Image-guided surgery (IGS) involves patient-specific anatomical images pre-operatively acquired that spatially localize pathology, digitization technology that allows the identification and tracking of targeted points of interest in a patient's physical space in an operating room (OR), and alignments of the patient-specific images to the patient's physical space in the OR such that the digitization technology can be referenced to the patient-specific images and used for guidance during surgery. Central to the IGS is the method of registering an image space (a coordinate system corresponding to the pre-operative images) to a physical space (a coordinate system corresponding to the intra-operative anatomy of the patient). Once the registration is performed, all pre-operative planning and acquired data related to the patient's anatomy could be displayed intra-operatively to a surgeon and used for assistance in surgical guidance and treatment.
In the recent years, there have been several detailed studies that have illustrated the need for soft-tissue deformation correction within image-guided neurosurgery [1-5]. With respect to the extracranial environment quantitative data is limited, nevertheless, there is a growing acceptance that to translate image-guided interventions to the extracranial environment (e.g. abdominal organs such as the liver), the need to correct for soft tissue deformation may be essential. While intraoperative magnetic resonance (iMR) and computed tomography (iCT) are available, these approaches are somewhat cumbersome and are not economically scalable to mid-level medical centers.
On the other hand, hepatic tumors represent a major health care problem in the U.S. Along with hepatocellular cancer, many primary neoplasms metastasize to the liver. In fact, the most common tumor treated in the liver is metastatic colorectal carcinoma, a condition where hepatic metastasectomy can result in long-term survival in properly selected patients. This is a frequent problem and the incidence of hepatic resection for colorectal cancer metastasis is increasing [6, 7]. For example, it is estimated that 150,000 new cases of colorectal cancer will present each year in the United States, from which 50% will develop metastatic disease [8]. Of patients with colorectal metastases, 60% will have hepatic involvement. Liver metastases occur in many patients with other primary malignancies (e.g. breast cancer) and are a frequent cause of cancer-related deaths. Unfortunately, it is estimated that there are only 6,000 to 12,000 patients who would be deemed resectable using current techniques [9]. These figures do not include the nearly 19,000 patients with primary intrahepatic malignant tumors [10]. Interestingly, in reviewing the patient base with potentially resectable colorectal metastases, the number of candidates who actually undergo resection is surprisingly low. This discrepancy may result from many factors, but is most likely influenced by the magnitude and complexity of hepatic resections as it iscurrently performed. Surgical therapy does improve survival for patients with hepatic colorectal metastasis and is largely considered to be more effective than chemotherapy [8, 9].
In these procedures, a large incision through the abdomen is created to expose the anterior surface of the liver. Either wedge or segmental liver resections are performed to remove one or more hepatic metastases. In wedge resections, the tumor and a 2-3 cm surrounding region of the liver is removed, while in segmental resection, an entire anatomic segment of the liver is removed. Each of the eight segments of the liver is supplied by its own portal venous and hepatic arterial pedicle [11]. FIG. 1 shows the anatomy of the liver.
Because liver metastases are likely blood-borne via the portal vein, for many years it was assumed that there was no advantage in wedge over segmental resection; the type of procedure chosen depended greatly on the location and number of metastases [12]. However, a better understanding of liver anatomy and improvements in radiologic imaging and anesthetic techniques has led to the widespread use of segmental liver resections. Segmental hepatectomy (i.e. segmentectomy) based on the anatomic descriptions of Couinaud is appealing for several reasons. First, blood flow to a segment is often stopped prior to transection of the liver. The resultant change of the liver color indicates the boundaries of the segment and ensures an adequate margin of normal tissue throughout the procedure. Second, segmentectomy may be used to preserve liver substance in cases that would otherwise require a resection of the entire right or left lobe. For this reason, segmental resection is suitable for the treatment of colorectal liver metastases. Removal of the tumor with an adequate margin is sufficient because intrahepatic metastases (i.e. tumor satellites) from an established colorectal liver metastasis are rare [13]. Two factors contribute to inadequate tumor clearance following non-anatomic wedge resections. First, traction on the specimen during division of the liver parenchyma can produce a fracture at the interface of the soft liver tissue and the hard colorectal metastasis. Second, lack of vascular control during a wedge resection can cause bleeding at the base of the wedge resection. This bleeding may obscure the transection and compromise the final margin [14].
