Intraoperative US plays a pivotal role in hepatic surgery, including detection and localization of lesions and planning of hepatic resection. Ultrasound is essential when performing liver-directed therapy for resection or ablation. Although modern cross-sectional imaging such as CT scans and MRIs are highly sensitive, there is still an incidence of additional lesions found by intraoperative ultrasound that are not visualized by preoperative imaging. This incidence can range up to 20% of patients.
IOUS allows evaluation of the relationship between tumor and the hepatic veins, thus allowing for determination of the amount of liver parenchyma that must be resected. Such determination is particularly important in patients with cirrhosis, in whom it is critical to leave an adequate liver remnant postoperatively. In this setting, IOUS allows greater use of segmental resection rather than formal anatomic lobectomy.42,43 The value of IOUS in operative planning has been demonstrated by reports that have revealed changes in surgical management of hepatic tumors due to sonographic findings in 30% to 50% of operations.17,44-47 Facility with IOUS is particularly crucial when considering laparoscopic liver resection, and is considered a mandatory part of the surgeon’s skill set prior to starting a laparoscopic liver program. In addition to aiding in planning for hepatic resection, IOUS may also detect occult hepatic metastases during colorectal surgery in 5–10% of operations.17,48
US is also widely used for guidance in the ablation of hepatic tumors. Thermal ablation, either with radiofrequency (RF) or with microwave (MW) energy has become an important part of the treatment algorithm for patients with unresectable tumors. These procedures may be performed percutaneously, laparoscopically, or at the time of laparotomy. US has become the ideal method for proper localization of the ablation probes, as it gives real-time information on proper placement. Typically, lesions will become hyperechoic as ablation progresses, although assessment of ablation margins by US is limited. These approaches are highly operator dependent, and require a high degree of facility with US on the part of the surgeon.49
Intraoperative US (IOUS) of the hepatobiliary tree can be performed either laparoscopically or via an open approach. The transducers used for IOUS may vary depending on the technique and are typically of higher frequency than employed for transcutaneous ultrasound, generally in the range of 7–8 MHz (Figure 7-9).15-17 The use of such high-frequency probes allows for visualization of subcentimeter lesions. Probes for IOUS are typically either flat or cylindrical. The flat probes provide a wide field of view and are particularly well suited for imaging the liver. Probes with a flexible tip that can be articulated or flexed and extended to allow the user to maintain contact on the curved liver surface for optimal visualization are also particularly useful, especially in the laparoscopic setting.
Ultrasound probes for intraoperative ultrasound.
There are two basic techniques utilized. One is the “lawn mower” technique (Figure 7-10) where the ultrasound transducer is swept back and forth across each sector of the liver in a systematic fashion. The second is the “pedicle tracking” technique (Figure 7-11) where the transducer is used to identify the main portal pedicle and then the left and right branches of the portal venous system are followed out through their sectoral and segmental branching points. Ultrasound is then used to map out the location of the hepatic veins as they track and drain into the inferior vena cava. Rotation of the probe in clockwise or counterclockwise direction, along with superior and inferior angulation (“rocking”), allows for structures to be followed and masses outlined with precision.
Lawnmower technique for ultrasound imaging of the liver.
Pedicle tracking technique for intraoperative ultrasound.
US imaging of the gallbladder and extrahepatic biliary tree is best performed by a saline immersion technique. In this method, the subhepatic space is filled with saline solution. A 7.5 MHz probe is utilized for optimal resolution.
The gallbladder is imaged from the fundus to the infundibulum. Imaging of a normal cystic duct and common bile duct can be challenging by US. Use of the color flow Doppler assists in distinguishing between bile ducts and blood vessels. The presence of dilated ducts is also helpful to the sonographer as it may permit visualization of the cystic duct to its junction with the common bile duct. If this is the case, the ducts are best visualized with the probe and the hepatoduodenal ligament in parallel, allowing the duct to be seen in longitudinal section. The duct can be followed medially to the ampulla of Vater. Compression of the first part of duodenum may be necessary to avoid interference of bowel gas. The common bile duct is seen in either oblique or transverse section coursing through the pancreatic head.
