Keywords

1 Introduction

Indocyanine green (ICG) is the most commonly used fluorophore in fluorescence imaging. It is a water-soluble, tricarbocyanine dye that binds to blood lipoproteins and remains confined in the intravascular compartment until elimination. It is selectively taken up by hepatocytes and excreted into the bile. This fluorophore has tissue penetration up to 5 mm and a plasma half-life of 3–5 min with biliary excretion after 15–20 min, thus it is ideal for repeated applications [1].

ICG has several clinically excellent properties, which have been thoroughly verified during its long clinical use: (1) it is nontoxic and nonionizing and therefore has a good patient safety profile; (2) it binds efficiently to blood lipoproteins and does not leak from the circulation, which makes it ideal for angiography; (3) it has a short life-time in the blood circulation, allowing for repeated applications; (4) it offers a good signal-to-noise ratio since separate wavelengths are used for illumination and recording so that only the target, not the background, is visible; (5) it operates in tissue optical window (near infrared) so it provides deep imaging; (6) it is used with simple and cheap imaging devices.

2 First Applications

ICG was initially used in photography, developed by Kodak during World War II for color imaging purposes. Medical applications of ICG were approved by the United States Food and Drug Administration in 1959. In 1960, Fox reported the characteristics of ICG and the results of its use in the Mayo Clinic centers [2]. ICG was originally, and is still, used to determine liver function [3]. In 1963, Walker applied ICG to determine renal blood flow, owing to its fluorescent characteristics. In 1965, Huffman investigated its applicability in detecting cardiac murmurs [4, 5]. ICG has since been applied to evaluate physiological brain perfusion [6]. Several perfusion and angiographic applications have been implemented with the advancement of fluorescence imaging [7, 8]. ICG is also widely applied in off-label use for real-time imaging for abdominal surgery, plastic surgery, and in oncologic staging and treatment [3].

3 Technology and Clinical Rationale

Fluorescence is caused by incident light that excites the target and causes light emission of a particular wavelength. When ICG is excited between 750 and 800 nm, fluorescence is viewed around the maximum peak of 832 nm. The fluorescent emitted light passes through a sensor in the optical device, displaying the green light in real-time visualization. Firefly is da Vinci’s integrated fluorescence capability that uses near-infrared technology activated at the surgeon’s console [9].

Fluorescence-guided imaging has widely evolved the last few years. With the implementation of advanced technologies, surgeons became able to perform more complex interventions with a minimally invasive technique and started to develop an interest in intraoperative imaging applications. ICG has applications in several surgical fields enabling real-time visualization of structures of interest and giving information that normally is uncertain under naked eyes. Tissue perfusion assessment, anatomic distinction, lymphography and other implementations have been described in general surgery, gynecology, urology, colorectal surgery and surgical oncology practice [8].

Robotic-assisted surgery is spreading quickly and has shown to overcome the intrinsic limitations of traditional laparoscopic surgery. Fluorescence imaging was integrated in the da Vinci Robotic Systems (Firefly Fluorescence Imaging Scope; Intuitive Surgical, Sunnyvale, CA) in 2010. Robotic-assisted surgery combined with fluorescence imaging technology represents a logical evolution in image-guided surgery and its benefits are still being discovered.

In general surgery, one of the very first applications was biliary duct identification. Because ICG was excreted into the bile entirely by the liver, its mechanism of enhancement became obvious. In their study, Ishizawa et al. demonstrate that fluorescent cholangiography enables real-time identification of biliary anatomy during dissection of Calot’s triangle, suggesting that this simple technique may become standard practice for avoiding bile duct injury during laparoscopic cholecystectomy, replacing radiographic cholangiography, which is time consuming and may itself cause injury to the bile duct [10].

4 Perfusion Evaluation

Because of its ability to become fluorescent, ICG has been used in several clinical applications to evaluate real-time intraoperative organ perfusion. An amount of 25 mg of ICG is reconstituted in 10 mL of aqueous solvent under sterile conditions and then diluted in 10 mL of an isotonic solution. ICG administration may be performed via a central or peripheral venous line by the anesthesiologist whenever asked by the surgeon and multiple doses can be administered as required, up to the maximum recommended total dose of dye, kept below 2 mg/kg. The literature reports variable doses of ICG being used depending on the patient’s body weight and ranging in most studies from 2.5 mg to 10 mg [3]. At the time of injection, the area of interest should be already exposed and targeted by the surgeon’s endoscope. On the console display, the surgeon activates the Firefly mode to enable infrared light emission and promote excitation and fluorescence of the desired tissue. Intensity peak and washout may be affected by the patient’s circulatory condition as well as by cardiocirculatory inotropes. The optimal time to detect a fluorescence signal varies between 25–60 seconds after administration and the signal peak is around 30–40 seconds after administration, losing intensity within 2 minutes [8] (Fig. 24.1).

