# Fractional Flow Reserve (FFR) Estimation from OCT-Based CFD Simulations: Role of Side Branches
## 1 Introduction  
Pressure wire-based fractional flow reserve (FFR) has been widely used to evaluate the functional significance of a stenosis in epicardial coronary arteries, which is defined as the ratio between the mean distal coronary pressure of the stenosis (Pd) to the mean aortic pressure (Pa) during maximal hyperemia [1]. FFR-guided percutaneous coronary intervention (PCI) has shown more favorable outcomes compared with angiography-guided PCI [2]. However, hyperemic FFR is associated with additional time, cost, and more importantly, unpredictable systemic blood pressure changes and vasodilation side effects causing chest discomfort [3]. Moreover, invasive FFR shows methodological deficiencies in assessing serial lesions and early post-stented acute coronary syndrome culprit lesions. Image-derived FFR estimation is more convenient and highly promising [4; 5; 6; 7; 8]. Precise model reconstruction of vessel geometry is paramount in image-derived methodology, as the ambiguity in the lumen diameter has proven to have the largest impact on FFR computation [9].  
Intravascular optical coherence tomography (OCT) provides much superior resolution compared to other imaging modalities [10] and has proven to deliver accurate lumen measurements with good reproducibility [11]. Several studies have employed OCT-derived vessel geometries to compute FFR by application of computational fluid dynamics (CFD) [12] or simplified fluid dynamics equations [5; 13] and delivered reasonable accuracies (~85-87%) in classifying the vessels conferring to the diagnostic threshold (FFR \(\leq\) 0.8). However, the quantitative agreement against the measured FFR was limited (with correlation coefficients ~0.7 and SD of Bland-Altman plots ~0.05-0.09), which can lead to difficulties in clinical decision making for vessels with intermediate FFR values. As OCT pullback apparently does not include the side branches' geometry, the effect of the side branches is essentially ignored [12; 13], or to a certain degree [5] in most of these studies. The study [5] assumed a varied flowrate along the vessel (derived proportionally to the reference lumen area) to indirectly account for the side branch flow and suggested an insignificant improvement in FFR prediction. Another study [14] combined CFD modeling of OCT-reconstructed vessel segments with a lump parameter model (LPM), which includes the effect of side branch flow and reported a better correlation coefficient (0.82) with FFR measurements. However, in these studies, the comprehensive analysis of the influence of the side branches on the FFR estimation is still lacking.  
This study aims to inspect the influence of side branches on FFR estimation using OCT-reconstructed vessel models. First, the influence of the side branches on FFR was systematically studied with three idealized artery models and corresponding circuit models (analogous to fluid domain). The mechanistic understandings from the idealized models were further validated with two representative patient-specific models against the clinical FFR measurements. The findings of this study can help to enhance the accuracy of FFR estimation based on OCT images.