Detection of Coronary Artery Disease With Myocardial Contrast Echocardiography
Comparison With 99mTc-Sestamibi Single-Photon Emission Computed Tomography
Background The purpose of this study was to determine whether myocardial contrast echocardiography (MCE) can be used to detect coronary artery disease (CAD) during rest and pharmacological stress in humans through the use of venous injections of contrast.
Methods and Results Thirty patients with known or suspected CAD underwent MCE and 99mTc-sestamibi single-photon emission computed tomography (SPECT) at baseline and after dipyridamole (0.56 mg · kg−1) infusion. Ten myocardial segments (5 each in the apical two- and four-chamber views) from the two sets of images using both methods were scored for myocardial perfusion as follows: 1=normal, 0.5=mildly reduced, and 0=severely reduced. The information from baseline and postdipyridamole images was then used to determine whether an abnormal segment was irreversible (similar abnormal perfusion at baseline and after dipyridamole) or reversible (perfusion better at baseline compared with after dipyridamole). Concordance between segmental scores was 92% (κ=.99) for both methods. Concordance between normal perfusion and reversible or irreversible segmental defects was 90% (κ=.80). Agreement between the two methods for each of the three vascular territories in each patient was 90% (κ=.77), while agreement for the presence or absence of CAD in each patient was 86% (κ=.86). In the 4 patients with disagreement, the perfusion scores were 0.5 for SPECT and 1.0 for MCE.
Conclusions This study shows that MCE, with venous injection of contrast, can define the presence of CAD during rest and pharmacological stress. The location of perfusion abnormalities and their physiologic relevance (reversible or irreversible) by MCE is similar to that provided by SPECT. MCE, therefore, holds promise for the noninvasive assessment of myocardial perfusion in humans.
The presence of myocardial perfusion abnormalities at rest or during stress are the hallmarks of coronary artery disease (CAD).1 Their spatial extent and magnitude have been correlated strongly with short- and long-term prognoses.2 3 4 5 6 Myocardial contrast echocardiography (MCE) uses microbubbles as contrast agents that scatter ultrasound during their transit through the coronary microcirculation.7 Heretofore, the assessment of myocardial perfusion in humans through the use of this technique has been limited by the need to inject microbubbles directly into the aorta8 or the coronary arteries.9 10 11 12 13
Recent advances have enabled the detection of myocardial perfusion with the venous injection of microbubbles. The first is the creation of microbubbles containing nondiffusible, high-molecular-weight gases with low solubility, which, unlike air-filled bubbles, are more resistant to change in size when mixed with blood. Since the backscatter from a bubble is related to the sixth power of its radius,14 these bubbles retain their acoustic properties during transpulmonary transit and reproducibly opacify not only the left ventricular cavity but also the myocardium.15 16 17 18
At a sufficient acoustic power, microbubbles can be made to resonate and break.19 These processes produce signals that contain not only the fundamental frequency to which the bubbles were exposed but also harmonics of that frequency.20 21 Ultrasound transducers have been designed to transmit at the fundamental frequency and receive at a harmonic frequency.22 Although backscatter is less during harmonic compared with fundamental imaging, since harmonic frequencies are generated almost exclusively from microbubbles rather than tissue, the signal-to-noise ratio after venous injection of microbubbles is greater with harmonic compared with fundamental imaging (in which backscatter occurs from both tissue and microbubbles). Improvement in myocardial opacification, however, is not seen during continuous harmonic imaging because the bubbles are continuously destroyed in tissue. If ultrasound is transmitted intermittently rather than continuously, bubble destruction in tissue is significantly reduced, resulting in a severalfold increase in myocardial opacification.19 22
In the canine model, we have previously demonstrated that during pharmacological stress, physiologically significant coronary stenoses can be detected when microbubbles containing high-molecular-weight gases are injected intravenously and intermittent harmonic imaging is performed.23 For the present study, we hypothesized that the same can be achieved in humans. We used 99mTc-sestamibi single-photon emission computed tomography (SPECT) as the “gold standard” for myocardial perfusion assessment.
Patient Population and Study Protocol
The protocol for this phase II study was approved by the Medicines Control Agency of the United Kingdom and by the Institutional Review Board at the Northwick Park Hospital in Harrow, where the study was undertaken. All patients gave written informed consent to participate in the study. Ambulatory patients with known or suspected CAD formed the study cohort. The inclusion criteria were age >18 years and echocardiographic images in which all myocardial segments could be discerned in the apical two- and four-chamber views. Exclusion criteria included pregnancy, lactation, unstable angina, known left main coronary disease, acute myocardial infarction or stroke within 6 months, and hypersensitivity to blood, blood products, or albumin. Coronary angiography was not necessary for inclusion in the study.
