The use of digitally reconstructed radiographs for three-dimensional treatment planning and CT-simulation

Recent interest in computed tomography-simulation (CT-simulation) suggests the possibility of a shift to digital images for field verification. This article examines the quality of Digitally Reconstructed Radiographs (DRRs) to determine if they can reasonably be substituted for conventional simulator films, and suggests techniques to improve these images.

Special developmental software and computer hardware allowed extremely rapid reformatting of CT data to produce images geometrically equivalent to treatment unit portal films. The technique uses a trilinear interpolation algorithm and gives a 512 x 512 DRR for any arbitrary beam direction. Resolution in line pairs/cm (lp/cm) for both simulator radiographs and DRRs was measured with a special test phantom. Patient data was reformatted to illustrate methods for improving the quality of the DRR.

The equipment used for this study reformats 50 512 x 512 CT scans in 8 s. The resolution for a DRR is limited by the voxel size of the CT scans. For typical voxel dimensions, the resolution was found to be 7 lp/cm transverse and 1.0 lp/cm longitudinal compared to 21.0 lp/cm for a simulator radiograph. Patient movement during the scan procedure further degrades the DRR. The reduced quality of this image makes it more difficult to discern structures, and it may not always be possible to perform essential tasks such as counting vertebral bodies. However, viewing the treatment field superimposed on a DRR displayed with a step function to include only bone aided in the identification of relevant landmarks. Switching between soft tissue, bone, or air windows takes less than 10 s on the equipment used for this study, and the use of different display techniques improved the viewer's ability to evaluate field positioning.

A DRR cannot match the spatial resolution of a radiograph taken with a short exposure and small focal spot, but the ability to change the display mode for the DRR increases the usefulness of these images. Fast reformatting is particularly important when evaluation of field position requires the comparison of each portal image to a series of DRRs.