DOI: 10.1148/rg.245035224
RadioGraphics 2004;24:1367-1380
© RSNA, 2004
Preoperative Hepatic Vascular Evaluation with CT and MR Angiography: Implications for Surgery1
Dushyant Sahani, MD,
Aparna Mehta, MBBS, DMRE,
Michael Blake, MD,
Srinivasa Prasad, MD,
Gordan Harris, PhD and
Sanjay Saini, MD
1 From the Division of Abdominal Imaging and Intervention, Departments of Radiology (D.S., A.M., M.B., S.P., S.S.) and 3D Image Processing (G.H.), Massachusetts General Hospital, White 270, 55 Fruit St, Boston, MA 02114. Presented as an education exhibit at the 2001 RSNA scientific assembly. Received December 12, 2003; revision requested January 20, 2004; final revision received February 25; accepted March 9. All authors have no financial relationships to disclose. Address correspondence to D.S. (e-mail: dsahani@partners.org).
 |
Abstract
|
|---|
Partial liver resection for living donor transplantations and treatment of hepatic tumors is a major surgical undertaking, and detailed knowledge of the hepatic angioarchitecture is essential to ensure safe and successful liver surgery. Noninvasive imaging techniques such as computed tomographic (CT) and magnetic resonance (MR) angiography have begun to replace conventional catheter angiography for evaluation of the hepatic vascular anatomy. Multisection CT angiography and MR angiography are complementary modalities that permit comprehensive, accurate preoperative delineation of the hepatic vascular anatomy and evaluation of the parenchyma in patients undergoing liver surgery, thereby obviating multiple invasive studies including catheter angiography. Understanding a surgeon’s perspective on liver surgery is critical so that the required information can be provided accurately with imaging. Both CT angiography and MR angiography have had a significant impact on the selection of candidates for liver surgery as well as on surgical technique.
© RSNA, 2004
Index Terms: Liver, anatomy, 761.92 • Liver, angiography, 761.12116, 761.12142 • Liver, surgery • Liver, transplantation
|
LEARNING OBJECTIVES
|
|---|
After reading this article and taking the test, the reader will be able to:
- Describe CT angiographic and MR angiographic techniques in evaluating the hepatic vasculature.
- Identify normal hepatic vasculature and anatomic variants that may influence patient selection or surgical technique for liver surgery.
|
Introduction
|
|---|
Despite refinements in hepatic surgical techniques, particularly microvascular reconstruction techniques, vascular complications still account for considerable morbidity and mortality. A detailed knowledge of the hepatic angioarchitecture is thus considered a prerequisite for successful, uncomplicated liver surgeries (1–3). Presurgical planning of vascular anastomosis is a key component of a variety of liver surgeries, including transplantation, tumor resection, and laparoscopic hepatobiliary surgery (1). A wide variety of imaging strategies are used to provide comprehensive preoperative information concerning the arterial, portal venous, and hepatic venous anatomy. Catheter angiography has long been considered the standard of reference for evaluation of the hepatic arterial anatomy. However, the morbidity and mortality associated with catheter angiography, coupled with the limitations of this procedure in demonstrating the hepatic venous anatomy, have provided impetus for the development of noninvasive methods of displaying the vascular anatomy. In this article, we discuss the role of computed tomographic (CT) and magnetic resonance (MR) angiography in the evaluation of normal and variant anatomy of the liver vasculature and the impact of such evaluation on surgical procedures including living donor liver transplantation, hepatic tumor resection, and intraarterial chemotherapy pump placement. In addition, we discuss the protocol for CT and MR angiography, with emphasis on the advantages and disadvantages of each modality.
|
Multi–Detector Row CT Angiography
|
|---|
Recent advances in helical CT technology, including multi–detector row CT, permit single-breath-hold volumetric data acquisition in addition to multiphase imaging. Helical multi–detector row CT permits thin-section coverage of large anatomic areas at speeds 3–7 times faster than those possible with single-detector helical CT scanners (4–8). Clinically useful information can be obtained even in patients who are having difficulty suspending respiration. Recent articles suggest excellent correlation between findings at helical CT angiography and those at catheter angiography (4–8). CT angiography is more convenient for the patient, without the morbidity and mortality associated with catheter angiography. In addition to obviating analgesia and periprocedural nursing care, use of CT angiography results in substantial reductions in cost and radiation burden. In addition, it is now possible to combine CT angiography with multiphase parenchymal evaluation in many abdominal organs.
The figures that appear in this article were obtained with a multisection CT scanner (Light Speed QX/I; GE Medical Systems, Milwaukee, Wis) after injection of 150 mL of nonionic iodinated contrast material with a concentration of 300 mg of iodine per milliliter (1). Contrast material was injected through an 18–20-gauge intravenous cannula at a rate of 5 mL/sec. In some cases, noniodinated contrast material (eg, water) was administered orally to label the stomach and small intestine.
