Nobuyuki Takemura , Kyoji Ito, Fuyuki Inagaki, Fuminori Mihara, Norihiro Kokudo

Department of Surgery, Hepato-Biliary Pancreatic Surgery Division, National Center for Global Health and Medicine, 1-21-1 Toyama, Shinjyuku-ku, Tokyo,162-8655, Japan

Keywords:Florescence imaging Indocyanine green Hepatocellular carcinoma Colorectal metastasis Hepatectomy Minimally invasive surgery

ABSTRACT Recently, indocyanine green (ICG) fluorescence imaging has been widely used as a substitute for cholangiography in hepatobiliary surgery, to detect hepatic tumors, for accurate anatomical hepatectomy, and to increase the safety and accuracy of minimally invasive (laparoscopic and robotic) hepatectomy.The clinical relevance of this method has been increasing gradually, as new procedures develop in this field.Various important roles and the latest added value of ICG fluorescence imaging in liver surgery are discussed in this report.

Introduction

The soluble dye indocyanine green (ICG) is used to estimate liver function using a feature that enables rapid fixation of plasma proteins, selective uptake by hepatocytes after intravenous injection, and then is excreted into the bile.The disappearance rate of the serum ICG concentration reflects the excretory function of the liver.ICG fluorescence imaging is another application of ICG.When ICG is illuminated by near-infrared light, it emits fluorescence with a peak wavelength of approximately 840 nm, which lies within the range of the absorbance spectra of hemoglobin (<600 nm)and water (>900 nm) [1].The wavelength of ICG excited by nearinfrared light is not visible to the human eye; therefore, an ICG fluorescence system equipped with interferential filters is applied to obtain fluorescence images.

ICG fluorescence imaging has been used for intraoperative navigation to detect lymph nodes [2], lymphatic flow [3], and blood perfusion during cardiovascular [4]/gastrointestinal [5]/cerebral surgery [6].ICG fluorescence imaging was first applied in hepatobiliary surgery as a substitute for cholangiography during laparoscopic cholecystectomy [7]and then applied to detect hepatic tumors located near the liver surface [8].In recent years, it has been widely applied for various purposes in hepatobiliary surgery,especially to ensure accuracy and safety in hepatectomy.The added value of ICG in liver surgery is reviewed in this article.

Tumor detection with ICG fluorescence imaging

ICG fluorescence imaging detects superficial liver tumors during hepatectomy.In 2009, Ishizawa et al.[8]first found that hepatic tumors or tissue surrounding tumors accumulate injected ICG, and the application of an ICG fluorescence system equipped with interferential filters enabled surgeon to detect these intraoperatively.They also distinguished the ICG accumulation pattern according to the tumor type and differentiation.Well-differentiated hepatocellular carcinoma (HCC) accumulates ICG, which is referred to as the total fluorescent type.In differentiated HCC, biliary excretion disorders lead to the retention of ICG in cancerous tissues.Poorly differentiated HCC and colorectal liver metastasis (CRLM) accumulate ICG in surrounding tissues, which is referred to as the rim fluorescent type [8].In these cases, biliary excretion in tumors surrounding non-cancerous tissue is disordered, which presents a rim fluorescent appearance in near-infrared light ( Fig.1 ).After their discovery of the newability of ICG fluorescence imaging, various studies have reported the effectiveness of ICG fluorescence imaging during hepatectomy.One of the greatest advantages of ICG fluorescence imaging is the ability to detect the disappearing small CRLM after chemotherapy ( Fig.2 ).Recently, this modality has been successfully applied to laparoscopic hepatectomy, which is referred to in the latter section named “Minimally invasive liver surgery with ICG fluorescence imaging”.

Another application of ICG fluorescence imaging for tumor detection is the search for extrahepatic metastases.Satou et al.[9]first applied ICG fluorescent imaging to detect extrahep-atic metastasis of HCC, including lung, lymph node, adrenal gland,and peritoneal metastases ( Figs.3 and 4 ).Inagaki et al.[10]applied ICG fluorescence imaging for the complete removal of metastatic lymph tissue in a case of HCC.Re-examination of the tumor removal site with ICG fluorescence imaging revealed metastatic HCC in the remnant lymphatic tissue, which enabled the surgeon to accomplish complete tumor excision.

Fig.1.Tumor detection with ICG fluorescence imaging (fusion imaging).ICG: indocyanine green.

Although ICG fluorescence imaging is an effective modality for detecting liver tumors near the hepatic surface, there are some limitations to this modality.With the addition of a restricted detection ability only for superficial tumors, Tanaka et al.[11]and Masuda et al.[12]reported limitations of ICG fluorescence imaging for tumor detection in cirrhotic livers.In a systematic reviewand meta-analysis conducted by Purich et al.[13], the combined usage of ICG fluorescence imaging with intraoperative ultrasonography was suggested because the sensitivity of intraoperative ICG fluorescence imaging for superficial tumors is high, but overall sensitivity is low.

