Essential updates 2020/2021: Current topics of simulation and navigation in hepatectomy

Abstract With the development of three‐dimensional (3D) simulation software, preoperative simulation technology is almost completely established. The remaining issue is how to recognize anatomy three‐dimensionally. Extended reality is a newly developed technology with several merits for surgical application: no requirement for a sterilized display monitor, better spatial awareness, and the ability to share 3D images among all surgeons. Various technology or devices for intraoperative navigation have also been developed to support the safety and certainty of liver surgery. Consensus recommendations regarding indocyanine green fluorescence were determined in 2021. Extended reality has also been applied to intraoperative navigation, and artificial intelligence (AI) is one of the topics of real‐time navigation. AI might overcome the problem of liver deformity with automatic registration. Including the issues described above, this article focuses on recent advances in simulation and navigation in liver surgery from 2020 to 2021.


| INTRODUC TI ON
Advances in perioperative care and surgical techniques have significantly improved the outcomes of liver resection during the last three decades. Liver surgery has inherent challenges, including difficult anticipation of complex and variable intrahepatic anatomy and the need for cognitive analysis by the surgeon to integrate preoperative imaging information into the operative field. Therefore, simulation and navigation techniques have been developed in this field.
This biannual review discusses the essential updates to simulation and navigation in Hx that occurred in the 2-y period from 2020 to 2021.

| PREOPER ATIVE S IMUL ATION
Preoperative 3D simulation has enabled surgeons to obtain a great deal of information, such as detailed anatomical visualization, the precise volume of each segment and each hepatic venous drainage area, and prediction of postoperative liver failure (POLF). As a result, more aggressive and complicated surgeries can be safely performed.
We herein summarize the recent refinements of preoperative simulation in liver surgery from 2020 to 2021 (Table 1).

| Anatomical visualization: 3D printing liver and extended reality
If a 3D liver model including the tumor, each vessel, and the liver parenchyma is created, it is meaningless to display that model on a 2D monitor or printed paper because of the lack of spatial awareness.
Therefore, many reports have described the usefulness of 3D printing of liver models for operative planning or medical education. [21][22][23] Because 3D printing of the liver results in a model of the patient's own liver, accurate information can be obtained regarding the vessel anatomy, the relationship between the tumor and vessels, and the parenchymal cutting plane. Another advantage of 3D printing is that the operator can freely pick up the patient's own liver. The material used for 3D printing is also being developed in various ways.
However, the high cost and complexity of the creation process are undeniable.
Recently, new technologies involving virtual reality (VR), augmented reality (AR), and mixed reality (MR), all of which can be referred to as extended reality (XR), have been developed and applied to various operative simulations. Head mount displays (HMDs) intrinsically provide the user with an egocentric viewpoint and allow the user to work hands-free without a monitor. Especially in VR, the surgeons can be immersed in the patient's own liver. The merits of the application of XR techniques to surgical support include no need for a sterilized display monitor, better spatial awareness, and the ability to share 3D images among all surgeons. 24 XR techniques are applied to preoperative planning or visualization of vessels in liver surgery, [25][26][27] and XR images are suitable for clinical presentation because of their sharing function. Huettl et al 25 compared 3D printed liver models and VR liver models and concluded that 3D VR liver models enable a better and partially faster anatomical orientation than 3D printed liver models. XR technology is still in its early stages.
HMDs should be refined into lighter, simpler, and easier to operate devices.

| Volumetry: Portal perfusion and venous drainage
Preoperative volumetry is essential to ensuring safe hepatic resec-

| Prediction of POLF
Aside from 3D reconstruction or simulation software, also simple liver function simulation is also critical in liver surgery. Preoperative volumetry can estimate the remnant liver volume and predict POLF. 30 Gadolinium ethoxybenzyl diethylenetriamine pentaacetic acid (Gd-EOB-DTPA)-enhanced magnetic resonance imaging (EOB-MRI) can also be used to evaluate liver functional reserve. 31 Functional remnant liver volumetry with signal intensity in EOB-MRI can precisely predict POLF of Hx involving more than one segment. 32 EOB-MRI is also useful for predicting POLF after major Hx for biliary malignancy. 33 In terms of a "one-stop shop" of preoperative simulation, EOB-MRI may be the most useful modality for detecting tumors, simulating vessel anatomy, and estimating remnant functional reserve. Fluorescent navigation using ICG has been clinically applied in various ways for liver navigation surgery. As described above for preoperative simulation, XR techniques have also been applied to intraoperative support systems. Furthermore, artificial intelligence (AI) technology has been introduced to intraoperative navigation.

