(Circulation. 1995;92:2723-2739.)
© 1995 American Heart Association, Inc.
Articles |
From the Departments of Cardiology (E.E.v.d.W., H.W.V., A.V.G.B.) and Radiology (A.d.R.), University Hospital Leiden, and the Interuniversity Cardiology Institute, Leiden, Netherlands.
Correspondence to Ernst E. van der Wall, MD, Department of Cardiology, Building 1, C5-P25, University Hospital Leiden, Rijnsburgerweg 10, 772333 AA Leiden, The Netherlands.
| Abstract |
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Key Words: magnetic resonance imaging coronary disease ischemia
| Introduction |
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| MR Imaging Techniques |
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Spin-Echo Imaging
Spin-echo imaging is the MR imaging
technique of choice in
assessing left ventricular mass by providing a
three-dimensional, direct visualization of the
myocardium with excellent mural edge discrimination (Fig 1
).
Cardiac and vascular anatomy
have been examined in normal subjects and in patients with various
forms of cardiovascular disease. Muscle mass and
chamber dimensions correlate well with those obtained with
echocardiography and contrast
ventriculography.10 11 12 13 14 15 16 17 18 19
Left and right
ventricular volumes, left ventricular ejection
fractions and regional left ventricular function can
accurately be
measured.10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
In addition, wall thickness
and thickening, as measures of myocardial viability, may be determined
with the use of MR spin-echo
imaging.25 26 27 At
present, MR spin-echo imaging is predominantly used for the
evaluation of static phenomena such as morphological appearance,
myocardial tissue characterization (with and without paramagnetic
contrast agents), assessment of wall thickness and left
ventricular mass, and left ventricular
thrombus, whereas cine MR imaging is the technique of choice for
evaluation of cardiac function (Fig 2
).
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Myocardial Tissue Characterization
Because of its capacities
for tissue characterization, MR imaging
is well suited for the evaluation of myocardial ischemia and
infarction. Myocardial ischemia and infarction are associated
with increased myocardial signal as a result of prolonged tissue
relaxation times T1 and T2 (T1, longitudinal relaxation time; T2,
transverse relaxation time).28 In several studies,
correlations were observed between increases in T1 and T2 and the
increase in tissue water content.29 30 Although it is
clear that tissue water content plays a major role in T1 and T2
changes, it has been suggested that other factors also may be important
such as changes in free radicals or changes in magnetic
susceptibility.31 32 33
Contrast Agents
The quality of MR imaging has been greatly
improved by the use of
paramagnetic contrast agents.34 Paramagnetic compounds
cause a shortening of both the T1 and T2 relaxation times, with the
reduction in T1 relaxation time predominating. The magnitude of the
change in relaxation time is influenced by the magnetic field strength,
the paramagnetic agent used, and the agent concentration. Local tissue
perfusion is thus delineated in a manner analogous to standard
indicator dilution methods, with more pronounced effects seen in areas
of highest contrast agent concentration. Despite the ability to
generate images with varying image contrast, when the relaxation
parameters T1 and T2 are used, the detection of acute
ischemia with unenhanced MR imaging does not occur until
several hours after coronary occlusion. Therefore, paramagnetic
contrast agents have been used to define functional and perfusion
abnormalities in the early stage of acute myocardial ischemia
and
infarction.35 36 37 38 39 40 41 42 43 44 45
The first studies of
contrast-enhanced MR imaging used manganese chloride to evaluate
ischemic and infarcted myocardial zones in canine
hearts.46 47 At present, gadolinium-containing
contrast agents (labeled with DTPA, DOTA, BOPTA, or albumin)
are being used
widely.48 49 50 51 52 53 54 55 56 57
Cine MR Imaging
With cine MR imaging the normal blood pool
shows high signal
intensity in all phases of the cardiac cycle and provides a constant
contrast with the less intense myocardium, unlike
spin-echo imaging in which intraluminal signal is usually low and
may be variable and inseparable from adjacent myocardial tissue
because of flow artifacts. Visual evaluation of global and regional
wall motion is facilitated by viewing the cinematic display of the MR
images. Basically, the cine MR imaging technique may be more accurate
for defining regional myocardial (dys)function than contrast
angiography, since the latter depends upon the evaluation of wall
motion only. Cine MR imaging appears to be the MR technique of choice
for assessing left ventricular function.58 59
Cardiac chamber volumes, ejection fraction, and regional wall motion
and thickening can be reliably determined by cine MR imaging, with high
reproducibility.60 All parameters correlated
well with two-dimensional echocardiography and
contrast
ventriculography.61 62 63 64
Pattynama et
al65 66 showed that cine MR imaging allowed the
accurate
assessment of right ventricular volumes and right
ventricular mass.
