Vascular and Interventional Radiology Original Research

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1 Vascular and Interventional Radiology Original Research Ibukuro et al. Hepatic rtery natomy of the Left Hemiliver Vascular and Interventional Radiology Original Research Kenji Ibukuro 1 Takaya Takeguchi 1 Hozumi Fukuda 1 Shoko be 1 Kimiko Tobe 1 Kazumi Tagawa 2 Ibukuro K, Takeguchi T, Fukuda H, be S, Tobe K, Tagawa K Keywords: 3D imaging, anatomy, fusion image, hepatic artery, liver DOI: /JR Received March 15, 2012; accepted after revision May 2, Department of Radiology, Mitsui Memorial Hospital, 1-Kanda Izumicho, Chiyoda-ku, Tokyo , Japan. ddress correspondence to K. Ibukuro (kj-ibkr@qd6.so-net.ne.jp). 2 Department of Gastroenterology, Mitsui Memorial Hospital, Tokyo, Japan. JR 2013; 200: X/13/ merican Roentgen Ray Society Spatial Relationship etween the Hepatic rtery and Portal Vein ased on the Fusion Image of CT ngiography and CT rterial Portography: The Left Hemiliver OJECTIVE. The objective of our study was to clarify the hepatic artery anatomy of the left hemiliver using the fusion image of CT angiography (CT) and CT arterial portography. MTERILS ND METHODS. CT and CT arterial portography were performed on a 64-MDCT scanner in 144 patients. ll images were transferred to a workstation for 3D analysis using the multiimage fusion mode. We classified the left hepatic artery (LH) and middle hepatic artery (MH) as type L when only the LH was present, type M when a medial branch from the LH was present, type LM when both the LH and MH were present, and type M when only the MH was present. The hepatic artery was classified into infraportal and supraportal groups on the basis of its relationship with the laterosuperior branch of the left portal vein. We also classified the branching pattern of the arteries to each segment. Pattern 1 was defined as when the LH divided into the laterosuperior segment artery (2), which then divided into the lateroinferior segment artery (3) and medial segment artery (4). Pattern 2 was defined as when the LH divided into 3, which then divided into 2 and 4. Pattern 3 was defined as when the LH divided into 4, which then divided into 2 and 3. Pattern 4 was defined as when the LH divided into 2, 3, and 4 simultaneously. RESULTS. The prevalence of each type was as follows: type L (n = 37, 25.7%), type M (n = 44, 30.6%), type LM (n = 53, 36.8%), and type M (n = 6, 4.2%). The number of cases classified as infraportal was 54 (37.5%) and supraportal, 73 (50.7%). The cases classified by branching pattern were as follows: pattern 1, 26 cases (18.0%); pattern 2, eight (5.6%); pattern 3, 93 (64.5%); and pattern 4, 13 (9.0 %). CONCLUSION. Three-dimensional fusion images based on CT and CT arterial portography can show the various anatomic patterns of the left hemiliver hepatic artery in relation to the left portal vein. T he 3D anatomy of the hepatic artery and portal vein in the right hemiliver based on CT fusion images was recently reported [1]. The results of that study suggested that the pathway of the right hepatic artery (RH) is not always the same as the homonymous portal vein in the hepatic hilum. The spatial relationship between the portal vein and the bile duct in the left hemiliver has been reported by several authors [2, 3]; precise recognition of the vascular anatomy of the left hemiliver is crucial, especially for living-donor liver transplantation [4]. However, there are few reports in the literature about the spatial relationship between the hepatic artery and the portal vein in the left hemiliver [3, 5]. Usually there are two hepatic arteries in the left hemiliver: the left hepatic artery (LH) and the middle hepatic artery (MH). The LH and MH feed the left lateral and medial sections of the left hemiliver, respectively. Michels [6] defined the MH as the artery entering the quadrate lobe (medial section of the left hemiliver) and as a branch of the RH (45%), LH (45%), and others (10%) [6]. However, the LH sometimes supplies part of the medial section of the left hemiliver, which is the case when the MH arises from both the RH and LH. ccording to this definition, it is possible to have two MHs. With this case in mind, we decided that it is necessary to define the LH and MH to avoid confusion in hepatic artery terminology. The purpose of this study was to define the LH and MH and to clarify the anatomic varieties of both arteries, including their spatial relationships with the portal vein, using a fusion image of CT angiography (CT) and CT arterial portography JR:200, May 2013

