How to Maximize Image Quality for theSonographic Evaluation of the Hind ProximalSuspensory Ligament

Johanna M. Reimer, VMD, Diplomate ACVIM

Author’s address: Rood and Riddle Equine Hospital, PO Box 12070, Lexington, Kentucky 40580-2070; e-mail: jreimer@roodandriddle.com. © 2010 AAEP.

1. Introduction

Lameness attributable to pathology of the proximalhind suspensory ligament is not uncommon and mayaffect horses of any performance discipline. Suchlameness varies in severity, may be bilateral, andcan be difficult to confirm with diagnostic analgesia.Radiography, nuclear scintigraphy, and ultrasonog-raphy have been used to investigate disorders of thehind suspensory ligament origin with variable re-sults.1–4 Sonographic evaluation of the hind sus-pensory ligament can be daunting and frustratingbecause of the location of the overlying superficialand deep digital flexor tendons, the artifacts createdby adjacent blood vessels, and the acoustic proper-ties of the muscle and fat within the ligament.Magnetic resonance imaging (MRI) is becoming in-creasingly available for equine imaging and hasbeen shown to be of great value in elucidating thenature of proximal metatarsal injuries in the horse.5

In a previous study, images obtained with MRwere found to be superior to those obtained withultrasound compared with histologic sections of nor-mal hind suspensory ligaments.6 A recent study ofhorses with lameness originating from the plantartarsal region showed far better detection of abnor-

malities with MRI over ultrasonography in horseswith proximal plantar metatarsal pain, includingthe ability to detect lesions of the suspensory origin.The conclusion of that study was that sonographywas of “limited value for detection of proximal sus-pensory desmopathy.”7

However, some of the sonographic images pub-lished in these reports could be considered subopti-mal because of either transducer positioning and/ortransducer-frequency selection. Transducer selec-tion and machine settings greatly impact imagequality and can potentially result in reduced detec-tion of abnormalities. Higher frequency transduc-ers have become more widely used, and althoughideal for imaging superficial structures such as thesuperficial digital flexor tendon and collateral liga-ments, they may not be ideal for imaging deeperstructures such as the suspensory origin and body.Additionally, factory-installed preset imaging appli-cations are available on almost all units but oftenneed to be modified by the operator. Although per-sonal preference is a factor in selection of image-processing settings, knowledge of the effects of eachprocessing function is important to achieve optimalimage quality.

AAEP PROCEEDINGS � Vol. 56 � 2010 239

IMAGING

NOTES

MRI may not be available to, or affordable for, anumber of veterinarians or horse owners. Generalanesthesia for studies of the hind proximal suspen-sory ligament is required, and follow-up studies maynot be affordable or practical. The ability to maxi-mize the quality of sonographic images of the prox-imal hind suspensory would be an asset to both tothe veterinarian and the horse owner. Ultrasonog-raphy may also provide a means to evaluate osseusabnormalities of the suspensory ligament attach-ment to the cortices of the third metatarsal andfourth tarsal bones, because it is superior to MRI forimaging bone margins.8 In the author’s experi-ence, horses with proximal suspensory desmitis as-sociated with an irregular, echolucent, pitted bonesurface at the suspensory origin have a poor prog-nosis for resolution of lameness.

The purposes of this paper are to review appropri-ate transducer positioning, show the impact of ma-chine settings on the appearance of lesions, andillustrate osseus abnormalities involving the originof the hind suspensory ligament that may be de-tected with ultrasound. This may result in im-proved diagnostic utility of ultrasonography for theevaluation of hind suspensory ligament disease.The viewpoints on image quality shared in this pa-per may also be applied to other musculoskeletal,abdominal, fetal, and cardiothoracic ultrasoundstudies.

