Myocardial scar tissue

Myocardial scar tissue is referred to as the final result and pathological correlate of a myocardial infarction, whichand develops from the infarcted tissue.

Terminology

Myocardial scar tissue isalso called ‘nonviablenon-viable myocardium’ even though the latter is a misnomer since it is known that even in myocardial scar tissue there are remainingresidual functional cells ^1,2.

Pathology

A prolonged reduction of blood flow results in myocardial cell death and eventually the replacement of the infarcted myocardial tissue with collagenous scar tissue 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
• remodelling 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 remodelling mechanisms, which are able to overpower local stresses by scar thinning.  

The final end 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 but not only in circumferential, concentric layers but in a variety of fibre orientations, collagen cross-linking but also surviving cardiomyocytes, which are widely separated by the fibrillary collagen fibres 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

• 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 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 volume (ECV) in a tissue mostly consistent of extracellular matrix protein as collagen constitutes an increased volume of distribution for extracellular contrast agents and thus to 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 sort of tissues ^3-6.

Cine-imaging

Scar tissue itself is non-contractile and shows a passive behaviour on pressure and during the systolic contraction of the remaining myocardium, which leads to thinning of the affected segment and to 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

LGE: varying degrees of subendocardial up to transmurallate gadolinium enhancement (LGE) in the affected area of the myocardium

ECV: increased

Differential diagnosis

• acute myocardial infarction

See also

• myocardial fibrosis
• myocardial tissue characterization
• myocardial mapping
• extracellular volume (ECV)
• late gadolinium enhancement (LGE)