For hepatic arterial phase imaging, the following parameters were used: scan delay, 20–25 seconds; pitch, 6:1; scanning time per rotation, 0.8 seconds; detector configuration, 4 x 2.5 mm; 140 kVp; and 220–260 mA. These parameters resulted in a table speed of 15 mm per rotation (18.7 mm/sec). Images with a section thickness of 2.5 mm were reconstructed every 1.25 mm, resulting in a 50% overlap of consecutive images.
Portal venous phase imaging was performed with the following parameters: scan delay, 65 seconds; pitch, 6:1; scanning time per rotation, 0.8 seconds; and detector configuration, 4 x 2.5 mm (table speed, 15 mm per rotation [18.7 mm/sec]). Images with a 2.5-mm section thickness were reconstructed every 2 mm. Other parameters such as kilovolt peak and milliamperage were kept constant. Each examination was performed during a single patient breath hold, and the approximate acquisition time was 10–15 seconds.
|
Gadolinium-enhanced MR Angiography
|
|---|
Gadolinium-enhanced MR angiography has been established as a safe, accurate, noninvasive method of evaluating the hepatic vasculature. This modality exploits the transient shortening in blood T1 following the administration of gadolinium-based contrast material. Improvements in gradient performance and coil design and refinements in MR angiographic techniques permit faster imaging with improved spatial resolution (9–11). Faster three-dimensional (3D) acquisition of MR imaging data, together with multiplanar imaging capability, allows excellent depiction of hepatic vessels with no saturation or turbulence-related artifacts. In addition, MR imaging is devoid of ionizing radiation and is safe for patients who are allergic to iodinated contrast material.
MR imaging protocol at out institution involves a 1.5-T magnet (Signa, GE Medical Systems) with a phased-array torso coil. First, we obtain axial breath-hold T1-weighted in-phase and opposed-phase gradient-echo images (repetition time msec/echo time [first echo] msec/echo time [second echo] msec = 150/4.2/2.1, 80° flip angle) and T2-weighted images of the liver. Next, an axial breath-hold 3D interpolated spoiled gradient-echo sequence (repetition time msec/echo time msec = minimum/15, 100° flip angle) is performed after intravenous administration of 40 mL of gadolinium chelate injected with a power injector (Medrad, Indianola, Pa) at a rate of 2 mL/sec (9–11). The 3D MR imaging parameters are as follows: field of view, 400 mm; effective section thickness, 2.4 mm; and matrix, 160 x 256. Hepatic arterial phase and portal venous phase images are obtained after a delay of 15–18 seconds and 70 seconds, respectively. Breath-hold images are obtained at end inspiration and usually require less than 30 seconds. Source images are used for multiplanar reformation and 3D reconstruction with maximum intensity projection (MIP) and volume rendering (VR) at a commercially available workstation (Fig 1).
View larger version (157K):
[in this window]
[in a new window]
[Download PPT slide]
|
Figure 1. Sagittal T1-weighted MR image demonstrates placement of a 3D slab. For MR angiography of the liver and mesenteric arteries, the 3D slab should be placed anteriorly covering the aorta to include the superior mesenteric and celiac arteries.
|
|
|
Image Postprocessing and 3D and Virtual Navigation System
|
|---|
The axial images can be processed on a commercially available workstation for multiplanar reformation as well as 3D reconstruction with MIP and VR. Three-dimensional reformatted images of the hepatic vessels are rendered using MIP, shaded surface display, and VR (4). The examples cited earlier were processed on ADW version 2.0 (GE Medical Systems) (Fig 2).
View larger version (134K):
[in this window]
[in a new window]
[Download PPT slide]
|
Figure 2. Three-dimensional MIP image from CT data demonstrates the hepatic venous confluence in which the left hepatic vein (LHV) (solid arrow) and middle hepatic vein (MHV) (open arrow) form a common trunk extrahepatically before joining to form the inferior vena cava (IVC). This configuration is important in left lateral resection, since the surgeon can simply clamp the LHV extrahepatically and then secure the venous drainage to the graft. The MHV is secured to preserve the venous drainage of segment IV.
|
|
The branching points of the relevant arteries and veins and their relation to the proposed site of incision can be viewed with little or no interruption between adjacent sections or on 3D images. Source images are most helpful for defining vascular anatomy, and 3D images are useful in understanding complex anatomy. The hepatic veins are best displayed in the axial plane, with the MHV determining the hepatectomy plane. In liver donors, special importance is attached to the presence of an accessory inferior right hepatic vein (RHV), which must be dissected separately during surgery.