2 Hepatic rtery natomy of the Left Hemiliver MH M (Lateral branch dominant) (Common) (Medial branch dominant) L M Materials and Methods From September 2004 to March 2011, 198 consecutive patients with primary or metastatic hepatic tumors seen on a previous CT or ultrasound examination were referred for CT and CT arterial portography at our institution. The patients were referred for precise evaluation of the size, number, and location of the hepatic mass or masses before possibly undergoing radiofrequency ablation, transcatheter arterial chemoembolization (TCE), or liver resection. CT and CT arterial portography were indicated if CT or ultrasound had detected no more than three masses or if the imaging characteristics of the hepatic mass or masses were atypical for hepatocellular carcinoma on dynamic CT. Of these 198 patients, 144 met the inclusion criteria for the study. The inclusion criteria were as follows: The left portal vein was sufficiently opacified on CT arterial portography and all hepatic arteries in both the medial and lateral sections of the left hemiliver were shown on CT. There were several exclusion criteria. Patients were excluded if imaging showed aberrant LHs, such as accessory or replaced LHs. The fusion images of the left hemiliver were composed of three CT series (CT arterial portography, CT for evaluation of the MH, and CT for assessment of aberrant LHs); in those cases, precise fusion of the three series would be difficult because of slight differences in the liver contour. Therefore, patients with the following types based on Michels [6] classification were excluded: type II (replaced LH, n = 19), type IV (replaced LH and RH, n = 3), type V (accessory LH, n = 1), type VIIIb (replaced LH and accessory RH, n = 1), and others (n = 7). Patients were also excluded if tumor displaced vessels and interofered with precise recognition of the vascular anatomy (n = 1), if the left portal vein was not clearly shown because of tumor thrombus (n = 1) or insufficient portal venous flow to the left hemiliver (n = 5), and if the artery of the left hemiliver was insufficient (n = 14) or was too dense to allow analysis of the individual arteries (n = 2). The 144 patients included in the study group were 105 men and 39 women who ranged in age from 45 to 85 years old (mean age, 68 ± 9 [SD] years). ll patients were sian. The patients had been diagnosed with liver diseases caused by hepatitis C (n = 85), hepatitis (n = 17), hepatitis and C (n = 2), alcoholism (n = 11), an unknown cause (n = 19), or liver metastasis (n = 10). The varieties of hepatic artery anatomy based on the classification proposed by Michels [6] that were seen on imaging of the study group were as follows: type I (n = 137 patients), type III (replaced RH, n = 2), type VI (accessory RH, n = 1), type IX (entire hepatic artery from superior mesenteric artery [SM], n = 2), type X (entire hepatic artery from left gastric artery, n = 1), and other (n = 1). The varieties of portal vein based on the branching patterns [7] were as follows: type I (right and left portal veins, n = 132), type II (right anterior branch, right posterior branch, and left portal vein, n = 3), and type III (independent right posterior branch, n = 9). * M2 4 PV4 LH (Supraportal) (Infraportal) L 3 2 PV3 PV2 Fig. 1 Classification of left hepatic artery (LH) and middle hepatic artery (MH) in relation to left portal vein. We defined LH and MH according to whether origin of split from proper hepatic artery was at left or right aspect of left main portal vein, respectively. When only LH or MH was present, case was classified as type L or M, respectively. When both LH and MH were present, case was classified as type LM. When LH was divided into two branches running along right (asterisk) and left aspects of left main portal vein, case was classified as type M2. When MH or medial branch of type M2 was supplying left medial section (segment 4) and LH or lateral branch of LH with type M2 was supplying left lateral section (segments 2 and 3), case was classified as common subtype. When LH had branch to segment 4, case was classified as lateral branch dominant subtype. When MH had branch to segment 2 or 3, case was classified as medial branch dominant subtype. When artery of segment 2 (2), artery of segment 3 (3), or common trunk of both from LH ran inferior to left laterosuperior branch of portal vein (PV2), case was classified as infraportal group and others were classified as