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.     

EFFECT OF DELAYED ACQUISITION TIMES ON GADOLINIUM‐ENHANCED MAGNETIC RESONANCE IMAGING OF THE PRESUMABLY NORMAL CANINE BRAIN

A delay in imaging following intravenous contrast medium administration has been recommended to reduce misdiagnoses. However, the normal variation of contrast enhancement in dogs following a delay has not been characterized. Contrast‐enhanced MR imaging of 22 dogs was assessed, in terms of identification of normal anatomic structures, to investigate the variation associated with 10‐min delay between contrast medium administration and imaging. All dogs had a normal brain MR imaging study and unremarkable cerebrospinal fluid. Specific regions of interest were assessed both objectively, using computer software, and subjectively using three observers. Mean contrast enhancement >10% was seen in the pituitary gland, choroid plexus, meninges, temporal muscle, trigeminal nerve, and the trigeminal nerve root. Structures with an active blood–brain barrier had minimal contrast enhancement (<6%). Enhancing structures had significantly more contrast enhancement at t=1 min vs. t=10 min, except in temporal muscle, the trigeminal nerve and the trigeminal nerve root. Interobserver agreement was moderate to good in favor of the initial postcontrast T1‐weighted (T1w) sequence. The observers found either no difference or poor agreement in identification of the nonvascular structures. Intraobserver agreement was very good with all vascular structures and most nonvascular structures. A degree of meningeal enhancement was a consistent finding. The initial acquisition had higher enhancement characteristics and observer agreement for some structures; however, contrast‐to‐noise was comparable in the delayed phase or not significantly different. We provide baseline references and suggest that the initial T1w postcontrast sequence is preferable but not essential should a delayed postcontrast T1w sequence be performed.     

DETERMINATION OF T1 RELAXATION TIME OF NORMAL EQUINE TENDONS USING MAGIC ANGLE MAGNETIC RESONANCE IMAGING

Seven isolated equine front limbs were used to establish the normal T1 relaxation time of equine superficial digital flexor tendon (SDFT), deep digital flexor tendon (DDFT), and suspensory ligament (SL) using magic angle magnetic resonance (MR) imaging. MR imaging of the metacarpi was performed with the limbs positioned at 55° (the magic angle) relative to the main magnetic field. Transverse spin‐echo proton density and inversion recovery images were acquired. T1 relaxation time was calculated based on ratios of signal intensity determined from the different pulse sequences. T1 relaxation times for SDFT, DDFT, and SL were 288 (±17), 244 (±14), and 349 (±16) ms, respectively. The difference in T1 values between SDFT, DDFT, and SL was statistically significant. T1 values of equine tendons can be determined with magic angle imaging on a clinical MR system using <10 min total scan time. The knowledge of the normal range of T1 values may be useful to identify horses with chronic tendinopathy, where based on the human literature, an increased T1 value may be expected.     

FEASIBILITY FOR DETECTING LIVER METASTASES IN DOGS USING GADOBENATE DIMEGLUMINE‐ENHANCED MAGNETIC RESONANCE IMAGING

Early detection of liver metastases may improve the prognosis for successful treatment in dogs with primary tumors. Hepatobiliary‐specific contrast agents have been shown to allow an increase in magnetic resonance imaging (MRI) detection of liver metastases in humans. The purpose of this prospective study was to test the feasibility for using one of these agents, gadobenate dimeglumine, to detect liver metastases in dogs. Ten consecutive dogs known to have a primary tumor were recruited for inclusion in the study. All dogs were scanned using the same protocol that included a T2‐weighted respiratory‐triggered sequence, T1 VIBE, diffusion‐weighted imaging, and 3D‐FLASH before and after dynamic injection of gadobenate dimeglumine contrast medium. Delayed imaging was performed less than 30 min after injection and up to 60 min in two cases. Histological analysis of liver lesions identified in delayed phases was performed for each case and confirmed metastatic origin. In all cases, lesion number detected in hepatobiliary contrast‐enhanced sequences was statistically higher than in other sequences. Optimal lesion detection occurred with a 3D‐FLASH sequence acquired in the transverse plane and less than 30 min after injection. Findings indicated that gabobenate dimeglumine enhanced MRI is a feasible technique for detecting liver metastases in dogs.     