Ultrafast MR Imaging
Ultrafast imaging refers to a group of
techniques developed with
the main purpose of acquiring images very rapidly. Because much of the
difficulty in cardiac MR imaging arises from respiratory motion,
imaging fast enough to be completed in a single breathhold reduces this
problem. Ultrafast gradient-echo pulse sequences have been used
recently for cardiac application and can be referred to as turbo fast
gradient-echo
imaging.67 68 69 70 71
In particular, the use of
commercially available rapid imaging sequences with fast low-angle
shot imaging has enabled more widespread study of myocardial perfusion
and wall motion with MR imaging.68 69 During fast
gradient-echo acquisitions, one can simulate cardiac gating by
reordering and segmentation of the MR image data acquisition
(so-called segmented k-space acquisition). This allows for
almost total elimination of motion by limiting the acquisition to 50 to
120 ms at end diastole.70 The fast
gradient-echo techniques also allow the acquisition of
end-systolic and end-diastolic phases of
the cardiac cycle to calculate cardiac volumes and mass.71
Other ultrafast MR imaging techniques, such as echo-planar MR
imaging, which obtain images in approximately 50 ms (one-shot
imaging), are also available.72 Within a single
breathhold, the entire heart at a specified time in the cardiac cycle
may be covered in approximately half a minute.73 Coupled
with the bolus administration of contrast media and the acquisition of
first-pass images of the left ventricular
myocardium, ultrafast MR imaging has great potential in the
assessment of regional myocardial wall motion and perfusion (Fig
3
). It enables the study of fast
physiological processes, such as cardiac
first-pass effects. The limitations of the current technique are
its inability to quantitate myocardial flow and the acquisition of
images at only a limited number of cardiac levels (currently maximum 3)
during a single bolus injection. Future technical improvements such as
echo-planar imaging should allow for more tomographic sections to
be acquired at several levels with each heartbeat during the first pass
of a contrast agent.74
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Myocardial Tagging
Recently, MR imaging methods have been
described that permit the
determination of the absolute motion and thickening of specific
myocardial segments. These involve the use of myocardial tagging as
first described by Zerhouni et al75 and Bolster et
al76 and subsequently enhanced by Axel and
Dougherty.77 78 Myocardial tagging involves localized
radiofrequency saturation of the myocardial tissue before acquiring
images, which allows the monitoring of the progressive distortion of
the myocardium during the course of the cardiac cycle. The
earlier tagging sequences (eg, DANTE79 ) have been modified
to increase the number of saturation bands, narrow the bands, and
improve their persistence throughout the cardiac cycle. The most
frequently used tagging sequence is called spatial modulation of
magnetization (SPAMM),77 78 which produces images
with a
regular pattern of stripes that move with the cardiac wall during the
cardiac cycle. It provides a unique method for analyzing regional
ventricular strain and the quantitation of regional
myocardial function such as the absolute motion and thickening of
specific myocardial segments.80 This can be used to
evaluate myocardial rotational deformation, ventricular
nonuniformity, and differences in subendocardial and epicardial wall
motion, both in normal and ischemic
myocardium.81 82 83 Based on these
findings, it
is likely that myocardial tagging will become a reference standard for
assessment of wall motion and wall thickening in the setting of acute
myocardial ischemia and infarction. Clinical studies in
patients with coronary artery disease are awaited.
MR Spectroscopy
MR spectroscopy is an exciting tool for
evaluation of cardiac
metabolism by direct measurement of
ischemia-induced changes of high-energy phosphates and
the intracellular pH in in vivo animal models with the use of surface
coils directly applied to the surface of the heart.84
During ischemia, adenosine triphosphate (ATP) and
phosphocreatine (PCr) levels decrease, whereas the level of inorganic
phosphate increases.85 After brief periods of
ischemia, PCr levels recover during reperfusion.86
After the onset of ischemia, the concentration of PCr decreases
much faster than that of ATP, resulting in a rapid increase of the
PCr/ATP ratio.87 At present, only a few spectroscopy
data are available that apply these experimental data to patients with
coronary artery disease.88 89 With the use of
isometric stress testing88 89 a significant decrease
in
PCr/ATP ratio was obversed in ischemic myocardial regions.