supraportal group. PV3 = lateroinferior branch of left portal vein, PV4 = medial branch of left portal vein, 2 = laterosuperior segment artery, 3 = lateroinferior segment artery, 4 = medial segment artery. CT and Workstation CT arterial portography and CT were performed before diagnostic angiography or TCE. One catheter was placed into the SM for CT arterial portography and another catheter was placed into the celiac artery (n = 2), common hepatic artery (n = 85), proper hepatic artery (n = 56), or LH (n = 1) for CT. For CT arterial portography, prostaglandin E 1 (5 µg) was injected via the catheter into the SM, and then contrast material (370 mg I/mL; Iopamiron 370, ayer Schering Pharma) was injected at a rate of 1.5 ml/s for a total of 45 ml. Data recording started 30 seconds after the contrast material injection was initiated. For CT, the contrast material (150 mg I/mL; Iopamiron 150) was injected through the catheter at a rate of 1.5 ml/s for a total of 35 ml. Data recording started 20 seconds after the contrast material injection was initiated. We used a 64-MDCT scanner (Sensation Cardiac, Siemens Healthcare) for CT arterial portography and CT. ll images were obtained with a 0.6-mm collimation, 1.2 pitch, 120 kv, 230 m, and 0.5-second rotation time. The axial images were reconstructed with a matrix at a 1-mm thickness and intervals. oth axial images were transferred to a workstation (ZIO Station System 1000, Ziosoft Inc., Japan), and volumerendered images were generated to analyze the spatial relationship between the portal veins and JR:200, May

3 Ibukuro et al. the hepatic arteries using the multiimage fusion mode. One of the authors manually adjusted and aligned the CT arterial portography and CT series on three different planes (axial, sagittal, and coronal) to obtain accurate fusion images of the left hemiliver. Definitions of the Classifications We defined segments 2, 3, and 4 of the left hemiliver using the risbane 2000 liver anatomy and resection terminology [8]; the arteries supplying segments 2, 3, and 4 were the laterosuperior segment artery (2), the lateroinferior segment artery (3), and the left medial segment artery (4), respectively. The proper hepatic artery usually splits into the LH, MH, and RH. We defined the LH and MH according to whether the origin of the split from the proper hepatic artery was at the site of the left or right aspect of the left main portal vein, respectively (Fig. 1). These arteries basically feed the lateral and medial sectors of the left D Fig. 2 Different combinations of left hepatic artery (LH) and middle hepatic artery (MH)., 78-year-old man with hepatitis C. Fusion image shows type L (infraportal group). There is only one hepatic artery from proper hepatic artery supplying whole left hemiliver. ecause this artery is running along left aspect of left main portal vein, this artery is classified as LH. Note that this LH (arrow) runs inferior to laterosuperior branch of left portal vein., 73-year-old woman with hepatitis C. Fusion image shows type L (supraportal group). LH from proper hepatic artery is supplying whole left hemiliver. Note that this LH (arrow) runs superior to laterosuperior branch of left portal vein. C, 72-year-old woman with hepatitis C. Fusion image shows type M2. LH (arrow) from proper hepatic artery is supplying left hemiliver. However, this LH divides into artery (arrowhead) running across anterior aspect of left main portal vein to medial section of left hemiliver and artery running along left aspect of left portal vein. D, 79-year-old man with hepatic disease of unknown origin. Fusion image shows type LM. Two hepatic arteries from proper hepatic artery are supplying left hemiliver. Proximal artery (arrow) is running along left aspect of left main portal vein, usually supplies lateral section of left hemiliver, and is called LH. Distal artery (arrowhead) is running along right aspect of left main portal vein, usually supplies medial section of left hemiliver, and is called MH. E, 69-year-old man with hepatitis C. Fusion image shows type M. Only one hepatic artery from proper hepatic artery is supplying whole left hemiliver. ecause this artery is running along right aspect of left main portal vein, this artery is classified as MH. Note that tumor was deleted from image because existence of tumor interferes with recognition of vessels. E hemiliver. ased on this definition, the MH does not arise from the LH, but a medial branch can arise from the LH. We classified the hepatic arteries supplying the left