The effect of low intensity ultrasound on Optison/sup TM/

It is known that exposure of microbubble contrast agent to ultrasound at the diagnostic level can result in their destruction. This effect has been used to its advantage in echocardiography for blood flow measurements and for drug delivery. To better understand the effect of low intensity ultrasound on a contrast agent Optison/sup TM/, the stability of Optison/sup TM/ diluted in saline solution insonicated by ultrasound was evaluated by measuring the attenuation coefficient and backscatter of the Optison/sup TM/ suspension. A container with four acoustic windows was constructed to allow simultaneous attenuation measurements at two frequencies using the through-transmission method with a flat reflector. The monitoring beams with negative peak pressures lower than 80 kPa are generated respectively by a 2 MHz transducer of 1.3 cm diameter and 55% -3 dB fractional bandwidth and a 3.5 MHz transducer of same diameter and 57% bandwidth. Ultrasound exposure of varying intensity is produced by varying the pulse repetition frequency (PRF) and pulse duration. The results show that the attenuation coefficient at 3.5 MHz of the contrast medium decreases as the PRF and the pulse duration increase, signifying the destruction of the bubbles as the ultrasound intensity is increased. However, the attenuation coefficient and integrated backscatter at 2 MHz show little change. These observations suggest that bubble destruction occurs even at the diagnostic level of ultrasound and it is related to the frequency of the insonicating ultrasound beam.