Rapid spatial mapping of the acoustic pressure in high intensity focused ultrasound fields at clinical intensities using a novel planar Fabry-Pérot interferometer

Abstract

Measurement of high acoustic pressures is necessary in order to fully characterise clinical high-intensity focused ultrasound (HIFU) fields, and for accurate validation of computational models of ultrasound propagation. However, many existing methods are unable to withstand the extreme pressures generated in these fields, and those that can often have high noise levels. Here, a robust sensor, based on a planar Fabry-Perot interferometer with hard dielectric spacer and mirrors, was used to measure acoustic pressure in the field of a 3.3 MHz single element spherically focused bowl transducer. In preliminary measurements, peak positive pressures of 27 MPa, and peak negative pressures of 14 MPa were measured. The noise equivalent pressure scaled with the adjustable dynamic range of the system between 50 kPa for pressures up to 8 MPa and 235 kPa for measurements up to 70 MPa. This makes the system suitable for measuring low pressure regions of the field as well as the high focal pressures. The −3 dB bandwidth of the sensor was 600 MHz, and the effective element size was 25 µm, which makes the sensor well suited to the measurement of the highly nonlinear and localised high-pressure focal regions generated in HIFU fields. Waveforms were acquired at a rate of 200 Hz, several orders of magnitude faster than can be achieved with a hydrophone scanning system. This sensor represents a critical improvement in measurement capability for HIFU fields in terms of dynamic range, bandwidth, noise equivalent pressure, and acquisition speed.