Basic analysis of optimal images for the development of 4D radiotherapy system

Understanding respiratory motion is necessary for accurate dose administration during stereotactic body radiation therapy. Although four-dimensional (4D) computed tomography (4DCT) is useful for understanding respiratory movements, 4D radiotherapy with a time axis is required. However, reports on the evaluation of 4DCT remain lacking. Therefore, this study was designed to evaluate the basic physical characteristics of a respiratory-gated 4DCT helical scanning system (4D-HS) using MATLABR. In this study, the physical characteristics were evaluated by examining the body-axis direction resolution, slice sensitivity profile (SSP), and volume change due to respiratory phase change and comparing multiplanar reconstruction profiles (MPRs) of slit phantoms. These characteristics were compared using normal helical scanning (N-HS). A 31% overestimation was observed in the volume fluctuation of 4D-HS based on the volume of N-HS. Furthermore, SSP with full width at half maximum (FWHM) and modulation transfer function (MTF) showed a similar resolution between 4D-HS and N-HS. The difference in FWHM is considered an offset by the voxel effect. In the MPR comparison, both 4D-HS and N-HS were visually identifiable up to a 0.5-mm slit. 4D-HS is a reconstruction system with excellent temporal resolution. Furthermore, its resolution follows respiratory dynamics and can continuously collect respiratory dynamics components. Our findings suggest the usefulness of 4D-HS in clinical practice.