A Robust Hydrophone for HIFU Metrology

Abstract

The high acoustic intensities generated by HIFU systems cause conventional hydrophones to fail before measurements can be reliably made. To address this challenge, we present a new piezoelectric needle hydrophone, which is resistant to cavitation while possessing a flat frequency response (+/-3 dB from 1 to 10 MHz) and a small effective aperture (400 micron effective diameter). This hydrophone has been used in a high intensity field (1.5 MHz tone burst of 30 microseconds and 3% duty cycle, with rarefactional pressures exceeding 4 MPa and positive pressures exceeding 15 MPa) without degradation in the hydrophone's performance, as indicated in before-and after calibration checks of the device.

Characterization of high intensity focused ultrasound transducers using acoustic streaming

“…Consequently, these techniques can accurately characterize HIFU transducers only at low power. For clinically relevant high powers, there are no alternative measurement standards available to accurately characterize medical ultrasound fields generated by HIFU transducers ͑Shaw and ter Haar, 2006;Harris 2005͒. Several new methods for measuring HIFU fields are being researched, including development of robust sensors and hydrophones ͑Wang et al, 1999;Shaw, 2004;Schafer et al, 2006;Zanelli and Howard, 2006;Shaw and ter Haar, 2006͒. An alternative approach to overcome the sensor-induced inaccuracies is to eliminate the use of sensors, and noninvasively measure the pressure field. One such commercially available noninvasive method is the schlieren imaging technique ͑Harland et Theobald et al, 2004͒, which utilizes changes in the optical index of refraction to qualitatively define the ultrasound field.…”

Pressure Pulse Distortion by Needle and Fiber-Optic Hydrophones due to Nonuniform Sensitivity

“…Compactness makes needle hydrophones preferable for in vitro testing of transcranial systems [2–4]. In addition, some needle [5] and fiber-optic [6–14] hydrophones are more robust for high-intensity therapeutic ultrasound (HITU) measurements than most membrane hydrophones, although robust membrane hydrophone designs also have been proposed [15, 16]. Fiber–optic hydrophones have been used extensively for calibration of HITU [17–23] and lithotripsy [24] systems.…”

Quality assurance for clinical high intensity focused ultrasound fields

“…Some needle-like devices have been developed to be more robust. These include the HNA and HGL series hydrophones of the Onda Corporation (Sunnyvale, CA) [35], and fibre-optic hydrophones which can measure very large pressure amplitudes (410 MPa, RP-Acoustics, Leutenbach, Germany) [36,40]. Other factors such as noise levels, stability and frequency response are factors which may also need to be considered.…”

“…Measurements are usually performed in water under so called 'free-field' conditions, in which the ultrasound beam propagates through a non-attenuating medium, in the absence of scattering structures. Ideally, the water should be degassed to avoid the formation of acoustic cavitation bubbles which can interfere with measurements, and also to minimise the risk of cavitation damage to the hydrophone [35]. It may also be filtered and de-ionised, the latter being a requirement for measurements with co-planar membrane hydrophones where the conducting electrodes are exposed to the water.…”