Myocardial regional blood flow: Quantitative measurement by computer analysis of contrast enhanced echocardiographic images

Abstract

Quantitation of regional myocardial blood flow constitutes the missing link between the anatomy of coronary obstruction and its physiological effect on regional oxygen supply. Microscopic air bubbles, introduced into the coronary circulation, were shown to produce a transitory enhancement of the myocardial tissue contrast, easily detectable with standard ultrasonic imaging equipment. This study presents a new approach linking the tissue blood flow with the time-dependent changes in the intensity of the ultrasonic reflections produced by the microbubbles. The tissue blood flow is evaluated using the well-known indicator dilution relation, according to which flow equals the ratio between the intravascular fraction of the tissue sample volume and the mean transit time of the contrast agent. We derive these two parameters from the time curves representing the contrast induced variations in the mean videointensity measured in two regions of interest, a reference region in the left ventricular cavity and the region of interest within the myocardial tissue. The intravascular volume fraction is computed as the ratio of the total power of the above two intensity curves, as each of these is assumed to be proportional to the total amount of tracer traversing the corresponding region of interest. The mean transit time is computed using combined time- and frequency-domain processing, involving Fourier deconvolution of the response function of the myocardial tissue sample. This approach was validated in an in vivo model in a series of animal experiments involving left atrial injection of albumin coated air microbubbles (Albunex®). Videointensity curves obtained during contrast enhancement of the myocardium were analyzed to provide values of regional myocardial blood flow (in mL/min/100 g) in 45 myocardial regions of interest defined in 7 experiments performed on 4 animals. The values obtained with our approach correlated well ( r = 0.77, p < 0.001) with standard reference measurements based on radiolabeled microspheres. The intertechnique variability was found to be smaller than the intersegment variability characterizing our technique. The difference between the mean flow values obtained with microspheres for segments of the entire heart and the mean flow obtained with our technique for all regions of interest ranged between 1 to 19% in the 7 experiments. In its present form, based on left atrial or left ventricular injection of contrast solution, this method may allow, for the first time, quantitative evaluation of myocardial regional blood supply in the cardiac catheterization laboratory or the operation theater. With further development of contrast agents suitable for transpulmonary enhancement of the myocardium by peripheral intravenous injection, this technique may provide the basis for the noninvasive quantitative measurement of myocardial tissue blood flow.