The acoustic field of a pulsed Doppler diagnostic ultrasound system near a pressure-release surface

Abstract

Extravasation of blood cells has been observed in the lungs of mice and monkeys exposed to pulsed ultrasound conditions comparable to diagnostic levels [Child etal., Ultrasound Med. Biol. 16, 817–825 (1990) and A. F. Tarantal and D. R. Canfield, Ultrasound Med. Biol. (in press)]. In order to understand the ultrasound exposure conditions under which damage occurs, lung was modeled as a simple interface from water to air. The acoustic pressure output of an Advanced Technology Laboratories Ultramark 9 High Definition ImagingR system with a 5-MHz linear array transducer in pulsed Doppler mode (4 MHz) was interrogated near the pressure release surface (≤3 mm) with a Marconi 0.5-mm bilaminar membrane-type PVDF hydrophone. For the nonlinear waveforms produced, peak negative pressures of 2.0 MPa measured in free filed were increased to 4.5 MPa due to constructive interference near the interface. The behavior of the pressure release surface was also simulated computationally as a low-pass filter with a 180° phase shift. The impulse response of the interface was constructed using experimentally measured spectra of the incoming and reflected waves. The pressure waveform was calculated as a function of position by superimposing the incoming and reflected waves and was compared to the hydrophone measurements. The implications of these observations on insitu acoustic exposure near lung will be discussed.