Speaker
Description
Radiation-induced Acoustic Computed Tomography is an imaging modality that combines the high penetrability of ionizing radiation with the flexibility of detection of ultrasounds. A single short pulse of electrons or x-rays locally heats an absorber inside the object of study, launching ultrasound waves in three dimensions that transducers outside the object detect. This enables image reconstruction and other analyses in one shot, thereby minimizing exposure to ionizing radiation. Efficient generation of acoustic waves in most materials require that the energy deposition happens in some nanoseconds. Only a few commercial x-ray generators operate at these durations, and they are limited in their energy output. Laser Wakefield Accelerators (LWFA’s), on the other hand produce femtosecond bunches and are energy-tunable, making them an attractive option for this application.
Here, we irradiate various targets with single ~100 MeV electron bunches from a laser-wakefield accelerator (LWFA). We calibrate deposited dose from the sound-wave amplitude, determine material-specific acoustic resonances from the Fourier transform of the acoustic waveform, and recover the absorber’s size and shape from 3D image reconstruction. First, phantom targets consisting of metal wires or slabs immersed in water were examined. Fourier decomposition of the pressure signal showed sharp frequency peaks that matched the expected eigenfrequencies from analytical solutions. These eigenfrequencies are linked to the density, speed of sound, and shape of the objects. Here, we used them to distinguish W, Cu, and Pb components of the targets, and accurately reconstructed the shape of wire grids and twisted wires. Next, electron irradiation of a bone sample immersed in water yielded ultrasound reconstructions that clearly distinguished the hard outer cortical bone, and matched the shape and size obtained from standard x-ray tomography. Finally, electrons irradiation of a kilogram of pork yielded ultrasound reconstructions that revealed preferential energy deposition in fatty tissue. Moreover, the acoustic response scaled linearly with number of shots, showing that this technique can provide real-time dosimetry in conjunction with very-high-energy electron (VHEE) therapy. All these cases are backed by simulations of the energy deposition using Monte Carlo simulations (GEANT4) and acoustic propagation (k-wave toolkit).
| Working group | WG6 |
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