Ultrasound contrast agents in radiological diagnosis

Contrast enhancement is widely applied to vascular imaging and has become a routine part of diagnostic X-ray, CT, MR and radionuclide methods for vascular diagnosis. Colour Doppler imaging, although offering a unique method for non-invasive vascular imaging, suffers limitations determined by the low velocity of blood in small vessels and the weakness of backscattered ultrasound from blood deep in the body. While the low velocity of blood in small vessels remains a fundamental obstacle, there is clear clinical potential for a method capable of enhancing the acoustic backscatter from blood itself, especially in the systemic arterial system, by means of an ultrasound contrast agent. In this paper, studies are made on a stabilized microbubble contrast agent capable of undergoing resonant oscillation in the ultrasound field used to create an image. The effect on this resonant behaviour is that very small quantities of the contrast agent in the systemic arterial circulation can produce outstanding enhancement of Doppler ultrasound signals from major and peripheral arteries. In addition, nonlinear oscillation of these bubbles makes possible an entirely new form of imaging, referred to here as harmonic imaging. It is felt that the use of these contrast agents will enable detection of smaller vessels in the arterial and venous circulation than is currently possible, of neovascular flow associated with malignant tumours, wound healing and possibly the measurement of blood flow rates in such tissues as the myocardium. Harmonic imaging adds to this capability by effectively subtracting Doppler signals from moving solid tissue, thus lowering the threshold of detectable velocity in the vascular system.