When colorectal metastases are confined to the liver, five year survival rates after resection range from 30-35%. While morbidity is affected by blood loss, OR time, and residual liver volume, the prognosis is dependent on the presence or absence of adequate margins, regardless of the type of resection chosen. If positive resection margins are present following surgery (i.e., tumor cells are still present in the remaining liver after the tumor was excised), five year survival rates range from 0-18%, and few patients remain disease-free beyond 20 months. Five year survival rates in patients with negative margins of less than 1 cm range from 1826%, significantly worse than the 44-50% survival rate seen in patients with negative margins greater than 1 cm. In further comparing wedge vs. segmental resection in one study with over 260 patients, the rate of positive margins present following wedge removal was 8 times higher than with segmentectomy [14].
As more liver is removed in a multi-segment or lobectomy procedure, however, there are complications. Although liver resection has shown promising survival rates and a perioperative mortality rate of less than 5%, a significant increase in postoperative morbidity due to hepatic dysfunction and infection has been reported, even by specialized centers [15-17]. It was suspected that as more major liver resections were being conducted, removal of more significant amounts of liver tissue could cause complications. The paradigm that at least one third of healthy liver volume should be left to avoid hepatic failure following resection has been a standard for many years, but until recently, data did not exist to support this claim. In a study of over 100 patients that studied hepatic dysfunction and infection after major liver surgery [18], Schindl et al. used a regression analysis to demonstrate that dysfunction increases significantly when the relative residual liver volume (RLV) was below 26.6%. Body mass index (BMI) was also a highly significant factor; note that although prolonged operating time and blood loss were not as significant, they did improve the regression model fit. In the 103 patients reviewed for this study, 69% underwent either a standard right lobectomy (48) or an extended right lobectomy (23). On average, these patients had an RLV value of 33.3%, which is above but within one standard deviation of the 26.6% cutoff. The authors believe that calculating a specific % RLV before major liver surgery from a virtual resection on segmented CT scans provided useful information for planning hepatic surgery. They also note that it should not be a barrier to undertake major liver resection when the chance for cure outweighs the risks. Interestingly, most patients who developed severe postoperative hepatic dysfunction also had additional predictive factors, including significant blood loss and long operating time.
Based on this comprehensive study, it is evident that image-guided liver surgery is potentially beneficial in several ways. Most importantly, it would allow surgeons to perform more specific procedures that they currently only perform in a virtual manner. By interactively viewing the accurate location of vasculature and tissue surrounding the tumor as they operate, surgeons can more confidently perform segmental and wedge resections and avoid the removal of extraneous healthy liver volume. Segment-oriented anatomical resection will be more hemostatic as surgeons will be more specific in the control of blood vessels due to enhanced anatomical information. Finally, as surgeons become more efficient at using 3-D imaging interactively during surgery, operating time will potentially decrease. Through an efficacy clinical trial to be conducted by PTI, IGLS will potentially be shown to positively affect the ability to perform specific operations with optimized RLV, decreased blood loss, and decreased overall operating time. These factors impact hepatic dysfunction following major liver transection and can be improved by more specific surgery as provided using IGLS.
It is known that determination of an accurate image-to-physical space registration is a fundamental step in providing accurate guidance information to surgeons via image-guided surgery. Since the use of rigid, point-based landmarks is not feasible in image-guided liver surgery (IGLS) applications, surface-based techniques were proposed to determine the registration between the preoperative images and the intraoperative patient space. Specifically, the iterative closest point (ICP) algorithm, developed by Besl and McKay, has traditionally been used to determine the transformation between the image space surface of the liver, derived from preoperative image segmentations, and the intraoperative liver surface. Intraoperative data were initially acquired using an optically tracked probe while more recent efforts have utilized laser range scanner (LRS) technology to provide spatially dense, textured delineations. The current protocol for the performance of a surface-based image-to-physical space registration first involves the selection anatomical fiducial points in the preoperative images sets prior to surgery. The homologous physical space location of these anatomical fiducials is then digitized during the surgery such that a point-based initial alignment registration can be performed. The point-based registration serves to provide a reasonable initial pose for the ICP algorithm. Being that the surface alignment provided by the ICP algorithm is highly dependent on the initial pose of the surfaces, gross errors in the initial alignment provided by the point-based registration can result in erroneous surface alignments. A failed surface-based registration not only compromises the guidance information relayed to the surgeon but also impairs deformation correction efforts due to inaccurate surface displacement data that are used to drive mathematical models. In IGLS, the quality of the initial alignment registration can be compromised by the large fiducial localization errors (FLE) inherent in using anatomical landmarks that undergo non-rigid deformation relative to the preoperative images. Additionally, gravity and the effects of the liver mobilization and packing performed prior to open liver resections can lead to liver deformations that compromise the results of a rigid ICP surface registration. Clinical data shows a poor initial alignment registration, due to high FLE of the anatomical fiducials, and large liver deformations resulted in the convergence of the rigid ICP algorithm to a gross misalignment.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.