ASYMMETRIC SIGNAL INTENSITY IN NORMAL COLLATERAL LIGAMENTS OF THE DISTAL INTERPHALANGEAL JOINT IN HORSES WITH A LOW-FIELD MRI SYSTEM DUE TO THE MAGIC ANGLE EFFECT

Increased signal intensity in one of the collateral ligaments of the distal interphalangeal (DIP) joint of sound horses in images acquired using a low-field magnet with vertical orientation of the magnetic field was investigated as a possible manifestation of the magic angle effect. Three isolated equine digits were imaged using the following pulse sequences: (1) spin echo T1, (2) turbo spin echo proton density and T2, and (3) 3D gradient echo T1, in different positions by mildly changing the orientation of the long axis of the digit, in the dorsal plane, relative to the magnetic field. The signal intensity in a ligament was significantly increased when the ligament orientation relative to the magnetic field was 55±10°. The signal intensity was markedly increased in pulse sequences with short echo time (TE) 5.0, 4.9, and 3.9 times increased, respectively, for 3D gradient echo T1, spin echo T1, and turbo spin echo proton density) and to a lesser extent with pulse sequences with a longer TE (1.8 times increased for turbo spin echo T2). These changes are characteristic of the magic angle effect. Because of the anatomic orientation of the collateral ligaments of the DIP joint, a slight deviation of the long axis of the digit in the dorsal plane, from the ideal horizontal position, will induce an increased signal intensity that can be confused with desmitis. Careful positioning of the foot in magnetic resonance imaging systems where B ₀ is perpendicular to the long axis of the digit is critical to prevent the occurrence of the magic angle effect.     

IS A MAGIC ANGLE EFFECT OBSERVED IN THE COLLATERAL LIGAMENTS OF THE DISTAL INTERPHALANGEAL JOINT OR THE OBLIQUE SESAMOIDEAN LIGAMENTS DURING STANDING MAGNETIC RESONANCE IMAGING?

Collagen fibers oriented at 55° to the static magnetic field (B₀) are characterized by an artifactual increase in signal intensity due to the magic angle effect. We hypothesized that there would be increased signal intensity in the collateral ligaments of the distal interphalangeal joint and oblique sesamoidean ligaments when these ligaments were at angles approaching 55° to a horizontal B₀ during standing magnetic resonance (MR) imaging. MR imaging was performed on four cadaver forelimbs in a 0.27 T standing system. Transverse and dorsal images were obtained using various sequences, with limbs angled at 0°, 4°, 8°, and 12° to the vertical. Images were analyzed and the angle of each ligament to B₀ determined. Mean signal intensity in the ligament and cortex of the adjacent phalanx was measured and ratios calculated. With subjective interpretation, there was increased signal intensity in the collateral ligaments of the distal interphalangeal joint and oblique sesamoidean ligaments over ranges of angles of 60–78° and 57–69°, respectively, to B₀. In fast spin echo (FSE) sequences, with a long echo time (72 ms), the effect was less pronounced. FSE sequences can help determine the significance of increased signal intensity within tissues. In spite of limited positions of a limb during standing MR imaging compared with horses under general anesthesia, deviation from a vertical posture sufficient to cause a magic angle effect can still occur in both ligaments tested. Conformation may contribute to the occurrence of the magic angle effect during standing MR imaging. Effort should be made to position horses squarely and to minimize leaning during image acquisition.     

Effect of glucocorticoids on cows suspected of subclinical infection with M. paratuberculosis

Summary

Glucocorticoids were administered to 10 heifers suspected of subclinical infection with Mycobacterium paratuberculosis. Three animals remained untreated.

M. paratuberculosis was isolated from the internal organs of 2 animals after this treatment but not from any of the control group. Delayed type hypersensitivity and lymphocyte reactivity towards Johnin and purified protein derivates of M. avium and M. bovis were depressed. A sharp increase in total leucocyte count, due loan increase in neutrophil numbers, occurred. In the three untreated animals these parameters did not change during the experiment.

A decrease of specific immunological reactivity towards M. paratuberculosis occurred, but not to such an extent that clinical disease developed.     

Influence of the position of the foot on MRI signal in the deep digital flexor tendon and collateral ligaments of the distal interphalangeal joint in the standing horse

Reasons for performing study: Hyperintense signal is sometimes observed in ligaments and tendons of the equine foot on standing magnetic resonance examination without associated changes in size and shape. In such cases, the presence of a true lesion or an artifact should be considered. A change in position of a ligament or tendon relative to the magnetic field can induce increased signal intensity due to the magic angle effect. Objectives: To assess if positional rotation of the foot in the solar plane could be responsible for artifactual changes in signal intensity in the collateral ligaments of the distal interphalangeal joint and in the deep digital flexor tendon. Methods: Six isolated equine feet were imaged with a standing equine magnetic resonance system in 9 different positions with different degrees of rotation in the solar plane. Results: Rotation of the limb induced a linear hyperintense signal on all feet at the palmar aspect of one of the lobes of the deep digital flexor tendon and at the dorsal aspect of the other lobe. Changes in signal intensity in the collateral ligaments of the distal interphalangeal joint occurred with rotation of the limb only in those feet where mediolateral hoof imbalance was present. Conclusions: The position and conformation of the foot influence the signal intensity in the deep digital flexor tendon and in the collateral ligaments of the distal interphalangeal joint. Potential relevance: The significance of increased signal intensity in the deep digital flexor tendon and in the collateral ligaments of the distal interphalangeal joint should be interpreted with regard to the position and the conformation of the foot.     

