Ultrasonographic Evaluation of the Equine Limb: Technique

Chapter 16 Ultrasonographic Evaluation of the Equine Limb


Technique



Norman W. Rantanen, Joan S. Jorgensen, Ronald L. Genovese


Diagnostic ultrasonography was introduced in the early 1980s as a practical imaging modality to evaluate soft tissue injuries of the equine limb.1 It continues to be used extensively for evaluation of tendonous and ligamentous structures to identify, confirm, and monitor soft tissue injury (Box 16-1).2-8 Ultrasonography is now the imaging modality of choice for most soft tissue evaluation, but in the digit magnetic resonance imaging (MRI), if available, is most useful to evaluate soft tissues. However, it is user specific, and diagnostic information relies heavily on adequate equipment, limb preparation, and the scanning skills of the ultrasonographer. This chapter describes terminology and techniques and provides advice about interpretation based on our collective experiences. We describe our systematic approach, which includes both qualitative and quantitative ultrasonographic analysis of distal limb injuries. The resulting data can be used to categorize the severity of the injury and compare with subsequent examinations. The technology is still developing, and changes will evolve to improve the clinician’s ability to identify and substantiate clinical findings, leading to improved management of soft tissue injury in the horse.



BOX 16-1 Indications for Ultrasonographic Evaluation of Limbs



• Diagnosis of soft tissue injuries, including muscular, vascular, tendon, tendon sheath, ligament, joint capsule, or bursal defects

• Assessment of fluid accumulation

• Evaluation of bony surfaces

• Monitoring of the healing progress

• Monitoring of the effect of training on soft tissue structures, especially tendons and ligaments


Equipment


Ultrasound machines used for musculoskeletal imaging require a range of transducer frequencies to examine superficial and deep structures. Soft tissues within 5 to 7 cm of the skin surface should be examined using a transducer with a frequency of at least 7.5 MHz or higher. Tissues within 7 to 15 cm of the skin surface are best evaluated with a 5.0-MHz transducer, whereas deeper tissues require a transducer in the 2.5- to 3.5-MHz range.


Sound is attenuated at 1 dB/cm depth per megahertz. Higher frequencies are attenuated at higher rates, thus limiting penetration. Conversely, lower frequencies, attenuated at lower rates, have greater penetration. The highest frequency possible should be used to obtain maximum resolution.


Claiming that one instrument is the only one to perform musculoskeletal examination is naive at best and totally inaccurate. All state-of-the-art machines have similar focusing capabilities and resolving power. Image appearances made by different machines at the same frequencies have subtle differences; however, resolution is about the same. Images may appear more crisp from transducers with a greater number of elements. Operator proficiency (or lack of it) largely determines image quality.


Large muscle masses or bone surfaces, such as the pelvis, are more effectively scanned with phased array, convex array, annular array, or sector transducers. The small skin surface imprints and divergent beam are advantageous when tissues are imaged at greater depths. However, annular array and sector transducers have focal zones that usually favor the central portion of the beam and have beam divergence, which causes far-field, lateral beam width artifact. Convex and phased array transducers have electronic focusing, and the examiner can control focal zone number and placement. It is common for these transducers to penetrate up to 30 cm.


Linear array transducers are the most popular for tendon and ligament scanning because they facilitate anatomical recognition of structures and evaluation of longitudinal tendon and ligament fiber alignment parallel to the skin surface. Convex array and sector transducers, if used properly, work equally well. However, they are more difficult to use because it is easier to change the beam direction because of the smaller contact area. It is necessary to keep the beam at 90 degrees to the tendon or ligament fibers. Anatomical features may be more difficult to recognize. Standoff pads are essential with any transducer to image the most superficial structures.


Sector and microconvex array transducers are preferable for use on contoured skin surfaces. For instance, scanning between the heel bulbs is difficult with a flat-face, linear array transducer but simple with convex or sector transducers. It is easier to steer a divergent beam and look around from a smaller contact area compared with the flat-face, linear array transducer’s broad contact area and rectangular sound beam.


Multiple frequency scanheads are available from most manufacturers; these scanheads allow operators to change frequencies relative to tissue depths without changing transducers. Newer broadband scanhead technology ensures quality, and there are no disadvantages in using these scanheads for musculoskeletal scanning, provided that basic ultrasound principles are practiced.



