Dual-energy CT-based material extraction for tissue segmentation in Monte Carlo dose calculations

Abstract

Monte Carlo (MC) dose calculations are performed on patient geometries derived from computed tomography (CT) images. For most available MC codes, the Hounsfield units (HU) in each voxel of a CT image have to be converted into mass density (rho) and material type. This is typically done with a (HU; rho) calibration curve which may lead to mis-assignment of media. In this work, an improved material segmentation using dual-energy CT-based material extraction is presented. For this purpose, the differences in extracted effective atomic numbers Z and the relative electron densities rho(e) of each voxel are used. Dual-energy CT material extraction based on parametrization of the linear attenuation coefficient for 17 tissue-equivalent inserts inside a solid water phantom was done. Scans of the phantom were acquired at 100 kVp and 140 kVp from which Z and rho(e) values of each insert were derived. The mean errors on Z and rho(e) extraction were 2.8% and 1.8%, respectively. Phantom dose calculations were performed for 250 kVp and 18 MV photon beams and an 18 MeV electron beam in the EGSnrc/DOSXYZnrc code. Two material assignments were used: the conventional (HU; rho) and the novel (HU; rho, Z) dual-energy CT tissue segmentation. The dose calculation errors using the conventional tissue segmentation were as high as 17% in a mis-assigned soft bone tissue-equivalent material for the 250 kVp photon beam. Similarly, the errors for the 18 MeV electron beam and the 18 MV photon beam were up to 6% and 3% in some mis-assigned media. The assignment of all tissue-equivalent inserts was accurate using the novel dual-energy CT material assignment. As a result, the dose calculation errors were below 1% in all beam arrangements. Comparable improvement in dose calculation accuracy is expected for human tissues. The dual-energy tissue segmentation offers a significantly higher accuracy compared to the conventional single-energy segmentation.

A stoichiometric calibration method for dual energy computed tomography

“…The formalism of Bazalova et al [3] is an adaptation of the linear attenuation coefficient notation proposed by Torikoshi et al [47]. It was adapted from synchrotron radiation to polyenergetic x-ray beams to be suitable for clinical purposes.…”

“…Torikoshi et al [47] was further adapted by Bazalova et al [3] for commercial scanners which use polyenergetic photon spectra yielding an accuracy of 1.8% on average for ED and 2.8% on average for EAN, this using phantom measurements. The same formalism was used by Landry et al [23] to compare experiments with Monte Carlo simulations of a CT scanner, yielding an agreement within ±5% both for ED and EAN.…”

The potential of dual-energy computed tomography for quantitative decomposition of soft tissues to water, protein and lipid in brachytherapy

“…(Schneider, 1996). To improve the accuracy of tissue classification by utilizing the dual-energy CT scanners, Bazalova et al (2008aBazalova et al ( , 2008b proposed a method based on the separation of the LAC to photoelectric and Compton scattering components according to Torikoshi et al (2003). The method allowed the determination of an effective Z and electron density of each voxel; this information was used for a more accurate classification.…”

Nondestructive Quantitative Analysis of a Heavy Element in Solution or Suspension by Single-Shot Computed Tomography with a Polychromatic X-ray Source

“…4,5 Because X-ray absorption is sensitive to the atomic number (absorption increases with increasing atomic number), X-ray CT is a useful tool for the detection of heavy elements. 6,7 Conventional dual-energy X-ray CT using monochromatic [8][9][10][11][12] and polychromatic [13][14][15][16][17][18][19][20][21][22] X-ray sources is usually used for the quantification of heavy elements, but in the present study, we investigated the use of single-shot, or single-energy, CT using a polychromatic X-ray source. The advantages of our method are that: (i) single-shot CT reduces the imaging time, prolongs the lifetime of the expensive X-ray tube and detector, and reduces radiation exposure of samples or patients, (ii) polychromatic X-rays can be readily generated by conventional medical CT instruments, and do not require huge facilities, such as particle accelerators.…”