Institute for Research and Development on Advanced Radiation Technologies (radART), Paracelsus Medical University, Salzburg, Austria.
medPhoton GmbH, Salzburg, Austria.
Med Phys. 2024 May;51(5):3578-3589. doi: 10.1002/mp.16859. Epub 2023 Nov 28.
Misalignment or double-contouring artifacts can appear in high-resolution 3D cone beam computed tomography (CBCT) images, potentially indicating geometric accuracy issues in the projection data. Such artifacts may go unnoticed in low-resolution images and could be associated with changes in the focal spot (FS) position.
High-resolution 3D-CBCT imaging by a mobile imaging device with a large gantry clearance offers more versatility for clinical workflows in image-guided brachytherapy (IGBT), intraoperative radiation therapy (IORT), and spinal, as well as maxillofacial surgery. However, misalignment or double-contouring artifacts hinder workflow advancements in these domains. This paper introduces intrinsic calibration and geometrical correction methods as extensions to a well-established technique for addressing geometrical deviations resulting from factors such as gravity or mechanical inconsistencies. These extensions cover shifts and drifts of the FS depending on FS size selection, temperature, tube current, and tube potential. The proposed methods effectively mitigate artifacts in high-resolution CBCT images stemming from geometrical inaccuracies in projection data, without requiring additional equipment like a pinhole device.
Geometrical offsets and drifts of the x-ray tube FS were characterized on a mobile multi-purpose imaging system, the ImagingRing-m. A pinhole-like experiment was simulated by adjusting the movable collimation unit to a small rectangular aperture within the FS size range. The influence of filament selection, that is, FS size, temperature, the relatively low tube currents, as well as tube potential settings have been studied on two different monobloc types sharing the same x-ray tube insert. The Catphan 504 and an Alderson head phantom were used to assess resulting image artifacts.
Switching the FS size to one different from what was used for geometrical (gravitation, mechanical variations) calibration induced the most notable position changes of the x-ray FS, resulting in double-contouring artifacts and blurring of high-resolution 3D-CBCT images. Incorporating these shifts into a geometrical correction method effectively minimized these artifacts. Thermal drifts exhibited the second largest geometrical changes, comparable to FS size shifts across the thermal operating conditions of the x-ray system. The proposed thermal drift compensation markedly reduced thermal drift effects. Tube current and potential had little impact within the range of available tube currents, eliminating the need for compensation in current applications.
Augmenting the geometrical calibration pipeline with proposed FS drift compensations yielded significant enhancements in image quality for high-resolution reconstructions. While compensation for thermal effects posed challenges, it proved achievable. The roles of tube current and potential were found to be negligible.
在高分辨率三维锥形束计算机断层扫描（CBCT）图像中可能会出现未对准或双重轮廓伪影，这表明投影数据存在几何精度问题。这种伪影在低分辨率图像中可能不会被注意到，并且可能与焦点（FS）位置的变化有关。
具有大龙门间隙的移动成像设备的高分辨率 3D-CBCT 成像为图像引导近距离放射治疗（IGBT）、术中放射治疗（IORT）以及脊柱和颌面手术中的临床工作流程提供了更多的多功能性。然而，未对准或双重轮廓伪影阻碍了这些领域的工作流程的进展。本文介绍了固有校准和几何校正方法，这些方法是针对由于重力或机械不一致等因素导致的几何偏差的既定技术的扩展。这些扩展涵盖了 FS 的偏移和漂移，具体取决于 FS 尺寸选择、温度、管电流和管电压。所提出的方法有效地减轻了由于投影数据中的几何不准确性而导致的高分辨率 CBCT 图像中的伪影，而无需像针孔设备那样的其他设备。
在移动多用途成像系统 ImagingRing-m 上对 X 射线管 FS 的几何偏移和漂移进行了表征。通过将可移动准直单元调整到 FS 尺寸范围内的小矩形孔径，模拟了类似针孔的实验。研究了灯丝选择（即 FS 尺寸、温度、相对较低的管电流以及管电压设置）对两种具有相同 X 射线管插入件的不同单块类型的影响。使用 Catphan 504 和 Alderson 头部体模评估了产生的图像伪影。
将 FS 尺寸切换到与几何（重力、机械变化）校准不同的尺寸会引起 X 射线 FS 最明显的位置变化，从而导致双重轮廓伪影和高分辨率 3D-CBCT 图像的模糊。将这些偏移量纳入几何校正方法中可以有效地最小化这些伪影。热漂移显示出第二大的几何变化，与 X 射线系统的热工作条件下的 FS 尺寸变化相当。所提出的热漂移补偿显著降低了热漂移效应。管电流和电压在可用管电流范围内的影响很小，因此在当前应用中无需补偿。
通过为 FS 漂移补偿增强几何校准管道，显著提高了高分辨率重建的图像质量。虽然热效应补偿带来了挑战，但这是可行的。管电流和电压的作用被发现可以忽略不计。
