The assessment of microvascular flow and tissue perfusion using ultrasound imaging.

Imaging microvascular flow is of diagnostic value for a wide range of diseases including cancer, inflammation, and cardiovascular disease. The introduction of microbubbles as ultrasound contrast agents offers significant signal enhancement to the otherwise weakly scattered signal from blood in the circulation. Microbubbles provide maximum impedance mismatch, but are not linear scatterers. Their complex response to ultrasound has generated research on both their behaviour and their scattered-signal processing. Nearly 20 years ago signal processing started with simple spectral filtering of harmonics showing contrast-enhanced images. More recent pulse encoding techniques have achieved good cancellation of tissue echoes. The good quality contrast-only images enabled ultrasound contrast-imaging applications to be established in microvascular measurements in the liver and the myocardium. The field promises to advance the quantification of microvascular flow kinetics.