LOW-FIELD MAGNETIC RESONANCE IMAGING OF THE CANINE STIFLE JOINT: NORMAL ANATOMY

Low-field magnetic resonance imaging (MRI) was performed on the stifle joints of four normal adult mongrel dogs using a 0.064 Tesla scanner. Markers were placed on each stifle joint to serve as reference points for comparing gross sections with the images. A T1-weighted sequence was used to image one stifle joint on each dog in the sagittal plane and the other stifle joint in the dorsal plane. The dogs were euthanized immediately following MRI and the stifle joints frozen intact. Each stifle joint was then embedded in paraffin, again frozen, and sectioned using the markers as reference points. On T1-weighted images, synovial fluid had low signal intensity (dark) compared to the infrapatellar fat pad which had a high signal intensity (bright). Articular cartilage was visualized as an intermediate bright signal and was separated from trabecular bone by a dark line representing subchondral bone. Menisci, fibrous joint capsule, and ligamentous structures appeared dark. In the true sagittal plane, the entire caudal cruciate ligament was often seen within one image slice. The patella was visualized as an intermediate bright signal (trabecular bone) surrounded by a low intensity signal (cortical bone). The trochlea and the intercondylar notch were difficult areas to analyze due to signal volume averaging of the curved surface of these areas and the presence of several types of tissues.     

MAGNETIC RESONANCE ARTHROGRAPHY OF THE CANINE STIFLE JOINT TECHNIQUE AND APPLICATIONS IN ELEVEN MILITARY DOGS

The stifle joints of eleven military working dogs were evaluated using conventional magnetic resonance (MR) imaging and MR arthrography. A protocol optimizing MR imaging of the canine stifle joint is discussed, as well as potential uses for administration of intra-articular gadolinium. The technique for performing MR arthrography is described, and post-contrast image findings are reviewed. MR arthrography was performed by using an intra-articular injection of diluted gadolinium. Consistently good quality images were obtained, and no complications were clinically detected following MR arthrography. Cranial cruciate ligament abnormalities were seen in six dogs, meniscal abnormalities were visualized in nine menisci, and synovitis and medial ligament strain were seen in eight dogs. Surgical and post-mortem confirmation of these findings is discussed in seven dogs. Although MR arthrography adds an invasive procedure to conventional MR imaging, it can provide useful information on pathologic changes in the canine stifle joint.     

GROSS AND HISTOPATHOLOGIC CORRELATION OF LOW‐FIELD MAGNETIC RESONANCE IMAGING FINDINGS IN THE STIFLE OF ASYMPTOMATIC HORSES

With the recent introduction of a 0.25T rotating MRI system, clinical evaluation of the equine stifle joint is now possible in the average equine athlete. A recent publication described common abnormalities of horses with stifle lameness detected with a low‐field MRI system; however, postmortem corroboration of the lesions detected was not possible. Therefore, our objective was to compare postmortem findings with low‐field MRI findings in equine cadaver stifle joints. Ten fresh cadaver stifle joints from horses without clinical signs of stifle disease were evaluated using low‐field MRI, gross dissection, and histopathology. In eight stifles, either the lateral or medial cranial meniscotibial ligament had an irregular shape, fiber separation, or moderate abnormal signal intensity (SI) on all sequences. In five stifles, the medial femoral condyle had articular cartilage fibrillation with or without an osteochondral defect over the weight bearing surface of the medial femoral condyle. All stifles had abnormal SI on all sequences within the patellar ligaments that corresponded with adipose tissue infiltrating between the collagen bundles. Other abnormalities identified included articular cartilage fibrillation of the tibial condyles in three stifles, and articular cartilage fibrillation with chondral defects in the patella in three stifles. All abnormalities detected with low‐field MRI were corroborated by gross dissection. Findings from the current study supported the use of low‐field MRI for detection of stifle joint lesions in horses and demonstrated that some stifle joint pathologies may be subclinical in horses.     