Anatomical hepatectomy with ICG fluorescence imaging

Anatomical hepatectomy is a method for the systemic removal of hepatic segments confined by tumor-bearing portal tributaries in HCC [ 14 , 15 ].It is theoretically effective for the eradication of intrahepatic metastases of HCC, as cancer cells from HCC spread through the portal system.An importance of the ICG fluorescent imaging is increasing not only in anatomical hepatectomy for HCC but also in regional resection for CRLM to resect deeply located lesions that are difficult to identify due to shrinkage after chemotherapy [16].To identify the border of a segment in the liver,a blue dye (i.e.indigo carmine) is conventionally injected into a portal venous branch under ultrasound guidance.This technique accurately indicates the portal area on the liver surface, which can otherwise never be visualized.ICG fluorescence imaging applied to anatomical hepatectomy was first reported by Aoki et al.in 2008 [17], based on the same principle as indigo carmine injections in the portal branch.This maneuver is called positive staining method [18]( Fig.5 ).

Ishizawa et al.[18]reported another new method to use ICG for anatomical hepatectomy, which involved confirming the hepatic segmental boundary by applying the same method to confirm the ischemic area on the liver surface after clamping or ligating the portal and arterial blood flow before hepatic parenchymal resection.In this method, ICG is injected systemically via the peripheral subcutaneous vein after clamping the intrahepatic segmental blood flow.Hepatic segments wherein the portal and arterial blood supply is maintained are illuminated under ICG fluorescence imaging;ischemic segments are confirmed as non-illuminated areas.They named this maneuver the negative staining method ( Fig.6 ).In fact,ICG fluorescence imaging detects not only the hepatic segmental border line on the hepatic surface, but also the intrahepatic boundary plane during hepatic parenchymal resection.As Shindoh et al.advocated in 2020, the intersegmental plane of the liver is not always flat [19].The segmental border within the liver may be visualized using ICG fluorescence imaging, thus facilitating accurate anatomical hepatectomy.

In a dye staining method with indigo carmine, it is sometimes difficult to clearly show the segmental boundary on the liver surface in patients who undergo repeat hepatectomy and whose liver is covered with thick connective tissue or when the surface of a patient liver is irregular due to cirrhosis.Furthermore, the intensity of staining is inconsistent depending on the surgeon’s skill,and the dye staining disappears quickly with dilution because it is not taken up by the liver.The advantages of ICG fluorescence segmental staining are its high reproducibility and sensitivity.ICG remains in the injected segment for a fewhours because it is taken up by hepatocytes.The success rates of hepatic boundary identification with ICG fluorescence imaging are reported to be 94%-100% [ 17 , 20 , 21 ].This is superior to the success rates of the dye staining method, especially in patients who have previously undergone hepatectomy and/or have liver cirrhosis [20].On the contrary,the limitation of ICG fluorescence staining might be whole-liver staining due to re-circulation overdose of ICG.Once whole-liver staining occurs, it is difficult to erase the fluorescence.Therefore,re-staining with ICG is difficult.

Assurance of surgical quality with ICG fluorescence imaging

ICG fluorescent imaging was first used as a substitute for cholangiography, as it enabled rapid fixation of plasma proteins,is selectively taken up by hepatocytes, and then excreted into the bile.Kaibori et al.[22]reported that ICG fluorescence cholangiography after the removal of the hepatic parenchyma significantly decreases postoperative bile leak complications.They performed direct fluorescence cholangiography with intrabiliary injection of contrast materials via a catheter which was inserted through thestump of the cystic duct after removal of the hepatic parenchyma that the tumor located in.Confirmation of the bile leak point using ICG fluorescence imaging and repair of the bile leak point by suturing or ligation reduced postoperative bile leakage.ICG fluorescence cholangiography detected bile leakage that cannot be identified using a standard bile leak test ( Fig.7 ).Mizuno et al.[23]first applied ICG fluorescence cholangiography in donor hepatectomy for living donor liver transplantation to detect and preserve the aberrant posterior right hepatic duct originating from the left hepatic duct.Intraoperative ICG fluorescence cholangiography has nowreplaced the role of radiological fluoroscopy with the benefit of avoiding unnecessary radiation exposure of medical staff.

Fig.2.Shrunken small tumors (arrowheads) after chemotherapy visualized using ICG fluorescence imaging.The shrunken tumor that was difficult to identify in the normal light (arrowhead in the upper picture) was detected in the ICG fluorescent imaging (below one in the black and white mode).ICG: indocyanine green.

Regarding another unique application of ICG fluorescence imaging, Kawaguchi et al.[24]applied ICG fluorescence imaging to investigate the decreased functional reserve of the venous congestion area after the resection or clumping of drainage veins.They used the difference in fluorescence intensity, which was interrupted due to venous congestion and subsequent interrupted portal uptake function ( Fig.8 ).The effect of the venous congestion area on liver function remains unclear.Sano et al.[25]evaluated the venous congestion area in liver grafts for liver transplantation.They reported portal flowregurgitation in the venous congested area and the necessity of venous reconstruction in liver grafts.However, the opening of peripheral venous communication released venous congestion in some patients.Mise et al.[26]recommended subtracting the volume of the venous congestion area from the future functional remnant liver volume that is calculated before major hepatectomy.The function of the venous congestion area remains controversial; ICG fluorescence imaging may provide a better understanding of this condition.

