Linear Endoscopic Ultrasound Examination of the Biliary System and Its Clinical Applications

Abstract Endoscopic ultrasound (EUS) examination of the biliary system plays pivotal role in pancreatobiliary studies. EUS offers a safe and noninvasive method of the biliary tract disorder evaluation. Although radial EUS provides a straightforward orientation, practicing biliary system examination with a curvilinear echoendoscope is advisable because of its added therapeutic benefits. Linear EUS may pose challenge in understanding the orientation and tracing nondilated bile duct in the beginning. However, adopting a systematic station-wise approach can help in comprehending the orientation and effectively tracing the entire bile duct. In this review, we have discussed linear EUS examination of the bile duct and gallbladder from various stations and its clinical applications.


Introduction
Endoscopic ultrasound (EUS) is an important tool in the evaluation of the biliary system.It is safe and effective compared with the other imaging modality for the biliary tract disorder. 1,2Both radial and linear EUS can be used for the diagnostic purpose; however, only linear echoendoscope is used for therapeutic interventions.Radial echoendoscope can provide a comprehensive view of the entire bile duct (BD) from the duodenal bulb/antrum, while linear echoendoscope necessitates more maneuvering to achieve a complete view. 3,4The utility of forward view echoendoscope in the evaluation and management of biliary tract disorder is still under the assessment.A station-wise approach aids in understanding orientation as well as it will allows complete examination of the biliary system.EUS examinations of the biliary system are performed from the stomach, duodenal bulb, and duodenum part 2 (►Fig.1).Parts of biliary system examined from various station on EUS are mentioned in ►Table 1.

Technique of Imaging
Before the procedure, a preprocedural assessment including history, clinical examination, and indication of EUS was performed, along with interpretation of cross-sectional imaging.The procedure was performed with a curvilinear echoendoscope (GF180, Olympus, Tokyo, Japan) with the patient under deep sedation and in left lateral position.EUS image orientation is on the right side.The basic principles of linear EUS and related terminologies have been discussed in our previous article. 5Maneuvers used during EUS examination are clockwise and anticlockwise rotation, as well as pull and push movements of the scope to trace the structure."Maintain clockwise or anticlockwise torque" refers to stabilizing the scope position without excessive rotation in either direction while tracing the structure.and retroduodenal part of the CBD, so here we have divided CBD simply into suprapancreatic and intrapancreatic part.
CBD examination is performed from three stations; 1) the stomach, 2) the duodenal bulb, and 3) the duodenum part 2. CBD is situated closest to the duodenal bulb, and whole CBD can be traced from the ampulla to the liver hilum from the duodenal bulb.However, the left-sided segmental ducts are well visualized from the stomach, and the ampullary region is better seen from the duodenum part 2. Therefore, it is recommended to scan CBD from all three station.The CBD is long tubular anechoic structure, measuring 6 to 8 cm in length and 4 to 6 mm in diameter.As CBD is smaller diameter structure, initially it can be challenging to trace normal CBD.Dilated biliary system are more easily appreciated on EUS compared with the normal.Enrolling in hands on training program and adopting a systematic approach of documenting classical images from different stations can expedite the learning. 7,8We have discussed tracing of both dilated and nondilated biliary system to enhance understanding.

