Automatic quantification of the myocardial extracellular volume by cardiac computed tomography: Synthetic ECV by CCT

The quantification of extracellular volume fraction (ECV) by Cardiac Computed Tomography (CCT) can identify changes in the myocardial interstitium due to fibrosis or infiltration. Current methodologies require laboratory blood hematocrit (Hct) measurement - which complicates the technique. The attenuation of blood (HU_blood) is known to change with anemia. We hypothesized that the relationship between Hct and HU_blood could be calibrated to rapidly generate a synthetic ECV without formally measuring Hct.

The association between Hct and HU_blood was derived from forty non-contrast thoracic CT scans using regression analysis. Synthetic Hct was then used to calculate synthetic ECV, and in turn compared with ECV using blood Hct in a validation cohort with mild interstitial expansion due to fibrosis (aortic stenosis, n = 28, ECV_CT = 28 ± 4%) and severe interstitial expansion due to amyloidosis (n = 27; ECV_CT = 54 ± 11%, p < 0.001). For histological validation, synthetic ECV was correlated with collagen volume fraction (CVF) in a separate cohort with aortic stenosis (n = 18). All CT scans were performed at 120 kV and 160 mAs.

HU_blood was a good predictor of Hct (R² = 0.47; p < 0.01), with the regression model (Hct = [0.51 * HU_blood] + 17.4) describing the association. Synthetic ECV correlated well with conventional ECV (R² = 0.96; p < 0.01) with minimal bias and 2SD difference of 5.7%. Synthetic ECV correlated as well as conventional ECV with histological CVF (both R² = 0.50, p < 0.01). Finally, we implemented an automatic ECV plug-in for offline analysis.

Synthetic ECV by CCT provides instantaneous quantification of the myocardial extracellular space without the need for blood sampling.