MAGNETIC RESONANCE IMAGING OF SUBARTICULAR BONE MARROW LESIONS IN DOGS WITH STIFLE LAMENESS

A bone bruise is a magnetic resonance (MR) imaging sign thought to signify acute traumatic microfracture of trabecular bone with hemorrhage and edema in the marrow that may occur without grossly visible disruption of the adjacent cortices or overlying cartilage. In approximately 75% of people with acute anterior-cruciate ligament tears, bone bruises are detected in characteristic locations within the femur and tibia and are best seen as high-signal lesions using fat-suppression sequences. We questioned whether this is a component of naturally acquired stifle lameness in dogs and obtained short-tau inversion recovery (STIR) images of six dogs with stifle lameness. High-signal STIR lesions were detected in five of six (83%) dogs and eight of 12 (67%) limbs. We observed these lesions deep to the intercondylar fossa of the femur and intercondylar eminence of the tibia, which are atypical locations in people. High-signal STIR lesions were detected in dogs with only synovitis, partial tear of the cranial cruciate ligament (CCL) and complete tear of the CCL. One of these lesions was seen in the lateral tibial condyle, a typical location in humans with acute anterior cruciate ligament tear. As the MR imaging appearance of stress fractures and bone bruises are similar, and the high-signal STIR lesions are at attachment sites of the CCL, this finding may be due to stress disease or other unknown causes, rather than bone bruising. High-signal STIR lesions may be a common sign in naturally acquired canine stifle disease, but the pathogenesis, prognostic and diagnostic values need further investigation.     

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.     

IMAGING DIAGNOSIS—MAGNETIC RESONANCE IMAGING PULSATILITY ARTIFACT IN THE CANINE CERVICAL SPINE

Pulsatile venous flow in the internal vertebral venous plexus of the cervical spine can lead to vertical, linear T2‐hyperintensities in the spinal cord at the cranial aspect of C3 and C4 in transverse T2‐weighted images in large breed dogs that are not accompanied by ghosting. The artifact is more conspicuous in pre‐ and postcontrast transverse T1‐weighted images and is accompanied by ghosting in that sequence, typical of a pulsatility artifact. A flow‐related artifact was confirmed as the cause for this appearance by noting its absence after either exchange of phase and frequency encoding direction or by flow compensation. Care should be exercised to avoid misdiagnosing this pulsatility artifact seen in transverse T2‐weighted images of the midcervical spine in large dogs as an intramedullary lesion when T1‐images or phase‐swap images are not available to confirm its artifactual origin.     

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.     

THREE TESLA MAGNETIC RESONANCE IMAGING FINDINGS IN 12 CASES OF CANINE CENTRAL EUROPEAN TICK‐BORNE MENINGOENCEPHALOMYELITIS

Central European tick‐borne encephalomyelitis can be challenging to diagnose in dogs because the virus may not be detected in blood and cerebrospinal fluid (CSF) after the first viremic stage of the disease. The purpose of this retrospective case series study was to describe 3 Tesla magnetic resonance imaging (3T MRI) findings in a sample of dogs with a confirmed diagnosis of tick‐borne encephalomyelitis. Dogs were included if they had neurological signs consistent with tick‐borne encephalomyelitis, history of a stay in endemic areas for tick‐borne encephalomyelitis virus, 3T MRI of the brain and/or spinal cord, cerebrospinal fluid changes compatible with viral infection and positive antibody titers in cerebrospinal fluid or pathologic confirmation of tick‐borne encephalomyelitis. Twelve dogs met inclusion criteria. Ten out of 12 patients had 3T MRI lesions at the time of presentation. One patient had persistent lesions in follow‐up MRI. The 3T MRI findings included bilateral and symmetrical gray matter distributed lesions involving the thalamus, hippocampus, brain stem, basal nuclei, and ventral horn on the spinal cord. All lesions were hyperintense in T2‐weighted sequences compared to white matter, iso‐ to hypointense in T1‐weighted, nonenhancing, and had minimal or no mass effect or perilesional edema. Six patients survived while the remaining six dogs were euthanized. Necropsy revealed neuronophagia and gliosis of the gray matter of the affected regions seen in 3T MRI, in addition to the cerebellum. Findings from the current study indicated that tick‐borne encephalomyelitis should be included in the differential diagnosis list for dogs with the above described 3T MRI characteristics.     

