doi: 10.1161/CIRCULATIONAHA.107.747303
(Circulation. 2008;118:291-308.)
© 2008 American Heart Association, Inc.
New Drugs and Technologies |
Myocardial Contrast Echocardiography
A 25-Year Retrospective
From the Division of Cardiovascular Medicine, Oregon Health and Science University, Portland, Ore.
Correspondence to Sanjiv Kaul, MD, Division of Cardiovascular Medicine, Oregon Health and Science University, UHN62, 3181 SW Sam Jackson Park Rd, Portland, OR 97239. E-mail kauls{at}ohsu.edu
Key Words: contrast media • coronary disease • echocardiography • imaging
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Introduction |
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 Top Introduction Some Historical and Technical...MCE in Acute Coronary...MCE for CAD DetectionMCE for Detecting Viability...MCE for Site-Targeted ImagingMiscellaneous Applications and...Remaining Challenges for MCESummaryReferences |
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Obstacles are those frightful things you see when you take your eyes off your goals.— –Anonymous
In 1997, I wrote an invited review on myocardial contrast echocardiography (MCE) entitled "Myocardial Contrast Echocardiography: 15 Years of Research and Development" in the From-Bench-to-Bedside section of Circulation.1 Then, in 2003, Hiroshi Ito and I wrote a 2-piece invited review entitled "Microvasculature in Acute Myocardial Ischemia: Evolving Concepts in Pathophysiology, Diagnosis, and Treatment" in the Clinical Cardiology: New Frontiers section of Circulation.2,3 In this review for the New Drugs and Technology section of Circulation, I will provide a 25-year retrospective on MCE but with emphasis on more recent developments and remaining challenges for the field. I will limit myself to myocardial imaging and will not discuss imaging of other organs or the vascular system. Likewise, I will not address the therapeutic applications of microbubble-ultrasound interactions such as drug and gene delivery or sonothrombolysis.
I have organized this review into 7 sections. I begin by briefly describing some historical and technical elements so that the reader unfamiliar with MCE will be able to follow the rest of the review. I then describe the more recent studies relating to the role of MCE in the diagnosis and prognostication in acute coronary syndromes and chronic coronary artery disease (CAD), followed by assessment of myocardial viability in chronic CAD. After that, I describe advances in site-targeted or molecular imaging and certain miscellaneous findings. Finally, I discuss the remaining challenges of MCE from a clinical adoption point of view.
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Some Historical and Technical Elements |
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TopIntroductionSome Historical and Technical...MCE in Acute Coronary...MCE for CAD DetectionMCE for Detecting Viability...MCE for Site-Targeted ImagingMiscellaneous Applications and...Remaining Challenges for MCESummaryReferences |
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Initial MCE studies were performed in dogs to define in vivo the area at risk during acute coronary occlusion with the use of hand-agitated solutions.4–9 Shortly thereafter, the technique of sonication was described, which allowed the production of smaller microbubbles,10 and was rapidly adopted for intracoronary injections in animals and humans.11–13 Subsequently, the first commercial agent (Albunex) was developed by sonication of 5% human albumin solution, and it produced excellent myocardial opacification on intracoronary injection.14 It therefore came as a surprise to many when this agent was not that successful in opacifying the left ventricular (LV) cavity after intravenous injection.15
The gas contained in Albunex microbubbles was air that, being highly diffusible, leaked out of the microbubbles as they crossed the pulmonary circulation, thus reducing their size. Also being highly soluble, the air dissolved in blood very rapidly after leaking out. Because the acoustic backscatter from a bubble is related to the sixth power of its radius, even modest changes in bubble size can result in large changes in backscatter, which explained the poor LV cavity opacification after intravenous administration of Albunex. To overcome this limitation, the air in the bubble was changed to higher-molecular-weight gases (such as fluorocarbons) that resulted in more stable bubbles. Being insoluble in blood, the gas, even when it had escaped from the bubble, continued to produce effective ultrasound backscatter by acting as a free gas bubble.16,17 These new preparations were highly successful in opacifying the LV cavity and the myocardium from a venous injection. Table 1 lists the various commercially prepared microbubble agents. The 2 currently available in the United States (Definity and Optison) have been approved only for LV cavity opacification. No agents have thus far been approved for myocardial opacification in the United States.
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There are salient features common to most of the commercially produced ultrasound contrast agents. The microbubbles in these agents do not aggregate, are biologically inert and safe,18,19 remain entirely within the vascular space,20,21 have an intravascular rheology that is very similar to that of erythrocytes,20–22 respond nonlinearly to ultrasound,23–25 and are eliminated from the body via the reticuloendothelial system with their gas escaping from the lungs.
A key technical advance in MCE was online signal processing of ultrasound backscatter from insonified microbubbles.26 Before that, it was not possible to separate bubble signals from myocardial backscatter without offline image processing.27 Unlike tissue, microbubbles are compressible and oscillate in an ultrasound field. At even a low mechanical index, these oscillations become nonlinear, that is, during each oscillation the microbubbles expand more than they contract. The term nonlinear in this context means that the output of a system does not match the input. In other words, small perturbations can cause large effects.24,25 With the use of novel signal processing techniques, the nonlinear signals emanating from these oscillating microbubbles can be amplified, and the linear signals can be suppressed, resulting in myocardial opacification.26 With the use of these approaches, both high–mechanical index intermittent imaging (with the use of B-mode and power Doppler) as well as low–mechanical index continuous imaging are currently being employed for MCE.