hemiliver in the terms described in the next sections. The left hepatic artery and middle hepatic artery combinations For type L (Figs. 1, 2, and 2), there is no MH present. The LH, the origin of which is located at the left aspect of the left main portal vein, supplies the entire left hemiliver (segments 2 4). For type M (Figs. 1 and 2C), there is no MH present. When the entire LH traverses the left main portal vein to the right, it is classified as type M1. When the LH is divided into two vessels running along the right and left aspects of the left portal vein (medial and lateral branches), it is classified as type M2. Note that the branch running along the right aspect of the left portal vein is called the medial branch but not the MH. For type LM (Figs. 1 and 2D), both the LH and MH are present. There are three subtypes: the lateral branch dominant subtype, in which the MH supplies segment 4 and the LH supplies segment 4 as well as segments 2 and 3; the common subtype, in which the MH supplies segment 4 and the LH supplies segments 2 and 3; and the medial branch dominant subtype, in which the MH supplies segment 2, segment 3, or both segments 2 and 3 as well as segment 4 and the LH supplies segment 2, segment 3, or both. These subtypes can also be applied to type M2. For type M (Figs. 1 and 2E), there is no LH present. The MH, the origin of which is located at the right aspect of the left main portal vein, supplies the left medial (segment 4) and lateral (segments 2 and 3) sections. Relationships between the portal vein of segment 2 and the left hepatic artery Cases were classified into two groups: the infraportal group (Figs. 1 and 2), in which one of the arteries of segment 2 or segment 3 runs inferior to the portal vein of segment 2 and to the first branch of the left portal vein, C 1162 JR:200, May 2013

4 Hepatic rtery natomy of the Left Hemiliver and the supraportal group (Figs. 1 and 2), in which none of arteries of segment 2 or segment 3 runs inferior to the portal vein of segment 2 or the left portal vein. This classification can be adapted for the LH C in cases with type L (n = 37), type M2 (n = 37), or type LM (n = 53). Left hepatic artery branching patterns lthough the left main portal vein is usually first D divided into the portal vein of segment 2 and common trunk (lateroinferior branch and medial branch of the left portal vein) of segments 3 and 4, the LH has a variety of branching patterns. The branching pattern of the LH was classified on the basis of the first branching artery: pattern 1, LH divided into 2, which then divided into 3 and 4; pattern 2, LH divided into 3, which then divided into 2 and 4; pattern 3, LH divided into 4, which then divided into 2 and 3; and pattern 4, LH divided into 2, 3, and 4 simultaneously (Fig. 3). Fig. 3 ranching patterns of left hemiliver hepatic arteries. 2 = artery of laterosuperior segment, 3 = artery of lateroinferior segment, 4 = artery of medial segment., 70-year-old man with hepatitis. Fusion image shows pattern 1 in which left hepatic artery (LH) is divided into 2, which is then divided into 3 and 4., 51-year-old man with hepatitis. Fusion image shows pattern 2 in which LH is divided into 3, which is then divided into 2 and 4. C, 64-year-old woman with hepatitis C. Fusion image shows pattern 3 in which LH is divided into 4, which is then divided into 2 and 3. D, 76-year-old man with hepatitis C. Fusion image shows pattern 4 in which LH is divided into 2, 3, and 4 simultaneously. Note that tumor was deleted from image because existence of tumor interferes with recognition of vessels. TLE 1: Relationship of Left Hepatic rtery (LH) and Middle Hepatic rtery (MH) Combinations and ranching Patterns of the Segment rteries ranching Pattern a of the Segment rteries (No. of Patients) Total No. of LH and MH Combinations Pattern 1 Pattern 2 Pattern 3 Pattern 4 Other Patients Type L b Type M1 c Type M2 d Type LM e Common subtype Lateral branch dominant and medial branch dominant subtypes Type M f Other Total Note The prevalences of type L, type M2, and the common subtype of type LM are almost the same; however, type M is rare. ranching pattern 3 is more prevalent than the other branching patterns. a Pattern 1, LH divided into laterosuperior segment artery (2), which then divided into lateroinferior segment artery (3) and left medial segment artery (4); pattern 2, LH divided into 3, which then divided into 2 and 4; pattern 3, LH divided into 4, which then divided into 2 and 3; and pattern 4, LH divided into 2, 