SALINE ARTHROGRAPHY OF THE DISTAL INTERPHALANGEAL JOINT FOR LOW‐FIELD MAGNETIC RESONANCE IMAGING OF THE EQUINE PODOTROCHLEAR BURSA: FEASIBILITY STUDY

Abnormalities of the deep digital flexor tendon, navicular bone, and collateral sesamoidean ligament can be difficult to visualize using magnetic resonance imaging (MRI) if bursal fluid is absent. The use of saline podotrochlear bursography improves podotrochlear apparatus evaluation, however, the technique has disadvantages. The objective of this prospective feasibility study was to describe saline arthrography of the distal interphalangeal joint as an alternative technique for improving MRI visualization of the deep digital flexor tendon, navicular bone, collateral sesamoidean ligament, and podotrochlear bursa, and to compare this technique with saline podotrochlear bursography. Eight paired cadaver forelimbs were sampled. Saline podotrochlear bursography or saline arthrography techniques were randomly assigned to one limb, with the alternate technique performed on the contralateral limb. For precontrast and postcontrast studies using each technique, independent observers scored visualization of the dorsal aspect of the deep digital flexor tendon, palmar aspect of the navicular bone, collateral sesamoidean ligament, and podotrochlear bursa. Both contrast techniques improved visualization of structures over precontrast MR images and visualization scores for both techniques were similar. Findings from this study demonstrated that saline arthrography is feasible and comparable to saline podotrochlear bursography for producing podotrochlear bursa distension and separation of the structures of the podotrochlear apparatus on nonweight bearing limbs evaluated with low‐field MRI. Clinical evaluation of saline arthrography on live animals is needed to determine if this technique is safe and effective as an alternative to saline podotrochlear bursography in horses with suspected pathology of the podotrochlear apparatus.     

THE USE OF SMALL FIELD‐OF‐VIEW 3 TESLA MAGNETIC RESONANCE IMAGING FOR IDENTIFICATION OF ARTICULAR CARTILAGE DEFECTS IN THE CANINE STIFLE: AN EX VIVO CADAVERIC STUDY

Noninvasive identification of canine articular cartilage injuries is challenging. The objective of this prospective, cadaveric, diagnostic accuracy study was to determine if small field‐of‐view, three tesla magnetic resonance imaging (MRI) was an accurate method for identifying experimentally induced cartilage defects in canine stifle joints. Forty‐two canine cadaveric stifles (n = 6/group) were treated with sham control, 0.5, 1.0, or 3.0 mm deep defects in the medial or lateral femoral condyle. Proton density‐weighted, T1‐weighted, fast‐low angle shot, and T2 maps were generated in dorsal and sagittal planes. Defect location and size were independently determined by two evaluators and compared to histologic measurements. Accuracy of MRI was determined using concordance correlation coefficients. Defects were identified correctly in 98.8% (Evaluator 1) and 98.2% (Evaluator 2) of joints. Concordance correlation coefficients between MRI and histopathology were greater for defect depth (Evaluator 1: 0.68–0.84; Evaluator 2: 0.76–0.83) compared to width (Evaluator 1: 0.30–0.54; Evaluator 2: 0.48–0.68). However, MRI overestimated defect depth (histopathology: 1.65 ± 0.94 mm; Evaluator 1, range of means: 2.07–2.38 mm; Evaluator 2, range of means: 2–2.2 mm) and width (histopathology: 6.98 ± 1.32 mm; Evaluator 1, range of means: 8.33–8.8 mm; Evaluator 2, range of means: 6.64–7.16 mm). Using the paired t‐test, the mean T2 relaxation time of cartilage defects was significantly greater than the mean T2 relaxation time of adjacent normal cartilage for both evaluators (P < 0.0001). Findings indicated that MRI is an accurate method for identifying cartilage defects in the cadaveric canine stifle. Additional studies are needed to determine the in vivo accuracy of this method.