Haemolysis in vivo by therapeutic intensities of ultrasound

Therapeutic intensities of MHz ultrasound directed at the upper abdominal area of rats in vivo resulted in damage to the circulating red blood cells. This damage was detected as free haemoglobin in the plasma as well as microspheres and spherocytic cell fragments which are characteristically produced when blood is heated to more than 49°C. The magnitude of this effect increased with increasing frequency of the ultrasound and was dependent upon the time-averaged intensity, even when the ultrasound was delivered as bursts of high spatial peak intensities. Thermal lesions were found above 10 W/cm 2 SP at 1 MHz and above 3 W/cm 2 SP at 3.4 MHz. These results show that the observed blood cell damage is primarily a thermal effect which occurs as the blood perfuses anatomical structures which are being heated by the ultrasound beam.