# Fractional Flow Reserve (FFR) Estimation from OCT-Based CFD Simulations: Role of Side Branches
## 3 Results and Discussion
### FFR Analysis in OCT-Reconstructed Vessel Models
this region (Qin in Figure 7d) was significantly lower in comparison to the model with side branches. The simulated flowrate variation in the main vessel was also compared with flowrate variation derived using [PERSON]'s flow distribution law. [PERSON]'s law assumes the flow is proportional to the third power of the vessel diameter (Q\(\sim\)d\({}^{3}\)), which is derived on the basis of adaptive mechanisms whereby blood vessels remodel to maintain homeostasis according to the shear stress levels on the endothelial surface [29]. This remodeling process is empirically validated [30] and found to be completed within a few weeks [31], and has even been observed in atherosclerotic vessels [32]. The flow distribution derived using [PERSON]'s law showed good agreement with the simulated results, where the simulated flowrates were slightly lower (on average by \(\sim\)7%). This result indicates the applicability of morphometric flow rules to modify the flowrate in the main vessel accounting for the side branch flow for a more accurate FFR estimation. One study has analyzed the impact of side branches on FFR estimation using a computational model derived from angiography images [33]. A linearly distributed, simplified flow leakage from the main vessel was adopted in that study. A significant decrease in the inlet flowrate was reported when the side branch flow was ignored, which agrees with our findings. The same study reported relatively lower FFR values when the side branches were ignored but concluded an insignificant effect on FFR due to side branch flow. This is different from our findings; this is mainly due to the different generic distal resistance values used for the models with and without flow leakage, tuned separately to match the measured FFR values. In contrast, the current study imposed the same distal resistance at the main vessel exit in models with and without side branches for comparison purposes. Moreover, analytical findings in our study suggested that the effect of side branch flow is case-specific depending on the location of the stenosis in the main vessel. Another study [27] has investigated the impact of side branches on hemodynamic indices using vessel tree models reconstructed by the fusion of OCT and angiography images and reported a significantly higher distal coronary pressure to aortic pressure ratio (\(P_{d}/P_{a}\)) in vessel tree models compared to a single lumen model (without branches). However, that study imposed the same inlet mass flowrates in the vessel tree and single lumen model. Hence, the flowrate in the single lumen model is implicitly forced to be higher, which leads to higher pressure drops than the vessel tree model. In contrast, the more realistic distal resistance and inlet pressure boundary conditions were used in the current study to permit the flow to adjust to the model geometry based on distal resistances. This highlights the significance of choosing the appropriate boundary conditions for FFR prediction when the side branch flow is considered.