Quantification of metabolism in humans may be difficult
because volumes of interest are relatively large compared with
myocardial wall thickness. As a result, there are no reliable data
available in patients with a previous myocardial infarction who may
have considerable wall thinning, which precludes reliable acquisition
of MR spectra. At present, clinical applications of cardiac MR
spectroscopy in patients with coronary artery disease are
sparse, and its value remains to be proven in larger
studies.90
| Coronary Artery Disease |
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Acute Myocardial Ischemia and Infarction
Experimental
Studies in Acute Myocardial
Ischemia
MR imaging allows the assessment of infarct size based on
different T2 relaxation times between infarcted and normal
tissue.91 92 93 94 Serial MR
imaging of left
ventricular infarct size 3 and 21 days after
coronary artery ligation with the use of T2 measurements
correlated well with histopathologically assessed infarct
size.95 MR imaging of dogs with reperfused
myocardium showed a significant increase in signal
intensity and T2 relaxation times as early as 30 minutes after
reperfusion,96 97 98 99 100 101 102
indicating that MR imaging detects
ischemic myocardial areas soon after coronary
occlusion, thereby providing a method to discern reperfused viable
myocardium. Based on these experimental findings, a T2
strategy has been advocated to evaluate healing patterns in patients
following reperfusion after thrombolytic
therapy.103
The paramagnetic contrast agent Gd-DTPA has been shown to enhance contrast of ischemic and infarcted myocardium in animal experiments.41 43 46 47 The contrast enhancement in the ischemic area probably is caused by differences in wash-in and wash-out of Gd-DTPA from normal and ischemic myocardium. In acutely damaged myocardium, the increased accumulation of Gd-DTPA may be related to one or more of the following factors: decreased blood flow, increased tissue blood volume, enhanced size of the extracellular space, and increased permeability of the capillaries, all of which cause slow wash-out from the infarcted zone. By 10 to 15 minutes after Gd-DTPA injection, it has largely washed out of the normal myocardium, whereas it remains in the infarcted zone, suggesting that MR imaging in acute myocardial infarction should be performed more than 15 minutes after administration of Gd-DTPA. Gd-DTPA remains outside the cells and is excreted by glomerular filtration.36 Gd-DTPA has been studied in several experimental models of myocardial ischemia that primarily differ from each other in the duration of coronary artery ligation, the time period between contrast administration and imaging, and the presence or absence of reperfusion.49 50 51 52 53 These experimental studies with Gd-DTPA demonstrated that changes in relaxation times occur very early (2 minutes) after coronary artery occlusion, implying that Gd-DTPA allows the detection of early myocardial ischemia even before the onset of myocardial edema formation or the development of irreversible damage. These studies also suggest that Gd-DTPA may be useful to outline distribution of regional myocardial blood flow. In a study by Miller et al54 using Gd-DTPAenhanced MR imaging, it was possible to measure myocardial flow reserve during pharmacological dilatation by dipyridamole. There was a significant correlation between changes in Gd-DTPAenhanced MR signal and microsphere-determined myocardial blood flow. Further experimental studies have shown that the use of Gd-DTPA may discriminate between occlusive and reperfused infarcts, based on differences in signal intensities.55 56 57 Moreover, administration of Gd-DTPA early after reperfusion allowed the identification of the area at risk by selective concentration of Gd-DTPA in reperfused myocardium.104 105 Holman et al,106 in 21 isolated rat hearts, compared distribution of Evans blue staining and Gd-DTPAinduced contrast enhancement in ischemic and reperfused myocardium and showed the excellent capability of Gd-DTPA to identify ischemia and reperfusion by contrast enhancement. Nishimura et al107 measured infarct size in canine hearts both by Gd-DTPAenhanced MR imaging and indium-111 labeled antimyosin. Gd-DTPA showed significant contrast enhancement of the infarcted area, and the extent of the contrast enhancement expressed infarct size precisely. Van Dijkman et al108 showed that Gd-DTPAenhanced MR imaging identified infarcted myocardium with great sensitivity in an in vivo porcine model.
In summary, experimental studies using spin-echo MR imaging may identify both reperfused and nonreperfused myocardium by its tissue characterization capabilities. Gd-DTPAenhanced MR imaging shows improved contrast enhancement of ischemic and infarcted myocardium, especially on T1-weighted images. It allows the distinction between normal and ischemic myocardial tissue and can be used to delineate the infarcted area and to calculate infarct size.
Clinical Studies in Myocardial Infarction
Clinical studies in patients with documented myocardial infarction
have shown T1 and T2 alterations in infarcted
myocardium.109 110 111 112 113 114 115 116 117 118
Johnston et
al110 studied 34 patients 3 to 30 days after myocardial
infarction and showed that regional increase of signal intensity was
consistent with the electrocardiographic location of the
infarction and with the presence of hypokinetic segments on the left
ventriculogram. Fisher et al111 showed in 29 patients 3 to
17 days after myocardial infarction prolonged T2 relaxation times in
infarcted myocardial regions. On the other hand, they observed that
increased signal intensity on T2-weighted images may be very difficult
to distinguish from slowly moving intraventricular
blood flow. Ahmad et al112 showed that T2 prolongation
might not be a specific marker for acute myocardial infarction and can
also be observed in abnormally perfused myocardial segments of patients
with unstable angina. Been et al113 114 showed in 41
patients with acute myocardial infarction that maximum T1 values were
observed at 2 weeks after the acute onset, suggesting that the increase
of T1 reflects cellular infiltration as much as or more than tissue
edema. No differences in T1 values were observed between the patients
with or without reperfusion, indicating that alterations of T1 are
complex and may bear no relationship to specific
histological findings. In the absence of any
histological confirmation, these statements remain
purely speculative. Studies by Postema et al115 and Krauss
et al116 117 showed in patients with acute myocardial
infarction, who underwent MR imaging studies with a mean of 8 days
after the acute event, that regional T2 abnormalities in 82% of
patients correlated with the presence, location, and extent of
thallium-201 perfusion defects at rest. In a subsequent study by Krauss
et al,118 good agreements were found between enzymatic
infarct size, thallium-201 scintigraphy, radionuclide
angiography, and MR findings.
In addition to changes in T1 and T2
relaxation times as indices for
tissue characterization, other characteristics can be used to indicate
infarcted myocardial areas, such as increased signal intensity,
ventricular cavitary signal, and regional wall thinning.
Filipchuk et al119 showed increased myocardial signal
intensity in 88%, cavitary signal in 74%, and regional wall thinning
in 67% of 27 patients with acute myocardial infarction. Of the three
features, wall thinning was the most predictive of and specific for
acute myocardial infarction (Fig 4
).
