Is invasive fractional flow measurement accurate in intracranial stenosis? A computational simulation study

Background In intracranial atherosclerotic stenosis (ICAS), low fractional flow (FF) may indicate hemodynamic significance. Objective To investigate, using simulation models, whether invasive measurement could accurately reflect ‘true’ FF, when the catheter/pressure wire might disturb focal flow during measurement. Methods We recruited 5 patients with high-grade, symptomatic M1 middle cerebral artery stenosis in three-dimensional rotational angiography (3DRA). In each case, the stenotic severity was manually manipulated to 50%, 60%, 70%, and 80%. At each stenotic severity, we simulated four situations: no catheter/pressure wire insertion (‘in vivo’ status); only catheter inserted proximally to the ICAS; catheter inserted and pressure sensor placed at 1 cm and 2 cm distally to the ICAS lesion. The blood flow was simulated with computational fluid dynamics modeling, and FF measured as post-stenotic and pre-stenotic pressure ratio. We calculated relative differences of FFs simulated at other situations compared with in vivo status. Results Compared with in vivo status, catheter insertion had no significant influence on simulated FFs. With pressure wire passing through the ICAS, simulated FFs slightly decreased (mostly <10%) at 50% and 60% stenoses, which significantly decreased (up to 50% and 88%) at 70% and 80% stenoses. The effects of pressure wire on FFs were similar when the pressure sensor was placed at 1 cm and 2 cm distally to the ICAS. Conclusions Invasive measurement of FF may overestimate the hemodynamic significance of ICAS with severe stenosis, as the pressure wire may further reduce the flow across the small residual lumen. The findings warrant verification in larger-scale studies, with information on collateral circulation and validation with other imaging modalities.