1. Transducer Specifications for Thermoacoustic Range Verification
SK Patch, R Lindert, UW-Milwaukee
D Hoff, T Webb, LG Sobotka, Washington University Saint Louis
T Zhao, Washington University Medical Center
Best viewed in presentation mode on a Windows PC with audio enabled !!
Particle Therapy Cooperative Group North America Annual Conference
Chicago, October 23-25, 2017

2. Motivation
Transducers should be sensitive to thermoacoustic emissions
Outline
simulation assuming instantaneous deposition – Monte Carlo + acoustic
pulse envelope measured on Mevion S250
account for measured pulse envelope

3. Forward Simulation of Thermoacoustic Emissions - Instantaneous Deposition
SRIM/TRIM Monte Carlo using
water target
90k protons at 230 MeV
beam σ = 3.3 mm in air upon entry into target
position and energy recorded every 1 MeV
custom software distributed energy loss uniformly along line segments – 1 mm discretization
3D k-Wave software propagated thermoacoustic pressures due to instantaneous deposition, pδ, results below for transducer located distal along beamline

4. S250 Pulse Envelope Measurements
1 mm x 1 mm x 6” scintillating fiber attached to a PMT
1 mm x 6” face placed in the beam
triggered on the injection of hydrogen into the synchrocyclotron
beam intensity reduced until only a few spikes per trigger (blue)
inverted & summed 400 waveforms (red)
FWHM = 5.8 μs , nearly Gaussian
histogram of sum
1 of 400 realizations

5. Stress Confined TA Signal Generation
Positive (compressional) followed by weak negative (rarefactional)
Build up pressure faster than it runs away
6 μs pulsewidth →
Slight loss of pulse amplitude and bandwidth
initial pressure > 0

6. Analysis pδ bandlimited below 150 kHz, pI below 100 kHz for FWHM= 6 μs
Convolved TA emissions with Gaussian to simulate experimental emissions pI = pδ * I, where I is Gaussian with FWHM = 6 μs
pδ bandlimited below 150 kHz, pI below 100 kHz for FWHM= 6 μs
pI amplitude also diminished

7. Discussion CUSTOM ULTRASOUND HARDWARE REQUIRED!!
Pulse amplitudes very low, O(10 Pa), require sensitive receivers
Receive-only TA transducers should be sensitive to kHz, not MHz
Signals received at lateral transducer locations are even lower frequency
Heavier ions may generate broader band signal because Bragg peak is sharper – pulse envelope likely to limit bandwidth
Acknowledgements: Jeff Heine, Patrick Zerkel (Mevion Engineering)
- and many PT physicists for helpful discussions, including next speaker, Jiajian Shen

8. Discussion – heavy ions better, with stress-confinement
Heavier ions may generate broader band signal because Bragg peak is sharper – pulse envelope likely to limit bandwidth
TRIM simulations comparing C vs H in water, distal transducer location
carbon emissions stronger and broad band compared to proton