The portal venous anatomy is best displayed in the coronal plane. Portal venous variants (including the separate origin of the right posterior portal vein), which may affect surgical technique or the selection criteria for potential donors, are well displayed.
|
Hepatic Vascular Anatomy
|
|---|
Arterial Anatomy
The classic hepatic arterial anatomy, with the proper hepatic artery branching into right and left branches, is seen in approximately 55% of patients (Fig 3) (12–14). Thus, a significant proportion of patients have variant arterial anatomy. For example, in the Michel type II variant, the RHA, MHA, and LHA arise from the common hepatic artery and a replaced LHA arises from the left gastric artery (Fig 4). The Michel classification of the hepatic arterial anatomy with the approximate frequency of occurrence of each type of variant in the general population are shown in the Table (14).
View larger version (169K):
[in this window]
[in a new window]
[Download PPT slide]
|
Figure 3a. Reformatted images from CT (a) and MR imaging (b) data depict the normal course and branching pattern of the hepatic artery. Note that in a, the gastroduodenal artery (curved arrow) branches off from the common hepatic artery (CHA). The proper hepatic artery divides into the left hepatic artery (LHA) and right hepatic artery (RHA). CA = celiac artery, SMA = superior mesenteric artery.
|
|
View larger version (147K):
[in this window]
[in a new window]
[Download PPT slide]
|
Figure 3b. Reformatted images from CT (a) and MR imaging (b) data depict the normal course and branching pattern of the hepatic artery. Note that in a, the gastroduodenal artery (curved arrow) branches off from the common hepatic artery (CHA). The proper hepatic artery divides into the left hepatic artery (LHA) and right hepatic artery (RHA). CA = celiac artery, SMA = superior mesenteric artery.
|
|
View larger version (169K):
[in this window]
[in a new window]
[Download PPT slide]
|
Figure 4a. (a) Curved two-dimensional reformatted image from CT data depicts the entire course of the replaced LHA (Michel type II variant) (arrow) from the left gastric artery. (b) Catheter angiogram shows the same arterial anatomy. This anomaly in a donor does not influence pediatric split liver transplantation. However, in left lobe transplantation, surgical technique must be modified in the donor, since two left hepatic arteries are anastomosed. LHA = left hepatic artery, Left Gastric = left gastric artery.
|
|
View larger version (144K):
[in this window]
[in a new window]
[Download PPT slide]
|
Figure 4b. (a) Curved two-dimensional reformatted image from CT data depicts the entire course of the replaced LHA (Michel type II variant) (arrow) from the left gastric artery. (b) Catheter angiogram shows the same arterial anatomy. This anomaly in a donor does not influence pediatric split liver transplantation. However, in left lobe transplantation, surgical technique must be modified in the donor, since two left hepatic arteries are anastomosed. LHA = left hepatic artery, Left Gastric = left gastric artery.
|
|
Venous Anatomy
The normal hepatic venous anatomy consists of three main venous tributaries that drain into the IVC. The RHV drains liver segments V–VII, the MHV drains segments IV, V, and VIII, and the LHV drains segments II and III (15,16). In 60% of cases, the MHV and LHV form a common trunk that drains separately into the IVC (Figs 5, 6) (15,16).
|
Portal Venous Anatomy
|
|---|
The normal portal venous anatomy consists of the main portal trunk and its two branching vessels, the right and left portal veins (16–18).
|
Surgical Considerations
|
|---|
In adult-to-adult live donor liver transplantation, the hepatectomy plane is to the right of the MHV and involves preservation of the segment IV artery (MHA), left portal vein, and MHV (4). Thus, the source of the MHA is of surgical significance. In some patients, the MHA arises from the RHA, and it is important to ensure that the RHA segment distal to the MHA is of sufficient length to permit surgery. In general, it is important to ensure sufficient length of the graft vessels to prevent kinking, since the graft migrates downward over time. Arterial patency is vital to graft survival: The presence of multiple arteries increases the complexity of the surgery and is a relative contraindication for live donor liver transplantation (Figs 7–9).
View larger version (41K):
[in this window]
[in a new window]
[Download PPT slide]
|
Figure 7. Schematic illustrates the transection plane (arrows) used in right lobe liver transplantation. The surgical plane extends craniocaudally to the right of the MHV. The MHV drains segment IV and therefore is carefully preserved with the remnant liver of the donor.
|
|
View larger version (62K):
[in this window]
[in a new window]
[Download PPT slide]
|
Figure 8. Schematic illustrates pediatric left lobe liver transplantation, with a hepatectomy plane to the left of the MHV. Structures at the liver hilum, which include the bile duct (arrowhead), hepatic artery (solid arrow), and portal vein (open arrow), are carefully dissected.
|
|
View larger version (139K):
[in this window]
[in a new window]
[Download PPT slide]
|
Figure 9. Intraoperative photograph shows the resection plane used in liver donor surgery. Note the positioning of clamps along the portal vein (open arrow), hepatic artery (arrowhead), and hepatic vein (solid arrow) to secure the pedicle of the transplant graft.
|
|
|
Preoperative Evaluation of the Hepatic Vasculature
|
|---|
Living Donor Liver Transplantation
CT helps assess the vascular anatomy in potential liver donors so that those who are at risk for liver failure do not undergo hepatectomy. Other considerations in donor selection include the diameter of the celiac artery and the inflow arterial vessels; the origin, branching pattern, and variants of the hepatic arterial system; and portal vein patency and bifurcation.
