Unfocused/Weakly Focused Pressure Pulse Sources for Pain Therapy: Measurements in Water and in a Dry Test Bench

Unfocused/weakly focused pressure pulses (UPP) are generated by an "air-gun" like mechanism, with a projectile accelerated by pressurized air and impinging on a metal applicator. They were introduced in 1998 for the treatment of orthopedic soft tissue pain. The patient side of the applicator (a circular piston of 6-30 mm diameter) releases single pressure pulses of 2-10 MPa and 4-5 μs duration. Up today, there is no standard for the measurement of UPP sources (UPPS), so data are often reported on the basis of the focused lithotripter standard IEC61846. The purpose of this research is to establish methods to reliably measure UPPS acoustic parameters and establish a parameter set based on definitions from focused lithotripsy sources as applicable. Therefore, acoustic characteristics and wave fields of the pressure pulses of UPPS from different manufacturers were measured in a water bath and in a dry test bench. It was demonstrated by comparison with optical hydrophones that piezoelectric hydrophones are appropriate for the measurements of UPP. For on-axis measurements at a fixed distance, measurements in the dry test bench can replace water-bath measurements. Additionally, the dry test bench allows for reliable results at pulse rates >1 Hz.