Extracellular volume - myocardium

Extracellular volume (ECV) refers tothe space or volume of a tissue, which is not occupied by cells. Apart from the usual extracellular space, which surrounds the cells of a specific tissue it also includes the intracapillary plasma volume ^1,2. It measures the space, which is occupied by the extracellular matrix ^1-4.

Measurement

A series of native T1 and postcontrast T1 images an acquired and co-registered. This can be done by an equilibrium infusion or after a bolus injection after waiting for a sufficient amount of time (15min) ^1-5. T1 values can then be calculated pixel-wise from a signal intensity versus time curve fitting model ². The administration of gadolinium leads to a shortening of the T1 value. Extracellular volume can be then calculated using the following formula ⁴:

ECV = (1-hct) [(1/pT1_my - 1/nT1_my) / (1/pT1_bp - 1/nT1_bp)] 

nT1 = native T1,  pT1m postcontrastpT1 = postcontrast T1, my = myocardium, bp = blood pool, hct = haematocrit

Interpretation

It is of interest in the myocardium as a biomarker for interstitial disease. In the absence of any diffuse infiltrative disease or myocardial oedema, it can be also considered as a biomarker for myocardial fibrosis ^1-4.

Apart from fibrosis, it is elevated in the following myocardial diseases ^1-4:

• cardiac amyloidosis
  • ATTR-amyloidosis – prognostic value
  • AL-amyloidosis
• myocarditis
• acute myocardial infarction / myocardial necrosis
• myocardial scar tissue/replacement fibrosis 

Low values can occur in ⁴:

• lipomatous metaplasia/fat
• cardiac thrombus

Influencing factors

Unlike native T1 extracellular volume (ECV)seems not to be dependant on field strength, but similar to native T1 it varies between systole and diastole ^2,4. There are only minor differences in regard to the myocardial region ⁶.  

See also

• T1 mapping
• myocardial mapping