White et al120 121 showed in patients with recent
myocardial infarction a good correlation between MR imaging and
two-dimensional echocardiography for
demonstrating regional wall motion abnormalities; the extent of
regional wall thinning by MR imaging could be used to measure infarct
size. Wisenberg et al122 showed in 66 patients 3 weeks
after acute infarction that infarct size could very well be determined
by MR imaging based on signal intensity. They demonstrated that in the
41 patients who had received acute streptokinase therapy, a significant
reduction in MR-measured infarct size was observed compared with the
patients without thrombolytic therapy. Johns et
al123 assessed MR infarct size in 20 patients based on
signal intensity at a mean of 9 days after the acute onset of symptoms.
MR infarct size correlated very well with the extent of the region with
severe hypokinesia visualized by left ventricular
angiography. Turnbull et al124 compared MR imaging, based
on T1 maps, with enzymatic infarct size, technetium-99m
pyrophosphate scintigraphy, and radionuclide angiography in
patients 5 to 7 days after myocardial infarction. The authors found a
good agreement between infarct size detected by MR imaging and that
assessed by the radionuclide techniques.
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Acute myocardial infarction
also has been studied with the use of
contrast-enhanced MR imaging. Most clinical experience has been
obtained with Gd-DTPA, which can be safely used in patients with
coronary artery disease and generally provides a better image
quality than unenhanced T2-weighted images.48 Eichstaedt
et al125 were the first to show, in 26 patients with
subacute myocardial infarction, that the 11 patients who were
studied with Gd-DTPA 5 to 10 days after the acute event (at 10, 25, and
45 minutes after administration of Gd-DTPA) had a 70% average increase
of signal intensity within zones of infarcted myocardium,
while only a 20% increase of signal intensity in normal myocardial
tissue was observed. The other 15 patients were imaged later in the
course of infarction and did not show differences in intensity ratio
between infarcted and normal tissue. These findings were corroborated
by Nishimura et al,126 who studied infarct patients in the
subacute phase with MR imaging and Gd-DTPA 5 to 10 minutes after
administration, at an average of 5, 12, 30, and 90 days after the acute
event. Increased signal intensity in the infarcted area was observed at
5 and 12 days, implying that only subacute myocardial infarcts show
significant accumulation of Gd-DTPA. In our institution, preliminary
studies127 128 showed that the signal intensity of
infarcted versus normal myocardium was significantly
greater after Gd-DTPA administration than before Gd-DTPA both by visual
and computer-assessed analysis. Maximal contrast was
observed at 20 to 25 minutes after administration of Gd-DTPA (Fig
5
). Van Dijkman et al129
also showed that signal intensity of Gd-DTPA was significantly
increased in the infarcted areas of patients who were studied more than
72 hours after the acute onset, indicating increased accumulation of
Gd-DTPA in a more advanced stage of the disease process. In a
subsequent study by Van Dijkman et al130 in 84 patients
with acute myocardial infarction, it was shown that Gd-DTPA enhancement
improved visualization of infarcted areas up to 6 weeks after onset of
symptoms and had a maximal effect within 1 week after infarction.
Holman et al131 showed a good correlation between infarct
size measured with gadolinium-enhanced MR imaging and enzymatically
determined infarct size in 24 patients 3 to 7 days after the acute
event (Fig 6
).
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These encouraging results have led to
initiation of clinical studies to
determine whether the use of Gd-DTPA allows the discrimination of
reperfused versus nonreperfused myocardial areas. In an initial report
by Van der Wall et al132 using Gd-DTPAenhanced MR
imaging in 27 patients after thrombolytic therapy for acute
myocardial infarction, it was shown that signal intensities measured 25
minutes after Gd-DTPA administration did not differ between reperfused
and nonreperfused myocardial areas. Van Rossum et al133
studied patients with acute myocardial infarction after
thrombolytic therapy, and they measured intensity ratios 6
to 8 minutes after injection of Gd-DTPA. They did observe a significant
difference in signal intensity ratios between infarcted regions
subtended by occluded coronary arteries and reperfused vessel
regions, indicating that assessment of the early dynamics of contrast
enhancement using Gd-DTPA MR imaging may identify successful
reperfusion. In a study by De Roos et al134 it was
observed that the morphological appearance of contrast enhancement by
Gd-DTPA may provide some clues as to the presence or absence of
reperfusion; reperfusion goes along with a homogeneous
aspect, while lack of reperfusion may be visualized as a
heterogeneous enhancement of contrast. Apart from these
morphological characteristics, De Roos et al135 used MR
imaging with Gd-DTPA to show that infarct size was significantly
smaller in patients with documented reperfusion than in patients
without reperfusion (Fig 7
).
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In summary, acute myocardial infarction is associated with prolonged T1 and T2 relaxation times. In addition to increased relaxation times, other morphological MR imaging features are valuable, of which regional myocardial wall thinning is most specific. The use of paramagnetic contrast agents has considerably improved the detection of myocardial infarction. Particularly in patients within 1 week after myocardial infarction, the effect of thrombolytic therapy can be assessed with the use of the early dynamics of contrast enhancement after administration of Gd-DTPA and by the accurate determination of infarct size. Disadvantages include the difficulty of monitoring acutely ill patients, lack of ability to do bedside studies, and contraindications for patients with pacemakers.