Fig. 24.1
3 intra-operative photographs. A. Exposed abdomen region has a Sureform stapler that holds the left colon. B and C. Exposed abdomen region has colon held by a stapler under near-infrared and deep infrared vision.

Perfusion evaluation with indocyanine green before transecting the distal margin during left colectomy. (a) Direct vision under white light. (b) Vision under near infrared (NIR). (c) Deep vision under advanced NIR. The demarcation line is much more evident in NIR modalities

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ICG tissue angiography can guide the identification of the optimal resection site and help to estimate the blood supply; it is used to assess the perfusion of anastomoses and sites before deciding, for example in colorectal surgery, where to resect the bowel.

5 Lymphatic Navigation

Another characteristic of ICG is its lymphatic tropism. Because of its inherent properties, ICG may lend itself to improved mapping rates. After submucosal or subserosal injection, it follows the lymphatic vessels and accumulates in the lymph nodes. In oncologic surgery, this property can be used to map the draining lymph nodes to assure a more precise resection, staging or lymphadenectomy.

In this case, a 1.25 mg/mL solution of ICG should be prepared as for the intravenous application, and administered according to the area of interest. In gastric or colorectal surgery, for example, an endoscopy-guided injection should be performed in four sites of the submucosal tissue around the tumor the day before surgery, for a total of 4 mL. With the Firefly mode turned on, the lymphatic tissue draining from the lesion appears ICG-enhanced on the surgeon’s display and the lymph nodes will be recognized and harvested, providing accurate staging of the disease and better oncologic outcomes [11].

Intraoperative ICG administration through subserosal injection around the lesion is also possible; however, if any spill occurs during the injection, the surgical field becomes blushed and fluorescence image is compromised [8].

6 Clinical Application Experiences

Anastomotic leak is a serious complication in gastrointestinal surgery. Despite technical advances in colorectal surgery, the incidence of anastomotic leaks has remained steady over the past 25 years, occurring in 3–20% of patients who have colorectal surgery [12, 13]. The cause of anastomotic leaks is multifactorial, and these leaks have widespread effects that lead to a considerable clinical and economic burden on the patient and healthcare system, as well as a predisposition to local cancer recurrence [13, 14]. The diagnostic tests available are often unable to identify anastomotic leaks early enough to allow timely intervention and minimize morbidity and mortality [15]. Perfusion is vital for healing, and inadequate blood flow can result in the failure of anastomotic healing and leakage. Adequate perfusion of the anastomosis is commonly confirmed by subjective methods.

ICG perfusion assessment has found a broad field of application in colorectal surgery and is used mainly to assess the perfusion of anastomoses and sites before deciding where to resect the bowel [8].

Bowel perfusion around the anastomotic area can be successfully visualized and quantified using near-infrared fluorescence imaging. Prolonged T0 might be a useful parameter for predicting anastomotic leak in colorectal surgery [16].

ICG use should be divided in two different steps: first, the planned point of proximal and distal transection area just before the bowel resection and, second, after completion of the anastomosis, another course of ICG injection is encouraged to visualize the integrity of anastomosis and its vascularity.

In his study, Kuzdus states that fluorescence imaging is a method that may significantly reduce not only the rate of severe complications in colorectal surgery but also the length of hospital stay [17].

There are of course some open questions about the use of fluorescence imaging to check the anastomosis: (a) timing of fluorescence (before or after transection); (b) time of fluorescence [18]; (c) distance between tissue and camera (5 cm?); (d) camera system (Karl Storz, Olympus, Stryker, Intuitive, Mitaka); (e) ICG dose (0.25 mg/kg–10 mg); (f) anastomotic technique (side-by-side, end-to-end, hand-sewn, stapled); g) anastomotic site (ileocolic, colocolic, colorectal, coloanal).

As stated, ICG can be used in colorectal surgery also for lymph node mapping, in order to recognize and properly harvest lymph nodes, to provide a more accurate lymphadenectomy and achieve better oncologic outcomes [19, 20].

In rectal cancer, in addition to assuring a correct transection line and well-perfused remnant bowel and visualizing the integrity of the anastomosis and its vascularity, if a low colorectal anastomosis is performed, a third optional step by visualization of the rectum and anastomosis mucosa may be achieved with an additional Firefly integrated endoscope via proctoscopy [8] (Fig. 24.2).

Fig. 24.2
2 intra-operative photographs of the exposed rectum area. 2 tip-graspers retract the tissue. Lymph nodes are present in the retracted tissue. The photographs have the same views under 2 different light sources.

Lymph nodes at the root of the mesenteric artery. Vision under white light (a) and under fluorescence (b). Highlighting of the lymphatic basin and lymph nodes after perilesional submucosal injection of indocyanine green, administered transanally, in a case of rectal cancer

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Some studies about rectal cancer surgery confirm that ICG fluorescence imaging is a promising tool that could be of help in clinical practice. It may reduce the anastomotic leak rate in patients undergoing colorectal resection for cancer [21] and it is also associated with fewer postoperative complications and a lower rate of secondary surgery [22].

