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Technical Note: Exploring the limit for the conversion of energy‐subtracted CT number to electron density for high‐atomic‐number materials
Author(s) -
Saito Masatoshi,
Tsukihara Masayoshi
Publication year - 2014
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.4881327
Subject(s) - effective atomic number , atomic number , attenuation , physics , range (aeronautics) , photon , computational physics , nuclear medicine , photoelectric effect , radiation , dosimetry , limit (mathematics) , absorbed dose , atomic physics , mathematics , materials science , optics , mathematical analysis , medicine , composite material
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 (ΔHU– ρ e conversion), which provides a single linear relationship between ΔHU and ρ e over a wide ρ 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 ΔHU– ρ e conversion process. Methods: The authors performed numerical analyses of the ΔHU– ρ 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, ρ e cal , for the ICRU‐46 body tissues fully satisfied the IPEM‐81 tolerance levels in radiotherapy treatment planning. If a criterion ofρ e cal / ρ e − 1 is assumed to be within ±2%, the predicted upper limit of Z applicable for the ΔHU– ρ 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 ΔHU– ρ 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 ΔHU– ρ 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.