Background: Application of
computational fluid dynamics (CFD) to three-dimensional CTCA datasets has been
shown to provide accurate assessment of the hemodynamic significance of a
coronary lesion. Virtual Functional Assessment Index (vFAI) has been recently
validated against fractional flow reserve and proposed as a possible surrogate
marker of hemodynamic significance of coronary stenoses. Aims: To test the
hypothesis that vFAI is related with regional flow parameters derived by quantitative
PET and can be used to predict perfusion abnormalities downstream stenotic coronary
lesions at CTCA. Methods: Seventy one patients with mean age 62.2±7.9 years
old, stable symptoms and intermediate pre-test likelihood of coronary artery
disease (CAD) underwent CTCA and 15O-water or 13N-ammonia PET stress-rest quantitative
myocardial perfusion imaging. Three-dimensional reconstruction of CTCA datasets
was performed, followed by CFD blood flow simulations using dedicated finite
element software. PET studies were considered positive for significant CAD,
when more than one contiguous myocardial segments showed stress myocardial
blood flow (s-MBF) ≤2.3 ml/g/min for 15O-water or <1.79 ml/g/min for 13N-ammonia.
The corresponding myocardial flow reserve (MFR) thresholds were ≤ 2.5 and ≤ 2
respectively. The extent of perfusion defects in 13N-ammonia studies was also
computed as % of the left ventricle and a reversible perfusion defect involving
≥ 10% of the left ventricle was defined as significant. Results: s-MBF and MFR
measurements were available in 206 vessels’ territories. After exclusion of arteries
because of unsatisfactory image quality, vFAI was computed in 196 vessels. vFAI
was positively correlated with s-MBF measured either with 15O-water or
13N-ammonia (R=0.47 and R=0.53, p<0.001 for both). A positive correlation
was also demonstrated between vFAI and MFR (R=0.40, p<0.001 and R=0.39,
p=0.004) for 15O-water and 13N-ammonia measurements respectively. The optimum
threshold of vFAI for predicting concordant impairment of both s-MBF and MFR
downstream a coronary stenosis ≥30% was 0.8 with sensitivity and specificity values
91% and 82% (AUC: 0.834, 95% CI: 0.730 to 0.937, p<0.001) or 63% and 80%
(AUC: 0.788, 95% CI: 0.622 to 0.953, p=0.019) for 15O-water and 13N-ammonia PET
studies, respectively. The same threshold had sensitivity and specificity 75%,
for predicting a reversible perfusion defect ≥10% of the left ventricle (AUC: 0.813,
95% CI: 0.641 to 0.984, p=0.05). Conclusion: vFAI can be readily computed in
the majority of coronary vessels from CTCA datasets. It is correlated with PET
measured stress MBF and MFR and identifies with reasonable accuracy impaired coronary
flow capacity downstream a coronary lesion at CTCA.