Potential of dual-energy subtraction for converting CT numbers to electron density based on a single linear relationship

Purpose: 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 (Delta HU) by means of dual-energy CT (DECT) to the relative electron density (rho(e)) via a single linear relationship. Methods: The Delta HU-rho(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 Delta HU-rho(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 Delta HU-rho(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 rho(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. Results: The simulated Delta HU-rho(e) plot exhibited a predictable linear relationship over a wide range of rho(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 rho(e) values was -0.7%. The satisfactory linearity of Delta HU-rho(e) was also confirmed from analyses of the experimental CT data. In the experimental cases, the maximum difference between the nominal and simulated rho(e) values was found to be 2.5% after two outliers were excluded. When compared with the case without the tin filter, the Delta HU-rho(e) conversion performed with the tin filter yielded a lower dose and more reliable rho(e) values that were less affected by the object-size variation. Conclusions: The Delta HU-rho(e) calibration line with a simple one-to-one correspondence would facilitate the construction of a well-calibrated rho(e) image from acquired dual-kV images, and currently, second generation DSCT may be a feasible modality for the clinical use of the Delta HU-rho(e) conversion method. (C) 2012 American Association of Physicists in Medicine. [http://dx.doi.org/10.1118/1.3694111]