The method for assessing (and quantifying) myocardial perfusion during MCE was described and validated in a canine model a decade ago.28 For this approach, a dilute solution of microbubbles is administered intravenously as a constant infusion with the use of a pump device. In a few minutes, steady state is achieved, at which time the infusion rate can be adjusted depending on the degree of attenuation seen in the LV cavity. Ideally, no shadowing should be seen in the LV cavity on apical views because when shadowing is present, the relation between microbubble concentration in the myocardium and ultrasound backscatter is no longer linear.28 However, some shadowing should be noted in the left atrium to ensure that there is an adequate concentration of microbubbles in the myocardium at a dose at which the relation between microbubble concentration and backscatter from the LV cavity is still linear. Then high–mechanical index pulses are used to destroy microbubbles in the myocardium, after which their rate of myocardial replenishment is measured. Time versus acoustic intensity (AI) curves can be generated from different myocardial regions and fitted to an exponential function: y=A(1–e–βt), where y is AI at a pulsing interval t, A is the plateau AI, and β is the rate constant that represents the rate of rise of AI (and thus mean microbubble velocity).28
The unique advantage of this method over other modalities for assessing myocardial perfusion is that it can be used to measure both components of tissue perfusion: myocardial blood flow (MBF) velocity and myocardial blood volume (MBV), the latter being denoted by the value A. Normalizing A to the AI value from the LV cavity provides a measure of MBV fraction.29 The product of MBV fraction and MBF velocity provides an estimate of MBF. It is preferable to make these measurements with the use of end-systolic frames because at this point in the cardiac cycle most of the larger intramyocardial vessels have been emptied of blood, and the majority of MBV resides in capillaries, which is the site of nutrient MBF.30 A modified version of this approach was also validated against positron emission tomography (PET) in humans.31
Figure 1 depicts idealized curves based on previous animal and human studies. The top panel of Figure 1 illustrates time versus AI curves that are obtained at rest during normal (Figure 1A [top]) and reduced MBF (Figure 1B to 1D [top]), in which the reduction in MBF can be due to a decrease in MBV alone (such as after infarction when the infarct-related artery is patent with minimal stenosis; Figure 1B [top]), a decrease in blood velocity alone (such as during subtotal occlusion or total coronary occlusion with collateral flow or hibernating myocardium; Figure 1C [top]), or a combination of both a decrease in MBV and blood velocity such as is seen in an infarcted myocardium supplied either by an artery with a very severe flow-limiting stenosis or by collaterals (Figure 1D). The decrease in MBV in the presence of critical stenosis (Figure 1C [top]) is due to reduced capillary perfusion pressure that results in partial capillary collapse or derecruitment.32 The more severe the stenosis, the greater is the fall in perfusion pressure, and the greater is the capillary collapse. This changing capillary resistance with decreasing flow is the probable reason for the positive zero-filling coronary pressure.32 The decrease in capillary blood volume also results in a smaller capillary surface area and a resting perfusion defect on myocardial perfusion imaging with the use of any imaging modality, including nuclear imaging.33
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The bottom panel of Figure 1 illustrates time versus AI curves obtained from the normal myocardium during rest and different forms of stress. Figure 1A (bottom) depicts curves before and during intracoronary infusion of adenosine in which MBV remains constant and blood flow velocity increases.28 At rest, the myocardium replenishes in 4 to 5 seconds after microbubble destruction. In the presence of intracoronary adenosine, MBF increases 4 to 5 times solely because of an increase in MBF velocity without any change in MBV. Therefore, instead of taking 4 to 5 seconds to replenish, the myocardium now replenishes in 1 second.
Figure 1B (bottom) shows curves before and during intracoronary infusion of dobutamine (in which both MBV and velocity increase34). Figure 1C (bottom) illustrates curves obtained before and during venous administration of either a vasodilator or dobutamine. The increase in MBV during intravenous compared with intracoronary administration of adenosine occurs from the increase in myocardial oxygen demand that results from mild systemic hypotension and resultant reflex tachycardia.35 Similar curves can also be obtained during supine bicycle exercise.36
The comparative efficacy of dobutamine and dipyridamole for coronary stenosis detection was studied in chronically instrumented closed-chest dogs with multivessel coronary stenosis.37 In the presence of either drug, MBV increased more in the normal bed than in the abnormal bed, but the increase was higher in both beds with dobutamine than it was with dipyridamole. The slope of the relationship between MBF reserve and MBV reserve was greater during administration of dobutamine than with dipyridamole, whereas the converse was true for MBF velocity reserve. Consequently, the relationship of the ratio of either variable between the abnormal and normal beds was similar for both drugs. Thus, these 2 drugs can be used interchangeably for CAD detection by MCE.