Horse Preparation


The horse should be adequately restrained, because an anxious, fidgety horse may cause a hurried or inadequate examination, resulting in poor-quality images and interpretive errors. One author (RLG) prefers to sedate most horses that are not entered in drug-tested competition to ensure that the horse stands quietly during the examination. The horse should be held by a person who has responsibility for the safety of the ultrasonographer and the equipment. A calm, well-positioned horse allows a thorough, tension-free evaluation.


Hair, air, scurf, scabs, and dirt induce artifacts and reduce image quality. The hair should be clipped with a No. 40 or No. 50 blade then cleaned with an antiseptic scrub, followed by a generous rinse with water, taking care to follow the growth pattern of the hair. In some horses an additional close shave with a disposable razor may improve image quality. Acoustic coupling gel is applied to improve transducer contact. Excessive application of gel causes a lateral image artifact that may impair assessment of the structures being examined, especially if peritendonous or periligamentous neural, vascular, and connective tissue structures are being evaluated. Removal of the excess gel from the transducer head or the limb corrects the artifact.


If the hair coat is fine and clipping or shaving is not possible or desired, then the limb can be washed. The examiner should spend some time wetting and soaking the hair to improve transducer contact. Topical application of alcohol while stroking the hair along its growth path may improve transducer contact,5 but alcohol can be harmful to some transducers. The operator should contact the manufacturer of the equipment to ensure that it is safe to use the transducer with alcohol.



Medical Records


Ultrasonographic images should be recorded using thermal prints, video recording, or digital recording. Thermal images were used most commonly in the past, but digital recording is rapidly superseding them.



Thermal Print Storage Envelope


Practical hints and suggestions for medical record documentation that can simplify case records, enhance clinical information for serial studies, and improve documentation are provided in the following text. We recommend a simple form stamped on the outside of a storage envelope. The information on the envelopes can be quickly filled in using symbols or checkmarks and should include the following data: horse identification, date, owner, trainer, limb or limbs examined, and structure or structures evaluated (superficial digital flexor tendon [SDFT], deep digital flexor tendon [DDFT], accessory ligament of the DDFT [ALDDFT], main body of the suspensory ligament [SL], SL branch, pastern, hock, antebrachium, crus, and other). It is also useful to record exercise level, the reason the examination was performed, and a brief summary of clinical findings. This checklist indicates which structures have been targeted and assists in interpretation of an actual lesion versus an off-incidence angle artifact. This is especially helpful if the images are reassessed at a later time.


We have developed an alphanumerical system for ranking exercise levels (Box 16-2). The current exercise level may have an important impact on diagnosis, treatment, future exercise control, and prognosis. For instance, if a horse at pasture injured a SDFT, advising pasture rest would be contraindicated. A chart for ranking exercise levels can be duplicated easily, laminated, and taped to equipment for easy reference.



BOX 16-2 Exercise Level Grading Scale










































































Exercise Level Type of Exercise Allowed
0 Complete stall rest.
1A Handwalk for 15 to 30 minutes once a day.
1B Handwalk ≥30 minutes a day or walk on a mechanical walker. We believe horses usually are more active walking on a mechanical walker than in hand.
2A Thoroughbred (TB) and Standardbred (STB) racehorse, event horse (EV), and sports horse (SH): exercise level 1A/B plus trotting in hand for 5 to 10 minutes a day.
2B TB racehorse, SH, and EV: trot under saddle 10 to 15 minutes once a day or swim.
STB racehorses: walk only in the bike or swim.
This level also includes 10 to 15 minutes of trotting on a treadmill.
3A Small paddock turnout for all horses. Small paddock implies small enough not to be able to work up to a sustained canter or gallop.
3B Large paddock turnout for all horses.
4A TB and SH: 15 to 20 minutes a day of walk, trot, and canter under saddle, 3 days per week, plus any of the above levels. This level does not apply to most STBs.
4B TB: 20 to 30 minutes a day of walk, trot, and canter under saddle, 3 to 4 days a week.
TB racehorse: “ponying” (being led from another horse) on the racetrack. This level does not apply to most STBs.
5 TB racehorse and EV: all of the above plus normal galloping.
STB racehorse: all of the above plus jogging.
SH: all of the above plus normal arena flat work with limited low fence jumping where applicable.
Dressage horses: normal work minus lateral movements and special gaits.
6 TB racehorse: all of the above plus faster gallops.
EV: all of the above plus jumping.
STB racehorse: all of the above plus training miles ≥2 : 10.
SH: all of the above plus unlimited low fence jumping where applicable.
Dressage: all of the above plus lateral movements and special gaits.
Contest horses (reiners, cutters, and so on): all of the above plus practicing specific turns and movements.
7 In essence, this is the maximal work level for any type of athletic horse.
TB and EV: racing, fast works, and competing.
STB racehorse: training miles <2 : 10 and racing.
SH: showing, jumping, hunting, and so on.
Dressage and contest horses: competing.

