Cartilage and subchondral bone thickness distribution with MR imaging.

RATIONALE AND OBJECTIVES The authors mapped articular cartilage (AC) and subchondral bone (SB) thicknesses in human acetabula in vitro by using magnetic resonance (MR) imaging and validated AC measurements by using light microscopy. MATERIALS AND METHODS Left and right acetabula from a deceased patient who had undergone left hemiarthroplasty were imaged with fat-suppressed spoiled gradient-recalled acquisition in the steady state (repetition time = 55 msec, echo time = 15 msec, flip angle = 50 degrees, matrix = 256 x 256, field of view = 8 cm). AC and SB thickness maps were generated from image data by using analytic geometry, which enabled correction for thickness overestimation due to oblique sectioning. Cartilage bone plugs were extracted from the acetabula, and light microscopy was used to validate the thickness measurements obtained with MR imaging. RESULTS Standard errors between thickness measurements obtained with MR imaging and light microscopy were 0.37 and 0.33 mm for the left and right AC, respectively, which is consistent with the voxel resolution of the MR imaging sequence (0.31 x 0.31 x 0.8 mm). SB thickness of the cartilage plugs could not be reliably measured with light microscopy and, therefore, could not be validated. Contour maps showed that SB thickness gradients were rapid and focal compared with the rather smooth gradients in AC thickness; however, thicker AC was accompanied by thicker SB for left (r2 = .261, P = .0001) and right (r2 = .308, P = .0001) acetabula. Average thickness differences between left and right acetabular AC and SB were 0.13 mm (P = .015) and 0.11 mm (P = .026), respectively. Although it was the operated hip that had thicker articular tissues, the differences were within the pixel resolution (< 0.31 mm). CONCLUSION AC and SB thickness distribution can be accurately determined by combining noninvasive MR imaging and analytic geometry, which may also provide a means for quantitative, longitudinal assessment of focal AC defects.