 100 kVp and tin-filtered 140 kVp) enables an easy classification of the elementary chemical composition of the scanned tissue, allowing accurate characterization of uric acid (color coded in green) separately from the calcium and bone marrow (cortical bone color coded in blue and medullary bone in pink).

DECT has been shown in recent publications to adequately confirm the presence of urate crystals in tissue with a high specificity and moderate sensitivity both in retrospective [4, 5, 7, 28] and in one randomized prospective study [29]. In 2009, Choi and colleagues [4] found that DECT was more efficacious than clinical assessment in identifying gout. In that study, DECT identified 22 anatomic sites with urate deposits, in comparison with clinical assessment, which was able to confirm only six sites (p < 0.001). Thus, the authors concluded that DECT was four times more efficacious in identifying urate deposits than the clinical examination [4]. Glazebrook and colleagues [6] conducted a retrospective study of 94 patients undergoing DECT for painful joints 31 patients had successful joint aspiration 1 month after DECT, and 12 of these patients had successful joint aspirations in which uric acid was identified. The sensitivity of DECT was 100%, with nearly perfect interobserver reliability for confirmation of the presence of urate crystals. Specificity for the two readers was 79% and 89%, respectively [6]. On the basis of these results, the authors concluded that DECT is a sensitive, noninvasive, and reproducible method for detecting uric acid deposits in patients with suspected gout (Figs. 3A , 3B , 3C , 3D , 3E , and 3F ).

A, Two-material decomposition multiplanar reformatted images were obtained. Volume-rendered images of feet (A) and right hand and wrist (B) display numerous large urate tophi (green) along whole extent of feet, ankles, hand, and wrist.

B, Two-material decomposition multiplanar reformatted images were obtained. Volume-rendered images of feet (A) and right hand and wrist (B) display numerous large urate tophi (green) along whole extent of feet, ankles, hand, and wrist.

C, Color-coded planar (C) and volume-rendered (D) images of right elbow joint reveal extensive uric acid deposits (green) along whole extent of proximal elbow.

D, Color-coded planar (C) and volume-rendered (D) images of right elbow joint reveal extensive uric acid deposits (green) along whole extent of proximal elbow.

E, Color-coded planar image of right foot and ankle illustrates large urate deposits (green) at midfoot, metatarsophalangeal joints (right-hand arrow), and proximal and distal interphalangeal joints (left-hand arrow) and along Achilles tendon (arrowhead).

F, Corresponding MRI shows normal-looking Achilles tendon (arrowhead) and presence of soft ill-defined masses (arrow) in midfoot region, which is nonspecific finding and could be seen in rheumatoid arthritis, amyloid, gout, or infection. Because MRI is not as specific as DECT in identifying presence of MSU deposits, origin of these masses cannot be determined from MRI, whereas DECT accurately confirms diagnosis of gout.

Because DECT can confirm with high specificity the presence of MSU crystals separately from calcium, this allows the automated calculation of MSU crystal volumes within a tophus. Dalbeth et al. [28] and Choi et al. [29] and have recently shown that the utilization of automated volume software is more reproducible, with both high inter- and intraobserver reliability, than physical measurements in the determination of urate tophus size. The most clinically relevant—because it was prospective, randomized, and double blinded— validation study of DECT in the assessment of gout was published recently by Choi et al. [29]. That study showed a promising clinical utility of DECT not only in accurately confirming the presence of MSU deposits but also in being highly reproducible in assessing urate crystal volumes. The authors prospectively determined the sensitivity, specificity, and the interobserver and intraobserver reproducibility for DECT urate volume measurements [29]. There were 17 aspiration-proven gout cases and 40 control cases with no evidence of gout, and a blinded radiologist identified urate deposition to calculate the specificity and sensitivity of DECT for detecting gout. Interrater volumetric reproducibility was determined by two independent radiologists on 40 index tophi from the 17 patients with tophaceous gout using volume automated software for the calculation of urate volumes. Sensitivity and specificity of DECT for detecting gout were 84% and 93%, respectively. The urate volumes of 40 index tophi ranged from 0.06 to 18.74 cm 3 (mean, 2.45 cm 3 ). Interobserver and intraobserver intraclass correlation coefficients for DECT were perfect for urate volume measurements at 100% and 100%, respectively, indicating high reproducibility of DECT urate volume measurements [29]. The specificity in that study for MSU deposits was high however, the sensitivity was moderate, which the authors suggested might be potentially explained by the frequent incidence of urate-lowering therapy use in their patient population [29]. Examples of the clinical utility of DECT in the volume assessment of gout are shown in Figures 4A , 4B , 4C , and 4D .