2. Procedure

Sedation with both detomidine and butorphanol isrecommended for most horses, both for safety of thesonographer and to limit any patient motion thatcould prolong the study time. Clipping of the hairboth facilitates the procedure and improves image

quality in general. This is particularly true in thehindlimb because of the presence of cow licks, whichcan trap air and impede sliding and rocking of thehead of the transducer over the area being imaged.Hair and dead skin are removed with an alcohol-soaked sponge, and ample coupling gel is appliedboth to the limb and the transducer. Images areobtained in short and long axis, with attention paidto the appearance not only of the ligament but ofthat of the associated bony attachments (Fig. 1).Care should be made not to interpret acoustic arti-facts from overlying vasculature as lesions and toavoid mistaking any dorsally located vasculature asa lesion by following the course of the blood vessel orDoppler imaging, if necessary. If any abnormali-ties are detected, the contralateral limb should beexamined to aid in the interpretation of the signifi-cance of the findings.

Transducer Selection

In the past, transducers with a frequency of 7 MHzor higher have been advocated for evaluation of thesuspensory ligament. Increased availability of 12-to 13-MHz transducers over the past few years hasresulted in the widespread use of these higher fre-quencies for musculoskeletal imaging in the equine,including that of the suspensory ligament. Thehigher resolution provides superior image quality ofsuperficial structures such as the superficial digitalflexor tendon; however, deeper structures such asthe suspensory ligament may not be adequatelyimaged. Transducers now often have variable fre-quency options, enabling the use of lower frequen-cies with a 12- or 13-MHz transducer. However,use of a transducer with the appropriate center oroptimal frequency for the structure being evaluated

Fig. 1. (A) Proximal suspensory injury in the hindlimb associated with an irregular surface to the third metatarsal bone (suspectedavulsion; arrows) in a 2-yr-old Thoroughbred in training. Notice the thickened suspensory ligament and its hypoechoic dorsal halfoverlying the bone lesion. Lateral and proximal are to the right of each image. (B) Echolucent pitted bone surface (arrows) in anIrish Sport Horse with severe chronic suspensory desmopathy that failed to respond to surgical and intralesional therapies.

240 2010 � Vol. 56 � AAEP PROCEEDINGS

IMAGING

will result in a better image than one obtained bymanipulating the frequency of a transducer that isnot optimal for the region being studied.9 Variable-frequency transducer settings are also not exact andare not comparable between manufacturers or evenwithin the same manufacturer. For example, a 12-MHz transducer set to a frequency of 10 MHz oreven 8 MHz may not provide as adequate of animage of the suspensory ligament as an 8-MHztransducer set to a frequency of 10 MHz or even 13MHz. Also of interest, the calculated cross-sec-tional area of the same suspensory ligament maydiffer greatly depending on the transducer used(Fig. 2). Every brand of machine is different, andtrial and error is necessary to determine the idealtransducer and best frequency options to use forimaging the suspensory ligament.

Transducer PositionThe transducer should be positioned plantaromedi-ally just distal to the chestnut and over the deepdigital flexor tendon rather than the superficialflexor tendon.10 The transducer is held overhand,cupped in the palm of the hand, with the thumb andfingers of the scanning hand in contact with the limbto facilitate transducer control during the study(Fig. 3). A portion of the suspensory ligament alsooriginates from the fourth tarsal bone; therefore,images should be obtained proximal to the thirdmetatarsal bone, because injuries can occur in thislocation (Fig. 4). Off-angle imaging or imaging theligament in a non–weight-bearing position may also

Fig. 2. (A and B) Images of the same abnormal suspensory ligament obtained with two different transducers using the same unit andimage-processing options. (A) The image on the left was obtained with an 8-MHz transducer (set on 13 MHz), and the image on theright was obtained with a 12-MHz transducer (set on 10 MHz). Notice that the margins of the ligament, lesion, and bone are moreclearly defined in the image on the left using the dedicated lower-frequency transducer, even at a higher-frequency setting. Also,notice the disparity in the calculation of the cross-sectional area of the ligament. The lower-frequency transducer yields a largercross-sectional area. (B) Sonographic images of a localized hypoechoic lesion (arrows) in the dorsolateral corner of the suspensoryorigin in the same horse using different transducers (lateral is to the right of each image). The image on the left was obtained witha transducer with a center frequency of 8 MHz, whereas the image on the right was obtained with a transducer with a center frequencyof 12 MHz. The frequency of each transducer was then adjusted to frequencies of 13 and 10 MHz, respectively, to result in a balanceof optimum resolution and penetration for each image. Notice that the lesion and its margins are more clearly delineated in the imageon the left, which was obtained with the lower-frequency 8-MHz transducer at a frequency option of 13 MHz.

