Bëlohlávek, Marek

Modern commercial medical ultrasound instruments are capable of obtaining up to 40 real-time, two-dimensional images per second. One of the most difficult organs to evaluate with ultrasound is the heart. The heart is a complex organ that achieves its pumping function through cyclic motion of its chambers and valves. Computerized visualization of the heart is being developed to aid in training and to provide comprehensible images of this complex organ to the patient, the surgeon, and the researcher. To obtain the required data, the two-dimensional imaging plane of an ultrasound instrument is scanned through the heart from the surface of the chest, or more commonly, from within the esophagus. Sequences of digitized two-dimensional images taken at incremental positions through the heart are gated with the cardiac cycle and assembled by computer programs into volumetric images. Visualization methods of filtering, segmentation, and rendering are used to produce comprehensible, data dense, three-dimensional images of the heart. Sequences of the three-dimensional images obtained at sequential points through the cardiac cycle produce what are called ‘‘four’’-dimensional images. Mathematical projections of elements of these images onto selected two-dimensional planes can be recorded on videotape to produce movies of the structure or function of interest at various angles of view. Metrics such as distance, area, volume, or speed can be used on these data to produce quantitative measurements.

Background— Onset of myocardial relaxation is highly energy dependent. Perfusion and therefore energy substrate delivery are predominantly reduced in the subendocardial myocardium in the early stages of progressive ischemia. We hypothesized that delayed onset of subendocardial diastolic thinning will functionally identify regionally hypoperfused resting myocardium. Methods and Results— Progressive left anterior descending coronary artery stenosis was induced by an ameroid occluder and maintained for 1 or 2 weeks (end point) in 12 dogs. M-mode tissue Doppler images of the anterior apical and middle segments (testing region) and middle inferior segment (control region) were acquired selectively in the subendocardium and subepicardium. The time to the onset of thinning was measured with the use of tissue Doppler velocity (TOTv) and a thickness function (TOTt). At the end point in the testing region, myocardial flow was significantly lower in the subendocardial layer ( P <0.05) in all animals, whereas viability staining showed preserved transmural viability in 10 dogs and thin subendocardial necrosis in 2 dogs. Both TOTv and TOTt were significantly ( P <0.01) prolonged in the testing region. The mean difference between subendocardial and subepicardial TOTv values versus that in the control region identified the ischemic region, even when only dogs with hypoperfused but transmurally viable myocardium were considered ( P <0.05). Systolic and diastolic myocardial velocities did not identify subendocardial hypoperfusion. Conclusions— In resting myocardium subtended to progressive coronary stenosis, a delayed onset of subendocardial thinning suggests an early stage of hypoperfusion, before the development of local wall motion abnormalities.

Background — Regional myocardial function assessment is essential in the management of coronary artery disease (CAD). Tissue Doppler imaging (TDI) by depicting local myocardial motion can potentially quantify regional myocardial function. Strain rate imaging (SRI) that depicts regional deformation is less susceptible to cardiac translation and tethering and may be superior to TDI for regional function analysis. We examined regional myocardial function using TDI and SRI in a unique clinical model of a small, discrete myocardial infarction. Method and Results — Ten patients with severely symptomatic septal hypertrophy underwent basal septal ablation via intracoronary alcohol injection and had TDI and SRI pre- and postablation. Invasive hemodynamics showed no appreciable change in global function. Peak systolic strain rate was significantly lower postablation versus preablation (−0.5 versus −1.2 s −1 , P <0.001) and when comparing infarct and noninfarct areas (−0.5 versus −1.5 s −1 , P <0.001). In contrast, peak systolic tissue velocities were similar pre- and postablation (3.9 versus 2.9 cm/s, P =0.16) and between infarct and noninfarct areas (2.9 versus 2.2 cm/s, P =0.13). SRI analysis demonstrated reduced systolic function in the peri-infarct zone and preserved systolic function in the remote nonischemic zone. Conclusion — In the clinical setting of a small, discrete infarct unaccompanied by changes in global function, SRI accurately depicted changes in regional function. These data suggest that SRI may be the optimal method for objective, quantitative assessment of regional myocardial dysfunction.