Stopping power accuracy and achievable spatial resolution of helium ion imaging using a prototype particle CT detector system

Abstract

A precise relative stopping power map of the patient is crucial for accurate particle therapy. Charged particle imaging determines the stopping power either tomographically - particle computed tomography (pCT) - or by combining prior knowledge from particle radiography (pRad) and x-ray CT. Generally, multiple Coulomb scattering limits the spatial resolution. Compared to protons, heavier particles scatter less due to their lower charge/mass ratio. A theoretical framework to predict the most likely trajectory of particles in matter was developed for light ions up to carbon and was found to be the most accurate for helium comparing for fixed initial velocity. To further investigate the potential of helium in particle imaging, helium computed tomography (HeCT) and radiography (HeRad) were studied at the Heidelberg Ion-Beam Therapy Centre (HIT) using a prototype pCT detector system registering individual particles, originally developed by the U.S. pCT collaboration. Several phantoms were investigated: modules of the Catphan QA phantom for analysis of spatial resolution and achievable stopping power accuracy, a paediatric head phantom (CIRS) and a custommade phantom comprised of animal meat enclosed in a 2 % agarose mixture representing human tissue. The pCT images were reconstructed applying the CARP iterative reconstruction algorithm. The MTF_10% was investigated using a sharp edge gradient technique. HeRad provides a spatial resolution above that of protons (MTF10_10%=6.07 lp/cm for HeRad versus MTF_10%=3.35 lp/cm for proton radiography). For HeCT, the spatial resolution was limited by the number of projections acquired (90 projections for a full scan). The RSP accuracy for all inserts of the Catphan CTP404 module was found to be 2.5% or better and is subject to further optimisation. In conclusion, helium imaging appears to offer higher spatial resolution compared to proton imaging. In future studies, the advantage of helium imaging compared to other imaging modalities in clinical applications will be further explored.

Keywords: Computed Tomography; Helium; Particle Imaging; Proton; Radiography; Resolution; Stopping Power.

Figure 1:

Prototype particle CT detector system first developed by the U.S. pCT collaboration. The system consists of two tracking detector systems, precisely measuring position and direction information, and an energy detector measuring the particles’ residual energy.

Figure 2:

HeCT of A) the Catphan® CTP404 sensitometry module and B) the CTP528 high resolution module reconstructed using the DROP-TVS reconstruction algorithm with 512×512 pixel per slice and slice thickness of 1.25 mm. The phantom was scanned 6 from 90 projections consisting of ~2.5×10⁶ particles each.

Figure 3:

Vertical slice of a HeCT of A) the CIRS pediatric head phantom model HN715 (CIRS, Norfolk, Virginia, USA) and B) the custom build animal tissue phantom reconstructed using the DROP-TVS iterative reconstruction algorithm with 256×256 pixel per slice and slice thickness of 1.25 mm. The phantom was 6 scanned using 90 projections consisting of ~4×10⁶ particles each.