3, and 4 simultaneously. b Only LH is present. c No MH is present and entire LH traverses the left main portal vein to the right. d No MH is present and LH divides into two branches running along the right and left aspects of the left main portal vein (medial and lateral branches). e oth LH and MH are present. For type LM, there are three subtypes: the lateral branch dominant subtype, in which the MH supplies segment 4 and the LH supplies segment 4 as well as segments 2 and 3; the common subtype, in which the MH supplies segment 4 and the LH supplies segments 2 and 3; and the medial branch dominant subtype, in which the MH supplies segment 2, segment 3, or both segments 2 and 3 as well as segment 4 and the LH supplies segment 2, segment 3, or both. f Only MH is present. JR:200, May

5 Ibukuro et al. Image Evaluation The volume-rendered fusion images made by one of the authors (with 8 years of CT interpretation experience) were retrospectively interpreted by consensus based on the classifications defined by two other radiologists (with 26 and 22 years of CT interpretation experience, respectively) who are familiar with the anatomy of the liver. In cases of disagreement, the radiologist who generated the images determined the classification. ll patients gave signed informed consent to be examined by CT. The institutional review board (IR) exemption permits staff physicians to review medical records for clinical studies provided that subjects cannot be identified, either directly or through identifiers linked to the subjects, and the IR waived informed consent for the retrospective study design. Results The Left Hepatic rtery and Middle Hepatic rtery Combinations The prevalence of each type was as follows: type L (n = 37, 25.7%), type M (n = 44, 30.6%), type LM (n = 53, 36.8%), type M (n = 6, 4.2%), and other (n = 4) (Table 1). Other types included cases with two LHs (n = 3) and two LHs and one MH (n = 1). The total number of lateral branch dominant, common, and medial branch dominant subtypes (Fig. 4) among the cases with type M2 and type LM was 16 (11.1%), 67 (46.5%), and seven (4.9%), respectively (Table 2). The number of cases with the lateral branch dominant subtype was more than that of the medial branch dominant subtype in both types. Relationship etween the Portal Vein of Segment 2 and the Left Hepatic rtery The number of cases belonging to the infraportal group was 54 (37.5%) and supraportal group, 73 (50.7%) (Table 3). The prevalence of the infraportal and supraportal groups among type M2 and type LM was equal; however, the supraportal group was dominant among the type L cases. The medial branch of type M2 runs inferior to the left portal vein, so there is a hepatic artery located inferior to the left portal vein or portal vein of segment 2 in 73 (n = ) of 127 cases (57.4%). Left Hepatic rtery ranching Patterns The total numbers of the cases with patterns 1, 2, 3, and 4 (Fig. 3) were 26 (18.0%), eight (5.6%), 93 (64.5%), and 13 (9.0%), respectively (Table 1). lthough the left main portal vein is usually first divided into the portal vein of segment 2 and common trunk (lateroinferior branch and medial branch of the left portal vein) of segments 3 and 4, the most common branching pattern of the artery in the left hemiliver was pattern 3 (i.e., LH divided into 4, which is then divided into 2 and 3). Fig. 4 Lateral branch dominant and medial branch dominant subtypes., 79-year-old woman with hepatitis C. Fusion image shows lateral branch dominant subtype in which middle hepatic artery (MH) supplied posteroinferior part (arrow) of medial section of left hemiliver. Left hepatic artery (LH) supplied anterosuperior part (arrowhead) of medial section and lateral section of left hemiliver., 78-year-old woman with hepatitis C. Fusion image shows medial branch dominant subtype in which LH supplied laterosuperior segment (arrowhead), which is artery to segment 2. MH supplied lateroinferior segment (arrow), which is artery to segment 3, and medial section of left hemiliver. Note that tumor was deleted from image because existence of tumor interferes with recognition of vessels. Other types included cases where one LH is divided into two trunks, one is divided into 2 and 4, and another one is divided into 3 and 4; one LH is divided into 3, which is divided into 4, which is then divided into 2 and 3; two LHs, one is 3, and another is divided into 4, which is then divided into 2 and 4; and one LH is divided into 2 and 3, and one MH is divided into 2 and 4. Other Findings There