Peak Negative Acoustic Pressure Research Articles

Prediction of in situ exposure to ultrasound: An improved method

The acoustic output of medical ultrasonic equipment is usually measured in water but to determine the safety (or performance) of a machine it is necessary to know the in situ acoustic pressure levels inside a patient. At present, when estimates of these levels are made, a linear propagation model is used although in practice the propagation is nonlinear. This paper shows that such a model can lead to large errors (80% or more in pressure) and in particular that in situ predictions of the peak-negative acoustic pressure are too low. To describe the field in water and in tissue it is necessary to take account of diffraction, attenuation, and dispersion as well as nonlinear propagation. It is difficult to use acoustic output levels in water to predict in situ values because the amplitude in water approaches a limit, an effect known as saturation. Nevertheless, a method of making such predictions is presented in this paper and is validated by comparison with experiment. The method is relatively time-consuming to implement and has not yet been applied to pulsed fields, so there is a need for more complete and simpler methods.

Calibration of medical ultrasonic equipment-procedures and accuracy assessment

A beam-plotting facility has been developed to provide a reference measurement system for determining the acoustic output of medical ultrasonic equipment. It consists of two coordinate-positioning systems controlled by stepping motors and a minicomputer. One system is used for holding and manipulating an ultrasonic transducer and the other for a hydrophone. A membrane hydrophone made from polyvinylidene fluoride of 9-mum thickness with an active element of 0.5-mm diameter is used for most measurements. The hydrophone is connected to an amplifier and digitizer, also controlled by the minicomputer, and the whole system has a measurement bandwidth of 75 MHz (-3 dB). A detailed description of this system is given together with a full assessment of measurement uncertainties and the methods used to correct for the effects of nonlinear distortion and spatial averaging. Typical overall uncertainties (95% confidence) for the determination of the peak-positive acoustic pressure, peak-negative acoustic pressure, spatial-peak pulse-average intensity and spatial-peak temporal-average intensity are +/-13%, +/-8%, +/-17%, and +/-23%, respectively.

Ultrasonically induced gas bubble production in agar based gels: Part I, experimental investigation

Macroscopically visible gas bubbles can be produced in an agar based gel by irradiation with either continuous or pulsed ultrasound at frequencies from 0.75 to 3.0 MHz. The variation in the number of bubbles formed with frequency, acoustic pressure, pulse length, duty cycle, and temperature closely resembles that seen in vivo. Furthermore, the acoustic pressure required to initiate bubble formation is also close to that required in vivo. It has been observed that alterations in the concentration and pH of the gels can have a profound effect on the nature and quantity of bubbles. This suggests that not only is this gel model suitable for the representation of the macroscopic features of bubble formation in vivo, but can be used to gain information about the preexisting bubble nuclei. Based on the experimental results obtained it can be suggested that for peak negative acoustic pressures of up 1 MPa (equivalent, for a plane travelling sinusoidal wave, to a time averaged intensity of 30 W/cm 2) bubble formation can be avoided by the use of high frequencies, short pulse lengths and long duty cycles.