Preoperative imaging evaluation provides a vascular "road map," which is critical for the planning and execution of right lobe transplantation in adults and left lobe transplantation in children and for vascular anastomosis in all cases. The vascular variants that are of surgical importance include variants of the hepatic artery, hepatic vein, and portal vein.
Hepatic Arterial Variants.—
Not all hepatic vascular anomalies are surgically significant. The level of importance varies depending on whether the variants appear in the donor or the recipient. Some of the variants are significant for the donor in terms of reducing morbidity and increasing the safety window by assessing the vascularity of the remnant liver for the likelihood of its regenerating to its original size postoperatively. In addition, this vascular road map aids in preventing graft failure in the recipient (19–23).
One significant hepatic arterial variant in recipients that affects the recipient is a replaced hepatic trunk that courses behind the portal vein (Michel type IX variant), resulting in a change in the sequence of vascular reconstructions (19–23). Another significant variant is a replaced or accessory LHA (Michel type II & V variants, respectively), in which, during native liver removal, the artery has to be ligated at its origin from the left gastric artery. However, the presence of this variant in the donor is not important because the left lobe is not removed.
Other significant arterial anomalies in recipients that affect the recipient are entities that cause inflow compromise such as celiac artery stenosis, which puts the recipient at risk for graft infarction and biliary complications. Documentation of celiac artery stenosis may necessitate appropriate vascular reconstruction (eg, aortohepatic interposition graft) in selected cases. Likewise, preoperative identification of splenic artery aneurysm is important in recipients with cirrhosis, since this condition can be surgically treated to prevent rupture following live donor liver transplantation.
A significant hepatic arterial variant in donors that affects the donor is an artery to the medial segment of the left hepatic lobe (segment IV) that arises from the RHA. This variant is important in the donor because the hepatectomy line would cross the arterial supply in this segment. The variant in which the MHA arises from the LHA is commonly seen (>11% of cases). This variant particularly affects a donor who requires a full arterial supply to the left hepatic lobe. The other MHA variant, in which the MHA arises from either the common hepatic artery or the LHA, does not affect surgical technique in the donor. MHA variants are not important in the recipient because the entire native liver is resected (11–13). The bile ducts are perfused solely by the hepatic arteries; insufficient arterial perfusion may result in biliary stricture, cholangitis, and, finally, graft failure. This complication can result due to excessive dissection of the RHA, which occurs more commonly with a variant course in the RHA, and leads to ischemic damage. To avoid such devascularization of the right or common hepatic duct, there should be careful dissection of the RHA confined to the right side of the common hepatic duct. With an accurate vascular road map, the risk of these complications can be significantly reduced. In addition, it is important not to isolate the RHA too far toward the right side of the liver hilum, which may lead to devascularization of the right hepatic duct. Furthermore, only a short segment of the RHA may be available for a right lobe graft and should be carefully assessed as to whether it suffices for microvascular anastomosis; otherwise, the candidate may be disqualified as a donor. The anomaly in which the origin of the RHA or LHA from the common hepatic artery occurs before the origin of the gastroduodenal artery affects the donor him- or herself, since ligation or clamping of the common hepatic artery can compromise perfusion to the stomach and duodenum, thus disqualifying the candidate as a donor. Trifurcation of the common hepatic artery into the gastroduodenal artery and the right and left hepatic arteries disqualifies the candidate as a donor for the same reasons. When the LHA originates from the celiac artery, modification of surgical technique is required only in cases of left lobe transplantation.
A hepatic arterial variant that affects both the donor and the recipient is a replaced RHA branching from the superior mesenteric artery (Michel type III variant), since extra steps are required for both removal and reimplantation of the liver tissue.
There are other arterial variants that must also be taken into consideration prior to surgery because they can have a marked impact on donor selection and on the surgical procedure itself (19–23). These variants include a separate origin of the hepatic artery from the aorta and variations in the branching pattern of the celiac artery.
Significant vascular variants in donors that affect the recipient include a short RHA, which is of particular significance for donor selection. This anomaly can lead to a difficult anastomosis or may require extensive reconstructive surgery.
Although surgically important vascular variants differ for donors and recipients, the presence of variants in one candidate should prompt a closer look for similar or additional variants in genetically related candidates. The most common variant that has been noted in both related and unrelated donor-recipient pairs is an accessory inferior RHV (19–21).
