## 4 Discussion  
Four optical techniques were tested intraoperatively for a quantitative perfusion assessment of the gastric conduit in oesophageal cancer patients undergoing resection. The perfusion restriction towards the fundus was quantitatively assessed and was significantly objectified in 10 out of 14 parameters. At the fundus of the gastric conduit (Loc4), the FA parameters T\({}_{1/2}\) and mean slope significantly differed between the patients with or without anastomotic leakage. The gastric conduit is mainly vascularised by the right gastroepiploic artery, which usually terminates before the future anastomotic site, making the future anastomotic site prone to vascularisation problems. Therefore, the gastric conduit is an adequate model for perfusion, as perfusion restriction is expected towards the gastric fundus. Quantitative data from this study indeed showed perfusion restriction towards the fundus. Ten quantitative parameters significantly differed between Loc1 and Loc4, of which seven decreased towards the fundus. Comparing those to the literature, the Flux and F\({}_{\text{max}}\) are described to indicate perfusion restriction towards the fundus [19; 20]. Three parameters (\(\Delta\)%Flux, influx time point (\(\uptau\)), and T\({}_{1/2}\)) increased towards the fundus, indicating decreased perfusion as well. Influx time point (\(\uptau\)) and T\({}_{1/2}\) measured the time-dependent change in fluorescence. In the literature, the time-dependent change in fluorescence intensity is a potential method for FA quantification, and increased \(\uptau\) and T\({}_{1/2}\) might indicate inadequate perfusion [19; 21; 22; 23]. An increase in these values might be due to either diffusion through tissue instead of an intact arterial route or to resistance in the vascular network in the case of venous outflow obstruction. Quantitative parameters were compared between patients with and without anastomotic leakage at Loc4. In patients with anastomotic leakage, a significantly higher T\({}_{1/2}\) and significantly lower mean slope were found. T\({}_{1/2}\) and mean slope provide information about the ICG influx velocity, amount of vessel density, and potentially the outflow. T\({}_{1/2}\) and mean slope are potentially hands-on parameters for intraoperative perfusion evaluation and anastomotic leakage prediction [19]. At Loc4, in contrast to an expected decrease, the SDF parameters and Flux appeared to be higher for patients with anastomotic leakage compared with patients without leakage. This increase might be explained by venous outflow obstruction. SDF measures the velocity of RBCs using a space-time diagram. The Flux measured by LSCI is dependent on the amount of blood particles and movement of these particles. In the case of venous outflow obstruction at the fundus of the gastric tube, veins swell to 5-10 times their original size [24; 25]. The SDF parameters and Flux are possibly influenced by the number of RBCs or blood particles in swollen veins and therefore could be higher at Loc4 in the case of anastomotic leakage secondary to venous outflow obstruction. No differences were observed at Loc4 for the OCT parameter speckle %. This explorative study allowed the comparison of four optical techniques in terms of their quantitative assessment and imaging characteristics. Both OCT and SDF have a limited field of view focusing on microperfusion, while LSCI and FA are wide-field (considering the heterogeneity of the capillary network). Wide-field techniques allow the immediate overview of entire gastric conduit, and potentially show vascular confines visually, which is valuable for clinical determination of the anastomotic site. Besides, the time consumption for imaging seemed lower for wide-field techniques. OCT, LSCI, and FA are all non-contact techniques, thereby avoiding stabilising problems. Only OCT is depth-resolving (depicting individual overlying vessels). In terms of quantification, quantitative SDF, LCSI, and FA had acceptable success rates.