A, Images show clinical utility of dual-energy CT (DECT) in reproducible accurate assessment of knee urate volumes in gout. Initial pretreatment axial source (A) and volume-rendered (B) DECT images show urate tophi volume of 20.2 cm 3 . After urate-lowering therapy, axial source (C) and volume-rendered (D) DECT images show urate tophi volume of 0.6 cm 3 . Axial source images (A and C) show automated tophus urate volume values based on datasets obtained from DECT that are then loaded into multimodality workplace (Leonardo, Siemens Healthcare). Volume application, which allows regions of interest to be drawn around visualized urate deposits with subsequent urate volume automated calculated values. Volume-rendered images of knees (B and D) nicely show extensive reduction of monosodium urate deposits (green) in and around knee joints after urate-lowering therapy.

B, Images show clinical utility of dual-energy CT (DECT) in reproducible accurate assessment of knee urate volumes in gout. Initial pretreatment axial source (A) and volume-rendered (B) DECT images show urate tophi volume of 20.2 cm 3 . After urate-lowering therapy, axial source (C) and volume-rendered (D) DECT images show urate tophi volume of 0.6 cm 3 . Axial source images (A and C) show automated tophus urate volume values based on datasets obtained from DECT that are then loaded into multimodality workplace (Leonardo, Siemens Healthcare). Volume application, which allows regions of interest to be drawn around visualized urate deposits with subsequent urate volume automated calculated values. Volume-rendered images of knees (B and D) nicely show extensive reduction of monosodium urate deposits (green) in and around knee joints after urate-lowering therapy.

C, Images show clinical utility of dual-energy CT (DECT) in reproducible accurate assessment of knee urate volumes in gout. Initial pretreatment axial source (A) and volume-rendered (B) DECT images show urate tophi volume of 20.2 cm 3 . After urate-lowering therapy, axial source (C) and volume-rendered (D) DECT images show urate tophi volume of 0.6 cm 3 . Axial source images (A and C) show automated tophus urate volume values based on datasets obtained from DECT that are then loaded into multimodality workplace (Leonardo, Siemens Healthcare). Volume application, which allows regions of interest to be drawn around visualized urate deposits with subsequent urate volume automated calculated values. Volume-rendered images of knees (B and D) nicely show extensive reduction of monosodium urate deposits (green) in and around knee joints after urate-lowering therapy.

D, Images show clinical utility of dual-energy CT (DECT) in reproducible accurate assessment of knee urate volumes in gout. Initial pretreatment axial source (A) and volume-rendered (B) DECT images show urate tophi volume of 20.2 cm 3 . After urate-lowering therapy, axial source (C) and volume-rendered (D) DECT images show urate tophi volume of 0.6 cm 3 . Axial source images (A and C) show automated tophus urate volume values based on datasets obtained from DECT that are then loaded into multimodality workplace (Leonardo, Siemens Healthcare). Volume application, which allows regions of interest to be drawn around visualized urate deposits with subsequent urate volume automated calculated values. Volume-rendered images of knees (B and D) nicely show extensive reduction of monosodium urate deposits (green) in and around knee joints after urate-lowering therapy.

DECT has also been found to be relevant in the acute care setting as a problem-solving tool. Nicolaou et al. [30] found that DECT was helpful in differentiating gout from other challenging diagnoses. Examples in that study included the confirmation of gout and exclusion of septic arthritis in the hands, elbows, and feet. In that study, DECT was very useful in an immunosuppressed patient with leukemia, who presented acutely with a destructive lesion along the distal interphalangeal joint of his foot. DECT was performed, confirming the presence of MSU crystals along the distal interphalangeal joint, excluding septic arthritis or a chloroma, and therefore negating the need to aspirate the joint. The patient was successfully treated conservatively. Figures 5A , 5B , 5C , and 5D shows an example of the utility of DECT as a problem-solving tool in a patient who presented with either osteomyelitis or an acute attack of gout. The scientific evidence provided supports DECT as a promising noninvasive technique in both the diagnosis and management of gout.