Clinical findings may be recorded concisely as follows:











































Leg Which Limb
S Subcutaneous swelling on a scale of 0 to 5. This does not refer to tendonous or ligamentous enlargement.
L Lameness on a scale of 0 to 5.
T Thickening of a tendon or ligament on a scale of 0 to 5. This is independent of subcutaneous swelling.
Sen Response to digital palpation of a tendon or ligament on a scale of 0 to 5.
H Heat or skin temperature on a scale of 0 to 5.
TS Distention of the digital flexor tendon sheath (DFTS) (tenosynovitis) on a scale of 0 to 5.
AS Fetlock sinking (hyperextension of the metacarpophalangeal joint), either at rest or during movement, on a scale of 0 to 5.
Qualitative diagnosis Comments on preliminary qualitative ultrasonographic interpretation.
New Whether this is the first time this horse has been examined ultrasonographically for this complaint.
Re-Ck A recheck of a previous injury.
Normal leg This indicates which is the clinically normal limb, which may not be normal ultrasonographically. It is strongly recommended that both limbs be examined routinely.
Both abn Both limbs are clinically abnormal.


Image Labeling


All ultrasonographic images should be labeled with the date of examination, the horse’s name, the owner’s or trainer’s name, the limb being examined, and the location of the image. In addition, it is helpful to include the age, breed, and use of the horse and the current exercise level. Most machines automatically record the frequency of the transducer and the focal zone. If digital recording is used, a system of image backup is essential.



Exercise Levels


One of the basic concepts in the management of horses with tendon and ligament injuries is to relate increasing levels of exercise to increases in tendon and ligament loading. For instance, walking a Thoroughbred (TB) racehorse in hand results in far less SDFT loading (stretching) than racing at 35 mph. Understanding the current exercise level is important in interpretation of ultrasonographic information and advising management programs for controlled exercise during rehabilitation from an injury (see Box 16-2).


For example, if a TB racehorse with a SDFT injury of 4 months’ duration were reexamined and the SDFT showed little improvement, despite box or stall rest, the veterinarian would conclude that the repair was of poor quality with a poor prognosis for a return to racing. However, if the horse had been turned out for 6 hours a day with other horses that ran around, the delayed healing would be interpreted as being caused by too much tendon loading and the lack of improvement would be attributed to low-level ongoing injury. The client would be advised to reduce the exercise level and would be given a somewhat more positive outlook for future racing soundness.


In the rehabilitation of a tendon or ligament injury, the odd exercise level numbers result in the most salient changes in tendon or ligament loading. Exercise level 1 is limited to walking, resulting in little structure loading, whereas exercise level 3 is a major step up in exercise and structure loading because it allows free movement of the horse. Exercise level 5 signals the return to active training. Finally, exercise level 7 is the ultimate goal of rehabilitation and is the return to maximal athletic use. During rehabilitation the ultrasonographer assesses the current morphological status of a structure and advises changes in exercise levels consistent with those findings. Readily available and definable exercise levels make exercise decisions more efficient and consistent.



