Proton Sound Detector for Beam Range Measurement in FLASH Hadron Therapy

21 Sept 2021, 17:20
1h 20m
Poster Programmable Logic, Design Tools and Methods Posters Programmable Logic, Design Tools and Methods

Speaker

Elia Arturo Vallicelli (University of Milano Bicocca)

Description

Proton-Sound-Detectors (ProSDs) sense (at <1 ms latency) the thermoacoustic signal generated by the fast energy deposition at the Bragg peak of proton beams penetrating energy absorbers.
ProSDs are especially promising for experimental monitoring of high pulse rate (FLASH) hadron therapy treatments working in-sync with the beam.
This paper presents a mixed-signal detector capable of sensing and processing high rate (1k beam shots/sec) ionacoustic signals with <1 ms latency. The system was validated by measuring the dose deposition of a 20 MeV proton beam in water, achieving 3.43% precision (±2.75 GyRMS) after 50 ms acquisition (77.56 Gy total dose deposition).

Summary (500 words)

Background and aims
Proton Sound Detectors (ProSDs) sense the proton-induced thermo-acoustic pulse generated by particle beams penetrating a water energy absorber. This signal is directly proportional to the deposited dose.
Compared to nuclear imaging techniques, ProSDs have < 1 ms latency and sub-mm precision, making this technique particularly suitable in real-time monitoring of high dose depositions, as in the case of FLASH hadron therapy.
However, to quickly acquire and process this information, advanced and specific digital signal processing (DSP) stages are required.
For this reason, this paper presents a digital system on FPGA as back end element of the Detector, operating in sync with 1k shots/sec beam and finally providing digital domain proton induced acoustic tracks within 1 ms latency.

Methods
The Detector is composed by a piezoelectric acoustic sensor and low-noise analog front-end (60-80 dB low-noise amplifier, 4.5 MHz -3dB 3rd order low-pass filter and 10-bits 80 MS/sec A/D converter) and a DSP stage on a Xilinx Spartan 6 FPGA. It performs event-driven data acquisition (triggered by the beam shot) and a custom c++ GUI is used for signal processing and visualization. The Detector and the embedded DSP have been validated with a sub-clinical 20 MeV proton beam with ~106 protons/shot and 1000 shots/sec rate. The Detector sensitivity is 173 mV/Gy (w.r.t. Bragg peak).

Results
Measured acoustic sensor output signal is 276 mV0-peak corresponding to 1.59 Gy dose at the Bragg peak, consistent with 1.6 Gy dose/shot calculated from the beam current and Bragg peak physical size. By repeating the measurement 8000 times, the measured dose precision has been found equal to 388 mGyRMS (24.4%) due to random noise fluctuations. By characterizing the noise performances (62.9 mVRMS, equivalent to 363 mGyRMS, from sensor and electronics themal/flicker noise) it is possible to measure the dose deposition variations due to random beam fluctuations over time, equal to 138 mGyRMS or 8.6% of proton number/shot. The cumulative dose after 50 shots (50 ms acquisition) is 77.56 Gy ± 2.75 GyRMS (3.43%). Finally, the ionoacoustic signal amplitude has been recorded for 1 s. By low-pass filtering, a slow ~7 Hz fluctuation in the ionoacoustic signal amplitude becomes observable. Such slow fluctuation is compatible with the beam fluctuation around the horizontal beam axis visible in the CCD beam spot monitoring system.

Conclusions
Compared to nuclear imaging techniques, Proton Sound Detectors promise sub-mm and sub-ms monitoring of the Bragg peak location, dose deposition and beam characteristics. The low-latency, high precision and low instrument complexity (and cost) make this technique appealing for experimental verification/monitoring of the proton beam range for both medical (hadron therapy) and physics research.

Primary authors

Elia Arturo Vallicelli (University of Milano Bicocca) Andrea Baschirotto (Univ. Milan Bicocca) Dr Marcello De Matteis

Presentation materials