Abstract
Quantitative dual-energy computed tomography may improve the accuracy of treatment planning in radiation therapy. Of special interest are algorithms that can estimate material composition of the imaged object. One example of such an algorithm is the 2D model-based iterative reconstruction algorithm DIRA. The aim of this work is to extend this algorithm to 3D so that it can be used with cone-beams and helical scanning. In the new algorithm, the parallel FBP method was replaced with the approximate 3D FBP-based PI-method. Its performance was tested using a mathematical phantom consisting of six ellipsoids. The algorithm substantially reduced the beam-hardening artefact and the artefacts caused by approximate reconstruction after six iterations. Compared to Alvarez-Macovski’s base material decomposition, DIRA-3D does not require geometrically consistent projections and hence can be used in dual-source CT scanners. Also, it can use several tissue-specific material bases at the same time to represent the imaged object.
Highlights
The model-based iterative reconstruction algorithm DIRA-3D was designed to address the need for better estimation of material parameters needed for radiation transport and dose calculations in brachytherapy with low energy photons and proton therapy
The largest and the only clearly visible improvement was achieved for compact bone, where the linear attenuation coefficients (LACs) changed from approximately 59 m−1 in iteration 1 to approximately 79.2 m−1 in iteration 10, which is equal to the true value in table 1
Quantitative assessment of the improvement is presented in figure 5, which shows that relative errors in LACs averaged over region of interest (ROI) approached zero already after 5 iterations for the soft tissue ellipsoids and 10 iterations for the compact bone ellipsoid
Summary
The model-based iterative reconstruction algorithm DIRA-3D was designed to address the need for better estimation of material parameters needed for radiation transport and dose calculations in brachytherapy with low energy photons and proton therapy. Atomic composition is important since the photoelectric effect dominates even in soft tissues of low atomic number. With protons both the mean excitation energy (I-value) and the probability of nuclear reactions depend on the atomic number. The need for better characterization of the tissues has already been stated and is currently a bottle-neck for the use of model-based dose calculation methods for this modality, see e.g. The need for better characterization of the tissues has already been stated and is currently a bottle-neck for the use of model-based dose calculation methods for this modality, see e.g. (Beaulieu et al 2012, van Elmpt et al 2016)
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