EX VIVO MAGNETIC RESONANCE IMAGING OF THE DISTAL ROW OF EQUINE CARPAL BONES: ASSESSMENT OF BONE SCLEROSIS AND CARTILAGE DAMAGE

The distal row of carpal bones (C2, C3, and C4) from eight left intercarpal joints--four from Standardbred Trotters and four from Swedish Warmblood horses--were used to assess the potential of magnetic resonance (MR) imaging to detect cartilage and bone lesions. The joints used in the study were classified by macroscopic and radiographic examinations as having normal, mild, moderate, or severe articular cartilage lesions and bone sclerosis. Those classifications correlated well with the appearance of the MR images. Bone sclerosis in the MR images was observed as regions of decreased signal intensity. Upon quantitative analysis of the MR images there was a significant difference (p < 0.0001) in the MR signal intensity from areas where radiographic bone sclerosis was observed compared to areas of radiographic nonsclerotic bone. In addition, the MR images were used to pilot the location of histology slices through areas of interest that were then examined microscopically; hence, the lesions found from the MR imaging examination were verified microscopically. It was concluded that cartilage lesions and cartilage loss are related to the sclerotic state of the underlying bone. The MR protocols developed in this study were applied on five intact cadaveric carpal joints, and it was concluded that MR imaging could successfully be used in the intact joint to detect minor cartilage and bone lesions not visualized by either radiography or macroscopic examination. Hence, MR imaging can be used to delineate interactions between articular cartilage and subchondral bone over time and in vivo.     

3 TESLA MAGNETIC RESONANCE IMAGING OF THE OCCIPITOATLANTOAXIAL REGION IN THE NORMAL HORSE

The aim of this study was to describe the appearance of the ligamentous structures of the occipitoatlantoaxial (OAA) region in the normal horse by 3 tesla (3T) magnetic resonance imaging (MRI). The MRI images of the longitudinal odontoid ligament, tectorial membrane, dorsal and ventral atlantoaxial ligaments, dorsal atlantooccipital membrane with its reinforcing ligaments, and the lateral atlantooccipital ligaments of 10 horse cadavers were evaluated. All ligaments and membranes were identified in all planes, except for the lateral atlantooccipital ligament in the sagittal plane due to its cranioventrolateral course. All were iso to mildly hypointense to musculature of the neck in T1W with the exception of the tectorial membrane that was moderately hypointense; moderately hypointense in PD‐SPIR, and markedly hypointense (isointense to cortical bone) in T2W. The PD‐SPIR was the best sequence to identify all ligaments and membranes from their cranial and caudal attachments. The longitudinal odontoid ligament, ventral atlantoaxial ligament, and reinforcing bands of the dorsal atlantooccipital membrane presented a characteristic striped heterogeneous signal behavior thought to be due to fibrocartilaginous content. The remaining ligaments and membranes showed homogeneous signal intensity. Special anatomical features in this species such as the fan‐shaped longitudinal odontoid ligament, absence of the transverse ligament and presence of the ventral atlantoaxial ligament were documented. Ligamentous structures that stabilize the equine OAA region were described with MRI in this study and these findings could serve as an anatomic reference for those cases where instability of this region is suspected.     