Patients underwent a physical examination followed by a 12-lead ECG, blood chemistry, hematology, and urinalysis. These were repeated at 30 minutes and at 48 hours after the last injection of FS-069. MCE and SPECT were performed on separate occasions. Data were acquired at baseline and after the intravenous infusion of 0.56 mg · kg−1 of dipyridamole (Dupont-Merck) over 4 minutes.24 For SPECT, 99mTc-sestamibi was injected 6 minutes after the infusion of dipyridamole and imaging was initiated 1 hour later. For MCE, imaging was initiated 6 minutes after dipyridamole infusion and was completed within 10 minutes in all cases.
Myocardial Contrast Echocardiography
FS-069 (Molecular Biosystems, Inc), a second-generation ultrasound contrast agent consisting of perfluoropropane-filled albumin microspheres, with a mean size of 3.9 μm and a concentration of 5-8×108 · mL−1, was used for this study.14 This agent has been demonstrated not to cause any alterations in myocardial blood flow, left ventricular wall thickening, pulmonary gas exchange, and cardiac or systemic hemodynamics when injected intravenously in dogs.14 Its safety in humans has also been demonstrated.25 Intravenous injections of 0.5 mL (24 patients) or 1 mL (6 patients) of this agent were used in each view at baseline and after dipyridamole infusion followed by a 5-mL saline flush. A digital ultrasound system (HDI-3000-CV, ATL-Interspec) was used for the study. A prototype broad-band transducer that transmits at a mean frequency of 1.67 MHz and receives at a mean frequency of 3.3 MHz (harmonic) was used for imaging. A dynamic range of 60 dB was used, and the gains were optimized at the beginning of the study and held constant throughout. Images were stored on videotape.
Image acquisition in the apical two- and four-chamber views was begun just before injection of contrast and continued until contrast effect in the myocardium had dissipated. After venous injection of bubbles, attenuation was noted throughout the left ventricular cavity and over the myocardium. When it decreased sufficiently to involve only the mitral valve and left atrium and not the myocardium, imaging was switched from continuous to intermittent mode. In this mode, ultrasound was transmitted once every cardiac cycle. By gating to the peak of the T wave on the ECG, images were acquired when the left ventricular cavity was the smallest and contained the least number of microbubbles, resulting in the least amount of shadowing.
Our method of digitally processing MCE data has been described previously.26 In brief, the images were transferred from videotape to the image memory of an off-line computer (Mipron, Kontron) with custom-designed image analysis software. To improve signal-to-noise ratio, 4 to 6 end-systolic preinjection frames, corresponding to the gated frames acquired during contrast injection, were averaged. A similar number of postinjection frames were also averaged. The averaged preinjection and postinjection frames were aligned, and the preinjection frame was digitally subtracted from the postinjection frame. The video intensity scale in the resulting subtracted frame was expanded to 128 gray levels, whereby the pixel with the greatest contrast change was assigned a level of 128, and all others were assigned proportionally lower values. Reassigned values of ≤10 were considered to represent noise. Each pixel with a gray scale value of >10 was relegated a color based on the degree of contrast enhancement through the use of a heated object algorithm, in which shades of red, progressing to hues of orange, yellow, and white, represent incremental contrast opacification. The left ventricular cavity was masked out.
Single-Photon Emission Computed Tomography
A 2-day protocol (baseline and dipyridamole) was used for SPECT. In both instances, 600 MBq of 99mTc-sestamibi (Dupont-Merck) was injected for assessment of myocardial perfusion. Imaging was performed with the use of a large field-of-view gamma camera with a high-resolution collimator (DS7, Sophy Medical). Thirty-two 20-second images were acquired over a 180 degree orbit. The data were processed with Ramp and low-pass filters and backprojection, after which tomographic images were created in the horizontal and vertical long-axis views. Counts within an image were normalized to the highest counts and were color-coded with the use of an algorithm in which no counts to the highest counts were assigned colors of blue, green, yellow, orange, red, and black.27
MCE images were interpreted by two observers blinded to both clinical and SPECT data. Similarly, the SPECT data were interpreted by two observers blinded to both clinical and echocardiographic data. Differences in opinion were resolved by consensus. Perfusion in each of the five segments in each view (Fig 1⇓) was graded both at baseline and after dipyridamole infusion as either 1=normal, 0.5=mildly reduced, or 0=severely reduced. For interpretation of MCE data, both the video clips and the processed color-coded images were viewed before assigning a score. For SPECT, only the color images were viewed. The horizontal and vertical long-axis images closest to the echocardiographic apical four- and two-chamber views were used for interpretation.
