The INFN proton computed tomography system for relative stopping power measurements: calibration and verification

Abstract

Objective: This paper describes the procedure to calibrate the 3-dimensional proton stopping power relative to water (SPR) maps measured by the proton Computed Tomography (pCT) apparatus of the Istituto Nazionale di Fisica Nucleare (INFN, Italy). Measurements performed on water phantoms are used to validate the method. The calibration allowed for achieving measurement accuracy and reproducibility to levels below 1%. 
Approach: The INFN pCT system is made of a silicon tracker for proton trajectory determination followed by a YAG:Ce calorimeter for energy measurement. To perform the calibration, the apparatus has been exposed to protons of energies ranging from 83 to 210 MeV. Using the tracker, a position-dependent calibration has been implemented to keep the energy response uniform across the calorimeter. Moreover, correction algorithms have been developed to reconstruct the proton energy when this is shared in more than one crystal and to consider the energy loss in the non-uniform apparatus material. To verify the calibration and its reproducibility, water phantoms have been imaged with the pCT system during two data-taking sessions.
Main results: The energy resolution of the pCT calorimeter resulted to be σ(E)/E≅0.9% at 196.5 MeV. The average values of the water SPR in fiducial volumes of the control phantoms have been calculated to be 0.995±0.002. The image non-uniformities were below 1%. No appreciable variation of the SPR and uniformity values between the two data-taking sessions could be identified.
Significance: This work demonstrates the accuracy and reproducibility of the calibration of the INFN pCT system at a level below 1%. Moreover, the uniformity of the energy response keeps the image artefacts at a low level even in the presence of calorimeter segmentation and tracker material non-uniformities. The implemented calibration technique allows the INFN-pCT system to face applications where the precision of the SPR 3D maps is of paramount importance.

Meeting the detector challenges for pre-clinical proton and ion computed tomography

“…Also, they are efficiently integrated into detector assemblies by automated manufacturing processes developed for the electronics industry. The systems described in Johnson et al (2016), Esposito et al (2018, Scaringella et al (2023), and Ulrich-Pur et al (2020) all use silicon-strip technology for tracking.…”

“…A more significant problem is the relatively small size of individual detectors, typically not much larger than 10 × 10 cm 2 when cut from 15 cm diameter wafers. 5 Therefore, it is impossible to integrate them into large apertures without gaps or tiling, both of which can lead to image artifacts (although deleterious effects from tiling can be mitigated as described in Scaringella et al (2023)). Another drawback is that their high S/N is typically achieved through large integration times, leading to problems with signal pileup and inefficiency when used with pencil beams of the desired intensity.…”

“…Its readout amplifiers have a 1 μs peaking time, however, so that rate could only be practical in a cone beam spread out across multiple crystals. Civinini et al (2020) and Scaringella et al (2023) describe operation of a 14-crystal calorimeter in a 211 MeV beam, where an energy resolution of 0.9% was achieved, although while operating at a proton rate of only 80 kHz.…”

OPTIma: simplifying calorimetry for proton computed tomography in high proton flux environments

“…Systems have already been developed to perform proton computed tomography (pCT) (Johnson 2017, Scaringella et al 2023, however typically these are designed to deal with broad passively scattered beams and/or low beam currents (often limited to 1 proton in the system at a time) which limits the range of facilities they can be used at. Commercial clinical facilities are typically shifting from passively scattered beams to pencil beam scanning systems where minimum cyclotron treatment currents are of order 100 pA at the nozzle and are delivered in pulses at frequencies of the order of 100 MHz (Owen et al 2014, Durante andPaganetti 2016).…”