DIFFUSION‐WEIGHTED MAGNETIC RESONANCE IMAGING OF THE NORMAL CANINE BRAIN

Diffusion‐weighted imaging (DWI) MRI has been primarily reported as a method for diagnosing cerebrovascular disease in veterinary patients. In humans, clinical applications for diffusion‐weighted MRI have also included epilepsy, Alzheimer's, and Creutzfeld–Jakob disease. Before these applications can be developed in veterinary patients, more data on brain diffusion characteristics are needed. Therefore, the aim of this study was to evaluate the distribution of diffusion in the normal canine brain. Magnetic resonance imaging of the brain was performed in ten, clinically normal, purpose‐bred beagle dogs. On apparent diffusion coefficient maps, regions of interest were drawn around the caudate nucleus, thalamus, piriform lobe, hippocampus, semioval center, and cerebral cortex. Statistically significant differences in mean apparent diffusion coefficient were found for the internal capsule, hippocampus, and thalamus. The highest apparent diffusion coefficient (1044.29 ± 165.21 μm²/s (mean ± SD (standard deviation)) was detected in the hippocampus. The lowest apparent diffusion coefficient was measured in the semioval center (721.39 ± 126.28 μm²/s (mean ± SD)). Significant differences in mean apparent diffusion coefficients of the caudate nucleus, thalamus, and piriform lobe were found by comparing right and left sides. Differences between brain regions may occur due to differences in myelination, neural density, or fiber orientation. The reason for the differences between right and left sides remains unclear. Data from the current study provide background for further studies of diffusion changes in dogs with brain disease.     

CORRELATION OF ARTICULAR CARTILAGE THICKNESS MEASUREMENTS MADE WITH MAGNETIC RESONANCE IMAGING,MAGNETIC RESONANCE ARTHROGRAPHY,AND COMPUTED TOMOGRAPHIC ARTHROGRAPHY WITH GROSS ARTICULAR CARTILAGE THICKNESS IN THE EQUINE METACARPOPHALANGEAL JOINT

Osteoarthritis of the metacarpophalangeal joint is common cause of lameness in equine athletes, and is hallmarked by articular cartilage damage. An accurate, noninvasive method for measuring cartilage thickness would be beneficial to screen for cartilage injury and allow for prompt initiation of interventional therapy. The objective of this methods comparison study was to compare computed tomographic arthrography (CTA), magnetic resonance imaging (MRI), and magnetic resonance arthrography (MRA) measurements of articular cartilage thickness with gross measurements in the metacarpophalangeal joint of Thoroughbred horses. Fourteen cadaveric, equine thoracic limbs were included. Limbs were excluded from the study if pathology of the metacarpophalangeal articular cartilage was observed with any imaging modality. Articular cartilage thickness was measured in nine regions of the third metacarpal bone and proximal phalanx on sagittal plane MRI sequences. After intra‐articular contrast administration, the measurements were repeated on sagittal plane MRA and sagittal CTA reformations. In an effort to increase cartilage conspicuity, the volume of intra‐articular contrast was increased from 14.5 ml, to maximal distention for the second set of seven limbs. Mean and standard deviation values were calculated, and linear regression analysis was used to determine correlations between gross and imaging measurements of cartilage thickness. This study failed to identify one imaging test that consistently yielded measurements correlating with gross cartilage thickness. Even with the use of intra‐articular contrast, cartilage surfaces were difficult to differentiate in regions where the cartilage surfaces of the proximal phalanx and third metacarpal bone were in close contact with each other.     

ACCURACY OF MAGNETIC RESONANCE IMAGING FOR ESTIMATING INTRAMEDULLARY OSTEOSARCOMA EXTENT IN PRE-OPERATIVE PLANNING OF CANINE LIMB-SALVAGE PROCEDURES

The objective of this work was to compare the accuracy of radiographs and magnetic resonance imaging (MRI) for estimating appendicular osteosarcoma margins. The accuracy of computed tomography (CT) and bone scintigraphy was also assessed when these studies were available. Eight dogs with appendicular osteosarcoma underwent radiographic and MRI of affected limbs. In addition, bone scintigraphy was performed in six dogs and CT examination was performed in five dogs. Two observers jointly measured tumor length on all imaging studies. Correlative gross and histologic evaluation of all affected limbs was performed to determine tumor extent as measured from the nearest articular surface. Results from imaging studies were compared to gross and microscopic morphometry findings to determine the accuracy of each modality for determining tumor boundaries. MRI images were accurate with a mean overestimation of actual tumor length of 3 +/- 13%. T1-weighted non-contrast images were superior in identifying intramedullary tumor margins in most instances whereas contrast-enhanced images provided supplemental information in two dogs. Lateromedial and craniocaudal radiographs overestimated tumor length by 17 +/- 28% and 4 +/- 26%, respectively. Scintigraphy and CT overestimated tumor margins by 14 +/- 28% and 27 +/- 36%, respectively. MRI appears to be an accurate diagnostic imaging modality in determining intramedullary osteosarcoma boundaries. MRI should be considered as part of a pre-operative assessment of appendicular osteosarcoma, particularly when a limb-sparing procedure is contemplated.     

