What is myocardial ECV (Extracellular Volume)?

Myocardial Extracellular Volume (ECV): Definition and Clinical Significance

Myocardial extracellular volume fraction (ECV) is a quantitative measure of the proportion of heart tissue that exists outside of cells, which increases in conditions involving myocardial edema, inflammation, fibrosis, or infiltrative processes. It represents a biomarker that can detect both focal and diffuse myocardial abnormalities that may not be visible with conventional imaging techniques 1.

Technical Definition and Measurement

Myocardial ECV is typically measured using cardiovascular magnetic resonance (CMR) imaging through the following process:

• Calculated from T1 mapping measurements taken before and after administration of gadolinium-based contrast agents
• Calibrated using the patient's blood hematocrit value
• Expressed as a percentage of myocardial tissue volume
• Normal myocardial ECV ranges from 20.4% to 30.4% (mean 25.4 ± 2.5%) 2

The formula for ECV calculation is: ECV = (1-hematocrit) × (ΔR1myocardium ÷ ΔR1blood) where ΔR1 is the change in relaxation rate (1/T1) before and after contrast administration.

Clinical Applications

ECV mapping provides several advantages over traditional late gadolinium enhancement (LGE) imaging:

1. Quantification of diffuse myocardial processes: Unlike LGE which primarily detects focal abnormalities, ECV can identify diffuse myocardial fibrosis or edema 2, 3

2. Diagnostic utility across cardiac conditions:

   • Myocardial infarction: ECV in infarct regions averages 68.5 ± 8.6% 2
   • Hypertrophic cardiomyopathy: Shows elevated ECV that correlates with but exceeds the extent of LGE 2
   • Non-ischemic dilated cardiomyopathy: Can detect abnormal ECV (38.1 ± 1.9%) even when LGE appears normal 2
   • Myocarditis: ECV values range from 39-56% in affected regions 2
   • Cardiac amyloidosis: Shows markedly elevated ECV values (32-60%) 2

3. Component of Lake Louise Criteria II: ECV is included as a T1-based marker for myocardial injury in the updated criteria for diagnosing myocardial inflammation 1

4. Prognostic value:

   • Elevated ECV correlates with worse clinical outcomes, including higher mortality in heart failure 4
   • Independent predictor of adverse outcomes after aortic valve replacement 5
   • The extent of abnormal ECV predicts risk of malignant arrhythmia and heart failure 6

Emerging Technologies

While CMR remains the gold standard for ECV assessment, computed tomography (CT) has emerged as a viable alternative:

• CT-derived ECV shows excellent correlation with CMR-derived values (mean correlation: 0.90) 7
• Particularly valuable for patients with contraindications to MRI 4
• Dual-energy CT techniques demonstrate higher correlation with CMR than single-energy CT 7

Clinical Significance

ECV mapping represents an important advancement in cardiac imaging because it:

1. Provides physiologically intuitive measurements on an absolute scale
2. Detects subtle myocardial abnormalities not apparent on conventional imaging
3. Quantifies the severity and extent of myocardial disease
4. Helps monitor disease progression and treatment response
5. Identifies subclinical myocardial changes, such as age-related fibrosis or remodeling in remote myocardium following infarction 3

ECV measurement should be considered when evaluating patients with suspected diffuse myocardial disease, particularly when conventional imaging techniques show normal or equivocal results.