Effect of transducer standoff on the detection, spatial extent, and quantification of myocardial contrast defects caused by coronary stenoses

Intermittent harmonic imaging during a continuous infusion of microbubbles may be able to quantify myocardial perfusion abnormalities. Measurements of the spatial extent of these perfusion abnormalities depends on homogenous destruction of the microbubbles in the elevation plane of the transducer. W e hypothesized that uneven microbubble destruction caused by attenuation of beam intensity could alter quantitative measurements of perfusion abnormal ities during stress. To test this hypothesis, we measured the spatial exten t of perfusion defects at peak dobutamine stress with a continuous intraven ous infusion of perfluorocarbon-exposed sonicated dextrose albumin and inte rmittent harmonic imaging in dogs with nonflow-limiting coronary stenoses i n the left anterior descending artery. The spatial extent of perfusion defe cts was also measured during total occlusion of the artery. Measurements we re made at standoffs of 2- to 3-cm and 4- to 5-cm distance from transducer surface to myocardium. These spatial extents were correlated with risk area determined after death. The risk area during left anterior descending occl usion at a standoff of 2 to 3 cm was significantly larger at a 1500-ms puls ing interval(6.5 +/- 2.6 cm(2) for 2- to 3-cm standoff versus 3.7 +/- 1.4 c m(2) for 4- to 5-cm standoff; P =.01). The spatial extent at the 2- to 3-cm standoff more closely approximated risk area measured with Monastral Blue (7.8 +/- 2.7 cm())(2). Myocardial perfusion abnormalities during peak dobut amine stress were significantly smaller with the 4- to 5-cm standoff and un detectable in 4 of the 5 dogs. We conclude that ultrasound beam attenuation can reduce the size of a myocardial perfusion abnormality observed with in termittent harmonic imaging during a continuous infusion of microbubbles. T his may reduce the sensitivity of this technique when transthoracic imaging is used.