MAGNETIC RESONANCE IMAGING FINDINGS OF DESMOPATHY OF THE COLLATERAL LIGAMENTS OF THE EQUINE DISTAL INTERPHALANGEAL JOINT

We report the use of a low-field magnetic resonance (MR) imaging system for the detection of desmopathy of the collateral ligament of the distal interphalangeal joint and the long-term outcome. Twenty horses were studied and their medical records and MR images were reviewed retrospectively. Long-term follow-up information was obtained by telephonic questionnaires of owners, trainers, or referring veterinarians. Desmopathy of the medial collateral ligament (80%) and enthesopathy of the affected collateral ligament (80%) were common MR imaging features. Treatment consisted of stall rest followed by a rehabilitation period. Additional treatments included shoeing, extracorporeal shock wave therapy, application of a half limb or foot cast, and medication of the distal interphalangeal joint. Twelve (60%) horses returned to their previous level of exercise and maintained their previous level, whereas eight horses had a poor outcome. Low-field MR imaging in the standing patient can be used to detect collateral ligament desmopathy of the distal interphalangeal joint without a need for general anesthesia.     

ULTRASONOGRAPHY OF THE EQUINE TARSUS

Ultrasonography was used to delineate the sonographic anatomy of the equine tarsus with emphasis on the flexor tendons and plantar ligament at the plantar surface of the tarsus, the medial collateral ligaments and the dorsomedial joint capsule of the tibiotarsal joint. The cross-sectional gross anatomy of these tendons and ligaments was evaluated in 6 cadaver limbs. Using a 5.5 MHz mechanical sector scanner, the examined tendons and ligaments were easily identified as hyperechoic structures in 10 limbs of normal live horses. The normal ultrasonographic appearance of the dorsomedial tibiotarsal joint capsule was studied in 8 cadaver limbs. The inner surface of the tibiotarsal joint capsule was covered with villi; short and pointed in the mid region, resembling a small nodular mass proximally and a straggly beard distally. In 4 lame horses diagnostic ultrasound aided the diagnosis of plantar swellings, medial collateral joint ligamentous injury and hypertrophic synovitis.     

LATERAL COLLATERAL LIGAMENT AVULSION OF THE HUMERORADIAL JOINT IN A HORSE

This report describes traumatic avulsion of the lateral collateral ligament of the humeroradial joint in a horse. The history and diagnostic procedures are included with relevant radiographs and ultrasonographs. The poor prognosis associated with this injury is due to degenerative joint disease.     

CHARACTERIZATION OF THE MAGIC ANGLE EFFECT IN THE EQUINE DEEP DIGITAL FLEXOR TENDON USING A LOW-FIELD MAGNETIC RESONANCE SYSTEM

Three isolated equine limbs were imaged with a low-field magnetic resonance system with a vertical magnetic field. Each limb was scanned in multiple positions with mild variation of the angle between the magnetic field and the long axis of the limb. When the long axis of the limb was not perpendicular to the magnetic field, a linear hyperintense signal was present at the palmar aspect of one of the deep digital flexor tendon lobes, at the level of the navicular bone and collateral sesamoidean ligaments, in proton density and T1-weighted pulse sequences. With increased angulation of the limb, the palmar hyperintense signal extended farther distally and proximally and additional signal hyperintensity was present at the dorsal aspect of the distal part of the other lobe of the deep digital flexor tendon. Increased signal intensity was also present in the collateral ligament of the distal interphalangeal joint on the same side as the palmar hyperintense signal in the tendon. The changes in the deep digital flexor tendon are due to the specific orientation of fibers at the palmar and dorsal aspect of the tendon, which is responsible for focal manifestation of the magic angle effect. Careful positioning of the limb perpendicular to the magnetic field can prevent this phenomenon. The association of palmar increased signal intensity in the deep digital flexor tendon with increased signal in the collateral ligament of the distal interphalangeal joint on the same side should be recognized as manifestations of the magic angle effect.     

ULTRASONOGRAPHY OF THE EQUINE STIFLE

Ten stifles from 5 clinically sound adult horses were scanned by high-resolution ultrasound. Normal anatomic structures seen consistently included the subcutaneous tissue, the medial, middle and lateral patellar ligaments, the medial and lateral collateral ligaments, the femoral trochlear ridges, and menisci. Cruciate ligaments could not be visualized in the standing horse. Four stifle specimens obtained from 2 of these normal horses at necropsy were scanned in a water bath to create optimal technical conditions. The sonographic appearance of stifle specimens was similar to that found in live horses. Results from ultrasonographic examination of 3 horses with stifle abnormalities were described to illustrate some applications of ultrasonography in the evaluation of the equine stifle. In these horses, ultrasound was a valuable diagnostic tool to study joint effusion, synovial thickening, articular cartilaginous and subchondral defects, and soft tissue/bony injuries.     

ULTRASONOGRAPHY OF THE EQUINE CRUS

Ultrasonography was used to delineate the sonographic anatomy of the equine crus, emphasizing the peroneus tertius muscle and the Achilles tendon. The cross-sectional gross anatomy was studied in 6 cadaveric limbs. Using a 5.5 MHz mechanical sector scanner the examined Achilles tendon and per- oneus tertius muscle were easily identified as hyperechoic structures in 10 limbs of normal live horses. In 4 lame horses diagnostic ultrasound aided the diagnosis of peroneus tertius rupture, haematoma of the lateral digital extensor muscle and the presence or absence of Achilles tendon injury associated with intertendinous calcaneal bursitis.