Abstract and Introduction
Background: The aim of this study was to perform a quantitative comparison between myocardial contrast echocardiography (MCE) and single-photon emission computed tomography (SPECT) in patients with prior myocardial infarction (MI). We also wanted to determine the optimal method for the intravenous administration of an ultrasound contrast agent in the clinical setting.
Methods and Results: Seventeen patients with resting perfusion defects in a single vascular territory on SPECT were studied. MCE was performed with intermittent harmonic imaging during continuous infusions of a second-generation ultrasound contrast agent (Sonovue, Bracco Diagnostics) in all 17 patients and after bolus injection in 8 of them. During continuous infusions, the video intensity (VI) ratio between the abnormal and normal myocardium at a pulsing interval (PI) of 8 cardiac cycles correlated well with the activity ratio between these segments on SPECT (r = 0.73, P < .01). When information regarding microbubble velocity (MV) denoted as change in VI with increasing PIs was added, the correlation with SPECT activity ratio improved (P < .05) significantly (r = 0.87, P < .0001). Higher microbubble doses resulted in higher VI during continuous infusions with good myocardial opacification and no far-field attenuation until the highest dose was reached. With bolus injections, the VI ratio between the abnormal and normal myocardium at PI of 1 and 5 cardiac cycles showed a modest correlation (r = 0.46 and r = 0.48, respectively, P < .05) with activity ratios between these regions on SPECT. When a dose of microbubbles administered as a bolus produced adequate myocardial opacification, it invariably resulted in far-field attenuation.
Conclusions: In patients with prior MI, quantitative assessment of resting perfusion defects on MCE correlates well with regional activity on SPECT. Continuous infusions offer an advantage over bolus injections because they can provide an assessment of both relative VI and MV. Adjustment of the microbubble infusion rate produces adequate myocardial opacification without attenuation.
When the relation between myocardial microbubble concentration and amplitude of ultrasound backscatter is within the linear range, myocardial video intensity (VI) on myocardial contrast echocardiography (MCE) reflects the concentration of microbubbles in that region.[1] The 3 main determinants of myocardial microbubble concentration are (1) myocardial blood volume (MBV); (2) the fraction of the MBV within the ultrasound beam that is filled with microbubbles[2]; and (3) the concentration of microbubbles in blood.[1,2,3]
At sufficiently high acoustic pressures, ultrasound destroys microbubbles.[2,4,5] When the myocardial microbubble concentration is low, a single sweep of ultrasound can destroy enough microbubbles. On continuous imaging at 30 Hz, therefore, by the next frame (33 ms later) myocardial VI is the same as that of the precontrast image.[6] However, if more time is allowed before the next ultrasound frame, microbubbles present in the blood will partially or completely replenish the myocardium. During a continuous infusion, in which the blood concentration of microbubbles is constant, myocardial VI progressively increases with prolongation of the interval between the ultrasound frames (PI) until the MBV within the entire ultrasound beam is filled, at which time VI reflects MBV.[2] The rate of change of VI from lower to higher PIs represents microbubble velocity (MV).[2]
Myocardial VI is also independently influenced by the blood concentration of microbubbles.[2,3] During a continuous infusion, higher doses produce greater VI for any given MBV and PI. After a bolus injection, however, the blood concentration of microbubbles varies, not only with the dose but also the duration of injection and the cardiac output.[7] In this situation, VI truly reflects MBV only if microbubbles are not destroyed by ultrasound.[3] During a bolus injection, the blood concentration of microbubbles needs to be high to produce adequate myocardial opacification. Thus the myocardial concentration of microbubbles is usually higher during the brief period of opacification after a bolus injection compared with a continuous infusion. Because in this setting only a fraction of the microbubbles are likely to be destroyed by ultrasound, VI could still predominantly reflect MBV.[3] However, because the concentration of microbubbles in the blood changes rapidly after a bolus injection, changes in VI at higher PI do not represent MV.
We postulated that the assessment of regional myocardial perfusion with the use of MCE during continuous microbubble infusion will correlate with myocardial isotope activity on single-photon emission computed tomography (SPECT). We also wanted to determine the optimal method (continuous infusion vs bolus injection) for the intravenous administration of an ultrasound contrast agent in the clinical setting. Accordingly, we studied patients with prior myocardial infarction (MI) in whom MBV varies depending on the degree of microvascular destruction.[8,9] MV in these patients also varies, depending on the patency of the infarct-related artery and/or magnitude of collateral flow.[10,11]
Am Heart J. 2000;139(2) © 2000 Mosby, Inc.
Copyright © Mosby-Year Book, Inc. All rights reserved.
Cite this: Assessment of Resting Perfusion with Myocardial Contrast Echocardiography: Theoretical and Practical Considerations - Medscape - Feb 01, 2000.
