Real-time respiratory triggered four dimensional cone-beam CT halves imaging dose compared to conventional 4D CBCT

Four dimensional cone-beam computed tomography (4D CBCT) improves patient positioning and the accuracy of radiation therapy for patients with mobile tumours. Generally, 4D CBCT requires many hundreds of x-ray projections to measure target trajectories and the imaging frequency is not adapted to the patient's respiratory signal resulting in over-sampling. In contrast, respiratory triggered 4D CBCT (RT 4D CBCT) is an acquisition technique that has been experimentally implemented and has shown to reduce the number of x-ray projections and thus 4D CBCT dose with minimal impact on image quality. The aim of this work is to experimentally investigate RT 4D CBCT in situ and measure target trajectory mean position, image quality and imaging dose from this approach. A commercially available phantom with programmable target motion was programmed with nine target trajectories derived from patient-measured respiratory traces known to span the range of image quality when used for 4D CBCT reconstruction. 4D CBCT datasets were acquired for each target trajectory using the RT 4D CBCT acquisition technique and the conventional 4D CBCT acquisition technique. From the reconstructed 4D CBCT datasets, target trajectory mean positions, imaging dose and image quality metrics were calculated and compared between the two techniques. Target trajectory and mean position were measured by tracking the target's displacement in the phantom; imaging dose was measured by counting the total number of x-ray projections acquired; and image quality was assessed by calculating the contrast-to-noise ratio (CNR), signal-to-noise ration (SNR) and edge response width (ERW). For each of the nine cases, the target trajectory mean position as determined by RT 4D CBCT and conventional 4D CBCT varied from the reference source trajectory mean position by 0.7 mm or less except for one case where a conventional 4D CBCT mean position varied by 1.3 mm. On the average of these nine studies, RT 4D CBCT required half as many projections as conventional 4D CBCT giving a 50% reduction in imaging dose. Overall, the image quality metrics (CNR and SNR) were marginally worse for RT 4D CBCT; ERW metric showed no statistically significant difference between the RT 4D CBCT and conventional 4D CBCT reconstructed datasets. Respiratory triggered 4D CBCT couples the real-time respiratory signal to the 4D CBCT image acquisition system and requires less imaging dose than conventional 4D CBCT to determine target trajectory mean positions.