Quantitative computed tomography for more accurate radiotherapy

Radiotherapy with low-energy photons or high energy protons makes it necessary to obtain more information on the atomic compositions of the patient's tumour and neighboring healthy tissue. This information can be measured in images from dual-energy or spectral computed tomography (CT), provided that they are free from artefacts. Artefact-free images can be achieved by physical modelling and reconstruction of the images with iterative reconstruction methods. The mass fractions of bone, fat, protein and water can then be estimated from the CT images and hence the elemental fractions of, for example, hydrogen, carbon, oxygen and calcium can be obtained for more accurate patient dose distribution.

The quantity underlying computed tomography (CT) images is the linear attenuation coefficient; the weighted sum of the linear attenuation coefficients of the elements in a tissue material. Quantitative CT (QCT) measures absolute values of the attenuation coefficient and thus gives a unique possibility to classify imaged tissues in terms of element concentrations. QCT is the basis for dose calculations in radiotherapy.

New modalities of radiation treatment have been introduced: brachytherapy with low energy photons and radiation therapy with high-energy protons and heavier ions. For accurate dose calculations using these modalities knowledge of electron densities is not sufficient, as is the case in conventional radiation treatment using high energy photons. Detailed knowledge of patient specific elemental concentrations in the tissues is needed. Accurate use of QCT depends on the possibility to achieve images free from artefacts. The aims of this work is to reconstruct artefact free images – a prerequisite for quantitative CT.

To achieve this, we intend to develop a model-based iterative reconstruction algorithm (DIRA) based on dual-energy CT (DECT) to account for patient specific tissue compositions and extend the algorithm to a multi-energy reconstruction algorithm (MIRA) that includes information from multi-energy or spectral CT. By segmenting the organs and mathematically decomposing the soft tissues to water, fat and protein by using the dual-energy CT images, it is possible to estimate the elemental compositions of the tissues. The images show the mass fractions of water in a slice in the pelvis region.

In the prostate, for example, it is important to quantify the calcium content in order to account for the change in dose distributions that will occur with low energy photons. With information of elemental composition, it it possible to do more accurate individual dose plans. Such plans may reduce the side effects of radiation therapy.

Quantitative CT figure 1

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