COMPUTED TOMOGRAPHY AND MAGNETIC RESONANCE IMAGING OF THE NORMAL EQUINE CARPUS

BARBARA KASER-HOTZ. DR. MED. VET., SABINE SARTORETTI-SCHEFEK, DR. MED., ROGER WEISS, DR. MED. VET.

A normal equine carpus was used for computed tomography and magnetic resonance imaging. The structures outlined were identified and described. The two techniques were compared. This anatomic description could be helpful as a basis for clinical exams. Veterinary Radiology & Ultmsourirl, Vol. 35, No. 6 , 1994, p p 457461

Key words: equine, carpus, anatomy, CT, MRI.

HE EQUINE CARPUS is a complex structure with many T joints, ligaments and tendons. Radiography is of lim- ited value in soft tissue evaluation. Ultrasonography of the carpus allows visualization of tendons and collateral liga- ments. ' However, the ultrasonographic technique is limited by its small field of view. Each structure has to be imaged separately and a cross scction through the entire carpus is not possible. Computed tomography and magnetic reso- nance appear to be ideal methods for examining the carpal soft-tissue structures. Magnetic resonance anatomy of the equine digit and metacarpophalangeal joint has been de- scribed2, and a recent paper compares ultrasonographic findings of tendon lesions with MR images.3 In this paper, we describe the normal CT and MRI anatomy of the carpus and compare the two techniques.

Materials and Methods The left forelimb of a functionally normal 3-year-old

Thoroughbred horse euthanized for medical reasons was obtained. The limb was disarticulated at the shoulder and radiographed. The radiographs of the carpal region were normal. Because access to CT and MRI was not available the day of limb disarticulation, the limb was frozen: it was defrosted 12 hours before imaging. A centimeter ruler was placed on the lateral aspect of the limb to correlate images with anatomic cross sections of the limb. The limb was then first placed into the CT-scanner." Technical factors were

- .~

From the Section of Radiology, Vetennary School (Kaser-Hotz), the Departmcnt of Radioloyy. Mcdical School (Sartorctti-Schefer). and the Institute of Veterinary Anatomy (WC~SS) of the University of Zurich, Swit- zerland.

Address correspondence and reprint requests to Barbara Kaser-Hotz, Radiologie, Vcteriniir-Medizinische Klinik, Wintcrthurerstr. 260. CH- 8057 Zunch, Switzerland.

Received Scptember 2. 1993: accepted for publication November 19. 1993.

"Siemens. Somaton plus. Giittinzen. Gcrmany.

210 niA and 110 kVp. Continuous, 5-mm thick slices were obtained. A wide window was used to enhance bony struc- tures; for soft tissue structures a narrow window was used. The average CT numbers (x-ray attenuation numbers) of the interosseus (suspensory ligament), the accessory ligament of the deep digital flexor tendon and superficial and deep digital flexor tendons (SDF and DDF) were dctcrmined.

The images were printed using a matrix with 512 x 512 pixels.

The limb was then transferred to the MR scanner.? Transverse and sagittal slices were obtained in a 3D- acquisition with 1 .S-mm thick, continuous slices. Several gradient-echo sequences were used. Images obtained with a pulsc repetition time (TR) of 14 msec and echo delay time (TE) of 3.5 msec gave the best anatomic visualization of the desired structures and are presented in the accompanying illustrations. Image matrix size was 256 x 256 pixels. The limb was then refrozen and sectioned at I -cm intervals. The anatomic slices were compared with CT and MR images and various anatomic reference^.'-^

Results

When a soft-tissue window was used, bone appeared hy- pcrdense while tendons, ligaments and muscles were rep- resented by varying shades of grey, (Fig. 1) and synovial fluid and blood vessels were hypodense. Muscle tissue had the darkest grey shade; tendons and ligaments were of a lighter grey.