USE OF THE T2*‐WEIGHTED GRADIENT RECALLED ECHO SEQUENCE FOR MAGNETIC RESONANCE IMAGING OF THE CANINE AND FELINE BRAIN

T2*‐weighted magnetic resonance imaging (MRI) has been reported to help improve detection of intracranial hemorrhage and is widely used in human neuroimaging. To assess the utility of this technique in small animals, interpretations based on this sequence were compared with those based on paired T2‐weighted and fluid‐attenuated inversion recovery (FLAIR) sequences in 200 dogs and cats that underwent brain MRI for suspected intracranial disease. Two sets of images (T2 + FLAIR and T2*) were reviewed separately in random order unaccompanied by patient information and were interpreted as normal or abnormal based on whether intracranial abnormalities were seen. The number and location of intracranial lesions were recorded. Eighty‐five studies were considered normal and 88 were considered abnormal based on both sets of images, with good agreement (κ = 0.731) between the two. Susceptibility artifact was present in 33 cases (16.5%) on T2*‐weighted images. In 12 cases (6%) a total of 69 lesions were seen on T2*‐weighted images that were not seen on T2/FLAIR, all of which were associated with susceptibility artifact caused by presumed intracranial hemorrhage. Pseudolesions were seen on T2*‐weighted images in five cases, none of which were associated with susceptibility artifact. Abnormalities were seen on T2/FLAIR images that were not seen on T2*‐weighted images in 35 cases, confirming that T2* does not replace standard spin echo sequences. These results support inclusion of T2*‐weighted sequences in small animal brain MRI studies and indicate that that a large number of abnormalities (especially hemorrhagic lesions) can go undetected if it is not performed.     

VALIDATION OF DELAYED GADOLINIUM‐ENHANCED MAGNETIC RESONANCE IMAGING OF CARTILAGE AND T2 MAPPING FOR QUANTIFYING DISTAL METACARPUS/METATARSUS CARTILAGE THICKNESS IN THOROUGHBRED RACEHORSES

The purpose of this study was to determine whether delayed gadolinium‐enhanced magnetic resonance imaging of cartilage (dGEMRIC) and T2 mapping are accurate techniques for measuring cartilage thickness in the metacarpus3/metatarsus3 (Mc3/Mt3) of Thoroughbred racehorses. Twenty‐four Mc3/Mt3 cadaver specimens were acquired from six healthy racehorses. Cartilage thickness was measured from postintra‐articular Gd‐DTPA^2? images acquired using short tau inversion recovery (STIR), and proton density weighted (PDw) sequences, and compared with cartilage thickness measured from corresponding histologic images. Two observers performed each histologic measurement twice at three different sites, with measurement times spaced at least 5 days apart. Histologic cartilage thickness was measured at each of the three sites from the articular surface to the bone–cartilage interface, and from the articular surface to the mineralized cartilage interface (tidemark ) . Intra‐observer repeatability was good to moderate for dGEMRIC where Mc3/Mt3 cartilage was not in contact with the proximal phalanx. Where the Mc3/Mt3 cartilage was in contact with the proximal phalanx cartilage, dGEMRIC STIR and T2 mapping PDw cartilage thicknesses of Mc3/Mt3 could not be measured reliably. When measured from the articular surface to the bone–cartilage interface, histologic cartilage thickness did not differ from STIR or PDw cartilage thickness at the site where the Mc3/Mt3 cartilage surface was separated from the proximal phalanx cartilage (P > 0.05). Findings indicated that dGEMRIC STIR and T2 mapping PDw are accurate techniques for measuring Mc3/Mt3 cartilage thickness at locations where the cartilage is not in direct contact with the proximal phalanx cartilage.     