Kappa statistics were used to determine concordance between scores.28 κ of >0.4, >0.6, and >0.8 indicate fair, good, and excellent agreement, respectively.28 With the consensus opinion of two observers, perfusion in each of the 10 segments was scored at baseline and after dipyridamole infusion. After combining both baseline and postdipyridamole scores, each segment was labeled as being normal or having an irreversible defect (abnormal score that is similar at baseline and after dipyridamole) or a reversible defect (score lower after dipyridamole compared with baseline). Concordance between coronary artery territories for the presence or absence of CAD was also assessed by assigning a priori each of the 10 segments to one of the 3 coronary arteries as shown in Fig 1⇑. Finally, concordance for the presence or absence of CAD was determined in each patient.
To determine interobserver variability, each set of MCE and SPECT images was also scored separately by the observers who participated in the consensus reading. The interval between the consensus interpretation and readings to determine interobserver variability was >4 months. Each observer then repeated the reading to determine intraobserver variability. The interval between the two individual interpretations ranged from 2 weeks to 3 months.
Of the 30 patients entered in the study, 18 were men and 12 were women, with ages ranging from 36 to 75 years (median, 60 years). Eleven patients had prior infarction (6 anteroseptal or anterolateral and 5 inferior) and 4 were being treated for congestive heart failure. In 24 patients, SPECT was performed before MCE, whereas in 8 it was performed after. The median interval between the two tests was 31 days. There were no clinical events between the two studies in any of the patients. No adverse effects were noted with FS-069 either at baseline or after dipyridamole infusion. Specifically, the ECG, blood chemistry, hematology, and urinalysis showed no changes at 30 minutes and at 48 hours after injection of this agent compared with baseline values.
Myocardial perfusion was noted with the use of both techniques in all patients. Digital processing and color-coding of the MCE images were not performed in 3 patients for technical reasons (poor alignment, myocardial shadowing, and inadequate number of precontrast frames). In these patients, analysis for the presence or absence of myocardial perfusion was performed from the video clips alone.
Of the 600 possible segments (10 per patient at baseline and the same number after dipyridamole infusion), analysis was performed in 582. Ten segments could not be analyzed on SPECT (all in the same patient) because of artifacts and 8 segments could not be analyzed on MCE because of shadowing (6 segments in 1 patient and 2 in another). There was 92% concordance (κ=.99) when comparing the segmental perfusion scores with the use of the two methods.
After combining baseline and postdipyridamole images, 291 segments were labeled as having normal perfusion or irreversible or reversible defects. Using this classification, concordance for the two methods was 90% (κ=.80). SPECT and MCE identified 204 and 213 segments, respectively, as normal; concordance between the two methods was noted in 199 segments (91%, κ=.92). Of the 73 abnormal segments by both techniques, concordance for reversible and irreversible defects was 85% (κ=.68).
Fig 2⇓ depicts normal perfusion at rest and after dipyridamole and Fig 3⇓ illustrates examples of fixed defects from 2 patients. Although these images were obtained at baseline, there was no difference between these and the postdipyridamole images. Figs 4⇓ and 5⇓ depict examples of fully reversible defects involving large and small regions of the myocardium, respectively. Fig 6⇓ illustrates a partially reversible defect.
Using baseline and postdipyridamole images, we classified each vascular territory (Fig 1⇑) as normal or showing either a reversible or an irreversible defect by both imaging techniques. Of the 90 territories (3 in each of the 30 patients), 1 could not be analyzed because of incomplete SPECT data. Of the remaining 89 territories, 31 were considered abnormal at either baseline or postdipyridamole infusion by SPECT, 28 by MCE, and 25 by both (90% concordance, κ=.77). Of the abnormal territories, 22 were considered reversible by both techniques (concordance of 88%, κ=.71).
There was concordance in 25 of the 29 patients with adequate images (86%, κ=.71) for the presence or absence of CAD with the use of both methods. All 4 patients with discordance had mild defects (score of 0.5) on SPECT, whereas MCE was normal (score of 1). The defects on SPECT in all 4 patients who had no defects on MCE were localized to a single vascular territory (2 in the right coronary artery and 1 each in the left circumflex and left anterior descending coronary arteries).