Hepatic Venous Variants.—
A significant hepatic venous variant in donors consists of accessory hepatic veins draining separately into the IVC, thus complicating surgery. An accessory RHV occurs in 52.5% of patients, two accessory hepatic veins in 12%, and an accessory vein draining the caudate lobe in 12%. The most common hepatic venous variant is an accessory inferior RHV (15,16,19–22) noted in related or unrelated donors and recipients. Most affected patients have a single accessory inferior hepatic vein, although occasionally there can be two such veins, which makes the surgical technique more complicated. This variant is significant in the evaluation of both the donor and the recipient, but more so in the donor. The surgeon’s assessment of the size of the accessory vein is important for transplantation, especially if the vein is larger than 3 mm, since the surgical approach must then be altered to prevent undue complications such as bleeding and ischemic graft malfunction. In addition, the distance of its drainage site from the main hepatic venous drainage site along the IVC is important because the surgical technique must be modified to ensure appropriate anastomosis. If the distance between the opening of the confluence of the main hepatic vein into the IVC and the accessory vein is more than 4 cm, it may be difficult to surgically implant both veins in the recipient with a single partially occluding clamp on the IVC (Fig 10). At times, three accessory hepatic veins are identified, which significantly increases the time required for surgery.
View larger version (164K):
[in this window]
[in a new window]
[Download PPT slide]
|
Figure 10. Coronal reformatted image from CT data demonstrates an accessory RHV (open arrow) draining separately into the IVC (arrowhead). This accessory vein measured over 4 mm in diameter and was located more than 5 cm from the insertion site of the main RHV (solid arrow) into the IVC. An accessory RHV is separately anastomosed with the graft in a recipient, a procedure that increases the complexity of transplant surgery.
|
|
A significant hepatic venous variant in recipients is the early branching of the vein that drains the right superoanterior segment (segment VIII) into the MHV. Although uncommon, this variant has important implications. The transplantation surgeon must be aware of the presence of this anomaly because the segment VIII venous branch may have to be implanted in the recipient to prevent congestion of that segment. At times, early confluence of the hepatic veins may be seen on the right side in the donor. This anomaly may result in a small graft that may not be sufficient to maintain metabolic function in the recipient. Alternatively, accurate depiction of this vein with 3D VR allows the surgeon to localize, transect, and reattach the vein to the RHV.
Significant hepatic venous variants affecting both donors and recipients are seen in approximately 30% of patients, thus posing a challenge for the surgeon, and may result in a modification of the hepatectomy plane. The MHV is an important surgical landmark for the hepatectomy plane that is preserved in both right lobe and left lateral segment grafts. The surgical plane courses 1 cm to the right of the MHV in cases of right lobe dissection (19–23). Early bifurcation of the MHV and large branching veins draining into the MHV from the adjacent segments of the right lobe (segments V–VIII) could require alteration of the hepatectomy plane. In such cases, the vessels would have to be reanastomosed in the recipient; otherwise, the transplanted right hepatic lobe could become congested and potentially lead to organ rejection. Thus, the drainage pattern of the MHV must be thoroughly evaluated. In addition, accessory hepatic veins can be a source of excessive hemorrhage if they are not recognized before surgery (Fig 11). Graft regeneration depends on the venous drainage; thus, it is important to identify and reimplant in the IVC of the recipient all accessory hepatic veins larger than 2 mm; otherwise, graft congestion and organ rejection can result. Severe bleeding can occur at the deeper plane of transection near the MHV and is associated with a decrease in hepatic blood flow and ischemic injury. This bleeding can be reduced by knowing the exact normal and variant vascular anatomy of the MHV and implementing the Pringle maneuver (ie, hemihepatic vascular occlusion) to prevent the MHV from being severed (Fig 12). Venous anomalies such as drainage of the right inferoanterior and right superoposterior segments (segments V and VII, respectively) to the MHV require modification of the surgical procedure to prevent bleeding complications. Thus, knowledge of the vascular anatomy and variations therein helps avoid undue damage to the transplanted and remnant liver.
