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ST: Fachverband Strahlen- und Medizinphysik

ST 6: Medical Imaging Technologies

ST 6.1: Talk

Wednesday, March 20, 2024, 15:00–15:15, PC 203

Piezo-Optic Power Calibration of Nonlinear and Shocked Ultrasound in Real Time — •Milan Fritsche, Johannes Köppl, Luis Guillermo, and Florian Steinmeyer — Technische Hochschule Nürnberg Georg Simon Ohm, Deutschland

Calibration of ultrasound transducers - used in medical diagnostics and therapies like lithotripsy or thermal ablation - is cumbersome: Standard is a radiation-force-balance calibration with time resolution of 1s-2s, where a target is irradiated off-focus to avoid damage. Hydrophones measure acoustic pressure over hours. They are easily destroyed by cavitation, suffer from reflections and exhibit resonances in the frequency response.

The piezo-optic effect is the change of the refractive index under pressure. A Schlieren imaging system consists of a LED with a collimator illuminating the acoustic wavefield in water, a focussing "Schlieren lens", a knife-edge at the lens focus (blocking parts of the Fourier plane) and a camera. Light is refracted proportional to pressure gradients integrated along the light path. The camera records the wavefield in a real image. It is very sensitive to the precise knife edge position, thus not useful for calibration.

In our procedure a CMOS sensor replacing the knife edge records intensity over the full Fourier plane. The intensity variance displays a linear dependence on acoustic power - so far over more than two orders of magnitude (0,5W-150W). This allows for robust calibration of acoustic power even of shock waves with real time resolution of sub-milliseconds.

Keywords: piezo optic effect; therapeutic and diagnostic ultrasound; transducer; shock waves; calibration of acoustic power