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

Abstract

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.

Figure 1

Top- schematic of phantom used to assess the detection limits of MDCT in the transverse plane. The longitudinal (L) imaging plane is also shown. Simulated cartilage thicknesses of 4.0, 2.0, 1.0, 0.75, 0.5, and 0.25 mm with constant joint space of 2.0 mm were used in chambers 1-6, respectively. A constant thickness of 2.0 mm with joint spaces of 1.0, 0.5, and 0.25 mm were used in chambers 7-9, respectively. Middle- exploded view of chamber #1 detailing: A) nylon center cylinder to represent trabecular bone, B) 1 mm thick aluminum sleeve to represent cortical bone, C) polycarbonate sleeve to represent cartilage, D) joint space, E) polycarbonate four-pronged spacer for creating the joint space, and F) bulk of the phantom body made using nylon. Bottom- CT scan of the phantom with contrast agent and inset showing image details of chamber #1. Letter call-outs correspond to the same details provided above.

Figure 2

Simulated cartilage RMS (top) and mean residual (bottom) reconstruction errors for the transverse contrast enhanced scan datasets as a function of contrast agent concentration. RMS errors grew progressively as the contrast agent concentration increased (for thickness > 0.75 mm). The directionality of the error was dependent on the contrast agent concentration and simulated cartilage thickness.

Figure 3

Simulated cartilage RMS (top) and mean residual (bottom) reconstruction errors for the transverse contrast enhanced scan datasets at 50% concentration as a function of imaging plane direction and spatial resolution. Errors were greatest for the anisotropic longitudinal data reconstructions. The longitudinal isotropic reconstructions yielded errors more consistent with the transverse scan results.

Figure 4

Simulated cartilage RMS errors as a function of joint space thickness, contrast agent concentration, imaging plane direction, and spatial resolution. Errors increased as contrast agent concentration increased. Reconstruction errors from the isotropic longitudinal dataset were less than the anisotropic dataset in the same imaging plane.

Figure 5

Simulated cartilage RMS (top) and mean residual (bottom) reconstruction errors for the non-enhanced scan datasets at 200 mAs as a function of imaging plane direction and spatial resolution. RMS errors for the longitudinal isotropic datasets were consistently less than the anisotropic dataset for simulated cartilage less than 2.0 mm thick.