Assessment
of Myocardial Function
The cine or gradient-echo MR imaging technique
can be used for
detection of myocardial ischemia by analysis of global
and regional cardiac function. Abnormal wall motion and more
specifically abnormal wall thickening indicate diminished regional
myocardial function.136 In a study by Pflugfelder et
al,137 13 normal subjects and 15 patients with
coronary artery disease were studied by cine MR imaging to
document and quantitate regional left ventricular wall
motion abnormalities. Abnormal wall motion was observed in 40 of 90
segments in patients with coronary artery disease, which
correlated well with results of echocardiography or
contrast ventriculography. The overall systolic wall thickening
in the normal subjects was 48±28%, in the normal segments of the
patients 43±31%, in hypokinetic zones 6±18%, in akinetic zones
-4±24%, and in dyskinetic zones -13±25%. Thus, the
absence of
systolic wall thickening proved to be a very specific marker of
regional myocardial dysfunction. Lotan et al138 studied 59
patients with suspected coronary artery disease with both
biplane cine MR imaging and biplane cineangiography. In the right
anterior oblique view, there was agreement in 96% of 275 segments and
in the left anterior oblique view in 92% of segments. Meese et
al139 studied 25 patients within 7 days after acute
myocardial infarction using cine MR imaging, left
ventricular contrast angiography, and radionuclide
angiography. They showed that left ventricular ejection
fraction by cine MR imaging correlated better with the ejection
fraction by left ventriculography (r=.94) than ejection
fraction by radionuclide angiography (r=.82). Compared with
left ventriculography, the concordance in regional wall motion was
similar for both cine MR imaging (69%) and radionuclide angiography
(65%).
The capability of cine MR imaging to provide functional information about the state of pathologically altered myocardium in combination with assessment of diastolic wall thickness and systolic wall thickening makes it suitable for identification of myocardial viability.140 141 142 Data from Baer et al,143 144 who compared wall thickness measurements by cine MR imaging with technetium-99m methoxy-isobutyl-isonitrile (MIBI) tomographic imaging, showed a high concordance in patients with large chronic Q-wave infarcts, particularly in patients with anterior wall infarcts. In a recent study, Baer et al145 compared low-dose dobutamine MR imaging with positron emission tomography in 35 patients with myocardial infarction (>4 months old). They showed that MR imaging was very accurate in assessing myocardial viability. These findings are supported by Perrone-Filardi et al,146 147 who used positron emission tomography with fluorine18-fluorodeoxyglucose. In most regions with reduced end-diastolic wall thickness and absent wall thickening, absence of metabolic activity was shown, indicating the suitability of MR imaging in the evaluation of myocardial viability.
In recent years, pharmacological stress has been applied in MR imaging for detection of functional abnormalities in patients with coronary artery disease, since physical exercise during MR imaging is difficult because of motion artifact and space restriction. Compared with dipyridamole as a vasodilating agent for producing perfusion abnormalities, dobutamine appears to be a more appropriate agent for eliciting wall motion abnormalities.148 Pennell et al149 studied 22 patients with coronary artery disease both by dobutamine cine MR imaging and thallium tomography. Comparison of perfusion defects and wall motion abnormalities during stress showed 90% agreement, and dobutamine infusion was well tolerated in all patients.
Van Rugge et al,150 in 23 healthy volunteers, identified wall motion dynamics and provided calculations of segmental wall thickening and hemodynamic parameters by using dobutamine stress imaging. In 37 patients with coronary artery disease, Van Rugge et al151 showed an overall sensitivity of 81% and a specificity of 100% when using dobutamine MR imaging; in patients with single-, two-, and three-vessel disease the sensitivity values were 75%, 80%, and 100%, respectively. In a subsequent study in 39 consecutive patients with clinically suspected coronary artery disease referred for coronary arteriography and in 10 normal volunteers, it was shown that dobutamine cine MR imaging identified wall motion abnormalities by quantitative analysis using the centerline method152 ; the sensitivity, specificity, and accuracy were 91%, 80%, and 90%, respectively. These findings were corroborated by Baer et al,153 who studied 28 patients with dobutamine cine MR imaging and found an overall sensitivity of 87% and a specificity of 100% for the detection of coronary artery disease. In a recent study, Baer et al154 compared the findings of dobutamine MR imaging with findings of dobutamine with technetium-99m MIBI tomographic imaging in 35 patients with coronary artery disease; a high concordance between the two imaging modalities was found with respect to the detection of a dobutamine ischemic response. These studies illustrate the feasibility of cine MR imaging to perform stress imaging and to detect the functional sequelae of reversible myocardial ischemia.
In summary, determination of wall motion and wall thickening by cardiac cine MR imaging may play an important role in the accurate detection and functional characterization of patients with suspected or known coronary artery disease. Cine MR imaging with the use of pharmacological stress may constitute a new modality to detect coronary artery disease. With the use of centerline analysis, accurate quantitative information can be obtained from regions that show reduced wall thickness and thickening caused by stress-induced myocardial ischemia. Although MR imagingdetermined reduction in end-diastolic wall thickness and absence of wall thickening may identify most viable segments, it is currently not the optimum approach, and metabolic information obtained by the use of MR spectroscopic methods may improve the specificity of functional analyses alone. Furthermore, more advanced technical developments are required before pharmacological stress cine MR imaging becomes a serious challenge to pharmacological stress radionuclide perfusion imaging or to two-dimensional echocardiography.
