Classification of histologically scored human knee osteochondral plugs by quantitative analysis of magnetic resonance images at 3T

Abstract

This work evaluates the ability of quantitative MRI to discriminate between normal and pathological human osteochondral plugs characterized by the Osteoarthritis Research Society International (OARSI) histological system. Normal and osteoarthritic human osteochondral plugs were scored using the OARSI histological system and imaged at 3 T using MRI sequences producing T1 and T2 contrast and measuring T1, T2, and T2* relaxation times, magnetization transfer, and diffusion. The classification accuracies of quantitative MRI parameters and corresponding weighted image intensities were evaluated. Classification models based on the Mahalanobis distance metric for each MRI measurement were trained and validated using leave-one-out cross-validation with plugs grouped according to OARSI histological grade and score. MRI measurements used for classification were performed using a region-of-interest analysis which included superficial, deep, and full-thickness cartilage. The best classifiers based on OARSI grade and score were T1- and T2-weighted image intensities, which yielded accuracies of 0.68 and 0.75, respectively. Classification accuracies using OARSI score-based group membership were generally higher when compared with grade-based group membership. MRI-based classification--either using quantitative MRI parameters or weighted image intensities--is able to detect early osteoarthritic tissue changes as classified by the OARSI histological system. These findings suggest the benefit of incorporating quantitative MRI acquisitions in a comprehensive clinical evaluation of OA.

Keywords: cartilage matrix; classification; imaging; osteoarthritis; quantitative MRI.

Figure 1

Standardized femur harvest locations (LA = lateral anterior, LP = lateral posterior, MA = medial anterior, and MP = medial posterior). From each location, cylindrical osteochondral samples (diameter = 5 mm; height = 5 mm) were harvested for histology (1) and MRI (2).

Figure 2

Proton density weighted images (2D spin echo with TE = 10 ms) of the ULTEM sample holder containing six human articular cartilage plugs per well and a DPBS standard. These images are representative of the slice orientation acquired for each image acquisition; a)slice through well 1 and 2; b)slice through well 3 and 4. The sample orientation is designated relative to head-first supine patient positioning with “anterior” and “left” indicated on the images with the letters “A” and “L,” respectively, and the static magnetic field B₀ oriented into the image plane. Cartilage (arrow) was delineated from subchondral bone.

Figure 3

Example of superficial (top) and deep (bottom) ROI selections drawn on a proton density weighted image (2D spin echo with TE = 10 ms) to exclude pixels at the cartilage margins. The full-thickness ROI was defined as the average SI from the superficial and deep pixels combined.