View larger version (180K):
[in this window]
[in a new window]
[Download PPT slide]
|
Figure 11a. Venous phase CT scan (a) and axial T1-weighted MR image (b) reflect the limitations of CT in demonstrating small hepatic veins: The MR image demonstrates two small accessory RHVs (arrows) that are not visualized on the CT scan.
|
|
View larger version (174K):
[in this window]
[in a new window]
[Download PPT slide]
|
Figure 11b. Venous phase CT scan (a) and axial T1-weighted MR image (b) reflect the limitations of CT in demonstrating small hepatic veins: The MR image demonstrates two small accessory RHVs (arrows) that are not visualized on the CT scan.
|
|
View larger version (42K):
[in this window]
[in a new window]
[Download PPT slide]
|
Figure 12. Schematic illustrates variant venous drainage for segment IV, which is drained by a branch of the LHV (curved arrow). The MHV drains segments V and VIII. Therefore, for right hepatectomy, the resection plane will run to the left of the MHV.
|
|
Portal Venous Variants.—
Portal venous variants account for about 20% of all important variants (17,18). A significant portal venous variant in donors that affects the graft in the recipient is portal venous supply of the right superoposterior segment (segment VII). In most patients, the dorsal branches of segment VII also supply the dorsocranial area of the right lobe, posterior to the RHV (17,18). These branches should be taken into account during presurgical planning for liver transection on the right side of the MHV to avoid devascularizing the dorsocranial area of segment VII of the right lobe graft (portal venous supply). In addition, the angle of portal vein branching is significant for the recipient. If the angle is too acute, the liver may engulf and encase the vein during the regeneration process, which may lead to vascular insufficiency, ischemia, and infarct of the remnant liver. Thus, if the distance between the left and right portal veins at their bifurcation is small, vascular reconstruction may be required, which would have an impact on surgical management. The length of the portal vein is important for anastomosis, which, if not properly achieved, may lead to graft failure in the recipient. Patency of the portal vein is crucial for graft survival; thus, the creation of multiple venous anastomoses is avoided to increase the chances of graft survival. The presence and extent of portal vein thrombosis in transplant recipients can also determine the surgical technique. In recipients with acute portal vein thrombosis, a thrombectomy may be performed. In patients with chronic portal vein thrombosis or a small (<4-mm) portal vein (17,18), the donor portal vein is anastomosed to a large, patent vein in the portal circulation. Although a short interposition vein graft suffices in recipients with short-segment portal vein thrombosis, a long-segment venous jump graft created with the donor iliac vein may be required in patients with long-segment portal vein thrombosis. Other surgically significant anomalies include portal venous cavernous malformation.
A significant portal venous variant in donors that affects the donor consists of the portal venules to segment IV, which have been shown to be important for collateral pathways. Knowledge of this anatomy is important surgically for anastomosis and for preventing bleeding and ischemia.
Trifurcation of the portal vein is a significant portal venous variant in both donors and recipients. Trifurcation of the main portal vein into right anterior, right posterior, and left portal venous branches is a surgically significant anomaly that is seen in 6% of patients (18). In this variant, the right portal vein is absent. Instead, the main portal vein trifurcates into the aforementioned branches directly at the same level. Awareness of this variant anatomy is important for alternate surgical planning and also alerts the surgeon to the fact that there is no portal venous segment that can be clamped during surgery. In the donor, it is important to appropriately ligate the venous branches so as to prevent bleeding. In the recipient, this is important for vascular anastomosis (Fig 13).
View larger version (178K):
[in this window]
[in a new window]
[Download PPT slide]
|
Figure 13. Axial CT scan demonstrates the transection planes used in right lobe liver transplantation (black lines). The presence of portal vein trifurcation increases the complexity of the surgery. The right anterior (open arrow) and posterior (solid black arrow) veins would have to be ligated individually.
|
|
Hepatic Tumor Resection
The objective of vascular imaging in patients with liver neoplasms is to provide a vascular road map for understanding the relationship of the tumor to adjacent vessels, which is important for tumor resection that includes a peripheral tumor-free margin (24). Multiplanar reformation and 3D reconstruction are very helpful in demonstrating the relationship of liver tumors to the hepatic veins and IVC.
Detection of vascular anomalies is important because it may influence surgical technique in patients in whom tumor excision is feasible. In addition, knowledge of vascular anatomy helps prevent inadvertent injury to aberrant hepatic vessels. The relationship of tumor to adjacent vasculature is critical for identifying the vasculature of the remnant liver and preserving it for functional reserve. As mentioned earlier, vascular injury can result in hepatic infarction and biliary ischemia and can compromise functional liver reserve (24). For right lobe resection, the MHV and LHV should be preserved to prevent parenchymal damage to the remnant liver; for left lateral segment resection, the RHV and MHV should be left intact to prevent parenchymal damage to the remnant liver, whereas the LHV should be resected at or above the confluence of the MHV.
Liver resection may be affected depending on the type of arterial variant. In Michel type II variant anatomy, surgical technique must be modified for tumors in the left lobe but not for tumors in the right lobe. In the latter situation, the LHA is ligated at the point where it branches off from the left gastric artery (Fig 14). In Michel type III variant anatomy, the surgical technique must be modified only if tumor is in the right lobe. In type IV, the surgical technique must be modified whether the lesion affects the right or left lobe. In type V, if the lesion affects the left lobe, the accessory LHA must be ligated from the left gastric artery to avoid excessive hemorrhage. In type VI, if the lesion affects the right lobe, ligation of the accessory RHA must be considered. In type VII, additional surgical steps must be taken regardless of lesion location. In type VIII, modification of technique is required depending on which replaced artery is present, and additional ligating procedures are required depending on which accessory artery is present. In types IX and X, modification of technique is required because the entire hepatic trunk is replaced.