Bone marrow edema is best visualized with MRI techniques. Bone marrow edema is often described as areas of decreased signal intensity on T1-weighted MRI and increased signal intensity on T2-weighted fat-suppressed or STIR-weighted MRI within the bone marrow. However, Pache et al. [7] have recently been able to evaluate traumatic bone marrow edema using a DECT virtual non–calcium-subtraction technique. This technique allows the subtraction of calcium from cancellous bone, allowing the identification of bone marrow edema. This allows the assessment of posttraumatic bone bruises of the knee to be potentially detectable with DECT. Pache et al. prospectively evaluated 236 regions of the tibia and femur in 21 patients with acute knee trauma who underwent both DECT and MRI. The authors reported a sensitivity of 86.4% and high specificities of 94.4% and 95.5% for the two observers [7]. Pache et al. attributed the lower sensitivity rate to inexperience with the novel technique, which they suggested would improve with experience and practice. In addition, the authors suggested the use of color-coded virtual non–calcium-subtracted images for better visual detection of attenuation changes in the bone marrow [7]. Figures 6A , 6B , and 6C shows the use of DECT and color-coded virtual non–calcium-subtraction techniques to identify bone marrow edema in the knee. CT, now for the first time utilizing dual energy, has the future potential for the detection of bone marrow edema after trauma via the virtual non–calcium-subtraction technique. This could be especially useful in situations offering an alternative imaging option for patients with contraindications to MRI.

A, Radiograph shows soft-tissue prominence (arrow) adjacent to first MTP joint of right foot with presence of cystic lucencies, both subchondral and marginal. Cause may be infectious or inflammatory.

B, Bone scan revealed increased uptake (arrows) on delayed phase in first MTP and midfoot concern for osteomyelitis was raised, but inflammatory arthropathy could not be excluded. Serum urate levels were normal.

C, Volume-rendered image from DECT (C) and sagittal two-material decomposition multiplanar reformatted color-coded image (D) show presence of gouty monosodium urate tophi in green (arrows) with erosions at first metatarsophalangeal joint, midfoot, and ankle of right foot definitively confirmed.

D, Volume-rendered image from DECT (C) and sagittal two-material decomposition multiplanar reformatted color-coded image (D) show presence of gouty monosodium urate tophi in green (arrows) with erosions at first metatarsophalangeal joint, midfoot, and ankle of right foot definitively confirmed.

A, Insufficiency fracture (arrow) is noted in proximal medial tibia that is not easily visible on plain anteroposterior radiograph of knee.

B, Dual-energy CT coronal (left panel, B) and axial (left panel, C) virtual non–calcium-subtracted images and corresponding color-coded three-material decomposition (calcium, fat, and water) images (right panels) show presence of subchondral bone marrow edema at medial femoral condyle and proximal medial tibial plateau as high attenuation on virtual unenhanced images (arrows, left panels) and as dense blue on color-coded images (arrows, right panels). There is also presence of some subchondral sclerosis (color coded orange) and insufficiency fracture (arrowheads, B) that can be seen in medial proximal tibia on coronal color-coded images.

C, Dual-energy CT coronal (left panel, B) and axial (left panel, C) virtual non–calcium-subtracted images and corresponding color-coded three-material decomposition (calcium, fat, and water) images (right panels) show presence of subchondral bone marrow edema at medial femoral condyle and proximal medial tibial plateau as high attenuation on virtual unenhanced images (arrows, left panels) and as dense blue on color-coded images (arrows, right panels). There is also presence of some subchondral sclerosis (color coded orange) and insufficiency fracture (arrowheads, B) that can be seen in medial proximal tibia on coronal color-coded images.