Scanning Technique


The basic objective of an ultrasonographic evaluation is to characterize the morphological characteristics of the soft tissue structures and bony surfaces of each designated anatomical area. Although physical examination findings may direct the clinician’s attention to a specific structure, a thorough examination of all soft tissue and bone surface structures is imperative. In many instances, lesions in more than one anatomical region may be identified and often result in a completely different recuperation regimen. To avoid off-incidence artifacts, the ultrasound beam must be perpendicular to the target structure. Although more than one structure may be perpendicular to the ultrasound beam simultaneously in either transverse or longitudinal images, often it is necessary to redirect the ultrasound beam to adequately examine all structures at each level. Initially the limb should be systematically examined from proximal to distal to assess the morphological features of the vasculature, periligamentous and peritendonous tissue, tendons, and ligaments. Once specific abnormal or normal structures are identified, each tendon or ligament is analyzed systematically by ultrasonographically targeting the transducer to the structure at each zone and simultaneously documenting the scan on tape, on print record, or by electronic storage.



Artifacts


Artifacts can cause major problems with any imaging modality but are especially prevalent with ultrasonography because the operator steers the ultrasound beam and sets the instrument parameters. Three primary artifact sources are common with diagnostic ultrasonography: operator error, ultrasound–tissue interaction, and inherent instrument design artifacts. Nothing can be done about the limitations of the imaging system except to recognize the artifacts in special scanning situations. Sound–tissue interaction creates myriad artifacts, some of which are useful and help with diagnosis. Others are annoying and can be difficult to overcome. Readers are referred to several publications that address the majority of artifacts peculiar to ultrasonography.9-14 Operator error and ultrasound–tissue interactions are discussed further.



Operator Errors



Inadequate Skin Preparation


Excessively long or dirty hair and unclean skin attenuate the ultrasound beam and produce image artifacts. Improper scanhead coupling to the skin caused by a lack of gel or the presence of scabs or scurf is common (Figure 16-1). Artifacts created by improper skin preparation result from poor contact. Images may be dark even though gain settings may be set at the highest limits. The tendons do not have a fine texture and are more grainy or mottled than normal. Tendon margins may not be seen, and reverberation artifacts caused by trapped air within the hair may appear within images, especially if standoff pads are used. Hypoechogenic streaks may be present in the images.


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Fig. 16-1 Transverse ultrasonographic images of the palmar metacarpal region. Improper skin preparation causes artifacts within the image on the right.



Ultrasound Beam Angle


Reflection of the ultrasound beam is dependent on the sound-interface and tissue-interface geometry. Ideally the ultrasound beam should strike tissue interfaces at 90 degrees to produce the best echo reflection back to the transducer crystals, which also act as receivers. If the beam strikes a tissue interface at a smaller angle, a portion of the ultrasound is reflected away from the primary beam direction, and the interface is not seen as well or at all. This is especially important in the evaluation of tendons and ligaments in the metacarpal and metatarsal regions because the fibers usually are parallel to the skin. If the ultrasound sound beam is not perpendicular to a tendon in a transverse image, information is lost, and hypoechogenic areas, which mimic lesions, are created (Figure 16-2). The problem is not seen in longitudinal images of the metacarpal or metatarsal region obtained using a linear array transducer because its surface is parallel to the fibers. However, with convex array and sector transducers that have divergent ultrasound beams, there are only small areas within longitudinal tendon fiber images in which valid information is found (Figure 16-3). In the divergent area of the beam, sound is reflected away from the beam path and is lost to the image. It is important not to confuse these areas with pathological conditions of the tendon or ligament. The ultrasound beam must be positioned parallel to the tendon fibers. Normal tendon or ligament fibers should be seen as continuous linear echogenic structures across the image (Figure 16-4). If the transducer is turned slightly, the fibers appear as short linear segments as the beam cuts obliquely across the longitudinal axis. Because fiber alignment is an important criterion to assess in diagnosis and rehabilitation of tendon and ligament injuries, care must be taken to not create this artifact.


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Fig. 16-2 Transverse ultrasonographic images of the distal palmar aspects of the metacarpal region. The left deep digital flexor tendon image has a normal tendon fiber pattern. The right image has an apparent central core artifact (arrows) that was produced by slightly changing the angle of the ultrasound beam.


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Fig. 16-3 Longitudinal ultrasonographic image of the proximal metacarpal region. The arrows point to tendon and ligament sections where the ultrasound beam is at 90 degrees, which allows the fibers to be seen. The remaining parts of the image provide no diagnostic information.


