Technical Note: Exploring the limit for the conversion of energy-subtracted CT number to electron density for high-atomic-number materials
Technical Note: Exploring the limit for the conversion of energy-subtracted CT number to electron density for high-atomic-number materials
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DOI：
10.1118/1.4881327
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发表时间：
2014-07-01
期刊：
影响因子：
3.8
通讯作者：
Tsukihara, Masayoshi
中科院分区：
文献类型：
--
作者：
Saito, Masatoshi;Tsukihara, Masayoshi
目的：为了在放射治疗计划中精确校正组织不均匀性，作者提出了一种新的将能量减去的CT数转换为电子密度的方法(Delta Hu-Rho(E)转换)，该转换在很大的Rho(E)范围内提供Delta Hu和Rho(E)之间的单一线性关系。这项研究的目的是通过阐明部分光子相互作用在Delta Hu-Rho(E)转换过程中的作用来解决该转换方法在原子序数(Z)方面的局限性。方法：作者对ICRU报告46中列出的105个人体组织和Z=1-40的基本物质的Delta Hu-Rho(E)转换过程进行了数值分析。使用XCOM光子截面数据库计算了这些材料的总衰减系数和部分衰减系数。选择用于计算衰减的有效X射线能量来模拟一台工作在80-140千伏/锡的双源CT扫描仪，在校准良好和校准较差的条件下。结果：所得到的ICRU-46人体组织的校准电子密度Rho(Cal)(E)的准确度完全满足IPEM-81放射治疗计划的耐受水平。如果假设Rho(Cal)(E)/Rho(E)-1的标准在+/-2%内，则在充分校准的条件下适用于Delta Hu-Rho(E)转换的Z的预测上限是Z=27。在刻度较差的情况下，Z的上限约为16。对于Z较高的物质，偏离Delta Hu-Rho(E)线性度的主要原因是光电吸收分量的异常变化。结论：光子相互作用的三个部分分量之间的补偿提供了Delta Hu-Rho(E)转换的足够线性，即使在条件较差的扫描中也适用，即使在条件较差的扫描中，由于物体大小和校准体模之间的失配引起的束硬化效应，有效X射线能量存在较大的变化。(C)2014年美国医学物理学家协会。
Purpose: For accurate tissue inhomogeneity correction in radiotherapy treatment planning, the authors had previously proposed a novel conversion of the energy-subtracted CT number to an electron density (Delta HU-rho(e) conversion), which provides a single linear relationship between Delta HU and rho(e) over a wide rho(e) range. The purpose of this study is to address the limitations of the conversion method with respect to atomic number (Z) by elucidating the role of partial photon interactions in the Delta HU-rho(e) conversion process.Methods: The authors performed numerical analyses of the Delta HU-rho(e) conversion for 105 human body tissues, as listed in ICRU Report 46, and elementary substances with Z = 1-40. Total and partial attenuation coefficients for these materials were calculated using the XCOM photon cross section database. The effective x-ray energies used to calculate the attenuation were chosen to imitate a dual-source CT scanner operated at 80-140 kV/Sn under well-calibrated and poorly calibrated conditions.Results: The accuracy of the resultant calibrated electron density, rho(cal)(e), for the ICRU-46 body tissues fully satisfied the IPEM-81 tolerance levels in radiotherapy treatment planning. If a criterion of rho(cal)(e)/rho(e) - 1 is assumed to be within +/- 2%, the predicted upper limit of Z applicable for the Delta HU-rho(e) conversion under the well-calibrated condition is Z = 27. In the case of the poorly calibrated condition, the upper limit of Z is approximately 16. The deviation from the Delta HU-rho(e) linearity for higher Z substances is mainly caused by the anomalous variation in the photoelectric-absorption component.Conclusions: Compensation among the three partial components of the photon interactions provides for sufficient linearity of the Delta HU-rho(e) conversion to be applicable for most human tissues even for poorly conditioned scans in which there exists a large variation of effective x-ray energies owing to beam-hardening effects arising from the mismatch between the sizes of the object and the calibration phantom. (C) 2014 American Association of Physicists in Medicine.