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Effect of Varying CT Section Width on Volumetric Measurement of Lung Tumors and Application of Compensatory Equations¹

¹ From the Departments of Radiology (H.T.W.M., S.G.J., C.A.M., Y.L., A.M.A., R.D.T.) and Radiation Oncology (R.C.M.), Indiana University School of Medicine, Indianapolis. Received July 11, 2002; revision requested August 29; final revision received February 11, 2003; accepted March 3. Address correspondence to H.T.W.M., 11224 Clarkston Rd, Zionsville, IN 46077 (e-mail: hwinermu@iupui.edu).

View larger version (27K): [in a new window]   Figure 1a. Plots of mean magnification versus section width for the three simulated tumor volumes measured with (a) elliptical and (b) perimeter methods. Note the generally linear relationship for each tumor volume, the decrease in slope with increasing tumor volume, and how each line extrapolated to the left intercepts the y axis at a magnification of approximately 1 as the section width (on x axis) approaches 0.

View larger version (27K): [in a new window]   Figure 1b. Plots of mean magnification versus section width for the three simulated tumor volumes measured with (a) elliptical and (b) perimeter methods. Note the generally linear relationship for each tumor volume, the decrease in slope with increasing tumor volume, and how each line extrapolated to the left intercepts the y axis at a magnification of approximately 1 as the section width (on x axis) approaches 0.

View larger version (42K): [in a new window]   Figure 2. Depiction of decreasing partial volume effect with decreasing section width (shown along z axis). The tumor cross section (outlined by thick lines forming circles) is measured as the sum of the areas depicted by the gray-shaded voxels. The speckled voxels outside the circle on the right represent the decrease in measured volume as section width decreases from thick (left) to thin (right). These results indicate improved volume measurement accuracy with thin sections. The partial volume effect is most apparent on the z axis.

View larger version (45K): [in a new window]   Figure 3. Depiction of decreasing partial volume effect with increasing tumor size. The tumor cross section (outlined by thick lines forming circles) is measured as the sum of the volumes depicted by the light gray- and dark gray-shaded voxels. As tumor size increases from 8 units in diameter (left) to 14 units in diameter (right), the ratio of surface voxels (light gray) to interior voxels (dark gray) decreases from 28:60 (47%) to 52:172 (30%).

View larger version (17K): [in a new window]   Figure 4. Illustration of how tumors with diameters of less than one section width may have great variations in repeated volume measurements. The cross-sectional area of the tumor will be more accurately measured at time A than at time B. The z axis of the tumor at time A is contained within one section width, whereas the z axis of the tumor at time B is straddling two sections.