Myocardial scar tissue or myocardial scar is referred to as the final result and pathological correlate of myocardial infarction or myocardial injury and typically develops from the infarcted myocardium.
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Terminology
Myocardial scar tissue is the result of replacement fibrosis and is also called non-viable myocardium even though the latter is a misnomer since it is known that even in myocardial scar tissue there are residual functional cells 1,2.
Pathology
A prolonged reduction of blood flow results in myocardial cell death. The necrotic myocardial zone will subsequently undergo a healing or remodeling process, starting with an inflammatory response, where the necrotic tissue is degraded and resorbed, followed by a fibrotic phase, where myofibroblasts build up collagenous scar tissue and eventually by a remodeling phase, where the scar matures by the steady increase of collagen cross-linking and myofibroblasts undergo apoptosis 1.
This is a gradual dynamic process occurring over the days and weeks, following myocardial infarction and can be divided into three stages 1:
inflammatory phase
fibrotic phase
remodeling phase
In addition to changes in organization and composition, the infarcted tissue faces changes in tissue geometry, characterized by thinning in the radial and lengthening in a circumferential direction. In addition, decreases in infarct surface area and shrinkage were found during the first three weeks indicative of remodeling mechanisms, which can overpower local stresses by scar thinning.
The final result of this on a microscopic/biochemical level rather complex process is a mature myocardial scar.
Myocardial scar tissue consists mainly of fibrillary collagen 1,2, arranged mainly not only in circumferential, concentric layers but in a variety of fiber orientations, collagen cross-linking but also surviving cardiomyocytes, which are widely separated by the fibrillary collagen fibers and cells termed myofibroblasts 2.
Radiographic features
Myocardial scar tissue can be seen with echocardiography and in CT and in particular, be assessed with cardiac MRI.
CT
thinning and decreased contrast-enhancement or rather perfusion of the affected myocardial segment
Echocardiography
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thinning and wall motion abnormalities of the affected segment
hypokinesia or akinesia/dyskinesia depending on the transmural extent
tardokinesis
Strain imaging
decreased systolic shortening or even lengthening of global longitudinal and circumferential cardiac strain parameters
decreased or negative radial strain
MRI
Cardiac MRI or rather late gadolinium enhancement is the gold standard in the depiction of myocardial scar tissue. The increased extracellular space in a tissue mostly consists of extracellular matrix proteins such as collagen and constitutes an increased volume of distribution for extracellular contrast agents. Therefore it can be nicely depicted with late gadolinium enhancement or pictured with T1 mapping and extracellular volume (ECV) mapping since gadolinium-based contrast agents experience a prolonged wash-out period in these sorts of tissues 3-6.
Cine-imaging
Scar tissue itself is non-contractile and shows a passive behavior on pressure and during the systolic contraction of the remaining myocardium, which leads to thinning of the affected segment and akinesia or dyskinesia due to outward bulging and stretching of the scar in a transmurally infarcted segment or to hypokinesia in a subendocardial infarct, where the scar tissue overlying cardiomyocytes still contract.
Signal characteristics
T2/STIR: normal or hypointense
T2 mapping: normal T2 [ms]
T1 mapping: increased T1 [ms]
perfusion imaging: perfusion defect also under rest
IRGE/PSIR: varying degrees of subendocardial up to transmural late gadolinium enhancement (LGE) in the affected area of the myocardium
ECV: increased
Differential diagnosis
Practical points
The complete absence of myocardial scar tissue predicts myocardial recovery in 75-80%. The presence of myocardial scar with transmurality of up to 50% is less accurate in predicting contractile recovery of the affected segment (PPV ~50%), whereas transmurality exceeding 50% predicts a poor likelihood of complete functional recovery of <10% 8.
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