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Fig. 16-4 Longitudinal ultrasonographic images of the palmar metacarpal region. The left image was obtained with a linear array transducer parallel to the tendon fibers. The right image was obtained with the transducer slightly oblique to the tendon fibers, causing them to appear as short segments instead of continuous fiber strands (arrows).



Improper Gain Settings


Ultrasound machines have gain settings referred to as overall, near, and far gain. As ultrasound penetrates normal soft tissues, it is attenuated at the rate of 1 dB/cm of tissue thickness per megahertz. Obesity and dehydration cause greater attenuation and can limit penetration. Because energy is lost from the ultrasound beam as it passes into the tissues, the instrument gain must be increased to allow echo detection from the deeper tissue depths. Overall gain changes the brightness and darkness over the entire image. The near-gain adjustment affects the echoes closest to the transducer. Increasing the near gain brightens the echoes, and decreasing it darkens them. The far gain affects the deeper aspect of the image. Some machines display a time-gain compensation curve at the side of the screen. Adjustment of the gain settings should produce an image in which the gray scale is similar over the entire image. Bright whites in the near field should prompt decreasing the near gain; conversely, if echoes are difficult to see, increase in the gain is necessary (Figure 16-5). Improper near-gain settings are the most common error in settings. Visibility of far-field echoes is enhanced by far-gain increases.


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Fig. 16-5 A, Transverse sector scan ultrasonographic image of the palmar metacarpal region with the near gain set too high. The superficial digital flexor tendon (SDFT) echoes are too bright and cannot be differentiated. Acoustic shadowing (large arrow) is caused by flexor tendon and reverberation artifacts caused by the standoff pad (small arrows). B, The near gain was set too low, so the SDFT cannot be seen (arrows).



Improper Focal Zone Use


Before the advent of moveable and multiple focal zones, transducers had fixed focal points and well-defined focal zones. The ultrasound beam was narrowest in the focal zone in which the lateral resolution produced the best image quality. If the tissues of interest were outside the focal zone, image quality was not ideal. Use of transducers with appropriate focal zones became necessary. If a sector transducer is used to image the SDFT, a standoff pad is necessary to place the tendon in the focal zone. Variable focal zones have eliminated this problem and allow focal zone placement throughout the image depth. Modern linear array and convex array transducers have multiple and moveable focal zones. However, they need to be set in the proper locations to investigate the areas of suspected abnormality. If the focal zones are not placed properly, clinically significant tendon and ligament fiber damage can be overlooked (Figure 16-6).


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Fig. 16-6 Transverse ultrasonographic images of the proximal metacarpal region. A focal lesion in the suspensory ligament (arrows) is visible on the left image. The lesion disappears (circle) in the right image because the focal zones are improperly placed. Arrows in the left and right margins point out the focal zone levels.



Incorrect Frequency Transducer


Tissue depth dictates the optimal transducer frequency. Tissues within 5 to 7 cm of the skin surface should be examined with a 7.5-MHz or higher-frequency transducer. A 5.0-MHz transducer is required to examine tissues from 7 to 12 and up to 15 cm (depending on the instrument), and a 3.0-MHz or lower-frequency transducer is necessary for tissues 15 to 30 cm deep.


Imaging the digital flexor tendons or SL with a transducer of 5.0 MHz or less results in major compromise of image quality because of lateral beam width artifact. Two types of resolution are peculiar to ultrasound. Resolution along the beam axis, or axial resolution, is frequency dependent; the higher the frequency, the better the axial resolution. Resolution in the transverse plane, or lateral resolution, is dependent on the width of the ultrasound beam; the better the focusing or sound beam narrowing, the better the lateral resolution. Smaller crystals produce narrower sound beams, hence increased lateral resolution. A transducer with a minimum frequency of 7.5 MHz should be used for superficial tendons and ligaments for best axial and lateral resolution.



Recording Images


Thermal printers were the most popular method for recording ultrasonographic images, and some are still in use. Brightness may be set too high, causing echoes to have no differentiating characteristics (Figure 16-7), or set too low, causing the image to be too dark (Figure 16-8). The same problem occurs with contrast settings; too high a setting causes excessive contrast, and too low a setting causes the image to be washed out and too dark.


