Methodological accuracy of image-based electron density assessment using dual-energy computed tomography

Abstract

This is the pre-peer reviewed version of the article entitled "Methodological accuracy of image-based electron-density assessment using dual-energy computed tomography", which has been accepted for Purpose: Electron density is the most important tissue property influencing photon and ion dose distributions in radiotherapy patients. Dual-energy computed tomography (DECT) enables the determination of electron density by combining the information on photon attenuation obtained at two different tube voltages. Most algorithms suggested so far use the CT numbers provided after image reconstruction as input parameters, i.e. are imaged-based. To explore the accuracy that can be achieved with these approaches, we quantify the intrinsic methodological and calibration uncertainty of the seemingly simplest approach.Methods: The core of this approach under study is a one-parametric linear superposition ('alpha blending') of the two DECT images, which is shown to be equivalent to an affine relation between the photon attenuation cross sections of the two xray energy spectra. We propose to use the latter relation for empirical calibration of the spectrum-dependent blending parameter. For a conclusive assessment of the electron-density uncertainty, we chose to isolate the purely methodological uncertainty component from CT-related effects such as noise and beam hardening.Results: Analyzing calculated spectral-weighted attenuation coefficients, we find universal applicability of the investigated approach to arbitrary mixtures of human tissue with an upper limit of the methodological uncertainty component of 0.15%, excluding high-Z elements such as iodine. The proposed calibration procedure is bias-free and straightforward to perform using standard equipment. Testing the calibration on five published data sets, we obtain very small differences in the calibration result in spite of different experimental setups and CT protocols used. Employing a general calibration per scanner type and voltage combination is thus conceivable.Conclusion: Given the high suitability for clinical application of the alphablending approach in combination with a very small methodological uncertainty, we conclude that further refinement of image-based DECT-algorithms for electrondensity assessment is not advisable.2

A comparison of relative proton stopping power measurements across patient size using dual- and single-energy CT

“…For the dense bone, the elemental composition data provided by the manufacturer indicated that there was a small percentage of Barium in the insert. The presence of this high-Z material may not be compatible with our DE calculations [10,19]. Given these potential complications with the phantom measurements, further investigations are warranted using phantoms from other manufacturers and/or organic materials taken from animal models.…”

Reply to “Comment on Methodological accuracy of image‐based electron density assessment using dual‐energy computed tomography” [Med. Phys. 44, 2429–2437 (2017)]

“…The recent letter by Dr. Saito refers to the theoretical framework of electron-density determination by dual-energy computed tomography (DECT), as presented in the appendix of our article. 1 After performing some calculations in this framework, the author of the Comment concludes that "the form of Eq. (A1) [.…”

“…(A1). 1,5 Raising doubts about the applicability of Eq. (A1) would therefore reduce trust in the validity of Eq.…”

Comment on: Methodological accuracy of image‐based electron density assessment using dual‐energy computed tomography [Med. Phys. 44(6), 2429–2437 (2017)]

“…To the Editor, I write this letter in reference to the recent paper by M€ ohler et al 1 In that study, the authors evaluated the methodological uncertainties of the dual-energy computed tomography (DECT) method for electron-density calculations with a single material-independent parameter a, which was developed by Saito. 2 Given the high accuracy and robustness of the simple method, the authors concluded that no further improvement is expected from more complex image-based DECT-algorithms for electron-density calculations.…”

“…4 They have also given an alternative form of photon attenuation characterized by mixtures of two basis materials (e.g., water and bone), 5 which is currently preferred as a more practical method in DECT. In my previous study based on the two-basis material decomposition, 6 I have elucidated that the optimal a opt determined by a least squares fitting is equivalent to a weighting factor, which synthesizes a virtual monochromatic CT image acquired at 1 MeV [a 1 . At this energy, the photonmatter interaction for human tissues is exclusively Compton scattering, a process in which the photon attenuation is proportional to the electron density of the material but independent of its atomic number (i.e.,l l MeV ¼n).…”