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. We hypothesized that uneven microbubble destruction caused by attenuation of beam intensity could alter quantitative measurements of perfusion abnormalities during stress. To test this hypothesis, we measured the spatial extent of perfusion defects at peak dobutamine stress with a continuous intravenous infusion of perfluorocarbon-exposed sonicated dextrose albumin and intermittent harmonic imaging in dogs with nonflow- limiting coronary stenoses in the left anterior descending artery. The spatial extent of perfusion defects was also measured during total occlusion of the artery. Measurements were 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 occlusion at a standoff of 2 to 3 cm was significantly larger at a 1500-ms pulsing interval (6.5 ± 2.6 cm² for 2- to 3-cm standoff versus 3.7 ± 1.4 cm² 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 cm²). Myocardial perfusion abnormalities during peak dobutamine stress were significantly smaller with the 4- to 5-cm standoff and undetectable in 4 of the 5 dogs. We conclude that ultrasound beam attenuation can reduce the size of a myocardial perfusion abnormality observed with intermittent harmonic imaging during a continuous infusion of microbubbles. This may reduce the sensitivity of this technique when transthoracic imaging is used.