After injection of biocompatible fluids, contrast echocardiograms are obtained by ultrasound reflection from microbubbles in the streaming blood, which is thus opacified analogous to angiocardiography. As the microbubbles cannot pass the pulmonary capillaries, left heart contrasts are usually not seen after peripheral venous injection. Especially intracardiac shunts and valvular deficiencies may be recognized by this technique, which became an important diagnostic tool in the diagnosis of congenital heart disease and in critically ill patients. The blurred border between the opacified cavity and the myocardium may be abolished by digital image enhancement techniques, applied to contrast echocardiograms, thus making these pictures more suitable for volume determination and ventricular function analysis than non-contrast echocardiograms. Newer contrast agents such as mini-microbubbles are able to pass the pulmonary vascular bed after venous injection. Furthermore, myocardial perfusion studies are being conducted in animals by opacification of the myocardium with echo contrast agents in order to detect ischaemic areas.