Examination of Bile Duct from the Stomach
Tracing the biliary tract from stomach is anchored to the left lobe of the liver.From gastroesophageal (GE) junction locate the liver hilum and follow the portal vein (PV) to trace the CBD.In the liver, multiple structures including ligaments, PV tributaries, biliary radicles, and hepatic veins are visualized. 9,10Color Doppler flow helps in differentiation of dilated biliary radicles from hepatic veins and PV tributaries.
At the GE junction, the left lobe of the liver is visible on the EUS view in the neutral position (►Fig.2A-H).The peripheral branch of left hepatic vein (LHV) is seen traversing through segment 2 and 3. On clockwise rotation, the umbilical portion of the left portal vein (UPV) along with ligamentum teres and ligamentum venosum is visualized.Segment 3 is seen above the UPV and segment 4 is seen below it.Nondilated intrahepatic biliary radicles may not be clearly visible.On further clockwise direction (<45) from the UPV, a  round-shaped left portal vein (LPV) becomes visible, and the LHD can be seen adjacent to the LPV.From LPV, push down the echoendoscope to visualize liver hilum.At liver hilum, main portal vein (MPV), right hepatic artery, and CHD are visualized in sequence.From the CHD, push the echoendoscope while maintaining clockwise torque to trace the portal vein confluence (PVC).The CBD runs parallel to the MPV till the PVC.If the CBD is not visible from the PVC, follow the superior mesenteric vein to visualize intrapancreatic CBD by keeping clockwise torque and pushing down the echoendoscope.Full clockwise rotation of echoendoscope should be avoided when tracing the CBD from the PV.The pancreatic duct (PD) in the head of pancreas (HOP) can also be visualized on minimal clockwise rotation from intrapancreatic CBD.On further tracing, joining of the CBD and PD becomes visible in second part of the duodenum from the stomach body.
During CBD examination, the cystic duct and GB can also be seen from the stomach body.This method allows whole BD examination from the left liver segments to the ampulla (►Fig.2A-H). 11It is important to push echoendoscope down while tracing CBD as ampulla is located caudal to the liver hilum (►Fig.2E).
Variation in anatomy can be observed from patients to patient.Tracing dilated biliary tract is straightforward; however, landmarks and PV branches should be followed for accurate characterization.From GE junction on anti-clockwise rotation segment 2 of liver becomes visible.On clockwise rotation, segment 3 will start appearing.Segment 2 duct will be seen on right side and segment 3 duct on left side of the image.Continuing clockwise rotation, segment 2 and 3 ducts can be seen forming partial "V" pattern, with segment 3 duct as running from the left upper quadrant to the right lower and segment 2 duct running from the right upper quadrant to the left lower.This can be the ideal position for puncturing segment 3 duct for EUS-guided hepaticogastrostomy.Segment 3 duct puncture is recommended for EUS HGS as well as for EUS-guided antegrade stenting, while segment 2 duct puncture for antegrade stenting only. 12urther clockwise rotation, union of segment 2 and 3 duct can be seen below the UPV.From the union on minimal clockwise rotation, LHD and LPV become visible.Push down the echoendoscope to see the liver hilum, where the biliary confluence can be seen.In some cases, a trifurcation branching pattern is observed, with right posterior sectoral duct (RPSD) directly emptying into the confluence. 13Maintain clockwise torque and follow the CHD.On maintaining clockwise torque and pushing down echoendoscope large hypoechoic lesion was seen in HOP in this case.The mass was causing distal CBD obstruction and it was also abutting the PVC (►Fig.3A-H).The HOP mass can also be detected from the stomach by following the CBD from the liver (F) Once the LPV is round push down the scope to visualize the liver hilum.At the liver hilum, the common hepatic duct (CHD), main portal vein (MPV) division, and the right hepatic artery (RHA) are seen.A part of the gallbladder (GB) can also be seen from this point.(G) On pushing further down and maintaining clockwise torque (without excessive rotation), the whole MPV, along with common hepatic artery (CHA) and suprapancreatic portion of CBD, is seen.(H) Further push down the echoendoscope and maintain clockwise torque to see the intrapancreatic CBD, head of pancreas (HOP) and superior mesenteric vein (SMV).The pancreatic duct (PD) can also be seen joining the CBD in duodenum part 2 (with air within).This way, the entire bile duct can be visualized from the stomach.
hilum.This technique is particularly useful in patients with gastric outlet obstruction preventing the passage of scope in the bulb, as well as in patients with surgically altered anatomy.