EVALUATION OF THE WARP‐TURBO SPIN ECHO SEQUENCE FOR 3 TESLA MAGNETIC RESONANCE IMAGING OF STIFLE JOINTS IN DOGS WITH STAINLESS STEEL TIBIAL PLATEAU LEVELING OSTEOTOMY IMPLANTS

Susceptibility artifacts caused by ferromagnetic implants compromise magnetic resonance imaging (MRI) of the canine stifle after tibial plateau leveling osteotomy (TPLO) procedures. The WARP‐turbo spin echo sequence is being developed to mitigate artifacts and utilizes slice encoding for metal artifact reduction. The aim of the current study was to evaluate the WARP‐turbo spin echo sequence for imaging post TPLO canine stifle joints. Proton density weighted images of 19 canine cadaver limbs were made post TPLO using a 3 Tesla MRI scanner. Susceptibility artifact sizes were recorded and compared for WARP vs. conventional turbo spin echo sequences. Three evaluators graded depiction quality for the tibial tuberosity, medial and lateral menisci, tibial osteotomy, and caudal cruciate ligament as sufficient or insufficient to make a diagnosis. Artifacts were subjectively smaller and local structures were better depicted in WARP‐turbo spin echo images. Signal void area was also reduced by 75% (sagittal) and 49% (dorsal) in WARP vs. conventional turbo spin echo images. Evaluators were significantly more likely to grade local anatomy depiction as adequate for making a diagnosis in WARP‐turbo spin echo images in the sagittal but not dorsal plane. The proportion of image sets with anatomic structure depiction graded adequate to make a diagnosis ranged from 28 to 68% in sagittal WARP‐turbo spin echo images compared to 0–19% in turbo spin echo images. Findings indicated that the WARP‐turbo spin echo sequence reduces the severity of susceptibility artifacts in canine stifle joints post TPLO. However, variable depiction of local anatomy warrants further refinement of the technique.     

FUNCTIONALITY OF VETERINARY IDENTIFICATION MICROCHIPS FOLLOWING LOW‐ (0.5 TESLA) AND HIGH‐FIELD (3 TESLA) MAGNETIC RESONANCE IMAGING

The ability to read patient identification microchips relies on the use of radiofrequency pulses. Since radiofrequency pulses also form an integral part of the magnetic resonance imaging (MRI) process, the possibility of loss of microchip function during MRI scanning is of concern. Previous clinical trials have shown microchip function to be unaffected by MR imaging using a field strength of 1 Tesla and 1.5. As veterinary MRI scanners range widely in field strength, this study was devised to determine whether exposure to lower or higher field strengths than 1 Tesla would affect the function of different types of microchip. In a phantom study, a total of 300 International Standards Organisation (ISO)‐approved microchips (100 each of three different types: ISO FDX‐B 1.4 × 9 mm, ISO FDX‐B 2.12 × 12 mm, ISO HDX 3.8 × 23 mm) were tested in a low field (0.5) and a high field scanner (3.0 Tesla). A total of 50 microchips of each type were tested in each scanner. The phantom was composed of a fluid‐filled freezer pack onto which a plastic pillow and a cardboard strip with affixed microchips were positioned. Following an MRI scan protocol simulating a head study, all of the microchips were accurately readable. Neither 0.5 nor 3 Tesla imaging affected microchip function in this study.     

COMPARISONS AMONG RADIOGRAPHY,ULTRASONOGRAPHY AND COMPUTED TOMOGRAPHY FOR EX VIVO CHARACTERIZATION OF STIFLE OSTEOARTHRITIS IN THE HORSE

