TU‐A‐BRB‐10: Ultrasonic Imaging of Dose to Water from a 1.5 MeV Electron Beam

Abstract

Purpose: To use an ultrasonic sensor array to obtain thermal images produced by absorbed ‐dose distributions in a water phantom. Method and Materials: An array of 128 PVDF ultrasonic transducers arranged in a circle with radius 177.8 mm is used to image the heating of water in a phantom irradiated by a vertical, 1.5 ‐MeV electron beam capable of delivering dose rates ranging from Gy/min to kGy/min. Fan ‐beam sampling and reconstruction are managed by a double ‐multiplexer system that selects one transducer to emit a short pulse and scans the corresponding transducers across the tank to receive the propagated pulse. Time ‐of ‐flight measurements between transmitter/receiver pairs are then converted to temperature and passed to a filtered back projection algorithm to reconstruct the desired thermal distributions. Since dose from the beam is deposited mostly within the top cm layer of water, the array is located just below the water surface. A small, 4 ‐cm diameter Al mask is placed just above the water surface to aperture the beam. Results: Preliminary measurements involving therapy ‐level radiation were obscured by system noise, but higher doses, of order 1000 Gy/min, produced detectable images within seconds. Conclusion: Imaging of absorbed dose to water has been achieved for the first time, although at dose rates that are considerably above therapy level. Planned improvements in signal processing are expected to bring the detection limit into the therapy regime. Other uses of the array, e.g. triangulation of thermoacoustic disturbances caused by pulsed radiation sources, are also being investigated.