Assessment of Myocardial Perfusion
Most MR imaging studies on myocardial perfusion have been
performed with the use of ultrafast MR imaging. Atkinson et
al155 demonstrated that a T1-weighted ultrafast MR imaging
technique can provide adequate temporal and spatial resolution to
permit first-pass perfusion studies of the heart. In an isolated
perfused rat heart model with Gd-DTPA as contrast agent, marked
differences in contrast enhancement were observed between perfused and
nonperfused segments. The wash-in effects of Gd-DTPA occurred
during several seconds. Wilke et al,156 using a
turbo-FLASH sequence, studied the correlation between myocardial blood
flow at rest and during dipyridamole-induced
hyperemia in a closed chest dog model. They showed that
contrast-enhanced ultrafast MR imaging allowed the assessment of
myocardial perfusion both at rest and under stress circumstances.
Wendland et al,157 using echo-planar imaging in rats,
injected the contrast agent gadodiamide and observed a 63% increase in
signal in normal myocardial segments compared with ischemic
segments. In a recent study, Saeed et al158 showed in rats
that the transit of the contrast agent gadodiamide, monitored by
echo-planar imaging techniques, could be used to distinguish
between reversibly and irreversibly injured myocardium.
These experimental ultrafast MR imaging studies have stimulated the
application of these techniques in patients with coronary
artery disease.
Atkinson et al155 and Van Rugge et al159 evaluated the value of ultrafast MR imaging for the assessment of dynamic contrast enhancement and myocardial perfusion in healthy volunteers. Both studies showed progressively increasing signal intensities in the right ventricular cavity, the left ventricular cavity, and finally in the myocardial wall.
Manning et
al160 used ultrafast MR imaging for the
assessment of myocardial perfusion abnormalities in patients with chest
pain. Regional myocardium perfused by a diseased vessel
demonstrated a lower peak signal intensity and lower rate of signal
increase than did myocardium perfused by coronary
arteries without stenosis. Repeat MR imaging study after
revascularization showed an increase in peak signal
intensity. The patients with an area of myocardium perfused
by a diseased vessel and associated low peak signal intensity had the
greatest improvement in regional peak signal intensity after
revascularization. Van Rugge et al161
studied 20 patients with previous myocardial infarction using ultrafast
Gd-DTPA MR imaging. After Gd-DTPA administration, infarcted
myocardium demonstrated a signal intensity enhancement of
50%, whereas in normal myocardium an enhancement of 134%
was obtained. Myocardial perfusion abnormalities were clearly observed
in infarcted areas (Fig 8
). The infarct
site on MR imaging corresponded with the location of wall motion
asynergy determined by echocardiography. Schaefer
et al162 and Eichenberger et al163 studied
patients with coronary artery disease using Gd-DOTAenhanced
ultrafast MR imaging and dipyridamole stress. MR
imaging showed a sensitivity, specificity, and accuracy of 65%, 76%,
and 74%, respectively.163 Based on these studies it can
be concluded that contrast-enhanced ultrafast MR imaging allows
noninvasive assessment of myocardial perfusion in patients with proven
coronary artery disease.
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To summarize, ultrafast MR imaging provides the opportunity to acquire dynamic information related to the passage of a paramagnetic contrast agent through the coronary circulation and thus provides an indirect measure of myocardial perfusion. Applying this technique to patients with the use of pharmacological stress with dipyridamole, myocardial regions perfused by a severely stenosed coronary artery can be detected by a delayed increase in signal intensity and a decreased peak signal intensity. A limitation is the acquisition of only one tomographic slice in most of these studies, whereas higher speed methods would allow multislice tomography to generate a three-dimensional perspective. In this way, ultrafast MR imaging can provide important information on the functional significance of coronary artery lesions.
Sequelae of Myocardial Infarction
MR imaging is capable of
detecting short-term and
long-term sequelae of acute myocardial infarction. Higgins et
al164 showed that segmental wall thinning was highly
indicative of a sustained myocardial infarction in 9 of 10 patients
with chronic infarctions. McNamara and Higgins165 observed
regional wall thinning in 20 of 22 patients with prior infarctions; in
10 of 14 patients with sufficient residual wall thickness for
measurement of T2 relaxation times, decreased signal intensities and
shortened T2 values were measured at the site of the infarcted area. In
a study by Krauss et al,166 19 acute infarct patients were
studied by MR imaging at discharge, of whom 13 patients were reexamined
4 to 7 months later. In 10 patients, infarct site and size did not
change, and the T2 relaxation times remained prolonged, particularly in
the patients with anterior infarction. The finding of prolonged T2
values in chronically infarcted areas was also observed in animal
experiments by Checkley et al,167 who found
high-signal areas at 10 days in infarcted minipig hearts. After 2
weeks, no further change in signal intensity was detected, but
myocardial thinning became more evident. Recently, Hsu et
al168 studied chronic myocardial infarcts (6 months or
longer) in 10 formalin-fixed human autopsy hearts, and they showed
significantly increased T1 and T2 values in infarcted tissue versus
noninfarcted tissue. These studies indicate that detection of infarcted
areas is possible at the chronic phase of infarction both by
morphological appearance and altered signal intensities. Complications
of acute myocardial infarction including ventricular
aneurysm, ventricular septum perforation, mitral
regurgitation, and left ventricular
thrombus (Fig 9
) also can be readily
demonstrated by MR imaging.169 170 Left
ventricular septal defect is clearly visualized by
spin-echo MR imaging as absence of muscular tissue in the septal
area, and by cine MR imaging as a signal void in the right
ventricle.
|
To summarize, MR imaging provides an excellent means for detection of complications of myocardial infarction. Early detection of complications by MR imaging may be very important for guiding proper patient management.
