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Vertebral Shape: Automatic Measurement with Active Shape Models¹

¹ From the Departments of Medical Biophysics (P.P.S., C.J.T.) and Diagnostic Radiology (J.E.A.), University of Manchester, Oxford Rd, Manchester, M13 9PT, England. From the 1997 RSNA scientific assembly. Received January 6, 1998; revision requested March 18; revision received August 11; accepted October 26. Address reprint requests to J.E.A.

View larger version (19K): [in a new window]   Figure 1. Schematic of the PDM of the shape of the L1 vertebra shows the mean shape and the limits of the first three modes of shape variation (A, B, C), which describe the most common ways in which vertebral shape can vary. The range of shapes allowed with the model always remains realistic. 1, 2, 3 = rank order of mode of variation.

View larger version (24K): [in a new window]   Figure 2. Typical gray-level profiles for a vertebra imaged with DXA.

View larger version (15K): [in a new window]   Figure 3. PDM images show the three most common modes of normal spine shape variation. Top: Normal spinal movement. Middle: Kyphosis. Bottom: Strong correlation in vertebral heights (arrows) across vertebral levels.

View larger version (61K): [in a new window]   Figure 4a. ASM search on DXA images, initialized with three manually marked points. (a) Normal DXA scan, with manually placed points marked. (b) Starting ASM search position, initialized with three points. (c) Final ASM search position, after convergence.

View larger version (64K): [in a new window]   Figure 4b. ASM search on DXA images, initialized with three manually marked points. (a) Normal DXA scan, with manually placed points marked. (b) Starting ASM search position, initialized with three points. (c) Final ASM search position, after convergence.

View larger version (63K): [in a new window]   Figure 4c. ASM search on DXA images, initialized with three manually marked points. (a) Normal DXA scan, with manually placed points marked. (b) Starting ASM search position, initialized with three points. (c) Final ASM search position, after convergence.

View larger version (15K): [in a new window]   Figure 5. A, Measurement of point-to-line error from a true contour position to a proposed search solution. B, Measurement of manual reproducibility. Dashed lines show directions in which errors are measured. Shaded areas show distribution of manually marked points.

View larger version (143K): [in a new window]   Figure 6a. ASM search in fractured lumbar vertebrae, initialized with two manually marked points. (a) Lumbar DXA scan, with end plate fractures of L1–L4 vertebrae. (b) Starting ASM search position, initialized with two points. (c) Final ASM search position, after convergence.

View larger version (124K): [in a new window]   Figure 6b. ASM search in fractured lumbar vertebrae, initialized with two manually marked points. (a) Lumbar DXA scan, with end plate fractures of L1–L4 vertebrae. (b) Starting ASM search position, initialized with two points. (c) Final ASM search position, after convergence.

View larger version (124K): [in a new window]   Figure 6c. ASM search in fractured lumbar vertebrae, initialized with two manually marked points. (a) Lumbar DXA scan, with end plate fractures of L1–L4 vertebrae. (b) Starting ASM search position, initialized with two points. (c) Final ASM search position, after convergence.

View larger version (16K): [in a new window]   Figure 7. ROC curves represent the distinguishability of normal from fractured vertebrae with three descriptions of vertebral shape.