ACRF Image X Institute, Biomedical Building (C81), 1 Central Ave, Australian Technology Park, Eveleigh, NSW 2015, Australia. Department of Medical Physics and Radiation Engineering, The Canberra Hospital, Canberra, ACT 2605, Australia.
Phys Med Biol. 2019 Mar 27;64(7):07NT01. doi: 10.1088/1361-6560/ab065d.
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.
四维锥形束计算机断层扫描（4D CBCT）提高了移动肿瘤患者的放射治疗定位精度。通常，4D CBCT 需要数百次 X 射线投影来测量目标轨迹，并且成像频率不适应患者的呼吸信号，导致过采样。相比之下，呼吸触发的 4D CBCT（RT 4D CBCT）是一种已实验实施的采集技术，已证明可以减少 X 射线投影的数量，从而减少 4D CBCT 剂量，而对图像质量的影响最小。本工作的目的是实验研究 RT 4D CBCT 原位，并测量从该方法获得的目标轨迹平均位置、图像质量和成像剂量。使用可编程目标运动的商业可用体模，为每个目标轨迹采集 4D CBCT 数据集，使用 RT 4D CBCT 采集技术和常规 4D CBCT 采集技术。从重建的 4D CBCT 数据集中，计算并比较了两种技术之间的目标轨迹平均位置、成像剂量和图像质量指标。通过跟踪体模中目标的位移来测量目标轨迹和平均位置；通过计数采集的 X 射线投影总数来测量成像剂量；通过计算对比度噪声比（CNR）、信噪比（SNR）和边缘响应宽度（ERW）来评估图像质量。对于这九个病例中的每一个，RT 4D CBCT 和常规 4D CBCT 确定的目标轨迹平均位置与参考源轨迹平均位置的偏差不超过 0.7 毫米，除了一个常规 4D CBCT 平均位置偏差 1.3 毫米的病例。在这九个研究的平均值中，RT 4D CBCT 所需的投影数量是常规 4D CBCT 的一半，成像剂量减少了 50%。总体而言，RT 4D CBCT 的图像质量指标（CNR 和 SNR）稍差；ERW 指标在 RT 4D CBCT 和常规 4D CBCT 重建数据集之间没有统计学上的显著差异。呼吸触发的 4D CBCT 将实时呼吸信号与 4D CBCT 图像采集系统耦合，并需要比常规 4D CBCT 更少的成像剂量来确定目标轨迹平均位置。
