Division of Imaging, Diagnostics and Software Reliability, Office of Science and Engineering Laboratories, Center for Devices and Radiological Health, United States Food and Drug Administration, Silver Spring, MD, USA.
J Xray Sci Technol. 2023;31(5):865-877. doi: 10.3233/XST-230007.
Geometric calibration is essential in developing a reliable computed tomography (CT) system. It involves estimating the geometry under which the angular projections are acquired. Geometric calibration of cone beam CTs employing small area detectors, such as currently available photon counting detectors (PCDs), is challenging when using traditional-based methods due to detectors' limited areas.
This study presented an empirical method for the geometric calibration of small area PCD-based cone beam CT systems.
Unlike the traditional methods, we developed an iterative optimization procedure to determine geometric parameters using the reconstructed images of small metal ball bearings (BBs) embedded in a custom-built phantom. An objective function incorporating the sphericities and symmetries of the embedded BBs was defined to assess performance of the reconstruction algorithm with the given initial estimated set of geometric parameters. The optimal parameter values were those which minimized the objective function. The TIGRE toolbox was employed for fast tomographic reconstruction. To evaluate the proposed method, computer simulations were carried out using various numbers of spheres placed in various locations. Furthermore, efficacy of the method was experimentally assessed using a custom-made benchtop PCD-based cone beam CT.
Computer simulations validated the accuracy and reproducibility of the proposed method. The precise estimation of the geometric parameters of the benchtop revealed high-quality imaging in CT reconstruction of a breast phantom. Within the phantom, the cylindrical holes, fibers, and speck groups were imaged in high fidelity. The CNR analysis further revealed the quantitative improvements of the reconstruction performed with the estimated parameters using the proposed method.
Apart from the computational cost, we concluded that the method was easy to implement and robust.
在开发可靠的计算机断层扫描（CT）系统时，几何校准至关重要。它涉及估计获取角度投影的几何形状。当使用传统方法时，对于采用小面积探测器（例如，当前可用的光子计数探测器（PCD））的锥形束 CT 进行几何校准具有挑战性，这是由于探测器的有限面积所致。
本研究提出了一种用于小面积基于 PCD 的锥形束 CT 系统的几何校准的经验方法。
与传统方法不同，我们开发了一种迭代优化程序，使用嵌入在定制体模中的小金属球轴承（BB）的重建图像来确定几何参数。定义了一个目标函数，该函数包含嵌入 BB 的球形度和对称性，以评估给定初始估计的几何参数集的重建算法的性能。最优参数值是使目标函数最小化的值。使用 TIGRE 工具包进行快速断层重建。为了评估所提出的方法，使用放置在不同位置的各种数量的球体进行了计算机模拟。此外，使用定制的基于 PCD 的台式锥形束 CT 对该方法进行了实验评估。
计算机模拟验证了所提出方法的准确性和可重复性。对台式机的几何参数的精确估计揭示了在 CT 重建中对乳房体模的高质量成像。在体模中，圆柱孔、纤维和斑点组都以高保真度成像。CNR 分析进一步显示了使用所提出的方法使用估计参数进行重建的定量改进。
除了计算成本之外，我们得出的结论是，该方法易于实现且鲁棒性强。