Fig. 3. Transducer position and handling for imaging the prox-imal hind suspensory ligament (left limb shown). Notice thelocation of the transducer head just below the chestnut and thecontact of the thumb and fingers with the limb.

AAEP PROCEEDINGS � Vol. 56 � 2010 241

IMAGING

help delineate muscular tissue from ligament fibersand can be incorporated into the examination.8

Dynamic RangeDynamic range basically correlates to image con-trast (Fig. 5). A low dynamic-range setting resultsin more contrast (black and white), and a high dy-namic-range setting results in less contrast (moregray and less black and white). Too high of dy-namic range can reduce the contrast between lesionsand normal structures, making detection more dif-

ficult. If dynamic range is set too low (more blackand white), more subtle yet important lesions will bemissed. The author will adjust the dynamic rangeto a setting at which bone is nearly white and fluidor blood is nearly black, while trying to maximizethe shades of gray.

Edge EnhancementEdge enhancement accentuates the interface be-tween echoes or tissue layers. The author prefersno edge enhancement, because it may reduce theability to detect hypoechoic lesions by effectivelyadding echoes to the area of interest (Fig. 6).

Focal ZonesThe greater the number of focal zones, in general,the slower the frame rate. Low frame rates resultin poor temporal resolution, simply resulting in ablurred still image.

Maximum lateral resolution (ability to distinguishpoints that are side by side) is achieved within thefocal zone of the transducer. Although instrumen-tation allows for several focal zones to be placedthroughout the image, the best resolution at a givendepth is achieved by the use of one focal zone at thedepth of interest.9 This requires that the operatoractively adjusts the focal zone during the ultrasoundstudy of structures at different depths, such as thesuperficial digital flexor tendon and suspensoryorigin.

Power, Gain, and RejectThe power should be set as low as possible to obtainthe best resolution and limit artifacts. The gainand time-gain compensation controls can be in-creased to amplify returning echoes to enable a

Fig. 4. (A) Sonogram of an injury of the left hind suspensory ligament (LH, left image) originating at the attachment to the fourthtarsal bone (compare with the normal right hind, RH, on the right). The suspensory ligaments are delineated by arrows. The lesionin this horse extended through the lateral portion of the upper suspensory body. Lateral is to the right. SDF, superficial digitalflexor tendon; DF, deep digital flexor tendon. (B) Same ligament as depicted in A obtained with the transducer off angle to illustratethe irregular contour (arrow) of the fourth tarsal bone. Lateral is to the right. SDF, superficial digital flexor; DF, deep digital flexortendon; SL, suspensory ligament origin at the fourth tarsal bone; PL, plantar ligament. (C) Long-axis view of the fourth tarsal boneof the same horse as depicted in A and B. Notice the irregular surface to the bone (arrow) at the suspensory attachment. Proximalis to the right.

Fig. 5. Images of the same abnormal hind suspensory ligamentobtained with the same transducer. All settings are the sameexcept that the right image was obtained with a higher dynamicrange. Notice that the reduction in contrast with the higherdynamic-range setting provides less contrast, thereby reducingthe relative echogenicity of the bone and increasing the relativeechogenicity of the lesion. The lesion margins and bone contourare more readily apparent on the image to the left using the lowerdynamic-range setting.

242 2010 � Vol. 56 � AAEP PROCEEDINGS

IMAGING

lower power setting. The reject control eliminatesweaker echoes from all depths that may not contrib-ute significantly to the image; however, a high rejectsetting may also result in loss of detail. Mostsonographers prefer to use little or no reject.9,11

Frame AveragingFrame averaging blends information from one frameinto the next, resulting in a smoother appearance inreal time. However, frame averaging results inloss of detail within each frame as well as creates ablurred still image. The author prefers not to useany frame averaging.

