T2 mapping of cartilage in the equine distal interphalangeal joint with corresponding histology using 0.27 T and 3.0 T MRI

Background
Low field 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 system. Immediately after imaging, the DIPJs were disarticulated and samples collected for histology. Histological sections were graded using the OARSI scoring system. Fiji ImageJ software with the MRIAnalysisPak plugin was used to calculate T2 maps and draw the ROIs.

Results
There was close agreement between the nominal and 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
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.