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Fig. 16-7 Transverse ultrasonographic images of the metacarpal region obtained without a standoff pad showing the deep digital flexor tendon (DDFT), accessory ligament of the DDFT (ALDDFT), and the suspensory ligament (SL). The brightness is set too high, causing tendon fiber detail loss.


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Fig. 16-8 Transverse ultrasonographic images of the metacarpal region obtained without a standoff pad showing the deep digital flexor tendon (DDFT), accessory ligament of the DDFT (ALDDFT), and the suspensory ligament (SL). The brightness is set too low, causing the image to be too dark.


It is important to consult the instruction manual to ensure that the image parameters are set properly. Proper settings also are necessary for the paper type. Interpreting improperly recorded (suboptimal) images causes substantial diagnostic error. Images must be assessed for photographic quality to preclude overlooking of lesions or overinterpretation. Suboptimal images should be repeated to preclude inaccurate interpretation and misdiagnosis. Images should always be frozen before capture.


Recording digital images on floppy disks was an effective way to capture all of the information. Depending on the system, images were recorded in bitmap (BMP) or Joint Photographic Experts Group (JPEG) formats. These were archived on hard drives and could be sent by e-mail if desired. External image capture devices functioned well, and some instruments with internal floppy drives are still in use. Digital archiving is now becoming routine. Various video formats, including video home system (VHS), 8-mm, S-Video, cineloop, and videodisk formats, are used to record motion. Recording static tissue (e.g., tendon) studies while moving the scanhead during recording can produce errors if the transducer is moved too rapidly for the frame rate to keep pace. Tendon examinations should be performed at relatively slow frame rates, as low as 7 per second, if multiple focal zones are activated. Moving the scanhead too fast causes ghosting of the image that blurs the anatomy. Activating multiple focal zones and the resultant slow frame rate can cause a similar problem with minimal scanhead movement.



Standoff Pads


Standoff pads are necessary when scanning structures near the skin, such as the SDFT and SL branches. Artifacts are produced each time they are used and can compromise image quality. It is important to recognize them. Transducers with built-in fluid standoff devices are no different. The artifacts are caused by reverberation within the standoff material and occur at a depth equaling the pad thickness (see Figure 16-5), which may obscure detail. For instance, examination of SL branches with a standoff pad can place artifacts in or near the axial border of the SL. This technique can obscure small axial border tears that are fairly common in horses. If the standoff pad thickness produces interfering artifacts, the SL branch should be examined with and without a standoff pad.



Ultrasound Tissue Interaction Artifacts (Horse-Produced Artifacts)


Ultrasonographic images are composed of returning echoes from many tissue interfaces, regardless of whether they are real or artifact. Some artifacts are useful in diagnosis and provide clues to tissue composition. Others are annoying and do not contribute to the diagnosis. Echoes are generated at tissue interfaces because of differences in acoustic impedance. Acoustic impedance is the product of tissue density (grams per cubic centimeter) and sound propagation velocity (meters per second). Substantial changes in either parameter produce acoustic barriers proportional to the magnitude difference, which causes sound reflection at the interface. Higher (brighter) amplitude echoes are created by greater acoustic impedance differences at tissue interfaces.



Skin Surface Contact


Debris and air trapped in the hair create large acoustic impedance differences and do not allow adequate sound penetration (see Figure 16-1).



Acoustic Enhancement


Whenever ultrasound passes through nonattenuating tissues, such as fluid with few or no interfaces, a slight increase in ultrasound intensity occurs in addition to less attenuation. The adjacent tissues attenuate the ultrasound normally. This causes brighter (enhanced) echoes deep to the less attenuating tissues. A good example is the enhancement seen deep to the metacarpal and metatarsal blood vessels. The SL has brighter echoes deep to the blood vessels, whereas between the blood vessels it is less echogenic (darker); it is tempting to interpret such results as abnormal (Figure 16-9). This is an inherent, tissue-produced artifact that can lead to interpretation errors but may help identify less attenuating areas within tissues such as muscle.


