Established and Emerging Applications of Magnetic Resonance Late Enhancement Imaging in Cardiology
Frank Grothues Director, Echocardiography Service, University Hospital Magdeburg
Cardiovascular magnetic resonance (CMR) imaging, with its versatility
and ability for soft tissue characterisation in conjunction with the lack of
ionising radiation, has over the past decade evolved as a first-line
imaging tool for several diagnostic problems.1 Developments in this
field, especially the introduction of the ‘late enhancement’ (LE) imaging
technique by the working group of Kim and Judd2,3 in the late 1990s,
have led to a widespread clinical acceptance of CMR. For this technique,
gadolinium-based contrast agents are used that act via a shortening of
T1 relaxation and cannot enter normal myocytes with intact, selectively
permeable cell membranes; hence, they are restricted to the
extravascular interstitial space. The loss of cell membrane integrity, for
example due to irreversible ischaemic injury, enables the contrast agent
to enter the intracellular space and consecutively increases its volume of
distribution. In addition, contrast wash-out kinetics of damaged
myocardium are also delayed.4–7 By the use of a dedicated pulse
sequence (the so-called inversion recovery gradient echo technique)
10–30 minutes after contrast administration, it is possible to null the
signal of normal myocardium and exaggerate the contrast between
viable tissue and the gadolinium-enhanced scar region. While initial
applications of this technique focused exclusively on ischaemic heart
disease, other forms of cardiomyopathies and systemic disease have
recently been investigated. This review aims to cover both established
and still evolving applications of LE imaging.
Late Enhancement in Coronary Artery Disease
Global left ventricular (LV) systolic function is a prognostic factor in
patients with coronary artery disease.8–11 Revascularisation of
dysfunctional but viable myocardium has shown to improve global
function,12–13 clinical symptoms14 and patient outcome.15 In contrast, in
the absence of viable myocardium revascularisation, procedures carry a
risk of a higher rate of death and non-fatal events.15 Thus, the
discrimination of myocardial dysfunction due to infarcted myocardium
with fibrosis and scar tissue – due to chronically hypoperfused but viable
myocardium (so-called hibernating myocardium) is of pivotal clinical
importance. Various non-invasive and invasive techniques have been
evaluated for their usefulness to distinguish reversible from irreversible
damaged myocardium, with nuclear medicine techniques such as single
photon emission computed tomography (SPECT) having gained the
widest clinical acceptance. Positron emission tomography (PET) until
recently has been regarded as the gold standard in non-invasive viability
assessment.16–18 Nuclear techniques, however, carry important
limitations. They have limited spatial resolution, expose the patient to
substantial ionising radiation and, with regard to PET imaging, are not
widely available and are highly expensive. Apart from the lack of
radiation, CMR is becoming increasingly attractive because of its three- to
five-fold higher spatial resolution19 and its ability to allow for
simultaneous evaluation of regional wall motion, myocardial perfusion
and associated cardiac pathology such as valve disease, presence of
pericardial effusion, etc. Several studies have compared the LE technique
with SPECT20–23 or PET.24–26 Concordantly they showed a close agreement
to nuclear imaging with a superior performance of CMR in the detection
of small and very small subendocardial infarcts. For example, in a study
by Wagner et al.20 SPECT was unable to detect a fixed perfusion defect
in 47% of segments with less than 50% transmural extent of LE.
The close correlation between the extent of hyperenhancement and
infarct size in histopathology has been extensively validated.27–30
Furthermore Rehwald et al.31 could demonstrate that reversible injured
myocardium does not enhance on LE images. The LE technique has
shown an excellent reproducibility32 and several studies could
demonstrate its potential for predicting myocardial contractile reserve
after revascularisation.19,33,34 Kim et al. studied 50 patients with ischaemic
dysfunction before and after revascularisation with cine and LE. They
applied a segmental approach with grading of the transmural extent of
LE and wall thickening on a five-point scale. While a single cut-off point
for prediction of functional recovery could not be defined, an increasing
LE transmurality gradually reduced the likelihood of functional recovery
after revascularisation. Notably, none of the segments with at least severe
hypokinaesia and a transmural extent of hyperenhancement of 76–100%
showed improved contractility at follow-up.
Assessment of Infarct Tissue Heterogeneity
In addition to the sole detection of myocardial scarring, CMR LE imaging
can take infarct imaging a step further. Frequently hypoenhanced regions
surrounded by hyperenhanced tissue can be seen within the infarct zone,
which resemble vital myocardium (see Figure 2). These areas have been
identified as regions of microvascular obstruction (MVO) that at the time
of image acquisition have not yet been reached by gadolinium.35,36 Initial
studies have associated the presence of MVO with adverse outcome.35,37
Wu et al.35 followed 44 patients with myocardial infarction and observed that the 11 patients with MVO had more cardiovascular events (death,
reinfarction, congestive heart failure or stroke) than those without (45%
versus 9%; p=0.016). Furthermore, microvascular status remained a
strong prognostic marker even after control for infarct size. These results
have been confirmed by Hombach et al.37 in a CMR study of 110 patients
early after myocardial infarct. Multivariable analysis revealed LV enddiastolic
volume, LV ejection fraction and MVO as significant predictors
for the occurrence of major adverse cardiac events.
