Siewerdsen J H, Jaffray D A
Department of Radiation Oncology, William Beaumont Hospital, Royal Oak, Michigan 48073, USA.
Med Phys. 2001 Feb;28(2):220-31. doi: 10.1118/1.1339879.
A system for cone-beam computed tomography (CBCT) based on a flat-panel imager (FPI) is used to examine the magnitude and effects of x-ray scatter in FPI-CBCT volume reconstructions. The system is being developed for application in image-guided therapies and has previously demonstrated spatial resolution and soft-tissue visibility comparable or superior to a conventional CT scanner under conditions of low x-ray scatter. For larger objects consistent with imaging of human anatomy (e.g., the pelvis) and for increased cone angle (i.e., larger volumetric reconstructions), however, the effects of x-ray scatter become significant. The magnitude of x-ray scatter with which the FPI-CBCT system must contend is quantified in terms of the scatter-to-primary energy fluence ratio (SPR) and scatter intensity profiles in the detector plane, each measured as a function of object size and cone angle. For large objects and cone angles (e.g., a pelvis imaged with a cone angle of 6 degrees), SPR in excess of 100% is observed. Associated with such levels of x-ray scatter are cup and streak artifacts as well as reduced accuracy in reconstruction values, quantified herein across a range of SPR consistent with the clinical setting. The effect of x-ray scatter on the contrast, noise, and contrast-to-noise ratio (CNR) in FPI-CBCT reconstructions was measured as a function of SPR and compared to predictions of a simple analytical model. The results quantify the degree to which elevated SPR degrades the CNR. For example, FPI-CBCT images of a breast-equivalent insert in water were degraded in CNR by nearly a factor of 2 for SPR ranging from approximately 2% to 120%. The analytical model for CNR provides a quantitative understanding of the relationship between CNR, dose, and spatial resolution and allows knowledgeable selection of the acquisition and reconstruction parameters that, for a given SPR, are required to restore the CNR to values achieved under conditions of low x-ray scatter. For example, for SPR = 100%, the CNR in FPI-CBCT images can be fully restored by: (1) increasing the dose by a factor of 4 (at full spatial resolution); (2) increasing dose and slice thickness by a factor of 2; or (3) increasing slice thickness by a factor of 4 (with no increase in dose). Other reconstruction parameters, such as transaxial resolution length and reconstruction filter, can be similarly adjusted to achieve CNR equal to that obtained in the scatter-free case.
一种基于平板成像器（FPI）的锥束计算机断层扫描（CBCT）系统用于检测FPI-CBCT体积重建中X射线散射的大小和影响。该系统正在开发用于图像引导治疗，并且先前已证明在低X射线散射条件下，其空间分辨率和软组织可见性与传统CT扫描仪相当或更优。然而，对于与人体解剖结构成像一致的较大物体（例如骨盆）以及增大的锥角（即更大的体积重建），X射线散射的影响变得显著。FPI-CBCT系统必须应对的X射线散射大小通过散射与初级能量注量比（SPR）以及探测器平面中的散射强度分布来量化，每个都作为物体大小和锥角的函数进行测量。对于大物体和锥角（例如以6度锥角成像的骨盆），观察到SPR超过100%。与这种水平的X射线散射相关的是杯状和条纹伪影以及重建值准确性的降低，本文在与临床环境一致的一系列SPR范围内对其进行了量化。测量了X射线散射对FPI-CBCT重建中对比度、噪声和对比度噪声比（CNR）的影响，并将其作为SPR的函数进行比较，与一个简单分析模型的预测结果进行对比。结果量化了SPR升高使CNR降低的程度。例如，对于水中乳腺等效插入物的FPI-CBCT图像，当SPR从约2%变化到120%时，CNR降低了近2倍。CNR分析模型提供了对CNR、剂量和空间分辨率之间关系的定量理解，并允许明智地选择采集和重建参数，对于给定的SPR，这些参数是将CNR恢复到低X射线散射条件下所达到值所需的。例如，对于SPR = 100%，FPI-CBCT图像中的CNR可以通过以下方式完全恢复：（1）将剂量增加4倍（在全空间分辨率下）；（2）将剂量和切片厚度增加2倍；或（3）将切片厚度增加4倍（剂量不增加）。其他重建参数，如横向分辨率长度和重建滤波器，可以类似地进行调整，以实现与无散射情况下相同的CNR。