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MCE in Acute Coronary Syndromes |
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TopIntroductionSome Historical and Technical...MCE in Acute Coronary...MCE for CAD DetectionMCE for Detecting Viability...MCE for Site-Targeted ImagingMiscellaneous Applications and...Remaining Challenges for MCESummaryReferences |
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As stated previously, experimental work in MCE began by establishing its role in defining the presence and size of the risk area during acute coronary occlusion.4–9 It then progressed to establishing the success of tissue reperfusion as well as the residual infarct size5,38–41 (via the no-reflow phenomenon), which has been reviewed in great detail previously.2,3 MCE was used to document the heterogeneity in resting MBF and MBF reserve in the previously occluded bed and the appropriate timing of imaging for determining infarct size.41–43 Finally, it was used to assess the presence and extent of collateral perfusion during acute coronary occlusion and its influence on myocardial viability.44–46 These studies were followed by similar ones in humans with the use of intravenous injection of microbubbles.47–52
It had been noted for some time that the circumferential extent of abnormal wall thickening overestimated that of infarction. Several mechanisms were postulated to explain this behavior, ranging from mechanical tethering53 to inherent limitations in the methods of analysis used. With the use of MCE, it has been shown that this disparity is due to collateral-derived intermediate levels of MBF in the border zones of the occluded bed that have escaped necrosis and that the reduction in function in these border zones is commensurate with the reduction in resting MBF.54
Recent advances in the management of patients with acute coronary syndromes with MCE pertain to those patients presenting to the emergency department (ED) with chest pain syndromes and nondiagnostic ECGs. In the United States, 
5 million people visit the ED each year with chest pain. The vast majority is either admitted to the hospital or to a chest pain unit in the ED, but acute myocardial ischemia or infarction (AMI) is confirmed in only 20%. The initial ECG is diagnostic in fewer than half of the patients with ongoing AMI, and cardiac enzymes do not become positive for several hours after coronary occlusion. Meanwhile, valuable time is lost before definitive therapy can be offered.
A multicenter study compared MCE with single photon emission computed tomography (SPECT) to diagnose AMI in chest pain patients with a nondiagnostic ECG.55 Concordance between MCE and SPECT was 77% for all territories with a higher concordance for the anterior wall (84%). Thirty-eight of the 203 patients in the study (19%) had a cardiac event within 48 hours of ED presentation. On multivariate regression analysis, the number of abnormal segments on MCE and SPECT was a significant predictor of cardiac events. MCE provided 17% and gated SPECT provided 23% additional information for predicting cardiac events compared with routine demographic, clinical, and ECG variables that were available in the ED at the time of presentation. Another smaller single-center study also reported similar results.56
A large National Institutes of Health–funded single-center study showed even more remarkable results.57 Of the 1017 patients studied, 166 (16.3%) had early (within 48 hours) events. Adding regional function assessment by MCE increased the prognostic information of the clinical and ECG variables significantly for predicting these events. When myocardial perfusion assessment was added, further additional information was obtained (Figure 2A). Patients without early events were followed for a median of 7.7 months. Of these, 126 had events. The added prognostic value of both regional function and myocardial perfusion assessments on MCE was similar to that seen for the early events. Patients with normal perfusion and function had excellent outcome, whereas those in whom both were abnormal had the worst outcome. Intermediate outcome was noted in those with normal perfusion despite abnormal function (Figure 2B). These patients included those with spontaneous reperfusion (approximately one sixth of the AMI patients) and those with nonischemic cardiomyopathies.
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Although an elevated troponin level is the gold standard for AMI diagnosis, it may not be elevated or even available at the time of ED presentation. It is at this juncture that MCE has been shown to be most helpful.58 In this study, a modified Thrombolysis in Myocardial Infarction (TIMI) score was calculated from 6 immediately available variables, which did not include troponin because it was not known at that time. Follow-up was performed for early (within 24 hours), intermediate (30 days), and late (after 30 days) events. The modified TIMI score was unable to discriminate between intermediate-risk compared with high-risk patients at any follow-up time point, whereas only 2 of 523 patients with normal regional function on MCE had an early event. Regional function on MCE provided incremental information over modified TIMI scores for predicting intermediate and late events. In patients with abnormal regional function, myocardial perfusion further classified those into intermediate- and high-risk groups. The full TMI score (after troponin levels became available) could not improve on these results at any follow-up time point. Since this study was completed, another 1000 patients have been analyzed, and, interestingly, multivariate models derived from the first 1000 patients predict early events in these patients with the same accuracy.
On the basis of the results of these studies, 2 separate editorials have strongly recommended the use of MCE in the ED.59,60
Figure 3A illustrates a case of Tako-Tsubo syndrome in which apical ballooning (arrows) is clearly seen in the end-systolic image, but myocardial perfusion is normal. On the basis of this study in the ED, the patient was not taken to the catheterization laboratory, and the regional dysfunction resolved spontaneously. Although Tako-Tsubo syndrome may be associated with microvascular abnormalities, these may be subtle and occur early in the pathogenesis of the syndrome. Generally, by the time MCE is performed, myocardial perfusion is normal.61 Figure 3B shows a patient examined in the ED for chest pain who not only had an apical defect on MCE (thick arrow) but very nicely demonstrated an apical thrombus as well (thin arrow). Contrast echocardiography has become the gold standard for detecting LV cavity thrombi, which has obvious implications for therapy.
