Near-field coded-mask technique and its potential for proton therapy monitoring

Abstract

Objective. Prompt-gamma imaging encompasses several approaches to the online monitoring of the beam range or deposited dose distribution in proton therapy. We test one of the imaging techniques – a coded mask approach – both experimentally and via simulations.
Approach. Two imaging setups have been investigated experimentally. Each of them comprised a structured tungsten collimator in the form of a MURA mask and a LYSO:Ce scintillation detector of fine granularity. The setups differed in detector dimensions and operation mode (1D or 2D imaging). A series of measurements with radioactive sources have been conducted, testing the performance of the setups for near-field gamma imaging. Additionally, Monte Carlo simulations of a larger setup of the same type were conducted, investigating its performance with a realistic gamma source distribution occurring during proton therapy.
Main results. The images of point-like sources reconstructed from two small-scale prototypes’ data using the MLEM algorithm constitute the experimental proof of principle for the near-field coded-mask imaging modality, both in the 1D and the 2D mode. Their precision allowed us to calibrate out certain systematic offsets appearing due to the limited alignment accuracy of setup elements. The simulation of the full-scale setup yielded a mean distal falloff retrieval precision of 0.72 mm in the studies for beam energy range 89.5–107.9 MeV and with 1 × 108 protons (a typical number for distal spots). The implemented algorithm of image reconstruction is relatively fast - a typical procedure needs several seconds.
Significance. Coded-mask imaging appears a valid option for proton therapy monitoring. The results of simulations let us conclude that the proposed full-scale setup is competitive with the knife-edge-shaped and the multi-parallel slit cameras investigated by other groups.

Prompt gamma imaging system in particle therapy: a mini-review

“…Effective TOF measurement requires synchronization with the ion beam's nano pulses, which can vary during extraction, often necessitating the use of a fast-beam monitor. In addition, the Maximum Likelihood Expectation Maximization (MLEM) algorithm is also used for image reconstruction in physical collimation methods such as knifeedge or coded aperture [56,57].…”

“…Correlation analysis is generally used as a reconstruction algorithm that uses the auto-correlation features of coded-aperture arrays. Approaches to confirm dose distribution by measuring PG in 1D or 2D using the coded mask and MLEM image reconstruction algorithm are being considered [57]. In the beam energy range 89.5-107.9 MeV, the average distal fall-off retrieval precision was 0.72 mm, indicating that the distal fall-off location can be accurately determined.…”