The predominant (90%) component of myocardial blood volume resides within the capillaries[2]. The myocardial signal assessed visually as contrast intensity reflects the concentration of microbubbles within the myocardium[3]. When the entire myocardium is fully saturated during a continuous infusion of microbubbles, the signal intensity denotes the capillary blood volume[4]. Any alteration of signal in such a situation must, therefore, occur predominantly from a change in capillary blood volume.
Furthermore, it has been shown that following destruction, or depletion, of microbubbles in the myocardium during high-power imaging, replenishment of contrast within the myocardium can be observed[5]. The capillary blood velocity is 1 mm/s with an ultrasound beam elevation of 5 mm[6]. Thus, it takes 5 s for complete replenishment of the myocardium. Any decrease in myocardial blood flow (MBF) prolongs replenishment time in proportion to the reduction in MBF[5].
Myocardial perfusion is defined as tissue blood flow at the capillary level. The two components of tissue blood flow are capillary blood volume and red blood cell velocity. As microbubbles are red blood cell tracers, the product of peak microbubble intensity (representative of myocardial blood volume) and their rate of appearance (representative of blood velocity) equals MBF[5].
Therefore, myocardial contrast echocardiography (MCE) can detect capillary blood volume and, by virtue of its temporal resolution, can also assess MBF. This imaging technique of ‘destruction (or depletion) and replenishment’ requires the delivery of a series of high-energy ultrasound pulses to destroy (deplete) microbubbles in the myocardium. Ultrasound imaging is then continued either intermittently (during high-power imaging) or continuously (during low-power imaging) to observe contrast intensity and microbubble velocity.
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