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MR Assessment of Myocardial Viability
Tae-Hwan Lim, MD, PhD
Professor
Department of Radiology, Asan Medical Center
University of Ulsan College of Medicine
Sang Il Choi, M.D.
Assistant Professor
Department of Diagnostic Radiology
Seoul National University Bundang Hospital
Seoul National University College of Medicine
Myocardial Viability Imaging
Assessment of Myocardial Viability
The loss of cellular integrity is the final step in a cascade of cellular responses
to ischemia and marks the point of no return prior to myocyte death. Multiple
definitions have been used in the assessment of myocardial viability based on
the method used to detect the presence of viable myocytes. For example: 1)
recovery of contractile function following revascularization; 2) response to
inotropic stimulation such as dobutamine echocardiography; 3) presence of
glucose metabolism such as PET; and 4) presence of active cellular transport
mechanisms such as Tl-201 SPECT.
Detection of Myocardial Viability by MRI
MRI has the unique ability to evaluate several markers of myocardial viability
that are of proven value(1-6). Reliable and accurate assessment of myocardial
scar burden, myocardial perfusion, and contractile reserve by MRI are all
becoming well established. With the rapid evolution of MRI techniques,
advances in the assessment of coronary flow reserve and myocardial
metabolism continue to be made.
1. Delayed Enhancement MRI (DE-MRI)
Nonviable myocardium is well recognized with the use of segmented inversionrecovery (IR) prepared T1-weighted gradient-echo sequence from 10-30 minutes
after the intravenous administration of a gadolinium-chelate (Gd). This CMR
technique has been named delayed-enhancement (DE-MRI) and demonstrates
nonviable tissue as "hyperenhanced" or bright.
Animal Study
Kim et al (7) demonstrated that DE-MRI accurately depicts histologically defined
regions of myocardial necrosis in animal model. The majority of recent
published data support the notion that the hyperenhanced regions on DE-MRI
have sustained irreversible ischemic injury.
Human Study
The extensive evidence from animal models of acute infarction has provided a
foundation for numerous patient studies that have confirmed the presence of
hyperenhancement following acute myocardial infarction. The transmural extent
of acute infarction, defined by DE-MRI has been shown to predict the likelihood
of contractile improvement both on a segmental and global basis (8-9). Similar
findings were observed in patients with chronic ischemic disease undergoing
revascularization (10).
Comparison with Other Modalities
Comparison of DE-MRI with other modalities has been favorable. DE-MRI
defined viability correlates closely with that defined by FDG-PET (11-12). The
advantage of the excellent spatial resolution of DE-MRI is in its ability to detect
subendocardial infarction that might otherwise be missed using SPECT and
PET (13).
Clinical Impacts
The DE-MRI technique is rapidly assuming a prominent role in the assessment
of viability, as it has the advantages of being performed under resting conditions
and without patient exposure to radiation. The excellent spatial resolution and
tissue characterization afforded by DE-MRI makes it ideal for accurate
quantification of areas of scar and viable tissue. The clinical utility of DE-MRI in
the delineation of nonviable myocardium has been confirmed by direct
comparison with several clinically established markers of myocardial viability,
including contractile reserve, perfusion, metabolism, and most recently,
electromechanical mapping. The prognostic value for the prediction of functional
recovery has been shown in both acute and chronic myocardial injury.
Further Consideration
Adjunctive information from other markers of viability, such as contractile
reserve with low-dose dobutamine stress MR (DSMR), may help for predicting
functional recovery.
2. T2-weighted MRI
DE-MRI detects myocardial infarction, but cannot necessarily be distinguished
between acute and chronic infarction. T2-weighted MRI has a potential to
differentiate infarct-related myocardial edema as a marker of acute myocardial
injury and fibrosis as that of chronic myocardial injury. Therefore, imaging
approach combining DE-MRI and T2-weighted MRI accurately differentiates
acute from chronic myocardial infarction (14, 15).
3. Myocardial Perfusion MRI
Hypoenhancement in the first few minutes after contrast bolus (no-reflow
phenomenon) is often occurred in patients with patent infarct-related artery after
revascularization. Potential clinical relevance of transient hypoenhancement
after contrast injection (no-reflow phenomenon) has been suggested in terms of
both prediction of functional recovery and prognosis in patients with acute
myocardial infarction containing areas of microvascular obstruction. Rogers et al
(16) observed little long-term functional recovery in those segments with an
initial hypoenhanced pattern early after contrast injection. Wu et al (17) found
that hypoenhancement seen 1 to 2 minutes after contrast injection was a
significant marker of postinfarction complications.
4. Low-dose dobutamine stress MRI (DSMR)
Wall thinning or the absence of thickening at rest may not be a reliable marker
of viability in view of the potential for maintained viability in thinned and akinetic
myocardium in some clinical situations (18). Regional and global contractile
function can be readily assessed using MR methods. Due to its dimensional
accuracy, high resolution, and 3D properties, MR images are ideally suited for
the assessment of left and right ventricular function. Therefore, DSMR has the
advantage of full visualization of the myocardium, whereas echocardiography
suffer from impaired image quality in patients with poor acoustic windows (19).
The presence of contractile reserve can be accurately demonstrated by lowdose DSMR and is a marker for myocardial viability. Low-dose DSMR has also
been reported to be a reliable indicator of viability as defined by the presence of
F18-FDG uptake on PET (20). Segmental wall motion abnormality in
combination with >75% hyperenhancement strongly suggests that the segment
will not recover contractile function. However, the outcome after
revascularization is less clear in dysfunctional segments that show intermediate
degrees of hyperenhancement (>25% and <75%) (10). Recent study showed
that low-dose DSMR is superior to DE-MRI in predicting functional recovery.
This advantage is largest in segments with a delayed enhancement of 1% to
74% (21).
5. Tagged MRI
Tagged MRI has the ability to quantify myocardial deformation and strain
precisely, and to permit a true comparison of contraction not only from region to
region, but also at different levels of function. With DSMR, regional strain
mapping can be used to differentiate between viable but stunned myocardium
and nonviable myocardium (22-23). Recent study showed that DSMR with
myocardial tagging detected more wall motion abnormality compared with
DSMR without tagging (24).
6. Coronary Flow and Myocardial Perfusion Reserve
MRI may have a promising role in reliably measuring absolute coronary arterial
flow and flow reserve. MRI-determined myocardial perfusion reserve can be
assessed reliably and noninvasively in detecting significant coronary stenoses
(25)
Conclusion
MRI provides a unique tool to assess myocardial viability as “one-stop
examination”. (Its overall accuracy appears to be equivalent, and in several
reports, superior to the currently available techniques, including PET imaging.)
Considering the greater spatial resolution compared with PET and the wealth of
correlative pathological data, DE-MRI may well represent the new gold standard
in the detection of irreversibly damaged myocardium. However, the clinical data
to date consist of relatively small numbers of patients, and setting a convincing
new standard will require larger and more definitive clinical trials. Nonetheless, it
is apparent that the full potential of MRI has only just begun to emerge, and its
impact in the assessment of myocardial viability will continue to increase.
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