Fiber-Tip Surface-Micromachined Optical Ultrasound Transducer (SMOUT) Probe for Acoustic Detection Induced by Ultrahigh Dose Rate (UHDR) Electron Beam

Online dose mapping is essential for safe and accurate beam delivery in FLASH radiation therapy (RT). While ionizing radiation acoustic imaging (iRAI) enables real-time, deep tissue dose measurement and beam localization for RT, previous piezoelectric acoustic sensors have suffered from electronic noise and neutron damage from ultra-high dose rate radiation (UHDR) beam. Described here is an innovative single-element full-optical surface-micromachined optical ultrasound transducer (SMOUT) mounted on an optical fiber tip to detect acoustic waves induced by UHDR beam. The design and fabrication of the fiber-tip SMOUT probe (diameter = 400 μm) have been demonstrated, and its optical and acoustic characteristics, as well as transmittance to the FLASH electron beam and imaging capability have been characterized. Experimental results show that the fiber-tip SMOUT probe has optical resonance wavelength (ORW) ~ 773 nm, acoustic central frequency ~ 300 kHz and -6 dB acoustic bandwidth ~ 160 kHz, and dose rate independent single pulse noise equivalent dose ~ 5.19 cGy. More importantly, the fiber-tip SMOUT probe exhibits negligible disturbance to FLASH irradiation. After 1-D scanning the fiber-tip SMOUT probe, the 2-D dose distribution of the FLASH electron beam can be mapped with high consistency. These findings highlight the potential of SMOUT based iRAI for mapping the dose distribution under UHDR beam, which is an important step toward practical implementation of iRAI-based online dosimetry in FLASH RT.