were three cases with type LM for the arteries and type III for the portal vein. The MH was located posterior to the right anterior portal vein (Fig. 5) in two of the three cases. Discussion Knowledge of the vascular anatomy of the hepatic hilum is important for the preoperative evaluation of patients with hilar cholangiocarcinoma, living liver donors, and liver graft recipients [2]. Therefore, the spatial relationship TLE 2: Relationship of Left Hepatic rtery (LH) and Middle Hepatic rtery (MH) Combinations and Subtypes Subtype (No. of Patients) LH and MH Combinations Lateral ranch Dominant a Common b Medial ranch Dominant c Total No. of Patients Type M2 d Type LM e Total Note The prevalence of the lateral branch dominant subtype is slightly higher than that of the medial branch dominant subtype in patients with types M2 and LM. a MH supplies segment 4 and the LH supplies segment 4 as well as segments 2 and 3. b When MH or medial branch of type M2 is supplying left medial section (segment 4) and LH or lateral branch of LH with type M2 is supplying left lateral section (segments 2 and 3). c MH supplies segment 2, segment 3, or both segments 2 and 3 as well as segment 4 and the LH supplies segment 2, segment 3, or both. d No MH is present and LH divides into two branches running along right and left aspects of left main portal vein. e oth LH and MH are present JR:200, May 2013

6 Hepatic rtery natomy of the Left Hemiliver TLE 3: Relationship of Left Hepatic rtery (LH) and Middle Hepatic rtery (MH) Combinations and Groups LH and MH Group (No. of Patients) Combination Infraportal a Supraportal b Total No. of Patients Type L c Type M2 d Type LM e Total Note Data presented are numbers of patients. The LH in cases with type L tends to run superior to the first branch of the left portal vein (portal vein of segment 2). a One of the arteries to segment 2 or segment 3 runs inferior to the portal vein of segment 2 and to the first branch of the left portal vein. b None of the arteries to segment 2 or segment 3 runs inferior to the portal vein of segment 2 or to the left portal vein. c Only LH is present. d No MH is present and LH divides into two branches running along right and left aspects of left main portal vein. e oth LH and MH are present. Fig. 5 Pathway of middle hepatic artery (MH) in patients with anatomic combination of type LM for hepatic artery and type III for portal vein. Type LM has both left hepatic artery (LH) and MH. Type III for portal vein has common trunk of left and right anterior branch of portal vein., 81-year-old man with hepatitis of unknown origin. Fusion image shows MH (arrow) is located posterior to right anterior branch of portal vein., 57-year-old man with hepatitis of unknown origin. Fusion image shows MH (arrowhead) runs anterior to right anterior branch of portal vein. Note that tumor was deleted from image because existence of tumor interferes with recognition of vessels. between the portal vein and the bile duct in the left hemiliver has been well documented. The anatomic variations of the arteries in the left hemiliver have also been evaluated [9 13]; however, the combined anatomy of the portal vein and the hepatic artery has not been well documented. To accurately describe the arterial anatomy, it is important to define each artery based on the portal segmentation. We analyzed the relationship between the hepatic arteries and the portal veins using fusion images based on CT and CT arterial portography. We summarized the anatomy of the LH and MH using classifications described in previous studies as well as our study, as shown in Table 4. ecause the classifications from the reports are different from ours, we modified their classifications of the hepatic artery anatomy to match ours. lthough there is a slight difference among these results in terms of the LH and MH, the type M (MH only) is very rare. Hirano et al. [13] studied the anatomic variations of the MH and LH in relation to the umbilical portion of the portal vein. The prevalence of type R-1 (corresponds to type M or LM in our study), R-2 (type M), and L (type L) was 32%, 32%, and 32%, respectively. They also stated that patients with biliary malignancies and an MH that ran along the right aspect of the left portal vein (type LM or M2 in our study) should be candidates for combined resection of the MH with a right hemihepatectomy to obtain negative margins. lthough the reported prevalence was different, Mizumoto and Suzuki [3] also noted cases with an abnormal blood supply to the