103 Simultaneous fluorescence imaging of bowel perfusion and ureter delineation using methylene blue INTRODUCTION Anastomotic leakage (AL) is one of the most dreaded complications after colorectal surgery. Probably the most important cause of AL is impaired perfusion of the bowel. Assessment of bowel perfusion is therefore one of the crucial strategies in reducing the incidence of AL 1, 2. Another feared complication during colorectal surgery is ureteral injury. In order to prevent iatrogenic damage, the surgeon must be aware of the exact location of the ureter. Intraoperative near-infrared fluorescence imaging (NIRF) with preoperative optical dye administration is a technique for quick and easy intraoperative visualization of the ureter 3-5 and for an improved assessment of anastomotic perfusion 2, 6-11. However, to date there is no clinical study which evaluates simultaneous fluorescence-enhanced ureteral delineation and intestinal perfusion in the same surgical procedure over time and the possibility of using one single dye. Over the last decade, (pre-)clinical studies have been performed to visualize the ureter. Due to the exclusive clearance of indocyanine green (ICG) by the liver, it is not suitable for ureteral imaging since it is not cleared in the urine. Methylene blue (MB) on the other hand, a clinically approved and widely used dye, is excreted by the kidneys and can consequently be administered for non-invasive ureteral imaging. However, results of clinical and pre-clinical experiments investigating the feasibility of MB for ureteral imaging have shown conflicting results regarding its added clinical value 3, 4. This may be due to the characteristics of the dye itself, having only a weak fluorescent signal, or to the laparoscopic equipment used. The latter refers to a disadvantage of MB, which is excited at ~670nm, in contrast to other dyes such as ICG which is excited at ~800nm. As a result, the use of MB requires specifically developed equipment. The vast majority of imaging systems used in the studies with MB thus far were experimental and not commercially available for clinical use 5. In colorectal surgery, there is a need for simultaneous non-invasive ureteral imaging and bowel perfusion assessment. The latter can be achieved by finding a single dye that can simultaneously identify these structures, or an adequate NIRF imaging system that can simultaneously identify these structures with two different dyes. Our group has already successfully studied and reported on the first approach 12. However, this was a pre-clinical study that is not yet ready for clinical implementation. The use of two dyes simultaneously for ureteral imaging and bowel perfusion imaging has become potentially feasible now that a commercial imaging system is available for such an approach. The aim of this study was to investigate the feasibility of simultaneous intestinal perfusion and ureteral imaging using a single commercially available NIRF imaging system. 5
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