Examination of Bile Duct from the Duodenal Bulb
In the duodenal bulb, wedge the echoendoscope at the apex. 6he echoendoscope will be in the long position with transducer facing posteriorly (►Fig.4A-H).PV is the homebase in the duodenal bulb.When MPV is visible, CBD is visualized anterior to the MPV.If the CBD is not visible, releasing pressure on the transducer by turning big wheal up (away from you) and instilling some water can help in visualizing the CBD, as normal CBD is often compressed by the transducer. 5Occasionally, extreme clockwise rotation from the MPV may be required to visualize the CBD.
In the duodenal bulb suprapancreatic CBD, PV and CHA are seen stacked, presenting the reverse sequence of the image seen from the stomach (►Figs.2G and 4B).Once suprapancreatic CBD is visible, it can be traced to the ampulla on clockwise rotation and to the liver hilum on anticlockwise rotation.On clockwise rotation intrapancreatic CBD, PD, PVC and HOP will be seen.On further clockwise rotation, superior mesenteric vessels will start appearing along with the HOP.On continuing clockwise rotation, the distal most part of intrapancreatic CBD, PD, posterior part of the pancreas head, inferior vena cava (IVC), right renal artery (RRA) and aorta become visible.IVC and RRA are sometime mistaken for MPV and CHA, respectively.To differentiate, the echoendoscope should be pulled and trace the MPV to the PVC, and vice versa (►Fig.4B-G).On further clockwise rotation from the distal intrapancreatic CBD, the ampulla becomes visible along with duodenal lumen.
From the ampulla, the entire CBD can be traced to the hilum on anticlockwise rotation (►Fig.5A-H).Along with anticlockwise rotation minimal withdrawal of scope is required in some cases (long patient) as the liver hilum is positioned higher up to the ampulla.On anticlockwise rotation, structures that were seen during clockwise rotations become visible.While tracing the CBD, cystic duct insertion can be seen.Additionally, the hepatic artery proper and gastroduodenal artery emerge from the CHA, forming an "arterial vascular seagull" can also be visualized. 14Further anticlockwise rotation, division of the CHD into RHD and LHD will be seen, with the RHD runs closer to the transducer.The confluence is clearly visible when it is dilated.The division of CHD occurs in front of the right portal vein (RPV). 15A part of the GB can also be visualized near the liver hilum on anticlockwise rotation.
Tracing a dilated CBD from the duodenal bulb is simple.Dilated CBD due to ampullary tumor was traced to the liver hilum on anticlockwise rotation (►Fig.6A-H).The dilated suprapancreatic CBD is seen in front of the PVC.As CBD is situated closer to the bulb, accessing it is easier for EUS-guided biliary drainage.If there are no intervening vessel and cystic duct insertion is not nearby, EUS-guided choledochoduodenostomy can be performed with ease.EUS-guided rendezvous from the duodenal bulb is preferred over the intrahepatic route because the papilla is closely positioned, making guidewire maneuvering simpler. 12,16n anticlockwise rotation, the junction of cystic duct with CBD appears as "flask" shape.On further anticlockwise rotation, the cystic duct becomes more visible, separated from the CHD.The confluence also begins to appear.At the confluence, RHD is seen closer to the transducer and running from right to the left of the image; however, LHD runs downwards.This can be a suitable site for EUS-guided hepaticoduodenostomy. On further anticlockwise rotation, the right sectoral ducts-right anterior sectoral duct and RPSD will be seen arising from the RHD.PV branches can be followed for segmental identification.On anticlockwise rotation without withdrawal, the RPV can be followed to the branching of right anterior and posterior veins.The right anterior vein further divides into segmental branches 5 and 8, and right posterior vein into segment 6 and 7. Following LPV from hilum left lobe can be visualized from the duodenal bulb.