Extensor tendons, collateral ligaments, the long tendon of the ulnaris lateralis, and the flexor carpi radialis tendon all had a similar, bright grey shade (Fig. 2 ) . The short

+General Electnc. Sigma. Milwaukee. USA

457

4.58 KASER-HOTZ I T A L 1994

FIG. 1. Transverse C T iniagc at the level of the chcstnut (soft tissuc windo\?, \?indour level 35, window width 302): I -Eslensor carpi radiali\

dense). 2-Common digital extensor tendon. 3-Ulna. 4-Chestnut. 5-Median artcry and ncrvc.

F ~ ~ , 2, T~~~~~~~~~~ CT Image the level the trochlca l - ~ ~ . carpi radialis tendon, ~ ~ ~ o i l l n l o n digital extenhor tendon, 3 - ~ ~ ~ ~ ~ ~ l

dense) and 5ho,f hrarlch (mixed density), s - ~ l ~ ~ ~ ) ~ ,,lnaris muscle with tendinou\ portion (hyperdense) and muscular portion (more hypo- dense). &Superficial difital flcxor tendon. 7-Deep digital flexor tendon.

ITIUSCIt. IIloStI)’ tendinous (hyperdense). with sonic ITlu \CIC t i w e (hypo- collateral lisamcnt, 4.ulnaris lateralis il,uacle with long tendon (hyper.

8-Flexor carpi radialis tendon with tendon sheath portion of the ulnaris lateralis muscle had a mixed opacity above the accessory carpal bone. but becamc uniform just beforc it attachcd to the accessory carpal bone. The flexor carpi ulnaris was composed of a hyperdense palniar and a more hypodense dorsal portion. (Fig. 2) Thc decp digital flexor tendon (DDF) was almost as hyperdense as the ex- tensor tendons. but the superficial digital llexor tendon (SDF) was hypodense (Figs. 2 to 5 ) . The accessory liga- ment of the DDF was lighter grey than the superficial digital flexor tendon, and the interosseus (suspensory ligament) had a mixed hyper- and hypodensc appearance consistent with the histologic makeup of muscle and tendon fibers (Fig. 3). The average CT-number of the SDF was 115. the DDF 132. the accessory ligament of the DDF 81, and for the interosseus it varied between 76 and 109, depending upon where the cursor was placed.

The tendons of the extensor carpi radialis, extensor carpi obliquus, and common digital extensor were well delineated (Fig. 4). The comnion digital extensor tendon was seen to split into the large. main portion and a small lateral portion, the radial head. At the medial aspect of the common digital extensor tendon a small separate portion appeared at the level of the distal carpal row and continued distally without diverging from the main tendon. The lateral digital tendon was not clearly identified (Fig. 5 ) . This is due to its small

10-Medial collateral ligament. 1 I-Extensor carpi tissue

9-Flexor rctinaculuni. obliquus tendon. (soft

- size and close rc]ationship to the lateral collateral ligament, The long tendon of the ulnaris lateralis was best seen as it

FIG. 3 . Transverse c r iinape at the midlevel of the proximal carpal row: 1-Extensor carpi radialis tendon. 2-Common digital extensor tendon with si,lal, lateral branch, 3.Lareral collateral ligament, 4-Centimeter ruler at

traversed the lateral surface of the accessory carpal bone. skin surface. (soft tissue window)

VOL. 35, No. 6 C T A N D MKI ANATOMY OF THE EQUINE CARPUS 459

Further distally, it was at the palmar border of the lateral collateral ligament and inore difficult to outline separately.

The course of the small round flexor carpi radialis tendon could be followed to its attachment at the second metacarpal bone. The deep and superficial digital flexor muscles be- came tendinous just above the accessory carpal bone. Both tendons were visible through the carpal canal. The acces- sory ligament of the IIDF originates from the palmar carpal ligament. The origin of the interosseus was unifomily hy- perdense, but became of a mixed density.

The medial collateral ligament was rounder than the lat- eral collateral ligament at the lcvcl of the proximal carpal row: the lateral collateral ligament became roundcr and the medial collateral ligament broader at the level of the distal carpal row. The separate portions of these ligaments could not be outlined clearly.