Table 1⇓ lists the intraobserver and interobserver agreement for the MCE images on the basis of segmental scores and whether a segment was read as normal or abnormal. The concordance is very good to excellent. Table 2⇓ depicts similar results for SPECT. Whereas the intraobserver agreements were comparable between MCE and SPECT, the interobserver concordance was slightly better with SPECT than with MCE.
This study demonstrates that MCE can consistently produce myocardial opacification in humans through the use of venous injection of microbubbles. More importantly, it can define the presence of abnormal perfusion at rest and during pharmacological stress. The location of these perfusion abnormalities and their physiological relevance (reversible or irreversible) are similar to that provided by SPECT. The interobserver and intraobserver agreements for this method are acceptable. These preliminary results set the stage for the use of MCE in the assessment of myocardial perfusion in different coronary syndromes.
Relation Between MCE and SPECT Perfusion
Unlike other tracers used for assessing myocardial perfusion, microbubbles used during MCE reside entirely within the vascular space.29 They do not enter the extravascular space, nor are they extracted by myocytes. When injected intravenously, microbubbles mix with blood in the central circulation, and their concentration at any given time within a myocardial region of interest reflects the volume of blood within that region.30 When changes in myocardial blood flow are regulated by changes in myocardial blood volume (as occurs with dipyridamole, which causes vasodilation of the coronary microcirculation), relative video intensity ratios within different myocardial regions reflect relative ratios of both myocardial blood volume and blood flow in these regions.30
It is well known that in the presence of a stenosis, myocardial blood flow remains constant until the stenosis is very severe (>85% luminal diameter narrowing).31 Myocardial blood flow is maintained by vasodilation and/or recruitment of <300-μm microvessels distal to the stenosis. Thus, it is accepted that at rest, myocardial blood volume increases within the bed supplied by a stenotic artery.32 33 34 Because it causes vasodilation of all microvessels, it is also generally assumed that myocardial blood volume is equal between beds supplied by stenosed and nonstenosed vessels during exogenous hyperemia. This assumption has been questioned through the use of ultrafast cine–computed tomography35 and MCE,36 in which it has been demonstrated that during hyperemia, both myocardial blood volume and flow distal to a stenosis are less than in the normal bed. Since during exogenous hyperemia, myocardial blood volume and flow are closely coupled, the ratios of video intensities from different beds have been demonstrated to reflect the ratios of myocardial blood flow to those beds.36
From the blood pool, 99mTc-sestamibi diffuses into the extravascular space and passively enters the myocyte before binding to the negatively charged mitochondrial membrane.37 Its retention within the myocyte, therefore, is dependent on intact mitochondrial function. Up to about two times normal flow, the myocardial uptake of 99mTc-sestamibi during exogenously induced hyperemia is determined by flow to that region. The relative distribution of 99mTc-sestamibi in various myocardial beds, therefore, reflects differential blood flow to those beds.31 Consequently, the presence of a perfusion mismatch during hyperemia indicates the presence of physiologically significant coronary stenosis.38
During rest, 99mTc-sestamibi imaging provides information not only on baseline flow but also myocyte viability. Necrotic areas do not retain 99mTc-sestamibi and demonstrate reduced uptake.39 In comparison, absent or reduced contrast enhancement is seen within necrotic regions on MCE because of reduced myocardial blood volume consequent to microvascular disruption, plugging, and obliteration.40 Regions with these patterns have been shown to demonstrate lack of recovery of function despite the presence of open infarct-related arteries.10 11
Implications of the Current Study
Whereas the basis for normal and abnormal perfusion patterns on MCE have been described in detail in canine models30 36 40 as well as in humans during intracoronary9 10 11 12 13 or aortic root8 injections of microbubbles, it has not heretofore been possible to evaluate these patterns in the outpatient laboratory. The recent development of second-generation microbubbles,15 16 17 18 harmonic imaging,19 20 21 and an understanding of the interaction between microbubbles and ultrasound21 22 has made it possible to study myocardial perfusion with MCE using venous contrast agents. Thus, MCE is now a new method of noninvasively assessing myocardial perfusion.
The current gold standard for assessing myocardial perfusion is single-photon imaging.1 This technique has been used for two decades and has generated valuable data on the noninvasive detection of CAD and risk stratification.1 2 3 4 5 6 It has also provided unique insights into coronary physiology and pathophysiology and introduced the use of computers,1 3 image processing,1 2 3 4 5 6 and quantification1 3 into the field of cardiac imaging. Nevertheless, the technique is limited because of the need to inject radioisotopes, which requires special licensing, making it less available to cardiologists. It is also time consuming and expensive.