A better understanding of imaging characteristics of equine stifle osteoarthritis (OA) may allow earlier detection and improve prognosis. Objectives of this ex vivo, prospective, methods comparison study were to (1) describe the location and severity of naturally acquired OA lesions in the equine stifle using ultrasound (US), radiography (XR), computed tomography (CT), and macroscopic evaluation (ME); (2) compare the diagnostic performance of each imaging modality with ME; and (3) describe subchondral bone mineral density (BMD) in equine stifle joints with OA using CT. Radiographic, CT, and US evaluations were performed on 23 equine cadaver stifles and compared with ME. Significant associations were found between osteophyte global scores for all imaging modalities (CT, P ? 0.0001; XR, P = 0.005; US, P = 0.04) vs. ME osteophyte global scores. Osteophytes were detected most frequently in the medial femorotibial (MFT) joint. A specific pattern of osteophytes was observed, with a long ridge of new bone at the insertion of the MFT joint capsule cranially on the medial femoral condyle. A novel caudo‐10°proximo‐5°lateral‐cranio‐disto‐medial oblique radiographic projection was helpful for detection of intercondylar osteophytes. Multiplanar CT reformatted images were helpful for characterizing all osteophytes. Osteophyte grades at most sites did not differ among modalities. Low sensitivity/specificity for subchondral bone sclerosis and flattening of femoral condyles suggested that these signs may not be reliable radiographic and CT indicators of equine stifle OA. Equine stifle OA was associated with a decrease in BMD and specific sites of focal subchondral bone resorption/cyst formation were found in some specimens.     

THE USE OF MAGNETIC RESONANCE IMAGING IN EVALUATING HORSES WITH SPINAL ATAXIA

To determine the accuracy of magnetic resonance imaging for diagnosing cervical stenotic myelopathy in horses, 39 horses with spinal ataxia and 20 control horses underwent clinical and neurologic examinations, cervical radiographs, euthanasia, magnetic resonance (MR) imaging of the cervical spine and necropsy. Twenty‐four horses were diagnosed with cervical stenotic myelopathy, 5 with cervical vertebral stenosis, 7 with idiopathic ataxia, 3 horses had other causes of ataxia, and 20 were controls. The MR images were assessed for spinal cord intensity changes, presence of spinal cord compression, spinal cord compression direction, shape of spinal cord, and the presence of synovial cysts, joint mice, and degenerative joint disease. The height, width, and area of the spinal cord, dural tube and vertebral canal were measured. The identification of spinal cord compression on MR images was significantly different in horses with cervical stenotic myelopathy (P < 0.02), but in the cervical stenotic myelopathy group the identification of spinal cord compression on MR images had poor to slight agreement with histopathologic evidence of compression (κ = 0.05). Horses with cervical stenotic myelopathy were more likely to have a T2 hyperintensity in the spinal cord (P < 0.05). Horses with cervical stenotic myelopathy or cervical vertebral stenosis were more likely to have degenerative joint disease than control horses or horses with other or idiopathic ataxia.     

DYNAMIC CONTRAST‐ENHANCED MAGNETIC RESONANCE IMAGING OF CANINE BRAIN TUMORS

We evaluated dynamic contrast‐enhanced magnetic resonance imaging (DCE‐MRI) in canine brain tumors. Magnetic resonance data sets were collected on seven canine intracranial tumors with a 3 T magnet using a T1‐weighted fast spin echo fluid attenuated inversion recovery sequence after an IV bolus injection (0.2 mmol/kg) of Gd‐DTPA. The tumors were confirmed histopathologically as adenocarcinoma (n=1), ependymoma (n=1), meningioma (n=3), oligodendroglioma (n=1), and pituitary macroadenoma (n=1) The data were analyzed using a two‐compartment pharmacokinetic model for estimation of three enhancement parameters, E_R (rate of enhancement), K_el (rate of elimination), and K_ep (rate constant), and a model‐free phenomenologic parameter initial area under the Gd concentration curve (IAUGC) defined over the first 90 s postenhancement. Pearson's correlations were calculated between parameters of the two methods. The IAUGC has a relatively strong association with the rate of enhancement E_R, with r ranges from 0.4 to 0.9, but it was weakly associated with K_ep and K_el. To determine whether any two tumors differed significantly, the Kolmogorov–Smirnov test was used. The results showed that there were statistical differences (P<0.05) between distributions of the enhancement pattern of each tumor. These kinetic parameters may characterize the perfusion and vascular permeability of the tumors and the IAUGC may reflect blood flow, vascular permeability, and the fraction of interstitial space. The kinetic parameters and the IAUGC derived from DCE‐MRI present complementary information and they may be appropriate to noninvasively differentiate canine brain tumors although a larger prospective study is necessary.