| INTR AOPER ATIVE NAVIG ATION
We herein summarize the recent refinements of intraoperative navigation in liver surgery from 2020 to 2021.

| ICG staining
Indocyanine green emits a fluorescent wavelength and is clearly visualized when irradiated with near-infrared light (760 nm). In total, 73 articles were found in PubMed in 2020-2021 using the search terms "ICG" and "Liver surgery"; articles describing the intraoperative use of ICG were more limited and can be classified into liver area staining or tumor detection. Recently, ICG has also been widely used in laparoscopic surgery (Table 2).
In 2021, consensus recommendations were established for the use of fluorescence imaging with ICG in hepatobiliary surgery. 34  The usefulness of positive and negative staining of each segment has been fully reported. In a retrospective single-center study of 120 cases, Lu et al 35 reported that ICG staining contributed to a shorter operative time and lower amount of intraoperative blood loss and that it helped to achieve a wide surgical margin.
Furthermore, ICG staining was performed in special types of Hx, such as laparoscopic donor Hx, 36 robotic Hx, 37 and Hx for hepatolithiasis. 38 Kubo et al 39  In tumor detection, ICG is useful to identify not only intrahepatic tumors 43 but also extrahepatic metastatic tumors such as those in the adrenal gland 44 or abdominal wall. 45 Tumor detection with ICG contributes to the safe achievement of surgical margins during liver resection. 46

| XR
In XR techniques, VR is useful for preoperative simulation, allowing the surgeon to become immersed in the patient's own liver with  In total, 27 articles were found in PubMed in 2020-2021 using the search terms "VR/AR/MR" and "Liver surgery" ( Table 3) 55 Operators and assistants can share the same hologram from each angle with HMDs and observe the detailed biliary anatomy around the dissected bile duct (Video S1). A system called a "virtual session" was also recently introduced. Conductor (operator), two assistants and a remote participant, who is not in the operating room, can share the hologram in the metaverse. Conductor explains the operative plan to assistants and the remote participant.

TA B L E 3 XR navigation in liver surgery
We plan to apply this system in the field of remote medical care in the near future (Video S2). Strictly speaking, holograms contribute to "last-minute simulation," not navigation. However, the ho-

| AI
Artificial intelligence technology was recently introduced to surgical navigation. AI should provide image recognition, focusing on anatomical structures, image recognition focusing on the surgical procedure itself, and control against incorrect performance of the surgical procedure. AI can already reportedly recognize the surgical process, 57 surgical instruments such as laparoscopic forceps, 58 and anatomical landmarks 59,60 in cholecystectomy or colorectal surgery.
AI has also been applied to assessment of surgical skill. 61 Nazir et al 62 reported a new searching and tagging system that recognizes various anatomical landmarks in laparoscopic liver surgery. Only one article focused on intraoperative navigation with AI in liver surgery from 2020-2021. AI is not yet frequently used in liver surgery, but future technological applications are expected.
To date, AI has only been used for the recognition of anatomical structures based on information of surgical field images. In the future, some suggestions or attention on anatomical structure information that cannot be directly seen in the surgical field are expected. Furthermore, an integrated analysis of real surgical field images and preoperative modalities should be developed for AI navigation surgery.

| CON CLUS ION
The current status of simulation and navigation in hepatectomy from 2020 to 2021 has been reviewed. Preoperative simulation technology is already almost fully established; the next step is navigation in liver surgery. ICG staining is now widely used for area staining and tumor detection. Some refinements should be developed in terms of positive staining in laparoscopic liver surgery. XR techniques provide amazing new information regarding the liver anatomy, with better spatial awareness; however, the problems of registration and realtime liver deformity remain to be solved. Finally, the development of AI technology is ongoing. The establishment of various simulation and navigation technologies should help surgeons to perform safer liver resection.