A prominent hypcrdcnsc structure was the flexor rctinac- ulum of the carpus. Short carpal ligaments could not be identitied as scparatc structures.

The synovial lluid of the extensor carpi radialis tendon sheath, the antcbrachiocarpal joint and the tendon sheath o f the flexor carpi radialis muscle were hypodcnse and ClcarlY' visible. The carpal sheath was outlined, but contained little synovial fluid.

Images made with a bone window produced good dzlin- eation between cortex and niedulla. The trabccular pattern was well depicted.

l k . 5 . Transverse C'l image through the proximal third of the rneta- carpus: I - c ~ ~ ~ ~ ~ ~ ~ ~ digital extenbor tendon. 2-Latcrai ciigitai extensor ten- don 3-fnteros\eu\. 4-Accs>\ory liganlent o l the DDF 5-Deep digitd Ilexor tendon. 6-Superficial digital llexor tendon. 7-Medial palmar ~irtcry m d nerve (soft tissue window)

Frci. 6. Transverse MK image at the distal aspect of the radius, just proximal to the 'iccestoq carpal bone: 1-Extensor carpi obl iquu tendon. 2-Extcnsor carpi radialis tendon. 3-Common digital extensor tendon. 4-Lateral digital cxtcnsor tendon. 5-Long tendon of ulnaris lateralis mus- cle. h-Short tendon of ulnaris lateralis muscle. 7-Flexor carpi ulnarib mus- cle (tendinous portion). X-l'lexor carpi ulnaris muscle (muscular portion). 9-Supcrficial and deep digital flexor tendons and surrounding carpal sheath. 10-Flexor carpi radialis tendon. 11 -Flexor retinaculum.

FK. 4. Transverse CT image at the midlevcl of the distal carpal roiv: 1-Common digital citcnaor tendon with m a l l . \eparatil medial part and lateral branch. ?-Lateral collateral liganicnt. 3-Palinar carpal ligament. 4-Deep digital flexor tendon. S-Superficial digital flexor tendon. h-Medial palmar a r t q and nerve. 7-Flexor carpi radialis tendon. (soft tibsuc \%in- dow)

460 KASER-HOTZ ET AI 1994

MR-images

The TI weighted images used in this study depicted all tendinous structures as low-signal (black) areas. Collateral ligaments, the flexor retinaculum and compact bone also produced low-signal zones (Figs. 6 to 10). The accessory ligamcnt of the DDF was represented by a heterogeneous grey and the interosseus by an alternating black and white pattern (Fig 10). Fibrous connective tissue and muscle pro- duced various shadcs of grey, synovial fluid a high-signal intensity (white), and vascular structures a low-signal in- tensity area on the image. Some vessels also produced high- intensity signals. This is probably due to the postrnortcm status of the specimen used. The anatomic characteristics described for the CT-images arc also correct for the MK- imagcs.

Compcirisoii CT-MR IincigeA

Thc most obvious difference was the greater soft-tissue contrast demonstrated on MK images. Tendons and liga-

tissue texture was more readily appreciated on C'T images. ments were more defined in the MK but FIG. 8 . Tranhverse MK illlagc through the pro~ i lna l carpal ~ O Q : (-Ex.

ten\(y c;lmi radialis tendon. ~ - ~ o n l m ~ , n dirita1 extcnaor tendon. 3-Latsral

PIC;. 7. Transverse MR imape at or near level of antebranchiocarpal joint: 1 -Extensor carpi radialis tcndon. 7-Common digital cxtensor tendon. 3-Portion of the lateral colloterul l i~amcii t . 4-Lateral dizital cxtcn\or ten- don. 5-Long tendon ot the ulnaris lateralis muscle. &Flexor carpi ulnans muscle, with muscular (cranial) and tendinous portion (caudal). 7-Carpal flexor tendon rhcath, \vith superficial and deep digital flcsor tendons. &Medial palmar artery and nervc. 9-Flexor carpi radialis tendon. 10- Flcxor retinaculum. 1 1 -Medid collateral liganienl. 12-Extcnsor carpi ohliquus tendon.