Echocardiography, on the other hand, is readily available to cardiologists, often in their offices. Its use in the detection of structural as well as flow abnormalities within the heart and great vessels is well established. It is an efficient modality because it is relatively inexpensive and less time consuming. Consequently, the assessment of myocardial perfusion with this technique may provide important incremental information at low additional cost.
Limitations of the Study
Being a preliminary phase II study, the number of patients recruited was small. Multicenter studies with a larger number of patients are required before the clinical role of MCE for the assessment of myocardial perfusion can be determined. Postprocessing is a new concept in echocardiography, and the algorithms used for this study are not currently available for general use. Considerable expertise is required for selecting the appropriate images before averaging and digital subtraction. Malalignment can result in images that may not represent actual perfusion within myocardial regions. Because ultrasound backscatter is nonuniform in any field of view, it is also important to discriminate artifacts from actual defects. Although not truly quantitative because it assigns colors according to relative gray scales, color-coding is a semiquantitative technique. We believe, however, that truly quantitative methods will be necessary to obtain results on MCE that are as reproducible and reliable as those obtained on quantitative single-photon imaging.1 2 3
We used SPECT instead of coronary angiography as our gold standard for two reasons: First, we wanted to compare perfusion with perfusion and not with anatomy. The limitations of using anatomy as a gold standard to determine the physiological relevance of coronary stenoses are well known.41 42 Second, we did not want to include a highly selected patient population, which occurs when only those referred for cardiac catheterization are included. These patients have a much higher incidence of CAD than the average patient with chest pain, which biases the results in favor of the test. That only half of the patients in our study had abnormal perfusion by both methods indicates that our patient population had a medium probability of disease, which is precisely the population in which myocardial perfusion imaging is most useful.
We selected patients on the basis of the adequacy of their baseline echocardiographic images. Our intent was not to include patients with the best images but to exclude those in whom myocardial segments were not seen, since our comparison with SPECT was to be on a segment-by-segment basis. Future studies will be needed to determine in which patients transthoracic MCE is unlikely to provide useful results because of poor baseline images. In these, transesophageal echocardiography could be an option.
Although we had a large degree of concordance between MCE and SPECT, the causes of discordance need to be addressed. On a patient-by-patient basis, the discordance was always in favor of SPECT, but the abnormality involved only one vascular territory in all four such patients. The scores, however, were different by only a single grade: 0.5 (mildly reduced perfusion) for SPECT and 1.0 (normal perfusion) for MCE. There is no way of knowing which of the two methods provided the correct information because SPECT can also provide false-positive results. Even if we consider the findings on SPECT to be correct in all cases, the results of our study are very encouraging given that this was our first attempt at using MCE in humans.
The observer agreement in our study was good to excellent for both SPECT and MCE. The slightly better interobserver agreement for SPECT compared with MCE is to be expected. The observers who interpreted SPECT data have >15 years of experience with single-photon imaging in the clinical setting. In comparison, this was our first clinical experience with MCE in patients in whom microbubbles were injected intravenously.
The results of this study show that it is possible to detect CAD with MCE through the use of venous injections of microbubbles. These results form the basis for larger studies that are required to examine the role of this new technology in the detection of CAD. Studies are also needed to determine its value in risk stratification, diagnosis of acute myocardial infarction, determining the success of reperfusion, and assessing the extent of myocardial salvage. There is ample evidence from both the experimental30 36 40 and the cardiac catheterization9 10 11 12 laboratories that this technique could play an important role in the noninvasive assessment of these clinical entities.
This study was supported in part by grants from the National Institutes of Health (R01-HL-48890) (Dr Kaul), Bethesda, Md; Molecular Biosystems, Inc, San Diego, Calif; and an equipment grant from ATL-Interspec, Bothell, Wash. Dr Kaul is an Established Investigator of the American Heart Association, Dallas, Tex. We are grateful to Jeffrey Powers of ATL-Interspec for custom-designing a prototype probe for this study and assisting us in achieving optimal system parameters.
Presented in part at the 69th Annual Scientific Sessions of the American Heart Association, November 10-13, 1996, New Orleans, La.
- Received December 4, 1996.
- Revision received February 27, 1997.
- Accepted February 28, 1997.
- Copyright © 1997 by American Heart Association
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