LOW-FIELD MAGNETIC RESONANCE IMAGING OF THE EQUINE TARSUS: NORMAL ANATOMY

The objective of this study was to define the normal gross anatomic appearance of the adult equine tarsus on a low-field magnetic resonance (MR) image. Six radiographically normal, adult, equine tarsal cadavers were utilized. Using a scanner with a 0.064 Tesla magnet, images were acquired in the sagittal, transverse and dorsal planes for T1-weighted and the sagittal plane for T2-weighted imaging sequences. Anatomic structures on the MR images were identified and compared with cryosections of the imaged limbs. Optimal image planes were identified for the evaluation of articular cartilage, subchondral bone, flexor and extensor tendons, tarsal ligaments, and synovial structures. MR images provide a thorough evaluation of the anatomic relationships of the structures of the equine tarsus.     

Magnetic Resonance Imaging (MRI) Anatomy of the Ovine Lumbar Spine

Although the ovine spine is a useful research model for intervertebral disc pathology and vertebral surgery, there is little peer‐reviewed information regarding the MRI anatomy of the ovine spine. To describe the lumbar spine MRI anatomy, 10 lumbar segments of cadaver ewes were imaged by 1.5‐Tesla MR. Sagittal and transverse sequences were performed in T1 and T2 weighting (T1W, T2W), and the images were compared to gross anatomic sagittal and transverse sections performed through frozen spines. MRI was able to define most anatomic structures of the ovine spine in a similar way as can be imaged in humans. In both T1W and T2W, the signals of ovine IVDs were similar to those observed in humans. Salient anatomic features were identified: (1) a 2‐ to 3‐mm linear zone of hypersignal was noticed on both extremities of the vertebral body parallel to the vertebral plates in sagittal planes; (2) the tendon of the crura of the diaphragm appeared as a hypointense circular structure between hypaxial muscles and the aorta and caudal vena cava; (3) dorsal and ventral longitudinal ligaments and ligamentum flavum were poorly imaged; (4) no ilio‐lumbar ligament was present; (5) the spinal cord ended between S1–S2 level, and the peripheral white matter and central grey matter were easily distinguished on T1W and T2W images. This study provides useful reference images to researchers working with ovine models.     

Time-dependent low-field MRI characteristics of canine blood: an in vitro study

This study was conducted to assess time-sensitive magnetic resonance (MR) changes in canine blood using low-field MR. Arterial and venous blood samples were collected from eight healthy beagle dogs. Samples were placed in 5-mL tubes and imaged within 3 hours of collection at 1 day intervals from day 1 to day 30. The following sequences were used: T1-weighted (T1W), T2-weighted (T2W), fluid-attenuated inversion recovery (FLAIR), short tau inversion recovery (STIR), and T2-star gradient-echo (T2^*-GRE). Visual comparison of the images revealed that four relatively homogenous blood clots and twelve heterogeneous blood clots developed. The margination of the clot and plasma changed significantly on day 2 and day 13. On day 2, heterogeneous blood clots were differentiated into 2 to 3 signal layers in the T2W, T1W, and especially the STIR images. Hypointense signal layers were also detected in the blood clots in STIR images, which have T2 hypo, FLAIR hypo, and T1 hyper intense signals. In all images, these signal layers remained relatively unchanged until day 13. Overall, the results suggest that hematomas are complex on low-field MRI. Accordingly, it may not be feasible to accurately characterize hemorrhages and predict clot age based on low-field MRI.     

APPEARANCE OF CANINE ABDOMINAL TUMORS WITH MAGNETIC RESONANCE IMAGING USING A LOW FIELD PERMANENT MAGNET

The study was carried out to evaluate the applicability of magnetic resonance imaging (MRI) in detecting tumors in the abdomen of the dog. Abdominal ultrasound and MRI were performed on 8 dogs having a mass lesion on abdominal radiography. MR images were obtained in the transverse, sagittal and dorsal planes using T1- and T2-weighted spin echo pulse sequences. There was good visual correlation of the lesion site by MRI and ultrasonography (US).     

