In plasma, ICG has an absorption peak around 807 nm and an emission peak around 822 nm, which is within the NIR window (Fig. 1). After intravenous administration, ICG has a short half-time of 150 to 180 seconds and is cleared exclusively by the liver [41]. Relatively hydrophobic ICG molecules bind rapidly and almost completely to serum proteins. Protein binding reduces aggregation, increases brightness (i.e., quantum yield) by over 3-fold, and increases effective hydrodynamic diameter to that of the bound proteins [38,42–44]. Hydrodynamic diameter has important implications for distribution and transport of ICG for tumour visualisation and retention in the SLN as discussed below [6,45,46].

ICG is excreted exclusively into the bile, which allows real-time NIR fluorescence cholangiography of biliary anatomy during cholecystectomy and other hepatobiliary surgery [73–75]. This technique provides a reliable roadmap of the biliary tree, which enables the surgeon to avoid injuring the bile duct [73]. In hepatobiliary cancer, it is hypothesised that the NIR fluorescent signal in or around the tumour is caused by passive accumulation due to hampered biliary excretion, which, in the case of a colorectal liver metastasis, results in a fluorescent rim around the tumour (Fig. 3) [31]. To date, five Japanese studies have reported the use of ICG in NIR fluorescence imaging of hepatobiliary cancer including colorectal metastasis, hepatocellular carcinoma and cholangiocarcinoma [31,63–66]. To identify liver tumours, the best time window is beyond 24 hours after injection, when most ICG is washed out of the healthy liver parenchyma and is still present in and around the tumour tissue [31].

Eleven studies report the use of ICG as a NIR fluorescent lymphatic tracer in the SLN procedure in a total of 548 breast cancer patients [3,12–21]. Before the introduction of NIR fluorescence imaging systems, Motomura et al.[48] used only the intrinsic green colour of ICG and identified the SLN in 73.8% of patients. After the introduction of intraoperative NIR fluorescence imaging systems, higher identification rates of 87.5% to 100% (aggregate 98.6%) were obtained and an average of 3.4 (range 1.5 to 5.4) SLNs were identified. Two studies performed an axillary dissection irrespective of the SLN status and found an aggregate false-negative rate of 7.7% in 39 patients with a negative SLN [12,14]. Additionally, as a result of the capability of NIR fluorescence light to penetrate tissue, ICG offers non-invasive imaging of lymphatic flow (Fig. 2). Upon injection of ICG, travel time to the axilla is 1 to 10 min [13,16]. The small size of the ICG particle is probably responsible for this relatively high velocity, which has logistical advantages compared to relatively larger gamma ray-emitting radiotracers. Hojo et al. [15] compared ICG to patent blue in 113 patients and showed that ICG had a higher identification rate (100%) than patent blue (93%). Three studies compared the method of SLN detection by ICG fluorescence (71 out of 73 nodes) and the radiotracer (70 out of 73 nodes) [12,15,17]. Both techniques are used simultaneously in all studies; therefore, both identification rates were similar. However, no comparison can be made as to whether one is superior to the other.

As a result of great variation in perforating vessels and their perfusion zones, it can be challenging to select the optimal vessel to transfer a flap. NIR fluorescence angiography using ICG is able to successfully identify perfusion zones intraoperatively in various types of flaps, including transverse rectus abdominis musculocutaneous flaps, deep inferior epigastric perforator flaps and superficial inferior epigastric artery flaps (Fig. 4) [96–98,102]. In these studies, areas with fluorescence filling defects correlated with poor clinical outcome [97,101,102]. Therefore, intraoperative measurements of the flap perfusion allows intraoperative surgical decision making based on accurate assessment of flap vascularity.