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Fig. 16-9 Longitudinal ultrasonographic image (left) and transverse ultrasonographic image (right) of the metacarpal region obtained without a standoff pad showing the deep digital flexor tendon (DDFT), accessory ligament of the DDFT (ALDDFT), and the suspensory ligament (SL). The tissue brightness (short arrows) increases deep to the blood vessels. An anechoic line (arrows) extends deep to the blood vessels (arrowheads).



Refractive Scattering


If the ultrasound beam is not perpendicular to a tissue interface, hypoechogenic artifacts are caused by refraction. These artifacts can be a major problem in assessing the origin of the SL because of the anastomotic veins between it and the ALDDFT (Figure 16-10). The curved blood vessel walls create hypoechogenic lines that extend deep to the vessel walls in transverse images (see Figure 16-9). This is called refractive scattering and should not be misinterpreted as a lesion.


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Fig. 16-10 Longitudinal ultrasonographic images of the proximal palmar metacarpal region. Proximal is to the left. Refractive scattering (arrows) caused by the veins between the origin of the suspensory ligament (SL) and the accessory ligament of the deep digital flexor tendon (ALDDFT). DDFT, Deep digital flexor tendon.



Acoustic Shadowing


A high acoustic impedance difference blocks the ultrasound beam and causes an anechogenic shadow deep to the reflecting interface. Bone and other mineralized tissue are much denser than soft tissues, and the sound propagation velocity is much faster. This creates an impenetrable acoustic barrier, and characteristically the reflector surface has bright echoes and the deeper tissues cannot be seen. These artifacts are useful because they identify soft tissue mineralization (see Figure 16-5). Bone surfaces are easily recognized because of shadowing. Dense scar tissue may create incomplete shadowing.



Reverberation


Reflection of ultrasound back and forth to the transducer from high acoustic impedance interfaces produces reverberation artifacts. The classic example is an air-filled lung surface that produces characteristic concentric reverberation echoes. Reverberation artifacts are not a major problem in tendon and ligament scanning; however, they can be important in certain circumstances. Gas production by anaerobic bacteria and air accidentally injected during diagnostic analgesia are two examples in which reverberation can be found in soft tissues. Standoff pads also create reverberation artifacts (see Figure 16-5).



Mirror Image Artifacts


Mirror image artifacts are not common problems in musculoskeletal ultrasonography and are more common in thoracic and abdominal scanning. They usually are found deep to interfaces that are highly reflective, such as air-filled lung.



Future Technology


Speckle reduction software, cross-beam scanning techniques, and the use of tissue harmonics are changing the appearance of ultrasound images. The images are smoother and lack the historically present “grainy” appearance that is common to ultrasound images. These innovations are gradually being accepted in medical imaging but are still relatively new. Some machines allow viewing the cross-beam imaging in a split-screen format with the original “grainy” images. This allows users to compare the two and to become familiar with the newer innovations. Off-axis or off-incidence scanning techniques that help to differentiate fluid from scar tissue within connective tissue structures have recently been introduced into veterinary imaging as well. With these newer technologies, artifacts peculiar to them will be defined as they find their place in medical and veterinary imaging.



Terminology and Quantitative Measurements


The basis of diagnosis is to determine the morphological variation from normal, which is not always easy. The goal is to determine the size, shape, echogenicity, fiber pattern, and surrounding inflammatory reaction of any structure. These findings should be considered carefully in conjunction with clinical impressions and the current athletic use of the horse. Box 16-3 lists parameters used for characterizing tendon and ligament lesions.15



BOX 16-3 Parameters Used for Characterizing Tendon and Ligament Lesions



• Region or location of the lesion

• Length of the lesion

• Alteration in echogenicity

• Pattern of altered echogenicity (i.e., homogeneous, heterogeneous, focal, and diffuse)

• Alteration of fiber pattern in longitudinal images

• Percent of cross-sectional area of tendon injury

• Changes in the character of the lesion over time


Echogenicity


Echogenicity refers to the whiteness or brightness of a structure. Each tendon and ligament has a characteristic echogenic pattern at specific anatomical sites. Lesions vary in echogenicity depending on morphological consistency at the time of the examination. A scoring system can be used to improve objectivity when assessing the severity of an injury or the response to therapy (Box 16-4). Such a system may improve case management and illustrate to the client the changes in echogenicity that correlate with repair of an injury.



BOX 16-4 Type Scores



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