Direct methods for characterizing high-intensity focused ultrasound transducers using acoustic streaming

Two techniques are presented for noninvasively determining the intensity field of high-intensity focused ultrasound transducers in a liquid medium. The techniques are based upon the streaming velocity induced in the liquid by the absorbed ultrasound beam. The approaches are similar to an iterative streaming method previously reported, but the present approaches are direct: The differential operations of the Navier-Stokes equations are performed directly upon the experimentally measured streaming velocity, rather than through an iterative approach that minimizes the difference between a theoretical estimate of the streaming velocity and the one measured experimentally. As such, the direct methods are much faster than the iterative technique. The price paid for the increase in speed is smaller spatial coverage; the direct techniques are applicable only where accurate streaming velocity is available. Comparisons performed in the range 100-1000 W/cm(2) focal intensity showed differences between the direct methods and the iterative streaming technique to be less than 20%. Similar differences were observed in low-power comparisons with hydrophone measurements.