(Circulation. 1997;96:785-792.)
© 1997 American Heart Association, Inc.
Articles |
From the Cardiovascular Division, University of Virginia School of Medicine, Charlottesville, Va (S.K.); the Cardiology Department, Northwick Park Hospital, Harrow, UK (R.S., U.R., R.K., A.L.); and Molecular Biosystems, Inc, San Diego, Calif (H.D.).
Correspondence to Sanjiv Kaul, MD, Cardiovascular Division, Box 158, Medical Center, University of Virginia, Charlottesville, VA 22908. E-mail sk{at}virginia.edu
| Abstract |
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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% (
=.99) for both methods. Concordance between normal perfusion
and reversible or irreversible segmental defects was 90% (
=.80).
Agreement between the two methods for each of the three vascular
territories in each patient was 90% (
=.77), while agreement for the
presence or absence of CAD in each patient was 86% (
=.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.
Key Words: echocardiography coronary disease tomography imaging
| Introduction |
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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.
| Methods |
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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
5-8x108 · 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
Image Interpretation
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.
|
Statistical Methods
Kappa statistics were used to determine concordance between
scores.28
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.
| Results |
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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.
Segment-by-Segment Analysis
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 (
=.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% (
=.80). SPECT and MCE identified 204 and 213
segments, respectively, as normal; concordance between the two methods
was noted in 199 segments (91%,
=.92). Of the 73 abnormal segments
by both techniques, concordance for reversible and irreversible defects
was 85% (
=.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.
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Territory-by-Territory Basis
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,
=.77). Of the abnormal
territories, 22 were considered reversible by both techniques
(concordance of 88%,
=.71).
Patient-by-Patient Basis
There was concordance in 25 of the 29 patients with adequate
images (86%,
=.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).
Observer Variability
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.
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| Discussion |
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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 cinecomputed 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.
Conclusions
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.
| Acknowledgments |
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| Footnotes |
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Received December 4, 1996; revision received February 27, 1997; accepted February 28, 1997.
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