Siemens Healthcare GmbH, Forchheim, Germany.
Pattern Recognition Lab, Department of Computer Science, Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen, Germany.
Med Phys. 2022 Aug;49(8):5014-5037. doi: 10.1002/mp.15787. Epub 2022 Jun 16.
Various clinical studies show the potential for a wider quantitative role of diagnostic X-ray computed tomography (CT) beyond size measurements. Currently, the clinical use of attenuation values is, however, limited due to their lack of robustness. This issue can be observed even on the same scanner across patient size and positioning. There are different causes for the lack of robustness in the attenuation values; one possible source of error is beam hardening of the X-ray source spectrum. The conventional and well-established approach to address this issue is a calibration-based single material beam hardening correction (BHC) using a water cylinder.
We investigate an alternative approach for single-material BHC with the aim of producing a more robust result for the attenuation values. The underlying hypothesis of this investigation is that calibration-based BHC automatically corrects for scattered radiation in a manner that is suboptimal in terms of bias as soon as the scanned object strongly deviates from the water cylinder used for calibration.
The approach we propose performs BHC via an analytical energy response model that is embedded into a correction pipeline that efficiently estimates and subtracts scattered radiation in a patient-specific manner prior to BHC. The estimation of scattered radiation is based on minimizing, in average, the squared difference between our corrected data and the vendor-calibrated data. The used energy response model is considering the spectral effects of the detector response and the prefiltration of the source spectrum, including a beam-shaping bowtie filter. The performance of the correction pipeline is first characterized with computer simulated data. Afterward, it is tested using real 3-D CT data sets of two different phantoms, with various kV settings and phantom positions, assuming a circular data acquisition. The results are compared in the image domain to those from the scanner.
For experiments with a water cylinder, the proposed correction pipeline leads to similar results as the vendor. For reconstructions of a QRM liver phantom with extension ring, the proposed correction pipeline achieved a more uniform and stable outcome in the attenuation values of homogeneous materials within the phantom. For example, the root mean squared deviation between centered and off-centered phantom positioning was reduced from 6.6 to 1.8 HU in one profile.
We have introduced a patient-specific approach for single-material BHC in diagnostic CT via the use of an analytical energy response model. This approach shows promising improvements in terms of robustness of attenuation values for large patient sizes. Our results contribute toward improving CT images so as to make CT attenuation values more reliable for use in clinical practice.
各种临床研究表明，诊断 X 射线计算机断层扫描（CT）在尺寸测量之外具有更广泛的定量作用的潜力。然而，目前由于缺乏稳健性，衰减值的临床应用受到限制。即使在同一台扫描仪上，对于不同的患者体型和定位，也会出现这种稳健性不足的情况。衰减值缺乏稳健性有不同的原因；射线源光谱的束硬化是误差的一个可能来源。解决此问题的传统且成熟的方法是使用水筒进行基于校准的单一材料束硬化校正（BHC）。
我们研究了一种替代的单一材料 BHC 方法，旨在为衰减值提供更稳健的结果。这项研究的基本假设是，一旦扫描物体强烈偏离用于校准的水筒，基于校准的 BHC 会自动以对偏差不利的方式校正散射辐射。
我们提出的方法通过嵌入校正管道的分析能量响应模型来执行 BHC，该管道以患者特异性的方式高效地估计和减去散射辐射，然后再进行 BHC。散射辐射的估计基于平均最小化我们校正后的数据与供应商校准后的数据之间的平方差。所使用的能量响应模型考虑了探测器响应的光谱效应和源光谱的预过滤，包括束成形的蝴蝶结滤波器。首先使用计算机模拟数据来描述校正管道的性能。然后，使用两个不同的体模的真实 3-D CT 数据集进行测试，这些数据集具有不同的千伏设置和体模位置，假设圆形数据采集。结果在图像域中与扫描仪进行比较。
对于水筒实验，所提出的校正管道的结果与供应商的结果相似。对于带有延伸环的 QRM 肝脏体模的重建，所提出的校正管道在体模内均匀材料的衰减值中实现了更均匀和稳定的结果。例如，在一个轮廓中，中心和偏离中心的体模定位之间的均方根偏差从 6.6 降低到 1.8 HU。
我们通过使用分析能量响应模型，在诊断 CT 中引入了一种用于单一材料 BHC 的患者特异性方法。该方法在大患者体型的衰减值稳健性方面显示出有希望的改进。我们的结果有助于改善 CT 图像，以使 CT 衰减值在临床实践中更可靠地使用。