FIG. 9. Tran\versr MK image through the distal carpal row: 1-Esten\or carpi radialis tendon. 2-Common digital extensor tendon with small inedial part and latcral branch. 3-Lateral collateral ligamsnt. I-Flexor retinacu- lum. S-Flexor carpi radialis tcndon. 6-Medial collateral ligament.

VOL. 35, No. 6 CT AND MRI ANATOMY OF THE EQUINE CARPUS 46 1

FIG. 10. Transverse MR image at the proximal aspect of the metacar- pus; 1-Common digital flexor tendon. 2-Lateral digital extensor tendon after it received lateral branch from common digital extensor tendon. 3-1n- terosseus (bilobed). 4-Flexor retinaculum. 5-Deep digital flexor tendon. 6-Superficial digital flexor tendon. 7-Accessory ligament of the DDF.

on CT scans but was heterogeneous on MR images. The accessory ligament of the DDF was a structure of high- signal density and contrasted clearly with the flexor tendons on MR images.

Discussion

Computed tomography and magnetic resonance imaging are diagnostic techniques not yet widely used in equine medicine. The main reasons are cost and the need for gen- eral anaesthesia. This study gives anatomic information on the equine carpus, which can be useful in both clinical and research use. The carpus is a frequently injured region in the

REFERE

1. Tnibar M, Kaser-Hotz B, Auer JA. Ultrasonography of the dorsal and lateral aspect of the equine carpus: Technique and normal appearance. Veterinary Radiology & Ultrasound 1993;34:413425.

2. Park RD, Nelson TR, Hoopes PJ. Magnetic resonance imaging of the normal equine digit and metacarpophalangeal joint. Veterinary Radiology 1987;28:105-116.

3 . Crass JR, Genovese RL, Render JA, Bellon EM. Magnetic reso- nance, ultrasound and histopathologic correlation of acute and healing equine tendon injuries. Veterinary Radiology and Ultrasound 1992;33: 206-2 16. 4. Kainer RA. Functional anatomy of equine locomotor organs. In:

Stash& TS. Adams’ Lameness in Horses. 4th edition. Lea and Febiger: Philadelphia, 1987:2!-31.

horse. The carpal anatomy is complex and diagnostic radi- ography and ultrasonography are not always sufficient for the full assessment of carpal lesions. CT images with a bone window would be particularly helpful for the diagnosis of occult fractures and for better evaluation of sclerosis of the third carpal bone. All major carpal soft tissue structures are well outlined both on CT and MR images. These techniques allow a more specific diagnosis of ligamentous, tendinous and synovial abnormalities in the carpal region than possi- ble with ultrasonography or radiography. Particularly le- sions in the carpal canal, an area not readily accessible for palpation, could be diagnosed more precisely. The equine carpus would fit into a CT or MR scanner provided a suit- able scanner table were available to support an anaesthe- tized horse.

On CT-scans we did not expect to see a difference in density among the flexor tendons. Of the structures exam- ined, namely flexor tendons, accessory ligament of the DDF and interosseus, the accessory ligament had the lowest CT-number. This was surprising to us, as we expected it to be the highest in accordance with ultrasonographic findings. A marked difference in signal intensity between the flexor tendons and the accessory ligament of the DDF was also seen on MR images. This particular postmortem specimen underwent ultrasonographic examination and no abnormal- ities were identified. The animal had no history of previous injury. Therefore we have to assume that the fact we used a postmortem specimen, and the limb was frozen and de- frosted may have altered the consistency of the tendons. The use of postmortem specimen may be sufficient for the outlining of anatomic structures but may not be valid for the determination of signal-intensities in CT or MR images and live animals should be used.

CT and MRI provided excellent images of carpal anat- omy. The better soft tissue contrast seen on MR images may not be advantageous enough, if one has to decide between CT and MRI equipment, since CT scans gave good soft tissue and bone images at the same time.

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