WE AB 207A 11: Respiratory Motion Guided 4DCBCT A Step Towards Controlling 4DCBCT Image Quality.
PURPOSE We have developed a method, called respiratory motion guided 4DCBCT (RMG 4DCBCT) in which the gantry speed and projection frequency are varied in response to the patient's real time respiratory signal to eliminate streaking artifacts and to suppress duplicate projections in 4DCBCT images. In 2015, we realized RMG 4DCBCT on an Elekta Synergy linear accelerator with a mechanical relay to suppress projections and a potentiometer to adjust the gantry speed in response to the patient's real time respiratory signal. The aim of this study was to analyse the image quality to determine what can and cannot be controlled. METHODS Using RMG 4DCBCT, we acquired 40 (RMG 4DCBCT_40) and 60 (RMG 4DCBCT_60) equally spaced projections per respiratory phase of the CIRS dynamic thorax phantom with breathing periods from 2s to 8s and two breathing traces from lung cancer patients. The contrast to noise ratio (CNR) and edge response width (ERW) were used to compare image quality between RMG 4DCBCT and conventional 4DCBCT. RESULTS Regardless of the breathing period, for RMG 4DCBCT, the CNR is approximately 7 and 9 with RMG 4DCBCT_40 and RMG 4DCBCT_60 respectively. Conventional 4DCBCT has a CNR dropping from 20 down to 6 as the breathing period drops from 2s to 8s. With RMG 4DCBCT, the ERW, in the direction of phantom motion, ranges from 2.1mm to 2.5mm as the breathing period drops from 2s to 8s which compares to a higher range of 2.0mm to 2.5mm with conventional 4DCBCT. Images with similar quality to conventional 4DCBCT can be acquired with RMG 4DCBCT_40 which has a 70% reduction in imaging dose. CONCLUSION The image contrast can be controlled with RMG 4DCBCT regardless of the patients breathing rate. However, although the image sharpness is better with RMG 4DCBCT, image sharpness has a small dependence on the breathing period; the accuracy of registration and segmentation will therefore vary with the patient's breathing period. This project was supported by a National Health and Medical Research Council (NHMRC) project grant 1034060 and Cancer Australia grant number 1084566.
Medical physics