Evaluation of Coronary Artery Bypass Grafts
MR imaging has
been used to evaluate the patency of
coronary artery bypass grafts. Using the spin-echo
technique, the grafts appear as small circular structures with absence
of luminal signal, since blood moves rapidly through normal grafts.
However, sternal clips used in bypass grafting can lead to small
regions of signal dropout that may be mistaken for patent grafts. There
also must be sufficient flow to generate contrast between the graft
lumen and the wall. Generally, multislice multiphase imaging is
required to obtain the appropriate images for detecting rapid graft
flow at contiguous levels in the same phase. In a study by Rubinstein
et al171 using a multislice spin-echo technique in 20
patients after bypass surgery, the overall sensitivity and specificity
for evaluating bypass patency were 92% and 85%, respectively. Gomes
et al172 studied 20 patients with patent bypass grafts and
showed that 54 of 64 grafts (84%) were detected by spin-echo MR
imaging. Jenkins et al173 assessed graft patency by
spin-echo MR imaging in 22 patients and found 90% accuracy
compared with contrast angiography. Frija et al174 showed
in 28 patients that spin-echo MR imaging after bypass surgery
provided a correct diagnosis in 95% of cases. The major causes of
diagnostic inaccuracies were hemostatic clips, in
particular clips for internal mammary bypass grafts.
While the spin-echo technique shows lack of signal intensity in vascular compartments with rapid blood flow, the cine MR imaging technique depicts flowing blood as a bright signal. Therefore, the presence of a bright visible intraluminal signal is indicative of graft patency. First results of cine MR imaging by White et al175 for determination of bypass patency in 25 patients showed accuracies of 91% for patency and 72% for occlusion. A subsequent study by White et al176 in 10 patients showed for the determination of patency a sensitivity of 93%, a specificity of 86%, and an overall predictive accuracy of 89%. Aurigemma et al177 used cine MR imaging in 20 operated patients with a total of 45 grafts and showed a sensitivity of 88%, a specificity of 100%, and an overall accuracy of 91%.
In summary, although these studies are preliminary, it has been presaged that a combined use of a spin-echo examination and cine MR imaging will be the optimal approach for imaging bypass grafts. Such a screening is applicable in postoperative chest pain syndromes to exclude graft occlusion or in screening for late graft occlusion or stenosis. Future flow-sensitive techniques are needed to exactly quantitate graft flow, as quantitation of bypass graft flow directly reflects distal runoff, which seems more valuable than simply detecting bypass patency.178 179 180 181
Visualization of the Coronary Arteries
Noninvasive
visualization of coronary arteries by MR
imaging techniques may provide a tremendous tool for detecting
stenoses in the main left and proximal coronary
arteries in patients with coronary artery disease (Fig 10
).
However, these techniques are yet
in an experimental phase and need further technical development. MR
angiography is used routinely in many centers for evaluation of the
carotid arteries and intracerebral vasculature,
aortography, and assessment of the ileofemoral system. MR angiography
of the coronary arteries, however, is technically more
difficult because of the relatively small size of these arteries, their
complex three-dimensional anatomy, and their constantly
changing position within the thoracic cavity caused by cardiac motion
and respiration. Several approaches for coronary MR angiography
have been proposed. Initial attempts at MR imaging of the proximal
coronary arteries had limited success because of the occurrence
of artifact resulting from prominent cardiac and respiratory
motion.182 183 More recently, other MR imaging
techniques
have been used to image the proximal coronary arteries in
healthy subjects including MR imaging subtraction
methods,184 185 three-dimensional MR angiograms
formed
by stacking two-dimensional planar images,186
echo-planar imaging,187 and fast spiral MR
imaging.188 Until now, the best results have been obtained
with the use of an ultrafast MR angiographic technique during periods
of breath holding.189 Breath holding is essential to avoid
excessive blurring from respiratory motion. MR imaging methods to
reduce the bothersome signal related to fat ("fat suppression")
are used to improve contrast between coronary arteries and
surrounding epicardial fat. Transverse sections permit assessment of
the left main, left anterior descending, and proximal right
coronary arteries, whereas oblique imaging sections are best
for depicting the left circumflex artery and the more distal segments
of the right coronary artery. As many as 20 to 30 interleaved
segments are acquired, and scan times (breath-holding periods) are
15 to 18 seconds for each image acquisition. In an initial study by
Manning et al,190 19 normal subjects and 6 patients with
coronary artery disease were imaged with the use of
fat-suppressed breath-holding MR angiography (Fig 11
). Imaging
time was approximately 45
minutes. Mean vessel diameter visualized ranged from 2.6 mm (left
circumflex artery) to 6.2 mm (left main coronary artery), which
correlated with quantitative contrast angiography. Mean vessel length
ranged from 8 mm (left main coronary artery) to 122 mm (right
coronary artery). Occluded vessels appeared as absent flow
signal distal to the occlusion, and high-grade stenoses
appeared as signal loss in the area of stenosis with
visualization of the vessel distally. In a subsequent study, Manning et
al191 compared MR coronary angiography with
conventional angiography. In this study in 39 subjects who were
scheduled for elective cardiac catheterization with
coronary angiography, the major epicardial coronary