View larger version (166K):
[in this window]
[in a new window]
[Download PPT slide]
|
Figure 14. MIP image from MR imaging data demonstrates the Michel type II variant, in which the replaced LHA (long solid arrow) originates from the left gastric artery (straight open arrow) and the MHA (short solid arrow) arises from the proper hepatic artery (curved open arrow). In cases of left lobe tumor, the surgical technique would have to be modified: The LHA would have to be ligated at its origin from the left gastric artery.
|
|
Hepatic venous variants can also have an impact on surgical technique. Understanding the normal and aberrant anatomy of the hepatic veins in relation to the tumor is crucial for planning resection that includes an adequate tumor margin without excessive dissection of the hepatic veins (15–17,24).
At times, a large tributary vein may drain segment VIII into the MHV, and resection of the MHV during left hepatectomy may compromise venous drainage of this segment, resulting in congestive ischemia and atrophy. Thus, the surgeon would have to apply ligating sutures at the drainage point of this tributary vein, which is draining into the MHV. A similar situation exists for accessory inferior hepatic veins, which typically drain segments V and VI directly into the IVC. Thus, in cases of a right lobe tumor, it is important for the surgeon to apply additional clamps to or ligate the accessory veins draining into the IVC. Accessory MHVs that drain posteriorly and inferiorly into the IVC are also seen. With such accessory veins, additional ligating procedures must be performed during surgery, increasing the time required for and the complexity of surgery; however, this is true only in cases of right lobe hepatectomy.
Data concerning the portal venous anatomy are also crucial for presurgical planning (16,17, 24). In patients with trifurcation of the portal vein in which the right anterior portal vein originates from the portal vein directly, resection of the left portal vein proximal to the origin of the right anterior portal vein would compromise the portal perfusion of segments IV, V, and VIII, resulting in segmental ischemia and subsequent atrophy (Fig 15). In some patients, both the right and left portal veins supply segment VII. Thus, in cases of tumor in segment VIII, both branches must be ligated to avoid excessive blood loss. If the tumor surrounds the two branches, 3D reconstruction enables the surgeon to plan the precise resecting margin around the tumor and also helps clarify the relationship of the tumor to the veins.
View larger version (88K):
[in this window]
[in a new window]
[Download PPT slide]
|
Figure 15. Three-dimensional VR image (inferior oblique view) from CT angiographic data demonstrates trifurcation of the portal vein (curved solid arrow) into the right anterior (straight solid arrow), right posterior (arrowhead), and left portal (open arrow) veins. Thus, in cases of right lobe tumor, these three latter veins must be ligated individually to preserve the portal venous supply to the left hepatic lobe.
|
|
Intraarterial Chemotherapy Pump Placement
Hepatic metastatic lesions derive their blood supply principally from the hepatic artery; thus, hepatic arterial infusion delivers maximum doses of chemotherapeutic agents to hepatic malignancies while exposing normal liver tissue and other organs to a relatively reduced amount of agent. Exact preoperative mapping of the hepatic vasculature is imperative for placement of a hepatic floxuridine infusion pump. Such mapping is also important in deciding whether the patient is suitable for the procedure itself (25–27).
In patients with normal anatomy, an intraarterial chemotherapy pump is placed in the proper hepatic artery distal to the origin of the gastroduodenal artery. In patients with variant vascular anatomy, the location of the pump relative to the gastroduodenal artery origin and the relationship between the dominant artery perfusing the liver and accessory hepatic arteries must be well understood. Accessory arteries that supply a nontumorous liver must be ligated or embolized.
The presence of arterial variants may render a candidate unsuitable for placement of a floxuridine infusion pump (25–27). For example, in a Michel type III vascular variant in which the lesion is in the right hepatic lobe, a pump placed in the main hepatic artery will supply chemotherapeutic agent only to the left lobe and MHA. In a Michel type IV vascular variant, a pump inserted into the gastroduodenal artery supplies only segment IV, as in patients with an early right hepatic arterial branch that arises before the origin of the gastroduodenal artery (which itself originates from the LHA) and in patients with replaced right and left hepatic arteries.
In some patients with vascular variants that are considered marginal for insertion of a hepatic arterial infusion pump, surgery may not be contraindicated; however, modification of technique may be required in accordance with the variation in vascular anatomy. Other variants that are considered marginal for chemotherapy pump placement are a replaced RHA (Michel type III variant) (Fig 16) and replaced right and left hepatic arteries (types II and IV).
