Osteoarthritis of the metacarpo/metatarsophalangeal joint is a major cause of lameness in the horse. Magnetic resonance imaging and particularly delayed gadolinium enhanced imaging of cartilage (dGEMRIC) and T2 cartilage mapping in humans has been shown to visualize cartilage matrix changes in osteoarthritis early in the disease process. T2 mapping is a non-invasive technique characterizing hyaline articular cartilage and repair tissue. In dGEMRIC, the negatively charged administered Gd-DTPA2−, penetrates hyaline cartilage in an inverse relationship to the proteoglycan concentration thereof. In osteoarthritis, proteoglycan concentration is decreased with increased penetration of Gd-DTPA2− due to a relative decrease in negative charge of the proteoglycan-depleted cartilage.
This study was performed on normal cadaver limbs of twelve euthanized racing Thoroughbreds. Six horses’ midcondylar distal third metacarpals/metatarsals (Mc3s/Mt3s) underwent six precontrast inversion recovery (IR) sequences for dGEMRIC T1 relaxation time calculation, as well as T2 mapping sequences using a 1.5T machine. Gd-DTPA2- was injected intra-articularly and the same six IR sequences repeated at 30, 60, 120, and 180 minutes post-injection at the same midcondylar sites. The distal Mc3/Mt3 cartilage thickness was measured histologically and compared to selected images of the T1 and T2 weighted sequences. T1 and T2 maps were created by fitting the respective data into mono-exponential relaxation equations for each pixel, and mean values of certain regions of interest were calculated. A second group of six horses’ fore and hind limbs were randomly assigned to two groups and the limbs either chilled or frozen, allowed to return to room temperature and scanned similarly to the first control group. Chilling and freezing effects on dGEMRIC and T2 mapping results were evaluated.
The main conclusions from this study are that IR and proton density weighted (T2 mapping) sequences can measure distal Mc3/Mt3 cartilage thickness where the cartilage doesn’t overlap with that of the proximal phalanx. However, accurate measurement was hampered by the thin cartilage in this region. dGEMRIC mapping, using intra-articular Gd-DTPA2- is a feasible technique and T1 relaxation times decrease in a similar fashion to that of the human, with the optimal time of scanning after intra-articular Gd-DTPA2- injection being 60-120 minutes. There is little effect on T1 or T2 relaxation time and mapping images after chilling and freezing of the limbs except where the magic angle effect predominates in the T2 mapping sequences.
Limitations of this study include relatively coarse spatial resolution of the thin cartilage, the overlap of the distal Mc3/Mt3 cartilage with the adjacent phalanx and the relatively low number of limbs used, resulting in low statistical power, particularly in the frozen limbs’ study. In spite of these limitations, this study provides technical information and reference values of dGEMRIC and T2 mapping in the cadaver distal Mc3/Mt3 of the normal Thoroughbred horse of value for forthcoming studies.
Future studies need to evaluate intravenous administration of Gd-DTPA2- and cartilage mapping in live exercised vs. non-exercised horses. Ultimately, dGEMRIC and T2 mapping of horse metacarpo/metatarso-phalangeal joints with differing degrees of osteoarthritis should be used to attempt to diagnose early cartilage degeneration to endeavour to halt or delay its progression.