3. Discussion

Undoubtedly, MRI has increased the understandingof the ultrasound appearance of the normal andabnormal hind suspensory ligament. Its use inclinical practice has also enabled the discovery oflesions causing lameness unrelated to the suspen-sory origin.5,7 However, MRI is restricted by itsavailability, cost, and requirement for general anes-thesia for most studies. These factors also impactthe feasibility of follow-up examinations.

Also, the incidence of abnormalities of the hindsuspensory origin as detected by MRI in successfullyperforming individuals and the appearance of qui-escent healed or asymptomatic injuries is not yetknown to the author’s knowledge. Experience hasshown that an abnormal structure discovered dur-

ing an ultrasound study may not be clinically impor-tant. The same conclusion will likely follow withincreased application of MRI to the investigation ofmusculoskeletal disease in the horse: that an ab-normal ligament (thickened or abnormal signal)does not necessarily mean that it is a source oflameness in a particular individual. The ability todetect fluid within a ligament or bone with MRI maybe more clinically important. Sonographic detec-tion of anechoic fluid within the hypoechoic muscleof the suspensory ligament is obviously more diffi-cult. However, abnormalities of the surface of thebone at the suspensory origin can be appreciatedfairly readily with ultrasonography. With atten-tion to proper technique and image quality and inlight of its affordability and practicality for follow-upstudies, ultrasonography may not be so bad for im-aging the hind suspensory origin after all.

References1. Dyson SJ, Weekes JS, Murray RC. Scintigraphic evaluation

of the proximal metacarpal and metatarsal regions of horseswith proximal suspensory desmitis. Vet Radiol Ultrasound2007;48:78–85.

2. Dyson S. Proximal suspensory desmitis: clinical ultra-sonographic and radiologic features. Equine Vet J 1991;23:25–31.

3. Dyson S. Proximal suspensory desmitis in the hindlimb:42 cases. Br Vet J 1994;150:279–291.

4. Gibson KT, Steel CM. Conditions of the suspensory liga-ment causing lameness in horses. Equine Vet Edu 2002;14:39–50.

5. Brokken MT, Schneider RK, Sampson SN, et al. Magneticresonance imaging features of proximal metacarpal andmetatarsal injuries in the horse. Vet Radiol Ultrasound2007;48:507–517.

6. Bischofberger AS, Konar M, Ohlerth S, et al. Magnetic res-onance imaging, ultrasonography and histology of the sus-pensory ligament origin: a comparative study of normalanatomy of Warmblood horses. Equine Vet J 2006;38:508–516.

7. Labens R, Schramme MC, Robertson ID, et al. Clinical,magnetic resonance, and sonographic imaging findings inhorses with proximal plantar metatarsal pain. Vet RadiolUltrasound 2010;51:11–18.

8. Werpy NM, Charles B, Rantanen N. Should I throw awaymy ultrasound machine now that MRI is here?, in Proceed-ings. 54th Annual American Association of Equine Practi-tioners Convention 2008;439–446.

9. Reef VB. Physics and instrumentation. In: Reef VB, ed.Equine diagnostic ultrasound. Philadelphia, PA: Saun-ders, 1998;1–23.

10. Denoix JM, Farres D. Ultrasonographic imaging of theproximal third interosseous muscle in the pelvic limb usinga plantaromedial approach. J Equine Vet Sci 1995;15:346–350.

11. Nyland TJ, Mattoon JS, Wisner ER. Physical principles,instrumentation, and safety of diagnostic ultrasound. In:Nyland TJ, Mattoon JS, eds. Veterinary diagnostic ultra-sound. Philadelphia, PA: Saunders, 1995;3–18.

Fig. 6. Effect of edge enhancement on the appearance of a lesionin the hind suspensory ligament using the same transducer onthe same leg. All settings are the same except that the image onthe right was obtained with edge enhancement. Notice the re-duction in contrast between the ligament and abnormal areaswith edge enhancement. Interestingly, cortical irregularities tothe third metatarsal bone are also more prominent without edgeenhancement.

AAEP PROCEEDINGS � Vol. 56 � 2010 243

IMAGING