Examination of Bile Duct from Duodenum Part 2
In the descending duodenum, echoendoscope should be shortened to achieve straight position with the transducer facing posteriorly. 5,17The aorta is the homebase in descending duodenum (►Fig.7A-J).In deep descending duodenum, the IVC and aorta are visualized.Start withdrawing the scope and rotate scope in clockwise rotation, first mesenteric vessels and then HOP with uncinate process will start appearing.Further withdraw and maintain clockwise torque to visualize the ampulla (usually hypoechoic) along with HOP.At the ampulla, release the big wheel and inject 50 to 100cc of water.Carefully inspect the ampulla for any lesions or stones.After ampulla examination, keep the big wheel down (toward you) to visualize the CBD and PD, this position of echoendoscope referred as "kissing the papilla position."Often, both ducts are seen joining the ampulla.If either duct is not visible, slight maneuvering of scope is required to visualize them, such as anticlockwise rotation is required to see the CBD from the PD and clockwise rotation to see the PD from the CBD.Once the CBD is visible, gradually withdraw the echoendoscope while maintaining anticlockwise torque to trace the entire CBD from the ampulla to the hilum.Always keep the CBD in view while tracing.If it disappears, follow the HOP and mesenteric vessels to bring the CBD back into view.While withdrawing, the echoendoscope may slip back into the stomach, causing the CBD to disappear from view.When superior mesenteric vessels come into view, rotate the scope in anticlockwise direction to bring the CBD into view and begin withdrawing to trace it up to the liver hilum.While tracing the CBD, PVC, and IVC can also be seen.At liver hilum, division of PV into RPV and LPV becomes visible.Ampullary lesions are best visualized from duodenum part 2. A small ampullary tumor can be seen after distending the duodenum with 50 to 100cc of water.In this case, a small ampullary tumor was causing dilatation of the CBD and PD.A dilated CBD was traced from the ampulla to the left liver segment from duodenum part 2 by maintaining anticlockwise torque and gradually withdrawing the echoendoscope (►Fig.8A-H).

Gallbladder
The GB is a balloon-shaped, halo organ appears anechoic on EUS.GB has 3 parts: the fundus, body, and neck. 18A Hartmann's pouch is an outpouching of the GB wall at the junction of the neck.It is a frequent but inconstant feature of normal and pathologic human GB. 19xamination of the GB is typically performed from the antrum and duodenal bulb.However, it can also be visualized from the mid-body of stomach and the descending duodenum.The interesting aspect of GB examination is that it frequently appears in view when it is not intended focus, and it can be challenging to visualize when it is intended.joins the splenic vein (SV) and forms the portal vein confluence (PVC).Anechoic CBD is seen anterior to the main portal vein (MPV).(F) On anticlockwise rotation, the mid-CBD becomes visible, along with the MPV and the division of the common hepatic artery (CHA) the into gastroduodenal artery (GDA) and the hepatic artery proper (HAP), forming an arterial vascular seagull.At this juncture, on careful observation, the cystic duct (CD) (depicted with yellow dotted line) can be seen joining the CBD.(G) On anticlockwise rotation, the CD is clearly visible along with common hepatic duct (CHD).The neck of the gallbladder (GB) can also be seen here.(H) On continuing anticlockwise rotation, the division of CHD into the right hepatic duct (RHD) and left hepatic duct (LHD) becomes visible.The RHD runs toward the transducer along with right portal vein (RPV) and right hepatic artery (RHA).This is the area of liver hilum.At this level parts of GB can also be visualized.

Examination of the Gallbladder from the Duodenal Bulb
While tracing the CBD, locate the cystic duct.Once the cystic duct is visible, visualizing the GB is easy.The cystic duct appears as a spiral, corrugated, anechoic tube due to valves of Heister.The cystic duct insertion with CBD can vary and it can be right lateral (commonest), medial, proximal, low medial, and parallel insertion with common sheath (►Fig.9A-C). 13Low insertion of cystic duct is considered when it joins with the CBD in the HOP, which is observed in 10% of the population. 13Knowledge of the cystic duct insertion location is vital for choosing self-expandable metallic stents and the route of EUS-guided biliary drainage.
Once the cystic duct is visualized, maintain clockwise (without any rotation) torque and gradually withdraw echoendoscope to visualize the GB.(►Fig.9D-F).Scan the whole GB until it is no longer visible.Tracing the GB following the cystic duct visualization is advantageous, as it ensures that any pathology in the cystic duct or the neck, such as stones or malignancies, is not overlooked.
Examining the GB from the duodenal bulb reveals an image opposite to that seen from the stomach (►Fig.10A-F).A complete examination of the GB should be performed from all stations, as small stones or lesions in the fundus may be missed without a thorough examination.

