Quantification of blood flow

Traditionally, Doppler ultrasound has been used to estimate blood flow as t he mean velocity multiplied by the vessel area, but this is subject to sign ificant errors and may be difficult to perform accurately. Microbubbles, de veloped as contrast agents for ultrasound, were initially envisaged as usef ul for increasing the intensity of echoes and thus rescuing Doppler studies that were technical failures because of attenuated signals or very slow fl ow. However, they can act as tracers and, by analogy with isotope technique s, can be used to measure blood flow with transit-time methods which exploi t both arterial and venous; time-intensity data. An acceptable compromise i s to acquire both a tissue intensity curve and one from the feeding artery. The transit of microbubbles across an organ or tissue can be used to estim ate haemodynamic alterations, e.g. the arterialisation of the supply to the liver in malignancies and cirrhosis and the delayed arterio-venous transit in the transplant kidney during rejection. The fragility of microbubbles c an be turned to advantage by being exploited to create a negative bolus by exposing a tissue slice to a high power beam. The rate of refilling of this slice by circulating microbubbles can then be followed with a low-intensit y monitoring beam and the resulting rising exponential curve analysed to ex tract indices of both the reperfusion rate (the slope) and the fractional v ascular volume (the asymptote). The product of these is a measure of true t issue perfusion.