Ultrasound for Hepatic Resections
When planning a hepatic resection, intraoperative ultrasound can be used to determine the intrahepatic vascular anatomy and its relationship to the lesion or region of the liver that is undergoing planned resection. The inflow vessels should be tracked and identified, and the outflow vessels that are planned to be preserved or resected can also be mapped (Figure 7-12). The liver capsule can then be scored along the planned transsection line with an electrosurgical device and the ultrasound transducer can be tracked along the score line. An acoustic shadow from the cautery mark will be visualized on the ultrasound screen as a trajectory of the transsection plane, and this can then be correlated with the intrahepatic vascular anatomy, so that the appropriate vessels can be resected or preserved within that transsection plane and margins of the resection can be anticipated.
Scored liver capsule results in acoustic shadow of planned transection line. Here medial margin of the tumor was pre-ablated, left lobectomy with preservation of middle vein.
Case Report: HCC in Patient with Nonalcoholic Steatohepatitis (NASH)
The patient is a 55-year-old man with a long history of diabetes, obesity, and hyperlipidemia, s/p coronary artery bypass grafting who was found to have abnormal liver enzymes upon routine physical examination by his primary care physician. An ultrasound was then performed, which revealed a markedly fatty liver and the presence of a 2-cm hypoechoic mass in the right lobe of the liver. Follow-up triple phase CT scanning confirmed a hypervascular lesion on late arterial phase with “washout” in the portal venous phase of imaging. This was diagnostic for hepatocellular carcinoma. Additional findings included a normal-sized spleen and no evidence of portal hypertension.
Several treatment options were possible. A cardiac work-up revealed that the patient had an ejection fraction of 40% with areas of hypokinesis of the left ventricle. The patient was cleared for ablative therapies, but not for major surgical resection or transplantation.
The patient underwent laparoscopic radiofrequency ablation of the centrally located HCC under laparoscopic ultrasound guidance. The patient was able to be discharged in postoperative day one after the ablation.
Hepatic lesions are often targeted for either biopsy or treatment with radiofrequency, microwave, or other ablation devices, and intraoperative ultrasound is essential for most accurate targeting. Multiple variables need to be considered when performing real-time targeting with intraoperative ultrasound. These variables include transducer-positioning, hand-positioning, as well as angle of approach. Following identification of a lesion to be targeted, the transducer is usually placed over the lesion and the applicator is then aligned in line with the transducer plane so that it can be followed accurately into the target. It is important to align the transducer in an angle that allows for maximum degree of freedom of the applicator so that adjustments can be made for more accuracy of the puncture. Hand-positioning on the transducer in an open or hand-assisted case is also important. Not only is visual feedback important for the operator in terms of targeting, but haptic senses are as well (Figure 7-13). If one’s hand is placed on the transducer cable or on the back end of the transducer itself, there can be loss of the haptic sense. It is our recommendation that the operator’s fingers are placed directly over the transducer so that the target can then be positioned between the operator’s fingers or over a specific finger to utilize the operator’s haptic sense in addition to visual feedback for more accurate placement of an applicator or biopsy needle.
Holding the transducer cable or back of the transducer leads to less haptic feedback.
When performing a laparoscopic targeting procedure, the operator must fuse input from his haptic sense as well as visual input from the laparoscopic and ultrasound images to form a mental model in three-dimensional space to carry out targeting. It is also important to understand the angles of approach, since during a laparoscopic procedure the abdominal wall as well as the liver represents two fixed points along a line, and opportunities to readjust are limited. Thus, the initial angle of approach needs to be as accurate as possible when placing a biopsy needle or ablation applicator. As the target is approached it is usually easiest to approach the lesion in-plane with the ultrasound transducer, so that the applicator can be seen in its longitudinal access; however, this may not always be technically possible (Figures 7-14 and 7-15). Frequently the operator is forced to approach the lesion out-of-plane, and the more out-of-plane the ultrasound transducer is to the applicator, the less the visualization of the applicator. If the transducer is perpendicular to the plane of the applicator, only a single dot will be seen and the loss of depth and the angle of approach can be compromised. With experience an operator can be able to place applicators safely and accurately out-of-plane.
Fingers on transducer allow for improved haptic sense and more accurate targeting.
When obtaining a biopsy approaching a lesion in line with the transducer provides more accurate targeting.
In order to facilitate targeting of lesions, several advanced image-guidance systems have been developed. These systems have the ability to merge cross-sectional imaging data with real-time mapping of the surface of the liver as well as fusing with ultrasound to give the operator a three-dimensional real-time view of the target and the applicator for ease of approach.