Fig. 7 57-year-old man with open reduction internal fixation, internal lateral plate of distal tibia, and fibula fractures after trauma. Shown are representative coronal CT images obtained from dual-energy CT that are reconstructed with monoenergetic spectrum application class on multimodality workplace in monoenergies at, from left to right, 50, 70, 130, and 190 keV and simulated 120-kVp conventional CT image. Beam-hardening artifact due to metal plates and screws is clearly reduced at 130 and 190 keV when compared with simulated conventional 120-kVp image. Distal tibial and fibular fractures are more clearly delineated on 130 and 190 keV with tibial fracture lacking solid bony union, whereas fibular fracture displays solid bony union.

Fig. 8 23-year-old man with pain involving plantar aspect of left forefoot showing plantar plate injury. Coronal color-coded collagen decomposition image shows focal discontinuity of plantar plate of left second metatarsophalangeal joint (arrow). Intact plantar plate of contralateral right second metatarsophalangeal joint shows continuous collagen color coding (arrowhead) in keeping with uniform collagen distribution.

Imaging of patients with metal hardware artifacts with CT remains a challenge despite techniques devised to overcome this problem. Causes of metal hardware artifact include photon starvation, partial volume averaging, scatter, and beam hardening. The type of metal, size and shape of the implant, and the orientation of the hardware all contribute to the artifact. High-attenuation objects, such as metal, dramatically change this spectrum by attenuating the low-energy photons and shifting the spectrum toward higher-energy photons.

Many techniques to decrease such artifacts have been described in the literature, including the adjustment of image acquisition parameters. However, this change is not accounted for by the detectors, thus giving rise to reconstruction errors and beam-hardening artifacts. Current techniques to reduce metal artifact include increasing the peak kilovoltage, which reduces photon starvation and beam hardening increasing tube current, which reduces photon starvation thin beam collimation, which decreases scatter and reduces partial volume averaging reconstruction with thicker slices and smooth reconstruction algorithms. Newer methods include iterative reconstruction, which reduces noise, and projection interpolation and adaptive filtering methods.

A, Transverse color-coded collagen decomposition map of ankle shows lateral, flexor, and extensor tendons in patient with gout. Collagen color coding of peroneus brevis and peroneus longus tendons (enlarged and inflamed) on right is irregular and of decreased intensity (arrow) when compared with normal left peroneus brevis and peroneus longus (arrowhead).

B, Transverse two-material decomposition images at same level show presence of uric acid in green within peroneus brevis and peroneus longus tendons (arrow), in keeping with urate tendinopathy presumably causing of abnormality on color-coded collagen decomposition images. Arrowhead points to normal peroneal tendons in left foot, which do not show uric acid deposits with tendons themselves.

In 1986, Hemmingsson et al. [31] described using DECT techniques with one image postprocessing method after image construction to reduce the beam-hardening artifact in CT. Otherwise, DECT has not been previously discussed for use in metal artifact reduction. Recent literature has suggested the use of the monoenergetic spectral beam of DECT as a method to eliminate the reconstruction error, by utilizing the ability of DECT to extrapolate the beam hardening to generate images as though they had been acquired with a single–kiloelectron-volt monoenergetic high-energy technique. In 2011, Bamberg et al. [14] compared various monoenergetic reconstructions in 31 patients with metallic implants, showing subjective and objective evidence that high-energy monoenergetic–kiloelectron-volt images were superior to low-energy monoenergetic–kiloelectron-volt images both in image quality and diagnostic value. Although those authors were unable to completely eliminate the metal artifacts, the image quality was improved by 49%, diagnostic value was enhanced by approximately 44%, and density of artifacts was reduced [14]. In several examinations, they reported that the decisive diagnostic features were only discernible in the high–kiloelectron-volt energy reconstructions. In addition, the authors reported that the optimal kiloelectron-volt setting varied depending on the size and composition of the prosthesis thus, DECT shows the strength of being able to retrospectively allow individual photon energy optimization without prior knowledge of composition of the prosthetic material [14].