ICG fluorescence imaging can be used to secure surgical margins during hepatectomy for hepatic tumors.Zhang et al.[27]re-ported the efficacy of this method by identifying the fluorescence contrast between normal liver and tumor tissue.Tashiro et al.[28]evaluated the surgical margin of a specimen that was resected under ICG fluorescence imaging-guide hepatectomy with fluorescence microscopy.They reported that safe surgical margins were obtained from the tumor edge, and no malignancy was confirmed pathologically in the peritumoral area, demonstrating fluorescence in cases representing rim fluorescence patterns.In a pilot study conducted by Achterberg et al.[29], surgical margins were assessed using ICG fluorescence imaging during laparoscopic and robot-assisted hepatectomy for CRLM.Lu et al.[30]achieved wide surgical margins during laparoscopic hepatectomy with ICG fluorescence navigation compared to that performed without this technique.These studies showed the potential of ICG fluorescence imaging to achieve tumor-negative margins in minimally invasive hepatectomy.

Fig.3.Extrahepatic lymph node metastasis (arrowheads) detected using ICG fluorescence imaging.ICG: indocyanine green.

Minimally invasive liver surgery with ICG fluorescence imaging

Minimally invasive liver surgery, comprising of laparoscopic and robotic hepatectomy, has rapidly prevailed in the past decade.In contrast to open hepatectomy, it is impossible to palpate tumors from the hepatic surface.ICG fluorescence imaging detects superficial tumors with high sensitivity even in laparoscopic [ 31 , 32 ]and robotic surgery [ 29 , 33 ], and can replace a role equivalent to the tumor palpation of open hepatectomy in minimally invasive surgeries ( Fig.9 ).As mentioned in the previous section, ICG fluorescence imaging also helps secure surgical margins during laparoscopic and robotic hepatectomy [ 29 , 30 ].Xu et al.[34]reported the results of positive and negative staining methods for laparoscopic anatomical resection.They mostly applied positive staining in segmentectomy or sub-segmentectomy and negative staining for sectionectomy and hemihepatectomy; however, laparoscopic staining remains difficult and the success rate of staining was 53% [34].Kim et al.[35]indicated ICG fluorescence navigation in pure laparoscopic donor hepatectomy, confirming both hepatic mid-plane dissection and bile duct division with a single injection of ICG.Reduced operation time compared to that before the application of ICG fluorescence imaging as well as lower postoperative liver transaminase levels were obtained as a result of the application of this maneuver.In addition, the application of ICG fluorescence imaging in robotic hepatectomy might reduce postoperative complications and tumor remnant.Marino et al.[33]reported that the use of ICG fluorescence imaging reduces bile leakage, re-admission,and R1 resection.They concluded that ICG fluorescence imaging is a real-time navigation tool that enables surgeons to enhance the visualization of anatomical structures.The application of ICG fluorescence imaging for laparoscopic and robotic surgeries may compensate for some of the disadvantages of minimally invasive surgeries.

Fig.4.Numerous disseminated peritoneal metastases identified using ICG fluorescence imaging.ICG: indocyanine green.

Fig.5.Positive staining method for anatomical hepatectomy using ICG fluorescence imaging.ICG: indocyanine green.

Fig.6.Negative staining method for anatomical hepatectomy using ICG fluorescence imaging.ICG: indocyanine green.

Fig.7.Bile leak test using ICG fluorescence imaging performed after the removal of the specimen.ICG: indocyanine green.

Fig.8.Hepatic vein congested area (arrowheads) visualized as a relatively low-contrast lesion using ICG fluorescence imaging.ICG: indocyanine green.

Fig.9.Tumor detection using ICG fluorescence imaging during laparoscopic heopatectomy.ICG: indocyanine green.

Conclusion

The application of ICG fluorescence imaging has dramatically changed the safety and accuracy of hepatobiliary surgery, even in minimally invasive surgery.Currently, ICG fluorescence imaging has become an essential tool for hepatic surgery.Further improvements to overcome the disadvantages are expected in this procedure.

Acknowledgments

None.

CRediT authorship contribution statement

Nobuyuki Takemura:Conceptualization, Funding acquisition,Writing – original draft, Writing – review & editing.Kyoji Ito:Writing – review & editing.Fuyuki Inagaki:Writing – review &editing.Fuminori Mihara:Writing –review & editing.Norihiro Kokudo:Conceptualization, Writing –review & editing.

Funding

This work was supported by a grant from the Grants-in-Aid for Research from the National Center for Global Health and Medicine(21A1019 to N.T.).

Ethical approval

Not needed.

Competing interest

No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article.