RESUMO
Fatigue-related subchondral bone injuries of the third metacarpal/metatarsal (McIII/MtIII) bones are common causes of wastage, and they are welfare concerns in racehorses. A better understanding of bone health and strength would improve animal welfare and be of benefit for the racing industry. The porosity index (PI) is an indirect measure of osseous pore size and number in bones, and it is therefore an interesting indicator of bone strength. MRI of compact bone using traditional methods, even with short echo times, fail to generate enough signal to assess bone architecture as water protons are tightly bound. Ultra-short echo time (UTE) sequences aim to increase the amount of signal detected in equine McIII/MtIII condyles. Cadaver specimens were imaged using a novel dual-echo UTE MRI technique, and PI was calculated and validated against quantitative CT-derived bone mineral density (BMD) measures. BMD and PI are inversely correlated in equine distal Mc/MtIII bone, with a weak mean r value of -0.29. There is a statistically significant difference in r values between the forelimbs and hindlimbs. Further work is needed to assess how correlation patterns behave in different areas of bone and to evaluate PI in horses with and without clinically relevant stress injuries.
RESUMO
BACKGROUND: Low-field magnetic resonance imaging (MRI) is widely available to equine veterinarians yet is insensitive at detecting cartilage damage in the distal interphalangeal joint (DIPJ). T2 mapping is a quantitative imaging technique that can detect cartilage damage before morphological change is apparent. OBJECTIVES: Validation of a T2 mapping sequence on a low-field MR system. Correlation of the mean T2 relaxation time in sections of cartilage with varying levels of pathology using low- and high-field MRI. STUDY DESIGN: Cross-sectional study. METHODS: Eight phantoms with known (nominal) T2 values underwent low-field (0.27 T) MRI and 38 ex vivo DIPJs were imaged. A further 9 ex vivo DIPJs were imaged on both the low- and high-field MR systems. Immediately after imaging, the DIPJs were disarticulated and samples collected for histology. Histological sections were graded using the Osteoarthritis Research Society International (OARSI) scoring system. Fiji ImageJ software with the MRIAnalysisPak plugin was used to calculate T2 maps and draw the regions of interest (ROIs). RESULTS: There was close agreement between the nominal and the measured T2 values in the phantom study. Spearman's rank correlation demonstrated significant positive correlation between low- and high-field T2 measurements, rho 0.644 (p < 0.001). The intrarater agreement for T2 measurements was excellent, intraclass correlation coefficient (ICC) = 0.99 (95% confidence interval [CI] = 0.99-1.00), the inter-rater agreement was excellent, ICC = 0.88 (95% CI = 0.82-0.92) and there was good intrarater agreement for OARSI scores (к = 0.76). MAIN LIMITATIONS: Only a small number of histological samples were analysed. Both articular cartilage surfaces were measured within the ROI. There were no OARSI grade 0 control samples. CONCLUSIONS: A T2 mapping sequence on a low-field 0.27 T MR system was validated. There was a positive correlation between low- and high-field T2 measurements. The findings suggest a higher mean T2 relaxation time in pathological cartilage tissue examined in this study compared to normal equine cartilage tissue.