Development and application of spot-poled membrane hydrophones for measurements of high intensity therapeutic ultrasound fields

Abstract

The reliable characterization of high intensity therapeutic ultrasound (HITU, HIFU) fields is important regarding the safe and effective clinical application of the modality. However, the required acoustic output measurements pose several metrological challenges. Extreme pressure amplitudes easily cause damage to the typical sensors, pressure waveforms comprise a large number of higher harmonics, and a small sensing element size is desirable due to the strong focusing. Membrane hydrophones are widely used as reference sensors due to their advantageous and predictable characteristics. However, they are usually considered to be rather fragile instruments possibly not well suited for HITU field characterization. A membrane hydrophone previously developed at PTB was tested by means of successive measurements at focus with increasing driving voltage, and the pressure range detectable without destruction of the hydrophone was determined. Second, a novel hydrophone design comprising additional protective layers and a backing was developed to increase the robustness against cavitation. After calibration, measurements were performed using an HITU transducer with working frequencies of 1.06 and 3.2 MHz. The examples show the favorable applicability for HITU field characterization. The maximum detectable rarefactional and compressional pressure amplitudes were 15 and 77 MPa, respectively, with a detection bandwidth of 50 MHz.