DISTAL INTERPHALANGEAL ARTICULAR CARTILAGE ASSESSMENT USING LOW‐FIELD MAGNETIC RESONANCE IMAGING

The suitability of low‐field magnetic resonance (MR) imaging for assessment of articular cartilage has been questioned, based on insufficient image quality. The purposes of this study were to describe the MR anatomy of the normal distal interphalangeal (DIP) cartilage, and to evaluate the sensitivity and accuracy of low‐field MR imaging for identification of cartilage erosions that were created ex vivo. Imaging sequences included sagittal and dorsal multiple‐oblique T1‐weighted gradient‐recalled echo (GRE) and sagittal dual echo sequences. In the thickest regions, normal cartilage appeared as a trilaminar structure on high‐resolution T1‐weighted GRE sequences. All 8 mm large full‐thickness erosions were correctly identified (100% sensitivity and accuracy) using T1‐weighted GRE sequences. Sensitivity and accuracy ranged from 80% to 100% and 10% to 80%, respectively, for detecting focal full‐thickness erosions and from 35% to 80% and 35% to 60%, respectively, for detecting partial thickness erosions, using T1‐weighted GRE sequences. Superficial irregularities were not diagnosed using any sequence. Overall, fewer cartilage alterations were detected with sagittal dual echo sequences than with sagittal T1‐weighted GRE sequences. The dorsal multiple‐oblique plane was useful to detect linear dorsopalmar erosions. A combination of T1‐weighted GRE sequences in two planes has potential for identification of severe DIP cartilage erosion in anesthetized horses using low‐field MR imaging.     

EFFECTS OF STIFLE FLEXION ANGLE AND SCAN PLANE ON VISIBILITY OF THE NORMAL CANINE CRANIAL CRUCIATE LIGAMENT USING LOW‐FIELD MAGNETIC RESONANCE IMAGING

Low‐field magnetic resonance imaging (MRI) is commonly used to evaluate dogs with suspected cranial cruciate ligament injury; however, effects of stifle positioning and scan plane on visualization of the ligament are incompletely understood. Six stifle joints (one pilot, five test) were collected from dogs that were scheduled for euthanasia due to reasons unrelated to the stifle joint. Each stifle joint was scanned in three angles of flexion (90°, 135°, 145°) and eight scan planes (three dorsal, three axial, two sagittal), using the same low‐field MRI scanner and T2‐weighted fast spin echo scan protocol. Two experienced observers who were unaware of scan technique independently scored visualization of the cranial cruciate ligament in each scan using a scale of 0–3. Visualization score rank sums were higher when the stifle was flexed at 90° compared to 145°, regardless of the scan plane. Visualization scores for the cranial cruciate ligament in the dorsal (H (2) = 19.620, P = 0.000), axial (H (2) = 14.633, P = 0.001), and sagittal (H (2) = 8.143, P = 0.017) planes were significantly affected by the angle of stifle flexion. Post hoc analysis showed that the ligament was best visualized at 90° compared to 145° in the dorsal (Z = ?3.906, P = 0.000), axial (Z = ?3.398, P = 0.001), and sagittal (Z = ?2.530, P = 0.011) planes. Findings supported the use of a 90° flexed stifle position for maximizing visualization of the cranial cruciate ligament using low‐field MRI in dogs.     

THE EFFECT OF SEQUENCE SELECTION AND FIELD STRENGTH ON DETECTION OF OSTEOCHONDRAL DEFECTS IN THE METACARPOPHALANGEAL JOINT

Six cadaver forelimbs were imaged in two high‐field magnetic resonance (MR) systems and one low‐field MR system following the creation of osteochondral defects on the palmar distal aspect of the third metacarpal bone. The following sequences were performed using all three systems: proton density (PD) turbo spin echo, T2^* gradient echo (GRE), T2‐weighted fast spin echo, and short tau inversion recovery. In addition, 3D T1 GRE sagittal standard and motion insensitive sequences were obtained using the low‐field system. PD fat saturated and 3D T1‐weighted spoiled GRE images with and without fat suppression were acquired with the high‐field systems. Lesions were measured and assigned a confidence score. The images obtained using high‐field systems (1.0 and 1.5 T) more accurately represented the osteochondral defects when compared with low‐field system (0.27 T) images. The largest difference was observed when evaluating articular cartilage defects, which were not identified on the low‐field images. Sequence selection affected the appearance of the lesions. On all systems the turbo and fast spin echo sequences more accurately represented the lesion size and shape when compared with the GRE sequences. The T1 GRE sequence is the only sequence that appears to allow visualization of the articular cartilage on the low‐field images, but is limited in providing adequate cartilage visualization. Confidence scores were greater on the high‐field systems when compared with the low‐field system.     

