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Robust membrane hydrophone for the measurement of high-intensity therapeutic ultrasonic fields

13.06.2016

Exceptionally robust piezoelectric membrane hydrophones for measurement of extreme acoustic pressure waveforms were developed until ready for marketing in Working Group 1.62 Ultrasonics of PTB in cooperation with the Gesellschaft für Angewandte Medizinische Physik und Technik (Opens external link in new windowGAMPT) mbH, Merseburg.

The characterization of medical high-intensity therapeutic ultrasonic devices poses particular challenges for the hydrophones used to measure acoustic pressure in water. In order to allow measurements at clinical levels in the focal region of the emitting transducers which are used in applications such as tumor ablation within the scope of cancer treatment, extreme robustness, large detection bandwidth, large dynamic range, and small detection element size all are needed simultaneously. So far, the available measurement equipment, like fiber-optic hydrophones, has only been able to meet these requirements with certain limitations. Conventional piezoelectric membrane hydrophones are frequently preferred as measurement tools when it comes to determining broadband ultrasonic waveforms in the most objective manner possible. However, these sensors can be destroyed easily in the event that cavitation occurs at very large pressure amplitudes.

Starting with a joint MNPQ project (Messen, Normen, Prüfen, und Qualitätssicherung: measurement, standardization, testing, and quality assurance) between PTB and GAMPT mbH which was funded by the BMWi, and an invention which arose within the scope of that project, the development of especially robust membrane hydrophones [1,2] was successfully completed now to the point where they were ready for marketing. In this construction, the sensitive element, which is susceptible to damage, is protected on the front side by a thin stainless steel foil (see Figure 1), which gives the measurement device the appearance of a pocket mirror. This foil impedes the erosion of the electrode underneath it if cavitation occurs in the water; secondly, due to its smoothness, it increases the acoustic pressure threshold at which cavitation starts at the boundary surface. The rear electrode is protected by a special cavitation-withstanding oil to prevent damage to this part as well. The oil chosen features a similar characteristic acoustic impedance as water to minimize acoustic reflections from the rear side.

Figure 1: Novel, robust membrane hydrophones (“pocket mirror hydrophone“) for measurement of high-intensity therapeutic ultrasonic waves.

After having performed a broadband calibration of the “pocket mirror hydrophones” at PTB, they can be applied to ultrasonic field measurements. Figure 2 shows examples of measurement results in the form of acoustic pressure waveforms for various driving levels at typical working frequencies of approx. 1 MHz and 3 MHz at the focus of a therapeutic ultrasound transducer. It is possible to observe the increasing distortion with an increasing driving level due to nonlinear sound propagation and diffraction; this distortion progresses towards a sawtooth shape, including a peak compressional pressure amplitude which is significantly larger than the peak rarefactional pressure amplitude. The depicted measurements with peak compressional and peak rarefactional pressures of 90 MPa and 15 MPa, respectively, were successfully performed without any damage to the instrument. The active element size is only 0.2 mm in diameter, and frequency components of up to 100 MHz are included. Therefore, the hydrophone, as developed, significantly expands the variety of measurement instruments and tools available for ultrasonic exposimetry; the first purchase orders for the product have already been received by the cooperation partner.

 

Figure 2: Acoustic pressure waveforms for working frequencies of 1.06 MHz (left) and 3.32 MHz (right) and various driving levels measured at the focus of a high-intensity therapeutic ultrasonic field.

 

Literature:

[1] Wilkens, V., Sonntag, S., Georg, O., Robust spot-poled membrane hydrophones for measurement of large amplitude pressure waveforms generated by high intensity therapeutic ultrasonic transducers, J. Acoust. Soc. Am. 139, No.3, 1319-1332 (2016).

[2] Bessonova, O., Wilkens, V., Membrane hydrophone measurement and numerical simulation of HIFU fields up to developed shock regimes, IEEE Trans. Ultrason., Ferroelect., Freq. Contr. 60 No.2 (2013) 290-300.

Contact person:

Volker Wilkens, FB 1.6, AG 1.62, E-Mail: Volker Wilkens, FB 1.6, AG 1.62, E-Mail: Opens window for sending emailvolker.wilkens(at)ptb.de