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MCE for CAD Detection |
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TopIntroductionSome Historical and Technical...MCE in Acute Coronary...MCE for CAD DetectionMCE for Detecting Viability...MCE for Site-Targeted ImagingMiscellaneous Applications and...Remaining Challenges for MCESummaryReferences |
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In the absence of prior infarction, the detection of CAD on myocardial perfusion imaging is based on the occurrence of reversible perfusion defects during pharmacological or exercise stress. Experimental studies had previously demonstrated the ability of MCE to detect coronary stenosis and to quantify the degree of MBF mismatch during pharmacological stress.62–64 Studies also showed that coronary stenosis can be detected65,66 and abnormal coronary blood flow reserve can be accurately measured67 with MCE in humans with venous administration of microbubbles.
The conventional wisdom had been that a reversible perfusion defect results from MBF mismatch that is seen at stress and not at rest. With the use of MCE, it has been shown that reversible perfusion defects are actually caused by a decrease in MBV distal to a stenosis during stress.32 When flow increases through a stenosis during stress, the coronary perfusion pressure falls. To maintain a constant capillary hydrostatic pressure, capillary derecruitment occurs, leading to a decrease in MBV. In the case of nuclear tracers, the resultant decrease in capillary surface area causes less tracer uptake and hence a perfusion defect.33 Thus, the site of abnormal flow reserve in CAD is not at the level of the stenosis but actually at the level of the microcirculation.
The decrease in MBV during stress is seen only with moderate to severe stenosis. With less severe stenosis, the only abnormality seen on MCE is the inability of the MBF velocity to increase by the desired amount. As shown in the bottom panel of Figure 1, MBF velocity increases 4 to 5 times in the normal myocardium during stress. The inability of the MBF velocity to increase by this amount during stress indicates a reduction in MBF reserve. What discriminates the attenuation of flow reserve in the presence of a stenosis compared with other causes such as hypertrophy, and hyperlipidemia is its regional nature.67
Figures 4 and 5
demonstrate normal perfusion (top panels) and either a reversible (Figure 4) or a fixed (Figure 5) defect (bottom panels) in patients undergoing dipyridamole stress imaging with the use of Cardiosphere. The imaging protocol is based on the principles depicted in the bottom panel in Figure 1. At rest, microbubble replenishment should occur in 4 to 5 seconds if MBF is normal. Therefore, the rest images (left panels) are captured at the fourth heartbeat after bubble destruction. If MBF reserve is normal, then at stress the myocardium should replenish within 1 second. Hence, the stress images are captured at the first heartbeat after bubble destruction (right panels). In the normal setting, these 2 images (rest and stress) should look similar (panel A in both figures). If there is a significant stenosis in the absence of prior infarction, the stress image should show a relative defect compared with the rest image (indicated by arrows in Figure 4B). In the presence of infarction, in which MBV is markedly reduced due to capillary loss, a fixed defect (present at both rest and stress) should be noted (indicated by arrows in Figure 5B). The images shown here were obtained with the use of power Doppler, with each wall in each view imaged separately because of the narrow imaging sector necessary for a high-pulse repetition frequency required to minimize motion artifacts.
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Figure 6 illustrates rest and stress (dipyridamole) images obtained with the use of Imagify. Here the images are acquired with the use of real-time low–mechanical index imaging. Figure 6A depicts a normal study, whereas Figure 6B depicts a reversible lateral defect (denoted by arrows) in a patient with a proximal left circumflex artery stenosis. In this particular case, the nuclear scan was entirely normal.
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The top panel in Figure 7 depicts MCE and SPECT results from a multicenter study of 55 patients with an intermediate probability of CAD who did not have a prior infarction.68 Thus, their resting function and perfusion were normal. From a clinical perspective, it is in these patients that the ability to diagnose CAD is most relevant. It is evident from the top panel in Figure 7 that the sensitivity of MCE was higher than that of SPECT at mild to moderate levels of stenosis. This occurred because abnormalities in MBF velocity were noted on MCE even when MBV may not have decreased (see bottom panel in Figure 7 and later discussion), resulting in abnormal MCE images but a normal stress SPECT study. Only at severe levels of stenosis, in which MBV reduction is almost certain to occur during hyperemia, was SPECT equivalent to MCE. Another reason for the superiority of MCE over SPECT for detecting mild to moderate stenoses was the presence of subendocardial perfusion defects on the former that could not be visualized on the latter because of the poor spatial resolution of SPECT.
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On a coronary circulation basis (anterior and posterior myocardium), the sensitivity for MCE in this study was significantly higher than that of SPECT for the detection of CAD (86% versus 43%). The specificities of MCE and SPECT, however, were similar (88% and 93%). Both techniques were marginally more accurate in the anterior compared with the posterior circulation. On a patient basis, MCE had a higher sensitivity than SPECT for the detection of CAD (83% versus 49%), although specificity tended to be higher for SPECT than MCE (92% versus 58%). When CAD was defined as >40% rather than >50% coronary stenosis on quantitative coronary angiography, the specificity of MCE increased to 83% without any change in sensitivity.
