German Cancer Research Center (DKFZ), Division of Medical Physics in Radiation Oncology, Heidelberg, Germany.
National Center for Radiation Research in Oncology (NCRO), Heidelberg Institute for Radiation Oncology (HIRO), Heidelberg, Germany.
Med Phys. 2017 Jun;44(6):2429-2437. doi: 10.1002/mp.12265. Epub 2017 May 20.
Electron density is the most important tissue property influencing photon and ion dose distributions in radiotherapy patients. Dual-energy computed tomography (DECT) enables the determination of electron density by combining the information on photon attenuation obtained at two different effective x-ray energy spectra. Most algorithms suggested so far use the CT numbers provided after image reconstruction as input parameters, i.e., are imaged-based. To explore the accuracy that can be achieved with these approaches, we quantify the intrinsic methodological and calibration uncertainty of the seemingly simplest approach.
In the studied approach, electron density is calculated with a one-parametric linear superposition ('alpha blending') of the two DECT images, which is shown to be equivalent to an affine relation between the photon attenuation cross sections of the two x-ray energy spectra. We propose to use the latter relation for empirical calibration of the spectrum-dependent blending parameter. For a conclusive assessment of the electron density uncertainty, we chose to isolate the purely methodological uncertainty component from CT-related effects such as noise and beam hardening.
Analyzing calculated spectrally weighted attenuation coefficients, we find universal applicability of the investigated approach to arbitrary mixtures of human tissue with an upper limit of the methodological uncertainty component of 0.2%, excluding high-Z elements such as iodine. The proposed calibration procedure is bias-free and straightforward to perform using standard equipment. Testing the calibration on five published data sets, we obtain very small differences in the calibration result in spite of different experimental setups and CT protocols used. Employing a general calibration per scanner type and voltage combination is thus conceivable.
Given the high suitability for clinical application of the alpha-blending approach in combination with a very small methodological uncertainty, we conclude that further refinement of image-based DECT-algorithms for electron density assessment is not advisable.
电子密度是影响放射治疗患者光子和离子剂量分布的最重要组织特性。双能计算机断层扫描（DECT）通过组合在两个不同有效 X 射线能谱下获得的光子衰减信息来确定电子密度。迄今为止提出的大多数算法都使用图像重建后提供的 CT 数作为输入参数，即基于图像的。为了探索这些方法可以达到的精度，我们量化了看似最简单方法的内在方法和校准不确定性。
在研究的方法中，电子密度是通过对两个 DECT 图像的两个参数线性叠加（“alpha 混合”）来计算的，这与两个 X 射线能谱的光子衰减截面之间的仿射关系等效。我们建议使用后者关系来对谱相关混合参数进行经验校准。为了对电子密度不确定性进行明确评估，我们选择将仅与方法相关的不确定性分量与 CT 相关的影响（如噪声和束硬化）隔离开来。
分析计算的谱加权衰减系数，我们发现所研究的方法适用于人类组织的任意混合物，其方法不确定性分量的上限为 0.2%，不包括碘等高 Z 元素。所提出的校准程序无偏且易于使用标准设备执行。在五个已发表的数据集上测试校准，尽管使用了不同的实验设置和 CT 协议，但校准结果的差异非常小。因此，设想对每个扫描仪类型和电压组合进行通用校准是可行的。
鉴于 alpha 混合方法结合非常小的方法不确定性非常适合临床应用，我们得出结论，进一步改进用于电子密度评估的基于图像的 DECT 算法是不可取的。
