## 4 Discussion
However, recent developments such as manual gain settings, protocols for fluorescent assessments that include tracer dosage, and standardization of distance aim to quantify and calibrate the results to some extent in the future. As shown in this study, the periphery-center effect impacts quantification outcomes if one is not aware of this while choosing ROIs (i.e., the fluorescent intensity may decrease significantly in the periphery, also represented by the FSL%). In clinical use, even though the camera is likely to be focused on the ROI at its center, one should take this into account and possibly correct for it. As the number of commercially available optical imaging systems has greatly increased, this technology is now being used by more inexperienced surgeons who occasionally lack knowledge of the fundamental principles and pitfalls of optical imaging, making it difficult to interpret imaging data in a way that is accurate, precise, reproducible, and reliable. Raising awareness that using and comprehending optical imaging modalities requires training and is, therefore, a learning curve, could lead to the organic beginning of this new class of trained surgeons; the interventional imaging surgeon. This is not novel because it is comparable to traditional imaging techniques, which today each have their own class of medical professionals, namely the nuclear physician and the radiologist. However, being able to compare fluorescent thresholds between patients and/or systems is a future goal as well as a challenge. The flat field corrected illumination differencesand optical behavior of the NIR imaging devices in these studies can be incorporated together with patient data in (deep learning) algorithms to automate quantitative/artificial intelligent ICG perfusion angiogram classifications to facilitate its interpretation for all clinicians, not only the prior mentioned interventional imaging experts. These algorithms incorporated in robotic or laparoscopic consoles might propose resection lines after NIR assessment on the live tissue based on patient-, optical-, fluorescent characteristics. This study is limited first for not truly mimicking the clinical situation, as the phantom is entirely flat. However, correcting for different angulation, depth, and tissue morphologies is hard to summarize in one formula. Second, it was conducted using an ICG phantom, however, both the methodology as the message of this study apply to other fluorescent tracers of the same emission peak. In addition, during this study we chose to perform all the assessments with one fixed distance so as to have the entire phantom in the FoV. To achieve this, a distance of 16 cm was set, however, during laparoscopy, especially in patients with a low BMI, a smaller distance is conventional and usually in the range of 10 cm. That said, the scope of this paper was not to mimic clinical use but to demonstrate the light distribution and impact of angulation. The experiments were executed in a completely dark environment as is the case in an abdominal cavity, and we assume that this has no influence on the correction, given that the image would have only 'zero' intensity pixels. Ideally, a fluorescent phantom offers comprehensive information on multiple systems' parameters with only one snapshot, but this phantom only addresses illumination and camera sensitivity. This study is also limited in assessing only the commercial camera systems we had at our disposal, whereas clinical practice is dependent on the fluorescence systems hospitals have available. In conclusion, there was a considerable center-periphery effect within an FoV with sometimes not even peak intensity in the middle of the FoV, and this differs among systems. Other phantoms with only a few wells do not capture these illumination differences entirely, these features should be considered and if possible corrected for while interpreting the fluorescent signal clinically and before selecting ROIs for quantification.