EVALUATION OF MAGNETIC RESONANCE IMAGING TECHNIQUES IN THE EQUINE DIGIT

An anatomic study of the equine digit using magnetic resonance imaging (MRI) was performed. Seventeen isolated forelimbs and one hindleg of nine warmblood horses were imaged in transverse, sagittal, and dorsal planes with a 1.5 Tesla magnet using T1-, T2- proton density-weighted spin echo sequences as well as T2 gradient echo sequences. One scan plane in each horse was compared with corresponding anatomic and histologic sections. The best imaging planes to visualize various anatomic structures were determined. Fibrocartilage was visualized in the insertion of the deep digital flexor tendon and the suspensory ligament as well as in the distal sesamoidean ligaments. The correlation of MRI images with anatomic and histologic sections confirmed that all of the anatomic structures in the equine digit could be evaluated in PD and T2 studies.     

MAGNETIC RESONANCE IMAGING ANATOMY OF THE NORMAL EQUINE LARYNX AND PHARYNX

The purpose of the present study was to describe normal magnetic resonance (MR) imaging anatomy of the equine larynx and pharynx and to present the optimal protocol, sequences, and possible limitations of this examination technique. Using a 0.3 T unit, the laryngeal and pharyngeal regions was imaged in two horses. The protocol consisted of sagittal and transverse T2-weighted (T2w) fast spin echo, transverse T1-weighted (T1w) spin echo, and dorsal high-resolution T1w gradient echo (both pre- and postcontrast enhancement) sequences. Euthanasia was performed at the end of the imaging procedure. Macroscopic anatomy of the cadaver sections were compared with the MR images in transverse, midsagittal, and parasagittal planes. There was good differentiation of anatomic structures, including soft tissues. The laryngeal cartilages, hyoid apparatus, and upper airway muscle groups with their attachments could be clearly identified. However, it was not always possible to delineate individual muscles in each plane. Most useful were both T2w and T1w transverse sequences. Intravenous application of contrast medium was helpful to identify blood vessels. The MR images corresponded with the macroscopic anatomy of cadaver sections.     

Reliability of high‐ and low‐field magnetic resonance imaging systems for detection of cartilage and bone lesions in the equine cadaver fetlock

Reasons for performing study: To determine the reliability of 2 magnetic resonance imaging (MRI) systems for detection of cartilage and bone lesions of the equine fetlock. Objectives: To test the hypotheses that lesions in cartilage, subchondral and trabecular bone of the equine fetlock verified using histopathology can be detected on high‐ and low‐field MR images with a low incidence of false positive or negative results; that low‐field images are less reliable than high‐field images for detection of cartilage lesions; and that combining results of interpretation from different pulse sequences increases detection of cartilage lesions. Methods: High‐ and low‐field MRI was performed on 19 limbs from horses identified with fetlock lameness prior to euthanasia. Grading systems were used to score cartilage, subchondral and trabecular bone on MR images and histopathology. Sensitivity and specificity were calculated for images. Results: High‐field T2*‐weighted gradient echo (T2*W‐GRE) and low‐field T2‐weighted fast spin echo (T2W‐FSE) images had high sensitivity but low specificity for detection of cartilage lesions. All pulse sequences had high sensitivity and low–moderate specificity for detection of subchondral bone lesions and moderate sensitivity and moderate–high specificity for detection of trabecular bone lesions (histopathology as gold standard). For detection of lesions of trabecular bone low‐field T2*W‐GRE images had higher sensitivity and specificity than T2W‐FSE images. Conclusions: There is high likelihood of false positive results using high‐ or low‐field MRI for detection of cartilage lesions and moderate–high likelihood of false positive results for detection of subchondral bone lesions compared with histopathology. Combining results of interpretation from different pulse sequences did not increase detection of cartilage lesions. MRI interpretation of trabecular bone was more reliable than cartilage or subchondral bone in both MR systems. Potential relevance: Independent interpretation of a variety of pulse sequences may maximise detection of cartilage and bone lesions in the fetlock. Clinicians should be aware of potential false positive and negative results.     

Comparison of the navicular region of newborn foals and adult horses by magnetic resonance imaging

Magnetic resonance imaging (MRI) was tested for evaluation of the soft tissue structures of the equine digit in 16 limbs, derived from three adult warmblood horses and two newborn warmblood foals. The following measuring sequences were used in sagittal, transversal and coronal planes: spin echo, gradient echo, inversion recovery. The images were made with a 1.5 Tesla Siemens scanner in a CP-Helmholtz circular coil. To compare the visualization of the same tissue structures in adult and in newborn cases the limbs were imaged with the routinely used MRI sequences that are used for mature tissues. In newborn foals the bursa podotrochlearis could not be determined with the used sequences. For both the adult and newborn limbs the most visualization of the same tissue structures including the fluid spaces and growth plates were taken by the inversion recovery sequence. T2 sequence was very informative in adult cases but moderately in newborn foals.     

