Optimization and performance study of a proton CT system for pre-clinical small animal imaging

Proton computed tomography (pCT) promises to reduce or even eliminate range uncertainties inherent in the conversion of Hounsfield units into relative stopping power (RSP) for proton therapy treatment planning. This is of particular interest for proton irradiation studies in animal models due to the high precision required and uncertainties in tissue properties. We propose a dedicated single-particle tracking pCT system consisting of low material budget floating strips Micromegas detectors for tracking and a segmented time-projection-chamber with vertical Mylar absorbers, functioning as a range telescope. Based on Monte Carlo simulations of a realistic in silico beam and detector implementation, a geometrical optimization of the system components was conducted to safeguard an ideal operation close to intrinsic performance limits at 75 MeV. Moreover, the overall imaging capabilities relevant for pre-clinical proton therapy treatment planning were evaluated for a mouse model. In order to minimize extrinsic uncertainties in the estimated proton trajectories, a spacing of the two tracking planes of at least 7 cm is required in both tracking detectors. Additionally, novel in-house developed and produced aluminum-based readout electrodes promise superior performance with around 3mm-1 spatial resolution due to the reduced material budget. Concerning the range telescope, an absorber thickness within 500 µm to 750 µm was found to yield the best compromise between water-equivalent path length resolution and complexity of the detector instrumentation, still providing sub-0.5% RSP accuracy. The optimized detector configuration enables better than 2% range accuracy for proton therapy treatment planning in pre-clinical data sets. This work outlines the potential of pCT for small animal imaging. The performance of the proposed and optimized system provides superior treatment planning accuracy compared to conventional X-ray CT. Thus, pCT can play an important role in translational and pre-clinical cancer research

Applicability of Capacitive Micromachined Ultrasonic Transducers for the detection of proton-induced thermoacoustic waves

This study investigates the application of broadband capacitive micromachined ultrasonic transducers (CMUT) to ionoacoustics (i.e., the thermoacoustic emissions induced by the energy deposition of ion beam) over a wide frequency range from hundreds of kHz to a few MHz. A water tank was irradiated by a 20 MeV pulsed proton beam. The frequency and amplitude of the ionoacoustic waves were modulated by adding material before to penetrate into the water tank to change the beam energy and its spatial dimensions. The measurements were performed with a 12 MHz CMUT prototype and compared to ones obtained from commercial 3.5 MHz piezoeletric transducer as well as to in silico studies employing the k-Wave Matlab toolbox in combination with FLUKA Monte Carlo simulations to derive the dose (i.e., energy deposition per mass) and initial pressure distribution. Comparison of the experimental and in silico results show that the CMUT bandwidth is wide enough to measure the signal without any degradation or attenuation of the frequency content in the investigated frequency range, thus ensuring accurate reconstruction of the dose distribution and potential bi-modality system for the co-registration of ionoacoustic and ultrasound imaging

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

A novel irradiation platform for preclinical proton therapy studies foresees proton imaging for accurate setup and treatment planning. Imaging at modern synchrocyclotron-based proton therapy centers 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 detector pixels for several incoming beam energies (here, called probing energies) and applying a signal decomposition method that retrieves the water-equivalent thickness. A single planar 114 mm × 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 built-in energy layer switching and the other time, using a custom degrader wheel. To assess water-equivalent thickness 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 water-equivalent thickness error compared to the ground truth was about 1 and the spatial resolution was 0.16(3) mm and 0.47(2) mm, respectively. For the largest separation distance of 33 mm air gap, proton scattering had considerable impact and the water-equivalent thickness 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 water-equivalent thickness accuracy and spatial resolution for reasonable phantom-to-detector distances