Assessment of perfusion of the anastomosis is especially relevant in non-anatomic resections, whereby aberrant or altered vascular anatomy can impair perfusion to the remaining colon [14].

Similar considerations might be extended to upper gastrointestinal surgery. ICG tissue angiography might guide the identification of the optimal resection site and help estimate the blood supply of upper gastrointestinal tissue and visceral anastomosis.

Intraoperative evaluation with ICG fluorescence angiography offers a dynamic assessment of tubularized gastric graft perfusion and can guide anastomotic site selection during an esophagectomy, with a reduction of anastomotic leak rates [23, 24]. Zehetner et al. [25] also described lower leakage rates in patients following esophagectomy when the anastomosis was placed in an area of good perfusion after fluorescence imaging.

In gastric cancer, an accurate lymphadenectomy is a crucial prognostic factor.

ICG can noticeably improve the number of lymph nodes harvested and reduce lymph node noncompliance without increased complications in patients undergoing D2 lymphadenectomy [26]. It is helpful for the surgeon not only to identify node stations but also to better discriminate the borders of the dissection, enhancing the recognition of vascular structures and other organs.

In the treatment of achalasia, ICG-guided assessment of the mucosal layer after myotomy during a robot-assisted Heller-Dor procedure has been recently reported to exclude iatrogenic microperforations intraoperatively, but also to visualize ischemic areas caused by monopolar diathermy, which may develop into delayed esophageal perforations with life-threatening consequences [27]. This technique may have relevant potential advantages over intraoperative endoscopy. Moreover, this method may improve the identification of residual fibers, making a more accurate myotomy possible and thus preventing possible relapse of the disease (Fig. 24.3). Finally, its use was linked to shorter operating times.

Fig. 24.3
An intra-operative photograph of the exposed esophagus in an indocyanine green view. A robotic hand holds the structure. The layer over the structures appears slimy. Nerves are visible.

Use of indocyanine green after a Heller myotomy to evaluate the mucosal layer. This method has been proposed as an alternative to intraoperative endoscopy to assess the mucosal integrity and to better identify any residual muscular fiber

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In liver surgery, ICG is still mainly used as a reagent for the evaluation of hepatic function. Also, ICG accumulates in the cancerous tissues of hepatocellular carcinoma and in the noncancerous hepatic parenchyma around adenocarcinoma foci, which may be used to increase detection. Liver fluorescence may be achieved with intravenous peripheral or central access administration or by a local intraoperative injection into the portal vein or right gastric vessels.

A second manner of enhancing the visualization of lesions is by injecting the ICG the day before surgery, which will provide clearance of the substance in the normal hepatic parenchyma with residual stain in the altered tissue area. Also, for a better visualization of the hepatic transection line and perfusion enhancement in major hepatectomies, after clamping or ligation the portal pedicle and arterial branch, ICG administration will also enlighten the remnant liver tissue in contrast to the non-well perfused [8].

This imaging modality is emerging as a navigation tool for resection of metastatic hepatic tumors in laparoscopic hepatectomy. It might help surgeons to safely and accurately identify colorectal metastatic lesions and complete laparoscopic hepatectomies, compensating for the limitations in tactile feedback and intraoperative ultrasound of the hepatic surfaces [28, 29].

Regarding pancreatic surgery, ICG fluorescence can be useful to identify pancreas tumors in patients undergoing pancreas resection, specifically neuroendocrine tumors and cystic neoplasms. Neuroendocrine tumors are enhanced with a higher fluorescence signal compared to pancreatic tissue; on the contrary, cystic neoplasms will display lesser fluorescence intensity compared to normal tissue [8].

7 Future Perspectives

Even if its usefulness is increasingly recognized and its use is expanding in the field of surgery, some aspects of ICG fluorescence still need to be more thoroughly analyzed and developed in the future:

  • Time and volume

    The assessment of administration time and volume is under debate. In published series, there is a wide variety in the reported dilution, time of administration, timing of observation (before or after transection), visualization system used, distance between tissue and camera, anastomotic technique. A standardized protocol is far from being defined and there is a lack of consensus on how to objectively judge the effect of ICG [18].

  • Lymphatic mapping

    Lymphatic mapping and image-guided lymphadenectomy are interesting developments, but we need larger experiences. The number of noncompliant resections may be lowered with ICG fluorescence imaging, but it is still unclear why the dye fails to identify all the lymph nodes. A very interesting perspective could be in the field of molecular engineered selective dyes which will allow selective binding of tissues [30].

  • Artificial intelligence

    Key surgical decisions are traditionally made by human visual judgements. To exclude variability in the judgement of the ICG effect on tissues, spectrophotometric objective evaluation of tissues will be possible in digital platforms (artificial intelligence and augmented reality) [31].