Examination of the Gallbladder from the Antrum
Examination of the GB from the antrum is easy when it is distended.Occasionally, it can be challenging to visualize the GB when it is contracted or intrahepatic.In such cases tracing, the cystic duct can help in the GB visualization.
The GB can also be visualized from the mid-body of stomach while tracing the CBD from the liver hilum (►Fig.2F and G).In the descending duodenum, on slight anticlockwise rotation from the aorta and IVC, the GB may become visible.In descending duodenum, on anticlockwise rotation structures right to the IVC like right kidney and GB, and on clockwise rotation structure left to the IVC, that is, aorta and pancreas can be visualized.
EUS-guided GB drainage can be performed from the antrum or duodenal bulb by puncturing body region of the GB and placing a lumen apposing metal stent or tubular stent. 20Adequately distended GBs are easier to drain than minimally distended GB (►Fig.9E).

Fig. 2
Fig. 2 Examination of the bile duct from stomach, starting from gastroesophageal (GE) junction to the distal body.Locate the liver hilum from the GE junction to trace common bile duct (CBD).(A) Illustration showing the position of the echoendoscope at the level of GE junction.(B) In the neutral scope position, first the left lobe of the liver is visualized with segment 2 and 3 and the peripheral part of the left hepatic vein (LHV).(C) On clockwise rotation, umbilical portion of the left portal vein (UPV) is seen along with ligamentum teres (LT) and ligamentum venosum (LV) is seen.Segment 3 is visible above, and segment 4 is visible below the UPV.(D) On further minimal clockwise rotation, the left portal vein (LPV) is seen as round structure.(E) Illustration showing the maneuvers required to see different parts of the CBD from the GE junction: push down the echoendoscope and maintain clockwise torque.(F)Once the LPV is round push down the scope to visualize the liver hilum.At the liver hilum, the common hepatic duct (CHD), main portal vein (MPV) division, and the right hepatic artery (RHA) are seen.A part of the gallbladder (GB) can also be seen from this point.(G) On pushing further down and maintaining clockwise torque (without excessive rotation), the whole MPV, along with common hepatic artery (CHA) and suprapancreatic portion of CBD, is seen.(H) Further push down the echoendoscope and maintain clockwise torque to see the intrapancreatic CBD, head of pancreas (HOP) and superior mesenteric vein (SMV).The pancreatic duct (PD) can also be seen joining the CBD in duodenum part 2 (with air within).This way, the entire bile duct can be visualized from the stomach.

Fig. 3
Fig. 3 Examination of dilated biliary system from gastroesophageal (GE) junction.Dilated biliary systems are better perceived with endoscopic ultrasound (EUS).(A) Segment 2 is visualized on minimal anticlockwise rotation from GE junction.(B) On clockwise rotation and keeping big wheel down, segment 2 and segment 3 ducts are visualized.Understanding these segments is essential during EUS-guided biliary drainage planning.If an imaginary circle is drawn at this site, it resembles a face, with segment 2 as the right eye and segment 3 as the left eye.(C) On further slight clockwise rotation, segment 2 and segment 3 ducts are seen as long tubular anechoic structure forming incomplete "V" pattern.Segment 3 duct runs from the left upper quadrant to the right lower quadrant.This is the ideal position for puncture of segment 3 ducts for EUS-guided hepaticogastrostomy. (D) On further clockwise rotation, the union of both segment 2 and 3 can be seen below/at the level of umbilical portion of the left portal vein (UPV).(E) On further clockwise rotation, the left hepatic duct (LHD) is seen.(F) Push down the echoendoscope minimally and maintain clockwise rotation to see the liver hilum.The LHD is seen near to the transducer.The right posterior sectoral duct (RPSD) is seen joining at the confluence to the right anterior sectoral duct (RASD) and LHD.Branching pattern of hepatic ducts can vary from the patients to patients.(G) Push down the echoendoscope minimally and maintain clockwise torque to trace the common bile duct (CBD) from the confluence.(H) In this patient, on tracing the CBD from the stomach, a large hypoechoic mass was seen in the head of the pancreas (HOP).The entire bile duct was visualized from the stomach, starting from the left intrahepatic segmental duct until the CBD was cut off due to the HOP mass.