arteries were classified by MR angiography as being normal (or having
minimal irregularities) or as having disease that was moderately severe
to severe. The sensitivity and specificity of MR coronary
angiography, as compared with conventional angiography, for correctly
identifying individual vessels with
50% angiographic
stenoses were 90% and 92%, respectively. The corresponding
positive and negative predictive values were 85% and 95%,
respectively. The sensitivity and specificity of the technique were
100% and 100%, respectively, for the left main coronary
artery, 87% and 92% for the left anterior descending artery, 71% and
90% for the left circumflex coronary artery, and 100% and
78% for the right coronary artery. The entire procedure,
during which both transverse and oblique imaging were performed, took
about 20 minutes. Pennell et al192 studied 21 healthy
control subjects and 5 patients with coronary artery disease
with segmented k-space gradient-echo imaging, such that a
complete image was obtained in 16 cardiac cycles during one breathhold.
The left main stem (95%), left anterior descending coronary
artery (91%), and right coronary artery were identified in all
subjects, but identification of the left circumflex coronary
artery was more difficult (76%). A good correlation was found between
measurements made by MR imaging and contrast coronary
angiography. Duerinckx and Urman193 found a somewhat lower
overall sensitivity for detecting significant coronary artery
stenoses, which may be due to patient selection, independent
evaluation, and differences in the technique used and experience in
reading MR angiograms.
|
|
From these studies it was concluded that MR angiography provides a new approach to evaluate the patency of coronary arteries. It also was stated that MR imaging of the coronary arteries is at too early a stage to predict its future role. A present limitation of MR angiography is the requirement of a regular sinus rhythm and the need for breath holding during 15 to 18 seconds, although newer methods have already obviated the need for breath holding and possibly for a regular sinus rhythm.194 195 196 Besides, frequent ventricular extrasystoles result in the degradation of the quality of the image. The present spatial resolution and loss of signal due to turbulence preclude accurate prediction of stenosis severity. Recently it has been shown that it is not possible to identify stenotic vessels based on quantification of signal intensity197 ; no significant difference in signal was found between vessel segments of a normal coronary artery and vessel segments proximal to a significant stenosis. The use of faster and stronger gradient coils and improved surface coils may improve spatial resolution and the signal-to-noise ratio in the future. Poncelet et al198 demonstrated the potential of echo-planar MR imaging to detect flow velocity changes in coronary arteries during isometric exercise. These findings suggest the utility of this technique to evaluate coronary flow reserve. It is assumed that if all these advances can be made, MR imaging may become useful for screening the major coronary arteries for significant coronary artery disease.
| Conclusions |
|---|
|
|
|---|
|
At present, MR techniques allow the evaluation of anatomy and function (accepted use), perfusion and viability (development phase), and coronary angiography (experimental phase). A particular strength of MR imaging is that one single cardiac MR test may encompass cardiac anatomy, perfusion, function, metabolism, and coronary angiography. Consequently, MR imaging has greater potential than any diagnostic instrument yet conceived. The expected replacement of multiple diagnostic tests such as echocardiography, nuclear medicine procedures, and diagnostic contrast arteriography with one MR test may have profound effects on cardiovascular healthcare economics. The definite judgments about the relative importance of MR imaging as a valuable clinical diagnostic tool must be settled. For those situations in which MR imaging techniques can replace the conventional techniques, these judgments should be based on large prospective multicenter studies. For MR imaging to have its most substantial impact in detecting coronary artery disease, future technical developments should allow definition of accurate distribution of regional myocardial blood flow during stress in order to assess the ischemic area at risk and the visualization of the coronary arteries with quantitation of coronary flow at multiple cardiac levels. These advances include faster imaging sequences, automated quantification algorithms, three-dimensional angiography, and the development of conventional exercise devices. Many of these improvements are already developed or still under development. If these technical advances will be applied on a large scale, then MR imaging may be primarily useful for screening the major coronary arteries for significant coronary artery disease. In addition, MR coronary arteriography could be used clinically in screening for abnormal origin of the coronary arteries, in diagnosing coronary artery disease in patients who present with chest pain or other suggestive symptoms, in monitoring the progression of disease in patients with known coronary artery disease, and in making decisions about treatment. Because of its safety, it could be used in younger age groups and patients with contraindications to conventional contrast angiography. Particularly, early detection and flow assessment of stenosed coronary arteries by MR angiography using flow velocity will constitute a tremendous progress in clinical cardiology that would far outweigh the cost inherent to the MR imaging procedure.
| Acknowledgments |
|---|
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|---|
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