Understanding the Basics of Linear Endoscopic Ultrasound: A Step-by-Step Guide to Anatomy and Orientation

Abstract

Linear endoscopic ultrasound (EUS) has evolved into an essential modality for diagnostic and therapeutic gastrointestinal interventions. Despite its growing utilization, the learning curve for mastering linear EUS remains steep, largely due to the complexity of anatomical orientation and image interpretation. This article provides a systematic, step-by-step approach to understanding the basics of linear EUS, focusing on spatial anatomy and procedural technique. Core topics include image orientation, in vivo scope rotation with the “rule of 360,” organ and structure tracing, understanding EUS scanning planes, and a station-wise evaluation strategy. A structured checklist is proposed to enhance procedural quality and prevent anatomical oversight. Additionally, the manuscript emphasizes the often-overlooked aspect of image optimization, detailing key parameters such as frequency, gain, contrast, focus, and Doppler techniques. The article aims to demystify orientation and facilitate accurate structure identification through a systematic approach. The comprehensive yet practical framework outlined in this manuscript is designed to support both novice endosonographers in achieving procedural proficiency. Integrating these fundamental principles into routine EUS practice can enhance overall understanding of the linear EUS, and likely to reduce the learning curve.

 

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Keywords

echoendoscope - endoscopic ultrasound - anatomy - orientation - station-wise approach

Introduction

Endoscopic ultrasound (EUS) is a standard diagnostic modality used for upper gastrointestinal disorders. The advent of new accessories and techniques has significantly expanded the therapeutic applications of EUS. Although the use of EUS has increased immensely, the learning curve remains steep. Understanding the orientation on linear EUS is quite challenging.[1] Competence in EUS demands both cognitive and technical abilities to carry out the procedure independently. Although many resources are available to simplify the process, most lack a comprehensive approach.[2] [3] [4] [5] Understanding three-dimensional anatomy and developing the ability to interpret the images are essential foundations for linear EUS.[6] In this review, we present a simplified approach to understanding anatomy and orientation in linear EUS.

Procedure

Procedure was performed using a linear echoendoscope (GF-UCT 180, Olympus, with ME2 and ME3 processor, and EG-38 J10UT Pentax scope with Arietta 65). It was performed in the left lateral position under total intravenous anesthesia (propofol). The basic principles of linear EUS, including echoendoscope rotation and maneuvering required during the procedure, have been previously explained.[1] [7]

There are few basic fundamental steps that should be followed to understand and perform high-quality EUS ([Video 1]). In this article, we focus on:

Video 1 Video demonstrating understanding basics of linear EUS with step-by-step guide to anatomy and orientation. EUS, endoscopic ultrasound.

• Image orientation.

• Scope rotation in vivo and “rule of 360.”

• Organ/structure tracing.

• Scanning plane of EUS.

• Station-wise approach.

• Checklist during examination.

• Image optimization.

Image Orientation

In linear EUS, either right or left orientation can be used based on the endoscopist's preference. Right orientation means the cranial end is on the right side of the image, and the caudal end on the left ([Fig. 1A, B]). Since the caudal side is on the left, it is easier for the endosonographer to advance the echoendoscope and bring the left-side image to the center, which appears more physiological. Conversely, left orientation is typically used in transabdominal ultrasound, and is more intuitive for those trained in transabdominal ultrasound.

Fig. 1 EUS image showing right orientation (cranial to the right side). Blue dot word (F) is indicative of right orientation in the two images. EUS, endoscopic ultrasound.

 

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Scope Rotation In Vivo and Rule of 360

For complete organ/structure tracing, the linear echoendoscope should be rotated in clockwise and anticlockwise directions. Although it is easy to understand the scope rotation ex vivo, it is difficult to comprehend how the movements occur in vivo. The position of the transducer is better understood when the echoendoscope is in a straight position. The transducer is oriented in the opposite direction of the universal cord.[8] The different movements occur based on the transducer's orientation—whether it is facing anteriorly, posteriorly, to the right, or to the left.

The direction of the transducer is inferred from the visualized structures. For example, if the heart or left lobe of liver is visualized, the transducer is facing anteriorly. If descending aorta or pancreas is visualized, it means transducer is facing posteriorly.