Zhou et al. [12] have also recently compared various monoenergetic reconstructions in 47 patients who were imaged with DECT after metal orthopedic device implantation following fractures. Postprocessing was performed to produce six photon energies (40, 70, 100, 130, 160, and 190 keV), which were compared to each other and with average weighted 120-kVp images [12]. Those authors found that monoenergetic imaging of DECT significantly improved the quality of the images and that an optimal setting with the lowest metal artifact was 130 keV for total, internal, and external metal orthopedic devices [12]. It is important to note that both of these studies utilizing the monoenergetic spectrum of DECT successfully improved image quality while maintaining radiation dose neutrality compared with standard literature protocols [12, 14]. In addition, there was good interobserver agreement in both studies [12, 14]. Thus, this new technique offers the potential to successfully minimize metallic artifacts without exposing patients to increased radiation dose ( Fig. 7 ).

Additional techniques involving spectral CT have been suggested to help reduce metal-related artifacts. Liu and colleagues have reported the use of gemstone monoenergetic spectral imaging with metal artifact reduction software to markedly reduce metal-related beam-hardening artifacts and significantly improve visualization of the periprosthetic cortex, medullary bone trabeculation, and adjacent soft tissue in a retrospective analysis of 26 patients [13]. However, image quality was affected by prosthesis composition and size and should be considered when imaging with gemstone monoenergetic spectral imaging with metal artifact reduction reconstruction [13].

Conventional MDCT is excellent at osseous assessment of joint internal derangement but somewhat limited in assessment of joint soft-tissue structures. Tendons and ligaments are made up of elongated fibrils of type 1 collagen, elastin, proteoglycans, glycosaminoglycans, and glycoproteins [32]. Collagen’s specific dual-energy index values allow collagen to be decomposed from the surrounding tissue, presumably secondary to its densely packed nature and side chains of hydroxylysine and hydroxyproline [1]. In DECT, the two datasets are loaded into a multimodality workplace where a three-material collagen decomposition algorithm is performed (collagen, fat, and soft tissue). In our application, collagen is color coded as orange and yellow.

Johnson et al. [1] initially showed the ability of DECT to detect tendons and ligaments using three-material decomposition to differentiate collagen, water, and soft tissue from each other. They found that the tendons could be differentiated clearly in an unenhanced scan [1]. In 2008, Persson et al. [9] conducted a postmortem analysis and were able to use DECT to depict tendons and ligaments via differentiation of collagen for better visualization of wounds in the ankle and wrist region. However, one criticism was that, because the study was conducted on deceased patients, the datasets generated were of extremely high resolution because of the lack of limitations on radiation dose and, thus, were not representative or feasible for daily clinical application of DECT [9].

There have been conflicting data presented in the literature regarding the use of DECT for imaging of tendons and ligaments [8, 10, 11]. In 2008, Lohan et al. [8] compared single-energy CT and DECT for the depiction of lower extremity tendons in 11 healthy and seven clinically referred patients for qualitative and quantitative analysis on six tendons in each lower extremity. The authors found single-energy CT to have significantly better signal-to-noise and contrast-to-noise ratios compared with DECT (p < 0.0001), although the interobserver ranking was poor [8]. In addition, the DECT protocol in their study had a significantly higher dose (p < 0.05) [8]. In the same year, Sun et al. [11] retrospectively studied 24 knees in 12 patients who underwent DECT for imaging. Although they were able to clearly visualize the anterior and posterior cruciate ligaments, fibular collateral ligament, and patellar ligament, they were unable to satisfactorily visualize the tibial collateral ligament [11]. An additional issue was the inability to display more attenuated structures, such as the transverse ligaments and the lateral and medial patellar retinaculum, satisfactorily [11].

In 2009, Deng et al. [10] used DECT to scan the hands and feet of 20 patients at 140 and 80 kVp. The authors were able to successfully visualize most of the flexor and extensor tendons of the hands and feet without increasing radiation dose compared with their single-source high-resolution CT protocol (6.56 vs 10.98 mGy) [10]. Eight patients were found to have abnormal tendons that were clearly visualized with DECT pathologic abnormalities seen included circuitry, thickening, and adherence [10]. Adjusting the window width and level to best highlight the ligament or tendon of interest is critical, and fine adjustments are often needed when different ligaments or tendons are examined. Figure 8 shows a patient with a plantar plate injury visualized on three-material decomposition color-coded images utilizing the tendon dual-energy application. Figures 9A and 9B shows a patient with tenosynovitis of the peroneus tendons with inflammation from uric acid deposition.

