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Measurement of electron density in dual‐energy x‐ray CT with monochromatic x rays and evaluation of its accuracy
Author(s) -
Tsunoo Takanori,
Torikoshi Masami,
Ohno Yumiko,
Uesugi Kentaro,
Yagi Naoto
Publication year - 2008
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 - 10.1118/1.2987738
Subject(s) - monochromatic color , x ray , dual energy , electron , physics , optics , gamma ray , electron density , medical physics , dual (grammatical number) , nuclear medicine , materials science , nuclear physics , medicine , art , bone mineral , osteoporosis , literature , endocrinology
Information on electron density is important for radiotherapy treatment planning in order to optimize the dose distribution in the target volume of a patient. At present, the electron density is derived from a computed tomography (CT) number measured in x‐ray CT scanning; however, there are uncertainties due to the beam hardening effect and the method by which the electron density is converted from the CT number. In order to measure the electron density with an accuracy of ± 1 % , the authors have developed dual‐energy x ray CT using monochromatic x rays. They experimentally proved that the measured linear attenuation coefficients were only a few percent lower than the theoretical ones, which led to an accuracy within 2% for the electron density. There were three factors causing inaccuracy in the linear attenuation coefficient and the electron density: the influence of scattered radiation, the nonlinearity in the detector response function, and a theoretical process to derive the electron density from the linear attenuation coefficients. The linear attenuation coefficients of water were experimentally proved to differ by 1%–2% from the theoretical one even when the scattering effect was negligible. The nonlinearity of the response function played an important role in correcting the difference in the linear attenuation coefficient. Furthermore, the theoretical process used for deriving the electron density from the linear attenuation coefficients introduces about 0.6% deviation from the theoretical value into the resultant electron density. This deviation occurs systematically so that it can be corrected. The authors measured the electron densities for seven samples equivalent to soft tissue in dual‐energy x‐ray CT, and finally obtained them with an accuracy of around ± 1 % .