Cartilage thickness: factors influencing multidetector CT measurements in a phantom study.

PURPOSE To prospectively assess in a phantom the reconstruction errors and detection limits of cartilage thickness measurements obtained with multidetector computed tomographic (CT) arthrography, as a function of contrast agent concentration, scanning direction, spatial resolution, joint spacing, and tube current, with known measurements as the reference standard. MATERIALS AND METHODS A phantom with nine chambers was constructed. Each chamber had a nylon cylinder encased by sleeves of aluminum and polycarbonate to simulate trabecular bone, cortical bone, and cartilage. Varying simulated cartilage thicknesses and 10 joint space widths were assessed. On 3 days, the phantom was scanned with and without contrast agent administration and with the chamber axes both perpendicular and parallel to the scanner axis. Images were reconstructed at 1.0- and 0.5-mm intervals. Contrast agent concentration and tube current were varied. The simulated cartilage thickness was determined by using image segmentation. Root mean squared errors and mean residual errors were used to characterize the measurements. The reproducibility of the CT scanner and image segmentation results was determined. RESULTS Simulated cartilage greater than 1.0 mm in thickness was reconstructed with less than 10% error when either no contrast agent or a low concentration (25%) of contrast agent was used. Error increased as contrast agent concentration increased. Decreasing the simulated joint space width to 0.5 mm caused slight increases in error; however, error increased substantially at joint spaces narrower than 0.5 mm. Errors in measurements derived from anisotropic CT data were greater than errors in measurements derived from isotropic data. Altering the tube current did not substantially affect reconstruction errors. CONCLUSION The study revealed lower boundaries and the repeatability of simulated cartilage thickness measurements obtained by using multidetector CT arthrography and yielded data pertinent to choosing the contrast agent concentration, joint space width, scanning direction, and spatial resolution to reduce reconstruction errors.