Although spectral imaging has also been used to evaluate tendons and ligaments it does have several limitations. Some potential limitations of collagen decomposition include resolution, the reproducibility of window width and level adjustments, and the increased time it takes to load the study onto a workstation. Window width and level adjustments can accentuate noise, which can mimic disease. It can also be extremely time consuming to load a study into a workstation, perform the collagen decomposition, and then manually adjust the window width and level while scrolling through ligaments or tendons in multiple planes to assess for abnormality.

On the basis of the studies discussed here, DECT can potentially improve the soft-tissue characterization of the supporting structures of joints. With further advancement of technology, new prospective studies are needed to properly place the role of DECT in the evaluation of ligaments or menisci and tendons.

DECT has several applications for musculoskeletal imaging. One of the most important applications is the ability to detect uric acid with high specificity. This unique ability of DECT provides a highly reliable noninvasive means of diagnosing gout. Other DECT applications in gout can include monitoring the therapeutic response to urate-lowering treatment, because the determination of tophus size and its dissolution is one of the most important validated outcome measures in the evaluation of gout.

Additional musculoskeletal applications of DECT include the abilities to show traumatic bone marrow edema, minimize artifacts from imaging of metal prosthesis, and visualize normal and pathologic tendons and ligaments. Some future applications include the potential use of DECT to reveal calcium deposition for calcium pyrophosphate dihydrate and calcium hydroxyapatite disease, rotator cuff tears, iron deposition for diagnosing pigmented villonodular synovitis and metallosis, and DECT arthrography for the rapid detection of meniscal and labral tears.

Publication of this supplement to the American Journal of Roentgenology is made possible by an unrestricted grant from Siemens Healthcare.


The Achilles tendon (AT) is the largest and strongest tendon in the human body. The great tensile loads, occurring predominantly during its elongation or contraction, make it vulnerable to overuse injuries. Although the prevalence and incidence of midsubstance Achilles tendinopathy (i.e., in the middle third of the tendon) are high in athletes, cases are also frequently reported in sedentary individuals [1–6]. The aetiology and pathogenesis of AT tendinopathy have been the subject of much research, but with inconsistent findings [2, 3, 7]. Hence, treating people suffering from this pathology remains challenging for rehabilitation professionals and the success rate of conservative treatments is variable [8–10].

Ultrasound imaging allows in vivo visualization of the biological integrity of the tendon. It is a safe, rapid, non-invasive, relatively inexpensive and popular method used in the assessment of AT tendinopathy [11, 12]. When looking at ultrasound images (UIs) of healthy ATs, well-organized and parallel alignment of the collagen fibres (i.e., fibrillary striation) are highlighted by alternating parallel bright bands (hyperechoic) of collagen and dark bands (hypoechoic) of extracellular matrix [12, 13]. The paratenon of a healthy AT appears as an uninterrupted, well-defined bright line surrounding the tendon [12, 13] (Fig. 1). Conversely, in people with midsubstance AT tendinopathy, the fibrillar striation pattern is often altered as a result of a disorganization of the collagen fibres and a thickened and hypoechoic portion of the AT reflects an increase in the quantity of extracellular matrix and tenocytes [8, 14, 15]. This will typically translate to focal thickening along the AT, presence of dark (hypoechoic) intratendinous regions and sometimes irregular contours of the tendon on UIs [13] (Fig. 1).

a ROI of a healthy AT in longitudinal view b ROI of a healthy AT in transverse view c grayscale histogram derived from the ROI of image (b) d Pathologic AT in longitudinal view e Pathologic AT in transverse view with arrows indicating the AT’s thickness at different locations in the sagittal plane f grayscale histogram derived from the ROI of image (e)

Interpretation of an UI of the AT is generally semi-objective. The general appearance of the image is annotated based on the different contrasts observed (e.g., heterogeneous, homogenous, focal or diffuse abnormalities) and the maximum thickness of the AT is often measured using a two-point digital caliper function on the US machine. This interpretation is largely influenced by the evaluator’s experience with the recording technique and ability to interpret an UI [16, 17]. Recent technological advances have helped to promote the development of new quantitative ultrasound (QUS) outcome measures extracted from an UI, specifically from a particular region of interest (ROI). Digital UIs can now be broken down into a multitude of micro pixels, and numerical values (e.g., average thickness, tendon width and area) can be measured. The echogenicity of a ROI within an image can also be quantified by allocating a numerical grayscale value to each of those micro pixels [18, 19].