CLINICAL SIGNIFICANCE AND PROGNOSIS OF DEEP DIGITAL FLEXOR TENDINOPATHY ASSESSED OVER TIME USING MRI

Deep digital flexor (DDF) tendinopathy is one of the most frequent causes of foot lameness and the prognosis is guarded. The progress of lesion healing may be followed by magnetic resonance (MR) imaging to formulate a prognosis and to adapt the rehabilitation program. We assessed the correlation of outcome with total tendon damage and temporal resolution of MR abnormalities. Images from 34 horses with DDF tendinopathy that had undergone at least two low‐field standing MR examinations of the foot (mean 2.5 ± 1.3 times) were reviewed. No horse having a T1‐GRE hyperintense lesion over 30 mm in length or over 10% tendon cross‐sectional area returned to its previous activity level. Horses with concomitant lesions had worse outcome than horses with DDF tendinopathy only (P = 0.005). In all horses including those with excellent outcome, the lesion persisted, even mildly, in T1‐GRE and PD images. Horses with tendon lesion resolution on STIR‐FSE and T2‐FSE images on recheck examination had a better outcome (P = 0.0004 and P = 0.002, respectively), and all horses that returned to their previous level of performance had complete resolution of signal hyperintensity on the STIR‐FSE sequence. Although rehabilitation remains multifactorial, characteristics of DDF tendinopathy and concomitant lesions on first and recheck MR examinations allow refining the prognosis.     

ANATOMICAL STUDY OF CRANIAL NERVE EMERGENCE AND SKULL FORAMINA IN THE HORSE USING MAGNETIC RESONANCE IMAGING AND COMPUTED TOMOGRAPHY

For accurate interpretation of magnetic resonance (MR) images of the equine brain, knowledge of the normal cross‐sectional anatomy of the brain and associated structures (such as the cranial nerves) is essential. The purpose of this prospective cadaver study was to describe and compare MRI and computed tomography (CT) anatomy of cranial nerves' origins and associated skull foramina in a sample of five horses. All horses were presented for euthanasia for reasons unrelated to the head. Heads were collected posteuthanasia and T2‐weighted MR images were obtained in the transverse, sagittal, and dorsal planes. Thin‐slice MR sequences were also acquired using transverse 3D‐CISS sequences that allowed mutliplanar reformatting. Transverse thin‐slice CT images were acquired and multiplanar reformatting was used to create comparative images. Magnetic resonance imaging consistently allowed visualization of cranial nerves II, V, VII, VIII, and XII in all horses. The cranial nerves III, IV, and VI were identifiable as a group despite difficulties in identification of individual nerves. The group of cranial nerves IX, X, and XI were identified in 4/5 horses although the region where they exited the skull was identified in all cases. The course of nerves II and V could be followed on several slices and the main divisions of cranial nerve V could be distinguished in all cases. In conclusion, CT allowed clear visualization of the skull foramina and occasionally the nerves themselves, facilitating identification of the nerves for comparison with MRI images.     

Magnetic resonance imaging of bone marrow in the pelvis and femur of young dogs

The purpose of this study was to determine the magnetic resonance (MR) imaging characteristics of bone marrow in the pelvis and femur of normal, young dogs. Six greyhounds were imaged at 4, 8, 12, and 16 months of age. Sagittal images of the femur and dorsal images of the pelvis were obtained with T1-weighted, fast spin echo (FSE) T2-weighted, and short tau (T1) inversion recovery (STIR) sequences. On T1-weighted images areas with high signal intensity, similar to fat, included the femoral heads, mid-diaphysis of the femur, femoral condyles, and the body of the ilium. T2-weighted images were characterized by uniform intermediate signal intensity (less than fat, but greater than muscle) in the femoral head, high signal intensity, similar to fat, in the mid-diaphysis of the femur and ilial body, and intermediate to high signal intensity in the femoral condyle. By 16 months high signal intensity was seen in the diaphysis and distal metaphysis on both T1- and T2-weighted images. On STIR images the femoral head had intermediate to low signal intensity, compared with muscle. The mid-diaphysis of the femur was of low signal intensity, similar to fat, and the body of the ilium had mixed signal intensity at all ages. The femoral condyle had inhomogenous, intermediate to low signal intensity at 4 months, but was of uniform low signal intensity at 8-16 months.