Fig. 4
Fig. 4 Examination of the bile duct from the duodenal bulb.On clockwise rotation, the common bile duct (CBD) is traced to the ampulla.Very fine movements of echoendoscope are required in duodenal bulb to trace the CBD.(A) Illustration showing the position of the echoendoscope in the duodenal bulb.In the duodenal bulb, echoendoscope will be in a long position with the transducer facing posteriorly.(B) The portal vein is the home base in the duodenal bulb.If echoendoscope is properly wedged in the bulb, suprapancreatic CBD is seen.Small, nondilated CBD sometimes get compressed by the transducer.Releasing the pressure on transducer by turning the big wheel away and instilling some water can help in visualizing the CBD.(C) By releasing the big wheel, the CBD is visualized.The CBD, main portal vein (MPV), and common hepatic artery (CHA) appear in a stacked configuration.This sequence is the inverse of what is observed from the stomach (►Fig.2G).Rarely complete clockwise rotation is required to see the CBD (retropancreatic variant of CBD) from the portal vein.(D) By clockwise rotation from the mid-CBD in the duodenal bulb, the CBD can be traced to the ampulla.While tracing, head of the pancreas (HOP), pancreatic duct (PD), superior mesenteric artery (SMA), and superior mesenteric vein (SMV) are seen.(E) Illustration showing the direction of the transducer with imaging field on clockwise rotation in the duodenal bulb.(F) On further clockwise rotation, HOP, intrapancreatic CBD, PD, SMA, SMV, inferior vena cava (IVC), and aorta are seen.Left renal vein (LRV) is seen crossing the aorta.(G) On further rotation, the distal part of intrapancreatic CBD and PD is seen along with IVC and aorta.The right renal artery (RRA) is seen posterior to the IVC.(H) On further clockwise rotation, the ampulla is visible, along with the duodenal lumen.

Fig. 5
Fig. 5 Examination of the bile duct from the duodenal bulb: Tracing bile duct from the ampulla to the liver hilum on anticlockwise rotation.Occasionally, slight withdrawal of echoendoscope while rotating in an anticlockwise direction is required to trace the common bile duct (CBD) from the ampulla.(A) Endoscopic ultrasound (EUS) image showing the ampulla, head of pancreas (HOP), and duodenal lumen.(B) On anticlockwise rotation from the ampulla, tracing CBD-images are similar to those of the CBD seen on clockwise rotation.The intrapancreatic CBD, pancreatic duct (PD), inferior vena cava (IVC), right renal artery (RRA), and aorta are seen.(C) On further anticlockwise rotation parts of suprapancreatic CBD starts visualizing along with HOP, superior mesenteric artery (SMA), superior mesenteric vein (SMV), IVC, and aorta.(D) The suprapancreatic CBD starts to appear on anticlockwise rotation.(E) On further anticlockwise rotation superior mesenteric vein (SMV)joins the splenic vein (SV) and forms the portal vein confluence (PVC).Anechoic CBD is seen anterior to the main portal vein (MPV).(F) On anticlockwise rotation, the mid-CBD becomes visible, along with the MPV and the division of the common hepatic artery (CHA) the into gastroduodenal artery (GDA) and the hepatic artery proper (HAP), forming an arterial vascular seagull.At this juncture, on careful observation, the cystic duct (CD) (depicted with yellow dotted line) can be seen joining the CBD.(G) On anticlockwise rotation, the CD is clearly visible along with common hepatic duct (CHD).The neck of the gallbladder (GB) can also be seen here.(H) On continuing anticlockwise rotation, the division of CHD into the right hepatic duct (RHD) and left hepatic duct (LHD) becomes visible.The RHD runs toward the transducer along with right portal vein (RPV) and right hepatic artery (RHA).This is the area of liver hilum.At this level parts of GB can also be visualized.

