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Cartilage Thickness: Factors Influencing Multidetector CT Measurements in a Phantom Study¹

¹ From the Department of Bioengineering and Scientific Computing and Imaging Institute (A.E.A., B.J.E., J.A.W.); and Department of Orthopedics (C.L.P., J.A.W.), University of Utah, 72 S Central Campus Dr, Room 2646, Salt Lake City, UT 84112. Received December 26, 2006; revision requested March 1, 2007; revision received April 23; accepted May 25; final version accepted July 9. Supported by the Orthopaedic Research and Education Foundation. Address correspondence to J.A.W. (e-mail: jeff.weiss{at}utah.edu).

View larger version (62K): [in this window] [in a new window] [Download PPT slide]   Figure 1a: (a) Schematic illustration of phantom used to assess detection limits of transverse multidetector CT. The longitudinal imaging plane (L) is also shown. Simulated cartilage thicknesses of 4.00, 2.00, 1.00, 0.75, 0.50, and 0.25 mm were used with a constant joint space width of 2.0 mm in chambers 1–6, respectively. A constant cartilage thickness of 2.0 mm was used with joint space widths of 1.00, 0.50, and 0.25 mm in chambers 7–9, respectively. (b) Expanded view of chamber 1 shows nylon cylinder center (A) representing trabecular bone, 1-mm-thick aluminum sleeve (B) representing cortical bone, polycarbonate sleeve (C) representing cartilage, joint space (D), polycarbonate four-pronged spacer (E) used to create joint space, and bulk of phantom body (F) constructed of nylon. (c) CT scan of phantom filled with contrast agent; inset shows image details of chamber 1; letters correspond to those in b.

View larger version (86K): [in this window] [in a new window] [Download PPT slide]   Figure 1b: (a) Schematic illustration of phantom used to assess detection limits of transverse multidetector CT. The longitudinal imaging plane (L) is also shown. Simulated cartilage thicknesses of 4.00, 2.00, 1.00, 0.75, 0.50, and 0.25 mm were used with a constant joint space width of 2.0 mm in chambers 1–6, respectively. A constant cartilage thickness of 2.0 mm was used with joint space widths of 1.00, 0.50, and 0.25 mm in chambers 7–9, respectively. (b) Expanded view of chamber 1 shows nylon cylinder center (A) representing trabecular bone, 1-mm-thick aluminum sleeve (B) representing cortical bone, polycarbonate sleeve (C) representing cartilage, joint space (D), polycarbonate four-pronged spacer (E) used to create joint space, and bulk of phantom body (F) constructed of nylon. (c) CT scan of phantom filled with contrast agent; inset shows image details of chamber 1; letters correspond to those in b.

View larger version (168K): [in this window] [in a new window] [Download PPT slide]   Figure 1c: (a) Schematic illustration of phantom used to assess detection limits of transverse multidetector CT. The longitudinal imaging plane (L) is also shown. Simulated cartilage thicknesses of 4.00, 2.00, 1.00, 0.75, 0.50, and 0.25 mm were used with a constant joint space width of 2.0 mm in chambers 1–6, respectively. A constant cartilage thickness of 2.0 mm was used with joint space widths of 1.00, 0.50, and 0.25 mm in chambers 7–9, respectively. (b) Expanded view of chamber 1 shows nylon cylinder center (A) representing trabecular bone, 1-mm-thick aluminum sleeve (B) representing cortical bone, polycarbonate sleeve (C) representing cartilage, joint space (D), polycarbonate four-pronged spacer (E) used to create joint space, and bulk of phantom body (F) constructed of nylon. (c) CT scan of phantom filled with contrast agent; inset shows image details of chamber 1; letters correspond to those in b.

View larger version (10K): [in this window] [in a new window] [Download PPT slide]   Figure 2a: Graphs illustrate simulated cartilage (a) RMS and (b) mean residual reconstruction errors for transverse contrast-enhanced CT data sets, as a function of contrast agent concentration. (a) At cartilage thicknesses greater than 0.75 mm, RMS errors increased progressively as the contrast agent concentration increased. Direction of error was dependent on contrast agent concentration and simulated cartilage thickness.

View larger version (10K): [in this window] [in a new window] [Download PPT slide]   Figure 2b: Graphs illustrate simulated cartilage (a) RMS and (b) mean residual reconstruction errors for transverse contrast-enhanced CT data sets, as a function of contrast agent concentration. (a) At cartilage thicknesses greater than 0.75 mm, RMS errors increased progressively as the contrast agent concentration increased. Direction of error was dependent on contrast agent concentration and simulated cartilage thickness.

View larger version (12K): [in this window] [in a new window] [Download PPT slide]   Figure 3a: Graphs illustrate simulated cartilage (a) RMS and (b) mean residual reconstruction errors for transverse contrast-enhanced CT data sets obtained with 50% contrast agent concentration, as a function of scanning direction and spatial resolution. Errors were greatest for the anisotropic longitudinal data reconstructions. The isotropic longitudinal reconstructions yielded errors more consistent with transverse CT results.

View larger version (12K): [in this window] [in a new window] [Download PPT slide]   Figure 3b: Graphs illustrate simulated cartilage (a) RMS and (b) mean residual reconstruction errors for transverse contrast-enhanced CT data sets obtained with 50% contrast agent concentration, as a function of scanning direction and spatial resolution. Errors were greatest for the anisotropic longitudinal data reconstructions. The isotropic longitudinal reconstructions yielded errors more consistent with transverse CT results.

View larger version (14K): [in this window] [in a new window] [Download PPT slide]   Figure 4: Graph illustrates simulated cartilage RMS errors as a function of joint space width, contrast agent concentration, scanning direction, and spatial resolution. Errors increased as contrast agent concentration increased. There were fewer reconstruction errors in the isotropic longitudinal data set than in the anisotropic data set in the same imaging plane.

View larger version (11K): [in this window] [in a new window] [Download PPT slide]   Figure 5a: Graphs illustrate simulated cartilage (a) RMS and (b) mean residual reconstruction errors for the nonenhanced CT data sets obtained at 200 mAs, as a function of scanning direction and spatial resolution. There were consistently fewer RMS errors in the isotropic longitudinal data set than in the anisotropic data set for measurements of simulated cartilage less than 2.0 mm thick.

View larger version (11K): [in this window] [in a new window] [Download PPT slide]   Figure 5b: Graphs illustrate simulated cartilage (a) RMS and (b) mean residual reconstruction errors for the nonenhanced CT data sets obtained at 200 mAs, as a function of scanning direction and spatial resolution. There were consistently fewer RMS errors in the isotropic longitudinal data set than in the anisotropic data set for measurements of simulated cartilage less than 2.0 mm thick.