A novel calibration device for quality assurance of therapeutic ultrasound

Photosensitive acrylate polymers can be spatially patterned to produce layers of alternating optical contrast using multi-beam interference patterns of laser light. This technology is widely used for the fabrication of photonic devices including optical sensors and holographic elements [1] , [2] . Here, the use of photonic Bragg structures fabricated in a pressure-sensitive photopolymer [3] for the calibration of therapeutic ultrasound pressure output is presented. On exposure to ultrasound pressure waves, the Bragg structure is compressed, inducing a change in the spacing of the periodic layers of varying optical contrast and, thus, changing the devices readout wavelength i.e. colour. The device colour change due to pressure can either be visually assessed or spectroscopically interrogated for more accurate pressure quantification. This technology offers a fast, straight-forward and low-cost method for regular mapping and calibration of ultrasound pressure output. This is crucial for quality assurance of therapeutic ultrasound scanners used in Extracorporeal Shock Wave Lithotripsy and High Intensity Focused Ultrasound. To date, the photopolymer composition and Bragg device structure has been optimised for MPa range pressures. Results from the optimisation and characterisation of the pressure-sensitive devices are presented.