# 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  
The effects of the side branches on blood hemodynamics and FFR estimation were further investigated in the OCT-based vessel models from two patients (Figure 6). In patient 1, the side branches at the upstream of the stenosis did not contribute to a significant difference in FFR estimation. In line with the observations from idealized model 2, as most of the side branches are located at the upstream of the stenosed region (denoted by region 1 in Figure 6a), the flow into these branches is likely to cause a minimum effect on the FFR estimation. The flow into the side branch located at the downstream of the stenosed region is likely to have more influence on the FFR. However, due to the smaller diameter (<1 mm) of this branch compared to the diameter (~3 mm) of the main vessel, the effect on FFR is not significant. Figure 6: CFD-derived pressure distribution of OCT-derived vessel tree models with (top) and without (bottom) side branches for (**a**) patient 1 and (**b**) patient 2. Severely stenosed regions are enclosed with dashed lines. For patient 2, the FFR value decreased by 13% when the side branch flow was considered. This difference was more significant than the one for patient 1, which was 2%. There was a higher pressure drop for the stenosis region with side branches located at its downstream (region 2 in Figure 6b) compared with the one without side branches. As the side branches of patient 2 are located at the downstream of the stenosis (denoted by region 2 in Figure 6b), the pressure distribution in patient 2 is more likely to follow the trends of idealized model 3. As depicted using the idealized circuit model 3 (in Section 3.2 using Equation (15)), stenosis located before side branches is subjected to a higher flowrate, leading to an increased flow resistance \(R(q)\) (and pressure drop) across the stenosis. Moreover, the existence of multiple side branches after the stenosis will result in increasing the distal resistance ratio (\(R1/R2\)) between the main vessel and side branches in Equation (15), further contributing to a decrement in FFR compared to a model without side branches. Overall, the FFR (or pressure drop) differences between the models with and without side branches relates to the flowrate differences in the main vessel. When side branches are considered, due to the decreased total resistance, the vessel tree intakes a much higher flowrate, and the flow along the main vessel varies as the flow leaks to the side branches. In contrast, the model without side branches intakes a lower flowrate that remains constant along the vessel. The variations of FFR and flowrate along the main vessel for patients 1 and 2 are illustrated in Figure 7. The simulated FFR variation was compared against the measurements derived from FFR pressure wire pullbacks. For patient 1, the simulated FFR variation in the main vessel with or without the side branches showed a good agreement with the measurements. Here, a significant drop in FFR was seen across the severely stenosed section (denoted by region 1 in Figure 6a). The flowrate across this section (Q34 in Figure 7c) was similar in both models with and without the side branches. For patient 2, the FFR variation in the model with side branches showed a better agreement with FFR measurements than the one without side branches. The model neglecting the side branches at the downstream of the stenosis considerably underestimated the drop in FFR (stenosis region between Qin and Q12 in Figure 6b). Moreover, the flowrate across  
Figure 7: FFR variation in the main vessel compared with guidewire measurements for (**a**) patient 1 and (**b**) patient 2. Flow across the different sections (denoted in (**c**,**d**)) of the main vessel of (**e**) patient 1 and (**f**) patient 2 compared with the flow distribution derived from [PERSON]’s law. The flowrates in the models without side branches are indicated with dashed horizontal lines in (**e**,**f**).