Accuracy of MRI technique in measuring tendon cross-sectional area.

Magnetic resonance imaging (MRI) has commonly been applied to determine tendon cross-sectional area (CSA) and length either to measure structural changes or to normalize mechanical measurements to stress and strain. The ability to reproduce CSA measurements on MRI images has been reported, but the accuracy in relation to actual tendon dimensions has never been investigated. The purpose of this study was to compare tendon CSA measured by MRI with that measured in vitro with the mould casting technique. The knee of a horse was MRI-scanned with 1.5 and 3 tesla, and two examiners measured the patellar tendon CSA. Thereafter, the patellar tendon of the horse was completely dissected and embedded in an alginate cast. The CSA of the embedded tendon was measured directly by optical imaging of the cast impression. 1.5 tesla grey tendon CSA and 3 tesla grey tendon CSA were 16.5% and 13.2% lower than the mould tendon CSA, respectively. Also, 3 tesla tendon CSA, based on the red-green border on the National Institute of Health (NIH) colour scale, was lower than the mould tendon CSA by 2.8%. The typical error between examiners was below 2% for all the measured CSA. The typical error between examiners was below 2% for all the measured CSA. These data show that measuring tendon CSA on the grey-scale MRI images is associated with an underestimation, but by optimizing the measurement using a 3 tesla MRI and the appropriate NIH colour scale, this underestimation could be reduced to 2.8% compared with the direct measurements on the mould.