left lateral section from the MH (9.4%) or to the medial section from the TLE 4: Our Classification of Left Hepatic rtery (LH) and Middle Hepatic rtery (MH) pplied to Cases Presented in Previous Studies and Our Study LH and MH Combination (% of Patients) Study [Reference No.] Type L a Type L a and Type M2 b Type M2 b Type LM c Type M d Ikegami et al. [9] Wang et al. [10] e Jin et al. [11] e Healey et al. [12] Hirano et al. [13] Our study e Note ecause the classification of the hepatic artery anatomy in each of the studies was modified using our classification regarding the combination of LH and MH, the number or the preference is just a guide. a Only LH is present. b No MH is present and LH divides into two branches running along right and left aspects of left main portal vein. c oth LH and MH are present. d Only MH is present. e berrant LH anatomy cases were excluded. JR:200, May

7 Ibukuro et al. LH (12.5%); these types correspond to the medial branch dominant and lateral branch dominant subtypes in our study, respectively. Even if there are two distinct arteries, such as the MH and LH, in the hepatic hilum, the intrahepatic distribution of these arteries is not always to the medial and lateral sections, respectively. If a combined resection of the MH with a right hemihepatectomy was performed in patients with the medial branch dominant subtype, some part of the left lateral section could become ischemic. s a result, we think that the individual anatomy of the hepatic artery is important in deciding whether to sacrifice or preserve the vessel during surgery. The bile ducts of segment 2 and segment 3 usually run superior to the left portal vein [2], which is called the supraportal course [14]. However, Ozden et al. [14] reported that an infraportal course of the segment 3 bile duct was noted in approximately 6% of cases and that recognition of this anomaly is important for avoiding biliary injury in an extended right hepatectomy or during a living-donor liver transplantation. With this possible anomaly in mind, we added the spatial anatomic information of the hepatic artery to that of the portal vein and the bile ducts of the left hemiliver. We analyzed fusion images based on CT arterial portography and CT because recognition of the peripheral vessels on axial images is supposed to be superior to contrastenhanced CT using IV bolus injection and multiphasic techniques. In fact, Lee et al. [15] reported that the origins of the arteries supplying segment 4 were not clearly shown or were misinterpreted in 18% in the cases in their study based on evaluations with MDCT angiography using IV contrast material. However, a more increased injection rate of the contrast material, such as 5 ml/s, can make it possible to show the peripheral hepatic arteries on the MDCT using IV contrast material and we can analyze the vascular anatomy even in patients with an aberrant LH who were excluded in this study. limitation of our study was that we excluded patients with aberrant LH anatomy (the replaced left [n = 19] and accessory left [n = 3] hepatic artery). ecause the prevalence of the aberrant LH anatomy has been reported to be more than 18% [6], our results do not reflect the true incidence of different hepatic artery anatomies within the left hemiliver. However, we can classify the replaced LH type according to Michels definition of the hepatic artery into the common or lateral branch dominant subtype of the LM type and the accessory LH type according to Michels definition of the hepatic artery into the medial branch dominant subtype of the LM type. Therefore, it is possible to apply our classification to these anatomic variations as well. Type M1 and type M were similar except for the origins of the LH and MH. When the hepatic artery is elongated, it is difficult to define its origin at the right or left aspect of the left portal vein. s a result, the classification was somewhat dependent on the researchers and their interpretations of the images may have skewed the data. ecause most patients in the study group are male and all are sian, there is a potential for sexual dimorphism and racial variation in the left hemiliver arterial anatomy. Conclusion It is important to understand the individual variation of the left hemiliver arterial anatomy for planning surgical and interventional radiologic procedures to prevent complications and to achieve successful treatment results. References 1. Ibukuro K, Takeguchi T, Fukuda H, et al. 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