Opacity Transfer Function Optimization for Volume-rendered Computed Tomography Images of the Prostate 1

Abstract

The selection of an opacity transfer function is essential for volume visualization. Computed tomography (CT) scans of the pelvis were used to determine an optimal opacity transfer function for use in radiotherapy. On sample datasets (a mathematical phantom and a patient pelvis CT scan), standard viewing orientations were selected to render the prostate. Opacity functions were selected via (1) trapezoidal manual selection, (2) trapezoidal semiautomatic selection, and (3) histogram volume-based selection. Using an established metric, the errors using each of these methods were computed. Trapezoidal manual opacity function optimization resulted in visually acceptable images, but the errors were considerable (6.3-9.1 voxel units). These errors could be reduced with the use of trapezoidal semiautomatic selection (4.9-6.2 voxel units) or with histogram volume-based selection (4.8-7.9 voxel units). As each visualization algorithm focused on enhancing the boundary of the prostate using a different approach, the scene information was considerably different using the three techniques. Improved volume visualization of soft tissue interfaces was achieved using automated optimal opacity function determination, compared with manual selection.