Several other small studies have compared MCE with SPECT during vasodilator stress,69 and some others have been summarized in a meta-analysis showing an overall superiority of MCE to SPECT.70 More recently, the results of 2 large (a total of 662 patients from 28 centers) phase III clinical trials performed to determine the efficacy of Imagify for detecting CAD in patients with chronic stable chest pain demonstrated a similar performance for MCE and SPECT.71 Interestingly, in this study wall motion abnormality on dipyridamole stress was much more frequent than reported previously in literature published in the United States. Most probably this resulted from a much better assessment of regional function with contrast in the LV cavity. Because in most instances even low-dose dipyridamole (0.56 mg · kg–1) causes some degree of hypotension and reflex tachycardia, an increase in myocardial oxygen consumption is frequent, which can explain the occurrence of wall motion abnormalities,72,73 that are much better appreciated when contrast is present in the LV cavity.
The ischemic cascade had been demonstrated previously in supply but not demand ischemia.74 This phenomenon was more recently demonstrated in demand ischemia as well with the use of MCE.75 It was shown that in the presence of noncritical coronary stenosis, perfusion abnormalities occurred earlier than wall motion abnormalities with escalating doses of dobutamine. Abnormal perfusion (delayed rate of microbubble replenishment) was seen at the lowest dose of dobutamine irrespective of the stenosis severity, whereas wall thickening abnormality was seen only at high doses of dobutamine and was influenced by the stenosis severity. The bottom panel of Figure 7 illustrates an example of normal wall thickening (Figure 7A) despite abnormal perfusion (denoted by arrows in Figure 7B) in an animal with left circumflex artery stenosis during dobutamine infusion. Figure 7C depicts the time versus AI curves, showing delayed filling in the left circumflex coronary artery bed. In this study, even when wall thickening abnormality occurred in single-vessel stenosis, it was less in extent than the perfusion defect.
Another series of experiments demonstrated that the mechanism of the disparity in the circumferential extent of abnormal wall thickening and perfusion defect during demand ischemia (the former being significantly less than the latter) was due to the presence of collateral MBF in the border zones of the ischemic bed.54 It was shown that at higher levels of MBF (>3 times normal), the relation between MBF and wall thickening was flat (Figure 8A). Thus, regions at the borders supplied with collateral flow that did not experience as much hyperemia as the remote normal beds nonetheless continued to exhibit as much thickening.
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These principles were confirmed in clinical reports. In 1 study, 170 patients underwent dobutamine-atropine stress testing.76 The sensitivity of MCE was higher than that of wall motion at both maximal (91% versus 70%) and intermediate (84% versus 20%) doses of dobutamine. Specificity was lower for MCE compared with wall motion abnormality (51% versus 74%). Most of the perfusion defects occurred at intermediate levels of stress when side effects of dobutamine were minimal. Similar results were subsequently reported in patients with diabetes, a high-risk group in whom CAD detection is very important.77
In a retrospective analysis, 788 patients undergoing dobutamine stress were followed up for a median of 20 months. It was shown that myocardial perfusion assessment on MCE had significant incremental value over clinical factors, resting ejection fraction, and wall motion responses in predicting events.78 As shown in Figure 8B, the 3-year event-free survival was 95% for patients with normal perfusion and function, 82% for those with normal function but abnormal perfusion, and 68% for those with both abnormal function and perfusion. When these data are interpreted on the basis of results of previous experimental studies, it likely that patients with both abnormal perfusion and function have a higher incidence of multivessel CAD and compromised collateral perfusion. Similar results have been reported in elderly patients79 as well as in those with diabetes80,81 and advanced liver disease.82
As stated earlier, in the current practice of cardiology, some form of stress is required to detect CAD on myocardial perfusion imaging. Unless there is a previous infarction, resting perfusion remains normal despite the presence of up to 85% luminal diameter narrowing of the coronary artery because of autoregulation. In the presence of a noncritical stenosis, autoregulation causes an increase in total coronary blood volume (volume of blood in the entire coronary tree), primarily from enhanced dimensions of arterioles.83
Because the blood in myocardial arterioles comprises only a small proportion of MBV, microbubble signals from these vessels are usually negligible when the ultrasound beam is fully replenished after microbubble destruction. However, if imaging is performed with a very short interval between destructive ultrasound pulses, the signal obtained is derived only from vessels that fill in this short period of time, as neither capillaries nor venules have adequate time to fill.84,85 Thus, this approach can be used to image the blood volume of relatively larger intramyocardial vessels.
