Medical and Biomedical Applications of Shock Waves by Achim M. Loske

Author:Achim M. Loske
Language: eng
Format: epub, pdf
Publisher: Springer International Publishing, Cham

Fig. 6.13Schematic of the hand-piece of a ballistic radial pressure wave source. Compressed air fires the projectile within a guiding tube. As the projectile hits the metallic applicator, stress waves are produced and transmitted into the patient’s tissue

When using ballistic therapy heads, it should be kept in mind that there is no linear relationship between the air compressor pressure and the energy output, and that two different models operating at the same air compressor pressure may not generate the same pressure field. More acoustic cavitation is to be expected when increasing the air pressure setting and, according to Császár et al. (2015), less cavitation is produced operating the device at high frequency compared to low frequency. Furthermore, it is important to distinguish between the air compressor pressure (generally given in bar) and the pressure pulse amplitude generated by a ballistic device (generally given in MPa). For instance, the 5.5 MPa (55 bar) peak-positive pressure shown in Fig. 3.​11 could have been generated by adjusting the air compressor pressure of the radial pressure wave generator at 3 bar. In this case, 55 bar is the pressure measured at a specific distance from the applicator and 3 bar is the required air pressure to produce the 55 bar output.

Because of differences in experimental setups there may be considerable discrepancies between pressure measurements reported in scientific publications and those reported by manufacturers. Perez et al. (2013) recorded the pressure waveforms emitted by a Duolith SD1T-Top (Storz Medical AG). According to their measurements, the waveform consists of a leading positive pulse (p + = 8 MPa) with a duration of 5 μs, followed by a negative phase with a p − equal to about −5.7 MPa. The reported EFD is 0.115 mJ/mm2 and most of the energy is contained at or below 200 kHz. Cleveland et al. (2007) carried out pressure measurements on a DolorClast Vet (Electro Medical Systems, Nyon, Switzerland) ballistic pressure wave source. The pressure waves generated by this equipment were transmitted into a small water tank through a membrane and recorded with a polyvinylidenevfluoride (PVDF) hydrophone and an in-line preamplifier. A convex shaped applicator with a 15-mm diameter, referred to as unfocused applicator and a focused concave applicator with a 12-mm diameter were tested. Adjusting the air compressor pressure at 3 bar, the waveform at 10 mm from the unfocused applicator consisted of a positive pulse with p + equal to 5.6 MPa and a 3.8 μs t FWHMp+ duration (Fig. 3.​1), followed by a strong negative phase (p − = −9.2 MPa), demonstrating that, contrary to shock wave pressure waveforms, the amplitude of the negative phase of ballistic sources may be larger than the generated compressive pulse (Fig. 3.​11). The pressure returned to the baseline after approximately 15 μs. The EFD (obtained by integrating the intensity from 0 to 25 μs) was 0.234 mJ/mm2. The positive and negative phase produced by the focused applicator had a peak pressure of 3.4 MPa and −5.2 MPa, respectively. The t FWHMp+ duration of the positive pulse lasted 3.

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