Time-resolved radiation beam profiles in water obtained by ultrasonic tomography

This paper presents a practical ultrasonic system for near real-time imaging of spatial temperature distributions in water caused by absorption of radiation. Initial testing with radiation from a highly attenuated infrared lamp demonstrates that the system is able to map sub-millikelvin temperature changes, thus making it suitable for characterizing dose profiles of therapy-level ionizing radiation beams. The system uses a fan-beam tomographic reconstruction algorithm to invert time-of-flight data derived from ultrasonic pulses produced and detected by a circular array of transducers immersed in water. Temperature dependence of the speed of sound in water permits the conversion of these measured two-dimensional velocity distributions into temperature distributions that indicate the absorbed radiation dose. The laboratory prototype, based on a 128-element transducer array, is used to acquire temperature maps of a 230 mm × 230 mm area every 4 s with sub-millikelvin resolution in temperature and about 5 mm resolution in space. Earlier measurements with a single-channel version of this prototype suggest refinements in signal-conditioning electronics and signal-processing algorithms that would allow the present instrument to resolve temperature changes as low as a few microkelvin. Possible applications include real-time intensity profiling of radiation beams and three-dimensional characterization of the absorbed dose.