Abstract
Objective. The use of ion computed tomography (CT) promises to yield improved relative stopping power (RSP) estimation as input to particle therapy treatment planning. Recently, proton CT (pCT) has been shown to yield RSP accuracy on par with state-of-the-art x-ray dual energy CT. There are however concerns that the lower spatial resolution of pCT compared to x-ray CT may limit its potential, which has spurred interest in the use of helium ion CT (HeCT). The goal of this study was to investigate image quality of pCT and HeCT in terms of noise, spatial resolution, RSP accuracy and imaging dose using a detailed Monte Carlo (MC) model of an existing ion CT prototype. Approach. Three phantoms were used in simulated pCT and HeCT scans allowing estimation of noise, spatial resolution and the scoring of dose. An additional phantom was used to evaluate RSP accuracy. The imaging dose required to achieve the same image noise in a water and a head phantom was estimated at both native spatial resolution, and in a scenario where the HeCT spatial resolution was reduced and matched to that of pCT using Hann windowing of the reconstruction filter. A variance reconstruction formalism was adapted to account for Hann windowing. Main results. We confirmed that the scanner prototype would produce higher spatial resolution for HeCT than pCT by a factor 1.8 (0.86 lp mm−1 versus 0.48 lp mm−1 at the center of a 20 cm water phantom). At native resolution, HeCT required a factor 2.9 more dose than pCT to achieve the same noise, while at matched resolution, HeCT required only 38% of the pCT dose. Finally, RSP mean absolute percent error (MAPE) was found to be 0.59% for pCT and 0.67% for HeCT. Significance. This work compared the imaging performance of pCT and HeCT when using an existing scanner prototype, with the spatial resolution advantage of HeCT coming at the cost of increased dose. When matching spatial resolution via Hann windowing, HeCT had a substantial dose advantage. Both modalities provided state-of-the-art RSP MAPE. HeCT might therefore help reduce the dose exposure of patients with comparable image noise to pCT, enhanced spatial resolution and acceptable RSP accuracy at the same time.
Highlights
The necessity for highly accurate stopping power relative to water (RSP) voxel maps in modern particle therapy treatment planning spurred the investigation and development of new imaging modalities
One of those prototypes is the phase-II proton computed tomography (CT) scanner (Bashkirov et al 2016, Johnson et al 2016) located at the Northwestern Medicine Chicago proton center. This scanner, which provides comparable relative stopping power (RSP) accuracy to clinical dual-energy x-ray CT technology (Dedes et al 2019), was modelled in a Geant4 Monte Carlo simulation platform (Giacometti et al 2017b), which is a well-established approach for the development, characterisation and optimisation of new radiotherapy devices (Park et al 2021)
We presented a comparison of proton CT (pCT) and helium ion CT (HeCT) using the Monte Carlo simulation of the phase-II pCT scanner using both ideal and realistic scoring
Summary
The necessity for highly accurate stopping power relative to water (RSP) voxel maps in modern particle therapy treatment planning spurred the investigation and development of new imaging modalities. Since the idea was originally proposed by Cormack (1964) and first realised by Hanson (1979), several prototypes were recently developed and are under investigation for proton and heavier ion CT (Coutrakon et al 2013, Rinaldi et al 2013, Johnson et al 2016, Sadrozinski et al 2016, Meyer et al 2017, Pettersen et al 2017, Esposito et al 2018, Civinini et al 2020) One of those prototypes is the phase-II proton CT (pCT) scanner (Bashkirov et al 2016, Johnson et al 2016) located at the Northwestern Medicine Chicago proton center. This scanner, which provides comparable RSP accuracy to clinical dual-energy x-ray CT technology (Dedes et al 2019), was modelled in a Geant Monte Carlo simulation platform (Giacometti et al 2017b), which is a well-established approach for the development, characterisation and optimisation of new radiotherapy devices (Park et al 2021). The phase-II pCT scanner was further successfully used with helium ions and first experiments to investigate image artefacts, spatial resolution, RSP as well as range prediction accuracy have been performed (Volz et al 2018, Bär et al 2021, Volz et al 2021)
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