Fixed-gantry radiation therapy has been proposed as a low-cost alternative to the conventional rotating-gantry radiation therapy, that may help meet the rising global treatment demand. Fixed-gantry systems require gravitational motion compensated reconstruction algorithms to produce cone-beam CT (CBCT) images of sufficient quality for image guidance. The aim of this work was to adapt and investigate five CBCT reconstruction algorithms for fixed-gantry CBCT images. The five algorithms investigated were Feldkamp–Davis–Kress (FDK), prior image constrained compressed sensing (PICCS), gravitational motion compensated FDK (GMCFDK), motion compensated PICCS (MCPICCS) (a novel CBCT reconstruction algorithm) and simultaneous motion estimation and iterative reconstruction (SMEIR). Fixed-gantry and rotating-gantry CBCT scans were acquired of 3 rabbits, with the rotating-gantry scans used as a reference. Projections were sorted into rotation bins, based on the angle of rotation of the rabbit during image acquisition. The algorithms were compared using the structural similarity index measure root mean square error, and reconstruction time. Evaluation of the reconstructed volumes showed that, when compared with the reference rotating-gantry volume, the conventional FDK algorithm did not accurately reconstruct fixed-gantry CBCT scans. Whilst the PICCS reconstruction algorithm reduced some motion artefacts, the motion estimation reconstruction methods (GMCFDK, MCPICCS and SMEIR) were able to greatly reduce the effect of motion artefacts on the reconstructed volumes. This finding was verified quantitatively, with GMCFDK, MCPICCS and SMEIR reconstructions having RMSE 17%–19% lower and SSIM 1% higher than a conventional FDK. However, all motion compensated fixed-gantry CBCT reconstructions had a 56%–61% higher RMSE and 1.5% lower SSIM than FDK reconstructions of conventional rotating-gantry CBCT scans. The results show that motion compensation is required to reduce motion artefacts for fixed-gantry CBCT reconstructions. This paper further demonstrates the feasibility of fixed-gantry CBCT scans, and the ability of CBCT reconstruction algorithms to compensate for motion due to horizontal rotation.

固定机架放射治疗已被提议作为传统旋转机架放射治疗的低成本替代方案，这可能有助于满足不断增长的全球治疗需求。固定机架系统需要重力运动补偿重建算法来生成足够质量的锥束 CT (CBCT) 图像以进行图像引导。这项工作的目的是适应和研究固定机架 CBCT 图像的五种 CBCT 重建算法。研究的五种算法是 Feldkamp-Davis-Kress (FDK)、先验图像约束压缩感知 (PICCS)、重力运动补偿 FDK (GMCFDK)、运动补偿 PICCS (MCPICCS)（一种新型 CBCT 重建算法）以及同步运动估计和迭代重建（SMEIR）。获得了 3 只兔子的固定机架和旋转机架 CBCT 扫描，使用旋转机架扫描作为参考。根据图像采集过程中兔子的旋转角度，将投影分类到旋转箱中。使用结构相似性指标衡量均方根误差和重建时间对算法进行了比较。重建体积的评估表明，与参考旋转机架体积相比，传统的 FDK 算法不能准确地重建固定机架 CBCT 扫描。虽然 PICCS 重建算法减少了一些运动伪影，但运动估计重建方法（GMCFDK、MCPICCS 和 SMEIR）能够大大减少运动伪影对重建体积的影响。这一发现通过 GMCFDK 进行了定量验证，MCPICCS 和 SMEIR 重建的 RMSE 比传统 FDK 低 17%–19%，SSIM 高 1%。然而，所有运动补偿固定机架 CBCT 重建的 RMSE 比传统旋转机架 CBCT 扫描的 FDK 重建高 56%–61%，SSIM 低 1.5%。结果表明，需要运动补偿来减少固定机架 CBCT 重建的运动伪影。本文进一步论证了固定机架 CBCT 扫描的可行性，以及 CBCT 重建算法补偿水平旋转引起的运动的能力。结果表明，需要运动补偿来减少固定机架 CBCT 重建的运动伪影。本文进一步论证了固定机架 CBCT 扫描的可行性，以及 CBCT 重建算法补偿水平旋转引起的运动的能力。结果表明，需要运动补偿来减少固定机架 CBCT 重建的运动伪影。本文进一步论证了固定机架 CBCT 扫描的可行性，以及 CBCT 重建算法补偿水平旋转引起的运动的能力。