Quantitative assessment of four-dimensional CT image acquisition quality

Abstract

The purpose of the present work is to describe the development and validation of a series of tests to assess the quality of four-dimensional (4D) computed tomography (CT) imaging as it is applied to radiation treatment planning. Using a commercial respiratory motion phantom as well as a programmable moving platform with a CT phantom, we acquired 4D CT data sets on two commercial multislice helical CT scanners, which use different approaches to 4D CT image reconstruction. Data sets were obtained as the platform moved in different patterns designed to simulate various breathing patterns. Known inserts in the phantom were contoured and statistics were generated to evaluate properties important to radiation therapy, namely accuracy of phase-binning, shape, volume, and CT number. Phase-binning accuracy varied by as much as 5% for a 4D procedure in which images were reconstructed, then binned, but exhibited no variation for a 4D procedure in which projections were binned prior to reconstruction. The magnitude of geometric distortion was found to be small for both approaches, as was the magnitude of volume error. Partial volume effects in the direction perpendicular to the transverse planes of reconstruction affected volume accuracy, however. CT numbers were reproduced accurately, but 4D images exhibited more variation in CT number than static CT images. Characterization of such properties can be used to better understand and optimize the various parameters that affect 4D CT image quality.