AUTHOR=Schnürle Katrin , Bortfeldt Jonathan , Englbrecht Franz Siegfried , Gianoli Chiara , Hartmann Jens , Hofverberg Petter , Meyer Sebastian , Niepel Katharina , Yohannes Indra , Vidal Marie , Landry Guillaume , Hérault Joël , Schreiber Jörg , Parodi Katia , Würl Matthias 

TITLE=Development of integration mode proton imaging with a single CMOS detector for a small animal irradiation platform

JOURNAL=Frontiers in Physics

VOLUME=Volume 10 - 2022

YEAR=2023

URL=https://www.frontiersin.org/journals/physics/articles/10.3389/fphy.2022.1044156

DOI=10.3389/fphy.2022.1044156

ISSN=2296-424X

ABSTRACT=A novel irradiation platform for pre-clinical proton therapy studies foresees proton imaging for accurate set-up and treatment planning. Imaging at modern synchrocyclotron-based proton therapy centres with high instantaneous particle flux is possible with an integration mode setup. The aim of this work is to determine an object’s water-equivalent thickness (WET) with a commercially available large-area CMOS sensor.
Image contrast is achieved by recording the proton energy deposition in the detector pixels for several incoming beam energies (here called probing energies) and applying a signal decomposition method that retrieves the WET. A single planar 114 mm x 65 mm CMOS sensor (49.5 µm pixel pitch) was used for this study aimed at small animal imaging. In experimental campaigns at two isochronous cyclotron-based facilities, probing energies suitable for small-animal sized objects were produced once with the built-in energy layer switching and the other time using a custom degrader wheel.
To assess WET accuracy, a micro-CT calibration phantom with 10 inserts of tissue-mimicking materials was imaged at three phantom-to-detector distances: 3 mm, 13 mm and 33 mm.
For 3 mm and 13 mm phantom-to-detector distance, the average WET error compared to the ground truth was about 1%  and the spatial resolution was 0.16 \pm 0.03 mm and 0.47 \pm 0.02 mm, respectively. For the largest separation distance of 33 mm air gap, proton scattering had considerable impact and the WET relative error increased to 30% and the spatial resolution was larger than 1.75 mm. 
We conclude that a pixelated CMOS detector with dedicated post-processing methods can enable fast proton radiographic imaging in a simple and compact setup for small-animal-sized objects with high WET accuracy and spatial resolution for reasonable phantom-to-detector distances.