During systole, a change in myocardial elastance causes retrograde displacement of the arteriolar blood volume (aBV) into the larger intramyocardial arterioles, resulting in a small systolic signal from these vessels on MCE. In the presence of a stenosis, because aBV is larger as a result of autoregulation, greater retrograde displacement of microbubbles from smaller arterioles into the larger intramyocardial arterioles occurs, resulting in an increase in the systolic myocardial signal from these vessels. Because the larger intramyocardial arterioles do not participate in autoregulation, the diastolic signal from them remains unchanged. It was first shown in an animal model84 (Figure 9A) and then in humans85 (Figure 9B) that the systolic-to-diastolic aBV signal ratio measured at rest increases in the presence of a noncritical stenosis and that the degree of increase is proportional to coronary stenosis severity. Thus, by exploiting the microcirculatory compensatory mechanisms that are evoked to maintain a constant perfusion pressure distal to a stenosis, we can detect the presence of stenosis even at rest using a novel adaptation of MCE.86
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As can be seen from Figure 9B, a normal systolic-to-diastolic aBV ratio can sometimes be seen even in the presence of moderate to severe coronary stenosis. It was hypothesized that this is caused by collateral blood flow. In a canine model of coronary stenosis, measurements were made in collateralized and noncollateralized myocardium. The systolic-to-diastolic aBV signal ratio in the noncollateralized bed increased significantly with increasing stenosis severity, whereas in the collateralized bed it did not.87 Because extensive collateralization may indicate excellent prognosis, this ratio may provide a more appropriate assessment of the myocardium despite the presence of significant coronary artery stenosis and hence better prognostic information than that provided by the coronary anatomy.
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MCE for Detecting Viability in Chronic CAD |
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TopIntroductionSome Historical and Technical...MCE in Acute Coronary...MCE for CAD DetectionMCE for Detecting Viability...MCE for Site-Targeted ImagingMiscellaneous Applications and...Remaining Challenges for MCESummaryReferences |
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One of the earliest experimental and clinical applications for MCE was the detection of myocardial viability based on microcirculatory integrity after attempted reperfusion in AMI.5,38–43,47–49 Another aspect was the maintenance of myocardial viability in AMI based on adequate collateral MBF.44–46 More recently, attention has shifted to the assessment of viability in chronic CAD. MCE was compared with 201Tl imaging and low-dose dobutamine stress in patients with CAD and dysfunctional myocardium undergoing coronary bypass surgery.88 It was found that the sensitivity of MCE for recovery of function after bypass was 90% and was similar to that of 201Tl imaging (92%) and dobutamine echocardiography (80%). However, the specificity of MCE was higher (63%) than that of the other 2 techniques (45% and 54%). These patients also underwent myocardial biopsies during bypass surgery to correlate MCE parameters to histology.89 As expected, MBV ratios from different beds correlated very well with microvascular density and capillary area ratios from these beds (r=0.84 and r=0.87, respectively). Also not surprisingly, MCE-derived MBF was a better predictor of functional recovery than MBV alone.
Another study examined patients presenting with acute heart failure who had no previous infarction or history of CAD.90 Clinically, in these patients 2 questions need to be answered. Do they have CAD, and if they do, is the myocardium viable? In this study, the sensitivity, specificity, and positive and negative predictive values of stress MCE for the detection of CAD were 82%, 97%, 95%, and 88%, respectively. Quantitative MCE demonstrated significantly lower MBF velocity reserve in patients with CAD compared with those with normal coronary arteries. Almost all patients with CAD had reversible defects indicating the presence of viable myocardium. Thus, MCE was able to answer the 2 critical questions at the bedside without requiring a more cumbersome assessment through SPECT, PET, or magnetic resonance imaging.
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MCE for Site-Targeted Imaging |
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TopIntroductionSome Historical and Technical...MCE in Acute Coronary...MCE for CAD DetectionMCE for Detecting Viability...MCE for Site-Targeted ImagingMiscellaneous Applications and...Remaining Challenges for MCESummaryReferences |
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As stated earlier, microbubbles used for MCE have a rheology that is similar to that of erythrocytes.20–22 Therefore, it was unexpected when instead of flowing freely through tissue, microbubbles were found to be sticking in the myocardium after cold crystalline cardioplegia delivery both in animal models91 and in humans.92 This sticking was not related to any of the cardioplegia constituents (including oxygen content) or temperature and was not seen when the myocardium was reperfused by venous rather than arterial blood,93 in which free oxygen radical–induced injury is less. Similar microbubble sticking was noted during ischemia/reperfusion, and it was found that the microbubbles were in fact adhering to activated leukocytes via cell-surface integrins or complement-mediated opsonization depending on the microbubble used.94 Thus began the journey with site-targeted (or molecular) imaging with MCE.
The advantage of MCE over modalities such as magnetic resonance imaging is that because of their nonlinear behavior when exposed to ultrasound, very few bubbles need to be present in the target to produce signal. Therefore, the signal-to-noise ratio is very good. The disadvantage that MCE has over other modalities, especially PET/CT, is that targets limited only within the vascular space can be imaged. Table 2 lists the targets that have been imaged successfully with MCE with the use of either antibodies or small molecules and the different experimental conditions in which they have been shown to be useful. Many of these applications are outside the heart or even the cardiovascular system.
