Department of Radiological Technology, Niigata University, Niigata, Japan.
Med Phys. 2012 Apr;39(4):2021-30. doi: 10.1118/1.3694111.
The conversion of the computed tomography (CT) number to electron density is one of the main processes that determine the accuracy of patient dose calculations in radiotherapy treatment planning. However, the CT number and electron density of tissues cannot be generally interrelated via a simple one-to-one correspondence because the CT number depends on the effective atomic number as well as the electron density. The purpose of this study is to present a simple conversion from the energy-subtracted CT number (ΔHU) by means of dual-energy CT (DECT) to the relative electron density (ρ(e)) via a single linear relationship.
The ΔHU-ρ(e) conversion method was demonstrated by performing analytical DECT image simulations that were intended to imitate a second-generation dual-source CT (DSCT) scanner with an additional tin filtration for the high-kV tube. The ΔHU-ρ(e) calibration line was obtained from the image simulation with a 33 cm-diameter electron density calibration phantom equipped with 16 inserts including polytetrafluoroethylene, polyvinyl chloride, and aluminum; the elemental compositions of these three inserts were quite different to those of body tissues. The ΔHU-ρ(e) conversion method was also applied to previously published experimental CT data, which were measured using two different CT scanners, to validate the clinical feasibility of the present approach. In addition, the effect of object size on ρ(e)-calibrated images was investigated by image simulations using a 25 cm-diameter virtual phantom for two different filtrations: with and without the tin filter for the high-kV tube.
The simulated ΔHU-ρ(e) plot exhibited a predictable linear relationship over a wide range of ρ(e) from 0.00 (air) to 2.35 (aluminum). Resultant values of the coefficient of determination, slope, and intercept of the linear function fitted to the data were close to those of the ideal case. The maximum difference between the ideal and simulated ρ(e) values was -0.7%. The satisfactory linearity of ΔHU-ρ(e) was also confirmed from analyses of the experimental CT data. In the experimental cases, the maximum difference between the nominal and simulated ρ(e) values was found to be 2.5% after two outliers were excluded. When compared with the case without the tin filter, the ΔHU-ρ(e) conversion performed with the tin filter yielded a lower dose and more reliable ρ(e) values that were less affected by the object-size variation.
The ΔHU-ρ(e) calibration line with a simple one-to-one correspondence would facilitate the construction of a well-calibrated ρ(e) image from acquired dual-kV images, and currently, second generation DSCT may be a feasible modality for the clinical use of the ΔHU-ρ(e) conversion method.
将计算机断层扫描（CT）数转换为电子密度是确定放射治疗计划中患者剂量计算准确性的主要过程之一。然而，由于 CT 数取决于有效原子数以及电子密度，因此组织的 CT 数和电子密度通常不能通过简单的一一对应来关联。本研究的目的是通过使用双能 CT（DECT）通过单一线性关系从能量减去的 CT 数（ΔHU）呈现一种简单的转换为相对电子密度（ρ（e））的方法。
通过执行旨在模拟具有附加锡滤光片的第二代双源 CT（DSCT）扫描仪的分析 DECT 图像模拟来演示 ΔHU-ρ（e）转换方法，该滤光片用于高千伏管。从配备有 16 个插件的 33 厘米直径电子密度校准体模的图像模拟中获得 ΔHU-ρ（e）校准线，这些插件包括聚四氟乙烯，聚氯乙烯和铝；这三个插件的元素组成与身体组织的元素组成有很大不同。该 ΔHU-ρ（e）转换方法还应用于先前发表的实验 CT 数据，该数据是使用两种不同的 CT 扫描仪测量的，以验证本方法的临床可行性。此外，通过对两个不同滤波的 25 厘米直径虚拟体模进行图像模拟，研究了物体尺寸对 ρ（e）校准图像的影响，这两种滤波分别为高千伏管是否具有锡滤光片。
模拟的 ΔHU-ρ（e）图在很宽的 ρ（e）范围内（从 0.00（空气）到 2.35（铝））显示出可预测的线性关系。拟合数据的线性函数的决定系数，斜率和截距的结果值与理想情况非常接近。理想值与模拟 ρ（e）值之间的最大差异为-0.7％。从实验 CT 数据的分析中也证实了 ΔHU-ρ（e）的令人满意的线性关系。在实验情况下，在排除两个异常值之后，发现名义值和模拟 ρ（e）值之间的最大差异为 2.5％。与没有锡滤光片的情况相比，使用锡滤光片进行的 ΔHU-ρ（e）转换会产生较低的剂量和更可靠的 ρ（e）值，这些值受物体尺寸变化的影响较小。
具有简单一一对应关系的 ΔHU-ρ（e）校准线将有助于从采集的双千伏图像构建经过良好校准的 ρ（e）图像，目前，第二代 DSCT 可能是临床使用 ΔHU-ρ（e）转换方法的可行方式。
