Measurement of acoustic streaming using magnetic resonance

Magnetic resonance imaging (MRI) has been used to explore acoustic streamin g caused in water under ultrasonic exposure conditions similar to those use d for diagnostic applications. Streaming was established in an enclosed tub e with acoustically transparent end windows, using a pulsed, weakly-focused transducer of acoustic frequency 3.5 MHz, Phase-detection MRI was used to image and quantify streaming profiles in the region of the acoustic focus. Acoustic powers in the range 0.4 mW to 100 mW were used. The sensitivity of the technique enabled streaming velocities down to 0.1 mm s(-1) to be meas ured, generated by acoustic power less than 1 mW, In addition, acoustic str eaming generated within open meshes with minimum pore dimensions of 3.0 mm and 2.0 mm was measured. The flow velocity in the coarser mesh reached 0.9 mm s(-1) at 95 mW total acoustic power. These observations demonstrate that acoustic streaming is probably a much more general phenomenon in diagnosti c ultrasound (ultrasound) than previously recognised, The combination of ma gnetic resonance and ultrasound shows promise as a diagnostic method for th e differentiation of cystic lesions in vivo, and for their characterisation , with sensitivity significantly greater than using ultrasound alone. (C) 2 000 World Federation for Ultrasound in Medicine & Biology.