Fig. 6
Fig. 6 Examination of the dilated bile duct from duodenal bulb to the hilum on anticlockwise rotation.Common bile duct (CBD) was dilated because of the ampullary tumor.(A) A dilated suprapancreatic CBD is visualized along with the portal vein confluence (PVC).This could serve as a potential site for endoscopic ultrasound (EUS)-guided choledochoduodenostomy as there is no intervening vessel and cystic duct (CD) insertion is also not nearby.(B) During anticlockwise rotation, the CD will start appearing.Illustration showing the joining of the CD with the CBD, resembling a "flask."(C) A representative EUS image shows the junction of the CD with CBD.In the same frame, main portal vein (MPV), common hepatic artery (CHA) and inferior vena cava (IVC) are also seen.(D) On anticlockwise rotation, the CD becomes clearly visible along with the common hepatic duct (CHD).Hepatic artery proper (HAP) and MPV are also seen in conjunction with the CHD.(E) Illustration showing division of the CHD into the right hepatic duct (RHD) and left hepatic duct (LHD).The RHD extends toward the left upper quadrant and near the transducer.(F) A representative EUS image shows the division of the CHD, along with the right hepatic artery (RHA) and right portal vein (RPV).This configuration of hepatic duct could be a potential site for EUS-guided hepaticoduodenostomy. (G) On continued anticlockwise rotation (without any pull or push movements), the RHD can be seen dividing into the right anterior sectoral duct (RASD) and right posterior sectoral duct (RPSD).(H) On further anticlockwise rotation, right intrahepatic sectoral ducts can be followed to the segmental ducts following the portal vein branches.Right anterior portal vein (RAPV) is seen arising from the RPV.

Fig. 7
Fig. 7 Examination of the bile duct from duodenum part 2. (A) In duodenum part 2, the aorta is home base.In the deep descending duodenum (D2-D3 region), the aortic bifurcation can be seen.(B) Start withdrawing the echoendoscope scope while maintaining clockwise rotation, the superior mesenteric vessels along with a part of the head of the pancreas will start to appear in the view (superior mesenteric vein; SMV, superior mesenteric artery; SMA).(C) Illustration showing the position of the echoendoscope with the imaging field in D2 at the level of the ampulla.This is called "kissing the papilla position."(D) Endoscopic ultrasound (EUS) image showing the ampulla, the distal part of the common bile duct (CBD) and Pancreatic duct (PD).(E) To trace the CBD from the ampulla, maintain anticlockwise torque and gradually withdraw the scope.By doing so, the whole CBD can be traced from ampulla to the liver hilum.(F) While tracing the CBD from D2, the echoendoscope may slip back into the stomach.On slipping into the stomach, the superior mesenteric vessels and pancreas body are visualized.If CBD is not visible, rotate echoendoscope in anticlockwise direction from the superior mesenteric vessels to see the CBD.(G) Illustration showing the position of the echoendoscope.Position of echoendoscope when it slips into the stomach is straight with the transducer facing toward the liver hilum along the lesser curvature.(H) Maintain anticlockwise rotation and gradually withdraw the scope while keeping the CBD in view.The portal vein confluence (PVC) and the inferior vena cava (IVC) are seen along with the CBD.(I) On withdrawing echoendoscope minimally, the cystic duct (CD) is seen arising from CBD. (J) Do anticlockwise rotation and gradually withdraw the echoendoscope to see the liver hilum.The common hepatic duct (CHD), along with the portal vein division into left portal vein (LPV) and right portal vein (RPV), is visualized at the liver hilum.From the liver hilum, on anticlockwise rotation, the left lobe of liver is visible.

