Laparoscopic liver ultrasound anatomy
The liver is an organ poor of anatomical landmarks, not useful to perform modern hepatic surgery; nevertheless, the liver has favourable characteristics for ultrasonography, which thus becomes an indispensable instrument in the surgeon’s hands. Laparoscopic ultrasonography (LUS) transfers to laparoscopy the well known advantages of open intraoperative ultrasonography.
Preoperative imaging provides useful information on vasculobiliary anatomy, but only intraoperative ultrasonography can accurately highlight the Glissonean pedicles and hepatic vein anatomy. For any liver resection, ultrasonography should always represent the first step. LUS allows the surgeon to visualize the anatomical map of the liver that will be used to program and perform the required resection. In addition, any anatomic anomaly or variant in the patient’s liver anatomy must be identified and recognized.
The ultrasound must be carried out prior to any manoeuvring of the liver, as surgical dissection in the presence of pneumoperitoneum causes the diffusion of gas into the anatomical planes, causing the appearance of annoying artefacts. Unlike open surgery, sectioning of the round ligament is not required.
The anatomical exploration can be performed through 2 laparoscopic ports: one for the laparoscope and one for the LUS probe. Trocar positioning varies depending on the programmed resection. For a basic study, a median 10 mm access (umbilical or supra-umbilical) and a left subcostal access for the ultrasound probe are sufficient. By inverting the laparoscope and probe these two accesses allow for a complete liver exploration.
The anatomical study must be systematic and it is therefore advisable to follow a constant pattern. Exploration begins at the hepatocaval confluence. The hepatic veins are hypoechoic vascular structures, surrounded by a thin hyperechoic edge only in the larger-sized vessels. The probe is inserted cranially along the liver convexity until identifying the inferior vena cava and the confluence of the right hepatic vein (RHV) and the common trunk of the left (LHV) and median (MHV) hepatic veins. It is important to recognize not only the 3 main veins but also all the branches that may be involved in the programmed resection, such as the scissural vein, for which it is important to identify the confluence into the LHV or MHV, as well as the independent branches for Sg2 or Sg7, which may be involved in the extraparenchymal isolation of the LHV and RHV.
The LHV and RHV are studied through the left port with a transverse scan, while the LHV is viewed longitudinally. Moving the probe to the median port the MHV can be studied longitudinally. To visualize the RHV longitudinally an additional probe access from the right is required. The right hepatic vein has a relatively constant appearance; however, a small size is always suggestive of the presence of a huge right inferior hepatic vein, which must be constantly looked for; its typical position is posterior to the right posterior Glissonean pedicle. As known, the presence of a major right lower hepatic vein allows for good venous drainage of Sg6 even after ligation of the right hepatic vein.
Once completed the study of the hepatic veins, a scan is made of the vena cava and of its relations with Sg1. It can be studied longitudinally from the central port and with a transverse scan through the left port. Hypertrophy of the left part of Sg1 may be detected, which at times tends to surround the vena cava entirely merging with the parenchyma of Sg7, with a consequent situation of intra-hepatic vena cava which is of difficult isolation during right hepatectomy. After the vena cava, the scan moves on to the hepatic hilum. The vasculobiliary structures are surrounded by a thick hyperechoic rim (caused by the presence of the Glissonean capsule), unlike hepatic vessels, from which they are easily recognizable.
The portal bifurcation and its possible anomalies are easily recognisable, such as the presence of a portal trifurcation or the independent origin of one or more third order pedicles. The biliary convergence is always cranial to the portal bifurcation, and establishes a tight relationship with it. The arterial vessels can also be studied: a left accessory hepatic artery originating from the left gastric artery is easily recognizable within lesser omentum, while a right hepatic artery originating from the superior mesenteric artery runs on the right lateral margin of the hepatic pedicle. The left Glissonean pedicle is then scanned in its horizontal portion, with the characteristic presence of the straight left bile duct, as a cranial element of the triad, until the umbilical fissure and the Rex recess. At this level on the left margin it is possible to recognize the cranial Sg2 branches as well as those of Sg3. On the right margin of the umbilical fissure it is possible to recognize 2-3 pedicles for Sg4 which head cranially to the superior Sg4 (Sg4a) and caudally to the gallbladder for the inferior Sg4 (Sg4b).
To the right, the Glissonean pedicle is shorter and divides early on into the anterior and posterior branches. With the probe through the left trocar it is easy to move from the anterior pedicle to the Sg8 pedicle, which can be viewed longitudinally, while the Sg5 branches can be recognized transversally by sliding the probe towards the gallbladder. The Sg8 pedicles are quite variable, but the ventral and dorsal ones are in most cases recognizable. From the right pedicle it is possible to follow the posterior pedicle. With the probe through the left port it is often possible to visualize longitudinally the Sg7 pedicle. With small movements of the probe it is possible to detect the origin of the Sg6 pedicle, which can be followed until the periphery.
When a Glissonean pedicle is difficult to locate it may be useful to search for it with a retrograde scan, from the periphery towards the origin.