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In terms of the myocardium, the obvious potential applications of this approach are imaging of angiogenesis (especially in the context of cell and gene therapy)95–97; acute and chronic cardiac transplant rejection (in which it could supplant the more invasive cardiac biopsies)98,99; ischemia/reperfusion injury, especially if one can modulate it (see later)94,100–102; imaging microthromboembolism during percutaneous coronary interventions (to determine effective therapy for the no-reflow phenomenon)103,104; and ischemic memory (to differentiate new from old wall motion abnormality in the setting of chest pain).105,106 Early coronary atherosclerosis can also be detected with this approach.107 Microbubbles can also bind to early atherosclerotic lesions nonspecifically by complement-mediated adhesion.108
Described herein is an experiment in which MCE-based molecular imaging was used to determine the efficacy of therapy aimed at reducing infarct size in a setting of percutaneous coronary intervention performed for acute coronary thrombosis. The reason to present these results is to indicate possible ways in which molecular imaging could be used to determine the effect of treatments.
The thrombus burden influences the size of the no-reflow phenomenon as well as infarct size after attempted coronary intervention,104 suggesting that the beneficial effects of platelet glycoprotein IIb/IIIa receptor blockade may be due in part to reduction in microthromboemboli. Another consequence of ischemia/reperfusion is endothelial activation from oxygen free radicals and tissue factor, resulting in the expression of the 
vβ3, which, in turn, causes platelets to adhere to the endothelium. In the presence of microthromboemboli, prothrombin can bind to both IIb/IIIa and vβ3, resulting in additional thrombus formation. Activation of vβ3 is also associated with leukocyte entrapment within the platelet-fibrin mesh as well as monocyte adhesion to the endothelium. These effects could reduce microvascular perfusion and cause more ischemia.
Thus, the therapeutic aim of this experiment was the combined inhibition of vβ3 and IIb/IIIa with the use of a novel synthetic compound, CP-4715, whose action was compared with that of tirofiban, which predominantly blocks the IIb/IIIa receptor.109 A control group (saline) was also used. Myocardial activities of vβ3 and IIb/IIIa were measured with echistatin-labeled microbubbles95 and 99mTc-labeled DMP-444,104 respectively. Inflammation was assessed with phosphatidyl serine–coated microbubbles.102
Whereas the risk area size was similar between the 3 groups, the no-reflow zone was different, as demonstrated by the examples shown in the top panel of Figure 10. It was the largest in the group 1 dog receiving saline (Figure 10A [top]), intermediate in the group 2 dog receiving tirofiban (Figure 10B [top]), and smallest in the group 3 dog receiving CP-4715 (Figure 10C [top]). 99mTc-DMP-444 activity on autoradiography is shown in the bottom panel in Figure 10. It was higher in group 1 (Figure 10A [bottom]) compared with group 2 and 3 dogs (Figure 10B and 10C [bottom]), indicating some reduction in microthromboemboli with IIb/IIIa inhibition caused by both drugs. Background-subtracted color-coded images after administration of echistatin-conjugated microbubbles are shown in the top panel in Figure 11. There was the least backscatter in group 3 dogs (Figure 11C [top]), indicating effective inhibition of vβ3 by CP-4715. Similar images after administration of phosphatidyl serine–coated microbubbles are shown in the bottom panel of Figure 11. The backscatter was the lowest in the group 3 dogs (Figure 11C [bottom]), indicating suppression of inflammation by CP-4715. Overall, there was a 59% reduction in infarct size in the group 3 dogs receiving CP-4715 compared with controls and a 37% reduction compared with the group 2 dogs receiving tirofiban. Thus, the combined suppression of both IIb/IIIa and vβ3 markedly limits no-reflow and tissue injury.109
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Miscellaneous Applications and Findings |
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TopIntroductionSome Historical and Technical...MCE in Acute Coronary...MCE for CAD DetectionMCE for Detecting Viability...MCE for Site-Targeted ImagingMiscellaneous Applications and...Remaining Challenges for MCESummaryReferences |
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MCE has been found to be very effective in identifying the septal perforator arteries that supply the thickened muscle, which contributes to outflow track obstruction in hypertropic cardiomyopathy. Thus, selective intracoronary injections of microbubbles can be used to define the vessel through which alcohol needs to be administered for creating localized necrosis and reduction in the outflow tract gradient.110 MCE has also been used for visualization of the right ventricular myocardium111 and to demonstrate improved perfusion after stem cell therapy.112 A particularly innovative application has been in the evaluation of cardiac tumors. Malignant tumors have been shown to have higher MBF velocity and MBV at rest, which separates them from benign tumors.113 With the use of MCE, it has been demonstrated that the reason for the reversible defect in patients with left bundle-branch block on stress nuclear imaging is the partial volume effect caused by the poor spatial resolution of SPECT.114 An experimental study using MCE has shown that the cyclic variation in integrated backscatter seen during the cardiac cycle is caused by cardiac contraction–induced cyclic changes in MBV and not due to cyclic alterations in myofibrillar elastic element orientation and ultrasonic anisotropy, as has been suggested previously.115 MCE has also been used successfully to assess myocardial perfusion in both rats116 and mice.117
MCE has opened up an entire new frontier of research in the control of microcirculatory flow. For instance, whole blood viscosity plays a premier role in capillary resistance, and it was shown recently that hyperlipidemia increases whole blood viscosity and decreases coronary flow reserve.118 In this study, when serum triglyceride levels were increased during maximal hyperemia, MBF velocity decreased and myocardial vascular resistance increased (Figure 12A).