The usefulness of new UI analysis techniques has been demonstrated in various studies on animals and humans [20]. For example, these techniques have helped to quantify changes in the composition of an exercised muscle compared to an unexercised muscle in an elderly population [21–23]. These techniques have also revealed differences in the histological composition of the supraspinatus muscle and the quadriceps muscle in adults [24] and have been successfully used to detect structural changes in four key muscles in youths with neuromuscular disorders [25]. Moreover, new UI analysis techniques have enabled the differentiation of persons with Achilles tendinopathy from healthy individuals [26, 27] and have been effective in detecting focal and diffuse abnormalities in the AT [28].

Very few studies have been conducted to evaluate the reliability of QUS measurements of the AT. This is worrisome considering that the reliability of the QUS measurement of AT thickness, a key diagnostic criterion for Achilles tendinopathy, is rarely reported. To our knowledge, studies that have investigated test-retest reliability of QUS measurements of the AT have shown a moderate to good level of reliability [29–33]. In addition, it was shown that ultrasound image recording is greatly influenced by the evaluator, even among highly experienced ultrasonographers (weak inter-evaluator reliability [34]). Various factors such as the pressure applied on the probe and its alignment can influence recorded image properties and thus alter the quantitative values extracted [35, 36]. Information about the reliability and minimal detectable change is essential in order to develop evidence-based measurement taking protocols, empowering clinicians and researchers to quantify the tendinous changes observed in Achilles tendinopathy and incorporate these findings into clinical practice.

The primary objective of this study was to evaluate the reliability and minimal detectable change (MDC) of AT QUS measurements in people with symptoms consistent with midsubstance Achilles tendinopathy affecting at least one lower limb, as well as in completely asymptomatic individuals. The secondary objective was to recommend the best QUS measurement collection protocol possible, which could be subsequently used to characterize AT integrity in clinical practice or in research projects. It is anticipated that all QUS measurements, when collected by the same evaluator, will be reliable (Φ ≥0.75) and accurate (MDCNORMALIZED ≤ 15 %) and that a QUS measurement taking protocol in which a single evaluator averages the results of at least three images obtained during a single visit will be recommended in clinical practice.

Stefan Bruckner

Stefan Bruckner is a full professor of visualization at the Department of Informatics of the University of Bergen, Norway. He received his master's degree (2004) and Ph.D. (2008), both in Computer Science, from the TU Wien, Austria, and was awarded the habilitation (venia docendi) in Practical Computer Science in 2012. Before his appointment in Bergen in 2013, he was an assistant professor at the Institute of Computer Graphics and Algorithms of the TU Wien.

His research interests include all aspects of data visualization, with a particular focus on interactive techniques for the exploration and analysis of complex heterogeneous data spaces. He has made significant contributions to areas such as illustrative visualization, volume rendering, smart visual interfaces, biomedical data visualization, and visual parameter space exploration. In addition to his contributions in basic research, he has successfully led industry collaborations with major companies such as GE Healthcare and Agfa HealthCare, and has 7 granted patents.

He is a recipient of the Eurographics Young Researcher Award, the Karl-Heinz-Höhne Award for Medical Visualization, and his research has received 9 best paper awards and honorable mentions at international events. He was program co-chair of EuroVis, PacificVis, the Eurographics Workshop on Visual Computing for Biology and Medicine, the Eurographics Medical Prize, and serves on the editorial board of Computers & Graphics. He currently serves on the Eurographics Executive Committee and is a member of ACM SIGGRAPH, Eurographics, and the IEEE Computer Society.