View larger version (160K):
[in this window]
[in a new window]
[Download PPT slide]
|
Figure 16. MIP image from CT angiographic data shows a replaced RHA arising from the superior mesenteric artery (SMA). For chemotherapy pump placement, the replaced RHA is ligated beforehand to optimize the chemotherapy. CA = celiac artery, LHA = left hepatic artery.
|
|
Complications that can occur due to lack of exact presurgical data concerning the variant vascular anatomy include extrahepatic misperfusion and chemotoxicity of normal hepatic tissue, but these complications can be minimized by evaluating the variant hepatic and extrahepatic vascular perfusion (Fig 17). Chemical cholangitis, bleeding, and duodenitis can occur due to lack of data concerning a small duodenal artery that originates between the tip of the infusion catheter and the junction of the gastroduodenal artery and the proper hepatic artery (25–27).
View larger version (153K):
[in this window]
[in a new window]
[Download PPT slide]
|
Figure 17. Axial MIP image from CT angiographic data demonstrates early branching of the celiac artery (CA) into the LHA, RHA, and splenic artery (black arrow). The presence of this arterial variant necessitates modification of the surgical technique for intraarterial chemotherapy pump placement. The pump catheter is placed in the dominant hepatic artery, and the other branch is usually ligated.
|
|
The response of patients with normal anatomy to intraarterial infusion is better than that of patients with aberrant arteries. This difference is due to the relative ineffectiveness of the intrahepatic collateral circulation compared with the main hepatic artery in delivering the chemotherapeutic agent. Again, a vascular road map is important in determining the suitability of a candidate for pump placement.
|
Disadvantages of CT Angiography
|
|---|
CT angiographic technique works best with state-of-the-art technology, such as multi–detector row CT; however, such technology may not be universally available. In addition, costly and sophisticated computer equipment is required for optimal results. Other disadvantages include the need to train technologists and the increased time required for image processing. The total time required for image processing by the technologist is 15–20 minutes, and interactive examination by the radiologist requires an additional 10 minutes. Large data sets generated with CT angiography may place a considerable burden on the picture archiving and communication system in terms of networking and image storage. Typically, more than 500 images are generated from each CT angiographic study, and it becomes impractical to view such a large data set on film. Cone beam artifacts potentially associated with multisection CT scanners may be another disadvantage. Moreover, CT angiography involves ionizing radiation and the injection of iodinated contrast material.
|
Disadvantages of MR Angiography
|
|---|
The spatial resolution of MR angiography is inferior to that of CT angiography and catheter angiography. Smaller segmental vessels that are readily seen at catheter and CT angiography may not be consistently visualized with MR angiography. In addition, patients with pacemakers, metallic hardware, or claustrophobia may not be suitable candidates for MR imaging. Furthermore, the longer breath hold required for MR angiography may introduce unwanted motion artifacts.
|
Conclusions
|
|---|
Multisection CT angiography and MR angiography are complementary modalities that permit one-stop, comprehensive, noninvasive evaluation of the liver and its vasculature. These modalities allow accurate, preoperative delineation of the hepatic vascular anatomy and evaluation of the parenchyma in patients undergoing liver surgery, thus obviating multiple invasive studies including catheter angiography. Understanding a surgeon’s perspective on liver surgery is critical so that the required information can be provided accurately with imaging. Both CT angiography and MR angiography have had a significant impact on the selection of candidates for surgery as well as on surgical technique.
|
Footnotes
|
|---|
Abbreviations: IVC = inferior vena cava,
LHA = left hepatic artery,
LHV = left hepatic vein,
MHA = middle hepatic artery,
MHV = middle hepatic vein,
MIP = maximum intensity projection,
RHA = right hepatic artery,
RHV = right hepatic vein,
VR = volume rendering,
3D = three-dimensional
|
References
|
|---|
- Van Thiel DH, Wright H, Fagiuoli S, et al. Preoperative evaluation of a patient for hepatic surgery. J Surg Oncol Suppl 1993; 3:49-51.[Medline]
- Ozeki Y, Uchiyama T, Katayama M, et al. Extended left hepatic trisegmentectomy with resection of main right hepatic vein and preservation of middle and inferior right hepatic veins. Surgery 1995; 117:715-717.[CrossRef][Medline]
- Fan S, Lo C, Liu C, et al. Safety of donors in live donor liver transplantation using right lobe grafts. Arch Surg 2000; 135:336-340.[Abstract/Free Full Text]
- Sahani D, Saini S, Nichols S, et al. Using multidetector CT for preoperative vascular evaluation of liver neoplasms: technique and results. AJR Am J Roentgenol 2002; 179:53-59.[Abstract/Free Full Text]
- Prokop M. Multislice CT angiography. Eur J Radiol 2000; 36:86-96.
Top
Abstract
LEARNING OBJECTIVES