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Whole blood viscosity is, in large part, determined by erythrocyte charge and deformability that ultimately affects erythrocyte mobility in capillaries. In an animal model of myocardial ischemia, it was shown that augmented MBF in the ischemic microcirculation during nitroglycerin administration occurs in tandem with increased erythrocyte S-nitrosothiol content that increases both erythrocyte mobility and oxygen unloading in ischemic but not nonischemic tissue.119 Thus, when tissue is normoxic, hemoglobin binds to nitric oxide. In contradistinction, when tissue is hypoxic, hemoglobin releases nitric oxide. Thus, erythrocyte-mediated microvascular mechanisms rather than only reduction in preload and large coronary artery dilatation may contribute to the powerful anti-ischemic effects of nitroglycerin, especially during low-flow states.
Finally, it has been shown that the myocardial capillary hydrostatic pressure is tightly regulated by the body even when autoregulation is exhausted. When the coronary perfusion pressure was kept within the autoregulatory range, as expected, MBV remained unchanged (coronary blood volume increased but not MBV, which primarily reflects capillary blood volume).120 However, when perfusion pressure was decreased, capillaries derecruited to maintain a constant hydrostatic pressure (Figure 12B). More interestingly, when aortic pressure was increased with phenylephrine, capillaries again derecruited to maintain a constant capillary hydrostatic pressure.
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Remaining Challenges for MCE |
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TopIntroductionSome Historical and Technical...MCE in Acute Coronary...MCE for CAD DetectionMCE for Detecting Viability...MCE for Site-Targeted ImagingMiscellaneous Applications and...Remaining Challenges for MCESummaryReferences |
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Despite 25 years of research and development, MCE is not yet used routinely as a clinical tool, for which there are several reasons. First, no ultrasound contrast agent has yet been approved for myocardial opacification in the United States. The second and most important reason is that currently there is no reimbursement for MCE. In comparison, SPECT is compensated handsomely. It is also true that the learning curve for MCE is steep, and >1 person (a sonographer and an assistant/physician) is required to perform a good-quality study. Furthermore, it involves placing an intravenous line, which in many states requires the services of a registered nurse who may not be readily available. However, a fair compensation for performing and interpreting the study is likely to result in its rapid adoption.
The future of molecular imaging with MCE as a clinical entity appears uncertain at the moment. Until now, not a single human study has been performed with any of the strategies depicted in Table 2. A major reason is the reluctance to use antibodies that may inhibit activated molecules outside the myocardium and produce untoward effects. Second, even if we use other ligands instead of antibodies, a method to conjugate them to microbubbles that is safe in humans has not yet been identified. Currently, biotin-streptavidin is used for conjugating ligands to bubbles or liposomes. Streptavidin can also bind to biotin in the body (needed for fatty acid synthesis and gluconeogenesis) and prevent its function, leading to potentially deleterious effects. Conjugation of ligands to microbubbles through amine or sulfhydryl covalent bonds may be safe in humans. Although these methods may be developed soon, it will take years to approve any one of these site-targeting agents. What is not known is whether a generic "labeling bubble" to which any small molecule can be attached will be approved or whether approval of each specific bubble-molecule combination will be required. The former would be most advantageous and would follow the model of PET, in which isotopes can be produced "in-house" and labeled with compounds, whereas the latter may be too daunting to pursue.
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Summary |
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TopIntroductionSome Historical and Technical...MCE in Acute Coronary...MCE for CAD DetectionMCE for Detecting Viability...MCE for Site-Targeted ImagingMiscellaneous Applications and...Remaining Challenges for MCESummaryReferences |
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It is remarkable that there is already a vast literature, including large studies, showing the diagnostic and prognostic value of MCE even before a single ultrasound contrast agent has been approved in the United States for this purpose. The response of the US Food and Drug Administration to phase III study results of Cardiosphere and Imagify and subsequent reimbursement for MCE will determine its clinical adoption and success. Whether MCE develops into a universally applied clinical tool or not, it has been a valuable experimental tool for the discovery of new physiological and pathophysiological insights into the microcirculation and local control of MBF. Some of these have been described previously and some in this review. For a relatively inexpensive tool, MCE can provide rather sophisticated information in individual patients in various clinical scenarios. Its greater clinical adoption can only enhance patient care.
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Acknowledgments |
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The work cited herein reflects the hard work by many colleagues here at Oregon Health and Science University and elsewhere as well as many postdoctoral fellows who have worked in laboratories of several investigators over many years. The true acknowledgement of their work is in the references cited here and in previous reviews (page and reference limits precluded many important articles from being cited here).
Sources of Funding
This work was supported by grants (R01-EB-002069, R01-HL-65704, and R01-HL-66034) from the National Institutes of Health, Bethesda, Md.
Disclosures
None.
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References |
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TopIntroductionSome Historical and Technical...MCE in Acute Coronary...MCE for CAD DetectionMCE for Detecting Viability...MCE for Site-Targeted ImagingMiscellaneous Applications and...Remaining Challenges for MCESummaryReferences |
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