Purpose: Transitioning thermoacoustic range verification from benchtop experiments with cumbersome research equipment to clinical practice is the purpose of our work. A wireless device for thermoacoustic range verification with correlation to online ultrasound images has been developed for initial application to adaptive planning (Fig. 1).
Methods: A 7 x 5.5 x 3.5 cm detector (NucSafe) requiring 5V power supply replaces the 0.5 m long prompt gamma detector that required -2 kV (Fig. 2a). The new detector provides both analog and digital signals to quantify proton pulse shape and serve as a trigger. Oscilloscopes are replaced by a wireless data acquisition board (DAQ) with 1 MSPS ADC and vertical resolution of 38 uV. 6 acoustic channels are amplified. 2 channels for compact gamma detector inputs (analog and digital) are not amplified. Data is acquired continuously for up to 30 minutes and stored on a secure data card for retrieval via a serial port after the exam.Two different types of transducers provide broadband response in a compact form factor (Fig. 2e).
Results: A custom receive chain provides improved 10-18 dB greater sensitivity over the relevant frequency band (10-100 kHz, Fig 2b). Transducers positioned to either side of the ultrasound imaging array are up to 8 dB more sensitive to low frequencies (Fig 2c). The new DAQ provides 10 dB greater amplification. A wireless ultrasound imaging array (Clarius P4-1) provides greater imaging depth, 30 cm vs 7 cm, and smaller footprint (Fig. 2d).
Conclusion: This improved device provides greater ease of use, positioning flexibility and receive sensitivity which could enable range verification during pulsed delivery of a 2 Gy fraction. Although validation on phantoms in proton therapy vaults is required, nonintervention patient studies could quickly follow because "listening" for thermoacoustic signals generated during the normal course of therapy is entirely passive.
Funding Support, Disclosures, and Conflict of Interest: NIH-NCI SBIR #R43CA243764. Dr. Patch founded Acoustic Range Estimates and holds US patent #10,758,127. Mr. Narayanaswamy is founder of Swamy Enterprises.