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.

Experimental fluence‐modulated proton computed tomography by pencil beam scanning

“…In this experimental study, we have demonstrated that the phase II pCT scanner prototype could be operated in PBS mode at acceptable pileup levels. Previous mentions of scanner operation in proton PBS mode did not present pileup analyses . The chosen cyclotron current of 4 nA led to a scanner trigger rate of approximately 400 kHz.…”

“…The pCT Phase II scanner had been so far mostly used with broad proton beams: either a passively scattered cone beam at the Loma Linda University Medical Center, covering the whole field of view of the scanner, or at the Northwestern Medicine Chicago Proton Center with a wobbled proton beam of 4–7 cm FWHM spot size . The single report of scanner operation in PBS mode makes no mention of pileup . Before performing the calibration and imaging runs, a study was made on the detector performance for a 1.37 cm FWHM PB, concentrating especially on the resulting pileup in the silicon strips of the tracker modules.…”

“…10 The single report of scanner operation in PBS mode makes no mention of pileup. 29 Before performing the calibration and imaging runs, a study was made on the detector performance for a 1.37 cm FWHM PB, concentrating especially on the resulting pileup in the silicon strips of the tracker modules. The main concern was that the amplifier time-over-threshold for the silicon strips could be such that if two protons impinge on the same strip within a time window of less than about one microsecond, the second one will be missed.…”

“…PBS scans have been previously acquired for carbon ion CT with a flat panel detector as well as with an ionization chamber stack detector which functions in PBS mode by design . Helium CT results obtained from PBS have also been presented for a particle tracking scanner, and the same study reported the acquisition of PBS pCT as well, but no results were reported …”

“…9 Helium CT results obtained from PBS have also been presented for a particle tracking scanner, and the same study reported the acquisition of PBS pCT as well, but no results were reported. 29 In this study, we performed PBS pCT scans and first experimental realization of FMpCT by making use of the Phase II preclinical prototype pCT scanner of the Loma Linda University and U.C. Santa Cruz, together with the PBS capability of the Northwestern Medicine Chicago Proton Center.…”