Fig. 8
Fig. 8 Examination of the dilated common bile duct (CBD) from duodenum part 2. (A) The echoendoscope is in D2 at the level of the ampulla.Duodenum is distended with 50 to 100cc water for better visualization.(B) On withdrawing of echoendoscope, small hypoechoic lesion is seen at ampulla.An ampullary tumor is causing the dilatation of the CBD and the pancreatic duct (PD).(C) By maintaining anticlockwise torque, the CBD is traced from the ampulla.Right renal artery (RRA) and aorta are also visualized.(D) On tracing, the echoendoscope slipped back into stomach.This is the view from stomach.A dilated mid-CBD, along with the main portal vein (MPV) and the liver is seen.(E) On withdrawing and maintaining anticlockwise torque, the liver hilum starts appearing.The common hepatic duct (CHD), along with the portal vein division into right portal vein (RPV), is visualized.(F) On anticlockwise rotation from the liver hilum, left hepatic duct (LHD) is seen, along with the left portal vein (LPV).(G) On continuing anticlockwise rotation, the left lobe of the liver with segment 2 and 3 ducts can be visualized.(H) Anticlockwise rotation from the union of segment 2 and 3 duct shows umbilical portion of left portal vein (UPV), ligamentum teres, part of segment 3 and segment 4 of the liver.

Fig. 9
Fig. 9 Examination of the gallbladder (GB) following the cystic duct (CD) from the duodenal bulb.(A) Variations can be observed in the CD insertion with the common bile duct (CBD).In the duodenal bulb, CD is usually appeared near the transducer and anterior (to the left) of the CBD.This is due to the right lateral insertion of the CD.(B) Endoscopic ultrasound (EUS) shows a long CD running parallel to the CBD and joining within the head of the pancreas (HOP)-this suggests low medial insertion of the CD.(C) The CD appears as tortuous, corrugated structure due to the valves of Heister.When following the CD, maintain clockwise torque to trace GB. (D) On tracing the GB, first the neck of the GB and then body will start appearing.Behind the GB, segment 4 of the liver with umbilical portion of left portal vein (UPV) and ligamentum teres is seen.(E) Maintain clockwise torque and withdraw the scope gradually to trace the complete GB.If the GB disappears while tracing, rotate scope in an anticlockwise direction to bring it into the view.(F) Trace the GB until it disappears completely.Thorough tracing of the entire GB ensures that small pathologies such as small polyp and floating stones are not overlooked.

Fig. 10
Fig. 10 Comparison of gallbladder (GB) imaging from the duodenal bulb and the stomach.(A) Illustration showing image of the GB from the duodenum.The sequence of GB parts on endoscopic ultrasound (EUS) from the duodenal bulb is neck, body, and fundus, from left to right.(B) A representative EUS image showing the neck of the GB on left side of the screen with a stone inside and body of the GB.Segment 4 is visualized posterior to the GB.(C) On tracing the GB, the fundus is visualized.(D) Illustration showing image of the GB from the stomach body.The sequence of GB parts on EUS from stomach body is mirror image of what is seen from the duodenal bulb.(E) A representative EUS image showing the neck of the GB on the right side of the screen.Segment 5 is visualized posterior to the GB.(F) On anticlockwise rotation, the fundus of the GB is visualized.

Table 1
Parts of biliary system examined from the various station on EUS Abbreviations: CBD, common bile duct; CHD, common hepatic duct; EUS, endoscopic ultrasound; GB, gallbladder; LHD, left hepatic duct.