When transducer is oriented anteriorly, on clockwise rotation it goes to the right side and on anticlockwise rotation it goes to the left ([Fig. 2A, B]).[8] For instance, when the subcarinal window is visualized from the mid-esophagus, the transducer is facing anteriorly. During clockwise rotation, it initially moves to the right side, allowing visualization of right bronchus (on withdrawal) and the azygous vein. As the rotation continues, the echoendoscope moves posteriorly, enabling visualization of the descending aorta. Once the descending thoracic aorta is visualized, withdraw the scope until the arch of the aorta is seen, and perform further minimal clockwise rotation to visualize the aortopulmonary (AP) window. From the subcarinal space, the AP window can be located on clockwise and on anticlockwise rotation ([Fig. 3A–F]). This illustrates the rule of 360°, i.e., any structure visualized during clockwise rotation can also be visualized during anticlockwise rotation—though different structures are seen in each direction. On anticlockwise rotation from the subcarinal space, the left-sided structures (descending aorta) are visualized. If echoendoscope is minimally withdrawn (about 2 cm) from the subcarinal space during anticlockwise rotation, the left bronchus, AP window, and arch with descending thoracic aorta are seen progressively ([Fig. 4A–F]). Although the AP window can be visualized during anticlockwise rotation, it is recommended to locate it during clockwise rotation, as performing EUS-FNA (fine needle aspiration) is easier in this position.

Fig. 2 (A, B) Echoendoscope rotation in vivo when transducer is facing anteriorly. (A) When transducer is facing anteriorly, on clockwise rotation echoendoscope goes toward right and right-sided structures are seen. (B) When transducer is facing anteriorly, on anticlockwise rotation echoendoscope goes toward left and left-sided structures are seen. AV, azygous vein; DA, descending aorta; LA, left atrium.

Fig. 3 (A–F) Understanding clockwise rotation, when transducer is facing anteriorly (i.e., heart, subcarinal window) in mediastinum. (A) On clockwise rotation, scope goes to the right. Arrows pointing the movements of echoendoscope rotation on CT. (B) Transducer is facing anteriorly, when subcarinal window between left atria (LA) and right pulmonary artery (RPA) is seen. (C) On clockwise rotation from subcarinal space, right bronchus is seen. (D) On further clockwise rotation azygous vein is seen, just before the vertebra. (E) On further clockwise rotation descending aorta is seen. (F) Aortopulmonary window is seen between arch of aorta and left pulmonary window, visualized by withdrawing the scope by 2 cm (till the arch) and simultaneously rotating it in clockwise direction.

Fig. 4 (A–F) Understanding anticlockwise rotation, when transducer is facing anteriorly (subcarinal window) in mediastinum. Aortopulmonary (AP) window can be visualized on clockwise as well anticlockwise rotation from the subcarinal space. This also explains rule of 360, any structure which is seen on clockwise rotation can also be seen on anticlockwise rotation. (A) On anticlockwise rotation, scope goes to the left. (B) EUS image showing subcarinal window between left atria (LA) and right pulmonary artery (RPA). (C) On anticlockwise rotation, left ventricle (LV) along with mitral valve and pulmonary trunk (PT) is visualized first. (D) On further anticlockwise rotation, left bronchus is visualized. (E) On further anticlockwise rotation (withdrawing scope minimally by 2 cm), aortopulmonary window is visualized. (F) On anticlockwise rotation, descending aorta with arch is visualized. Perform minimal clockwise rotation to visualize AP window from the descending aorta, if not seen earlier. EUS, endoscopic ultrasound.

When the transducer is facing posteriorly, the echoendoscope movements are opposite to those when the transducer is facing anteriorly. When the transducer is directed posteriorly, on clockwise rotation it goes to the left side, and on anticlockwise rotation it goes to the right ([Fig. 5A, B]).[1] For instance, when the transducer is facing the pancreas body, clockwise rotation reveals the left-sided structure, including pancreatic tail, left kidney, and spleen ([Fig. 6F]). During anticlockwise rotation from the pancreas body, right-sided structures such as pancreatic genu and portal vein confluence are visualized ([Fig. 7A–F]).

Fig. 5 (A, B) Echoendoscope rotation in vivo when transducer is facing posteriorly (toward the aorta) at GE junction/proximal stomach. (A) When transducer is facing posteriorly, on clockwise rotation echoendoscope goes toward left and left-sided structures are seen. (B) On anticlockwise rotation, echoendoscope goes toward right and right-sided structures are seen.

Fig. 6 (A–F) Understanding clockwise rotation, when transducer is facing posteriorly in the stomach. (A) On clockwise rotation, echoendoscope goes toward the left and left-sided structures are seen. Arrows pointing the movements of echoendoscope rotation on CT. (B) Descending aorta is seen along with celiac artery (CA) and superior mesenteric artery with part of the pancreas body in the left upper quadrant. (C) From celiac artery, advance the echoendoscope to visualize of the pancreas body. (D) On bringing the pancreas body in the center and on clockwise rotation, tail of the pancreas, left kidney, and spleen are visualized. (E) Left kidney and spleen are hallmark of the pancreas tail. (F) On further clockwise rotation, splenic hilum is visualized. CT, computed tomography.

Fig. 7 (A–F) Understanding anticlockwise rotation, when transducer is facing posteriorly in stomach. (A) On anticlockwise rotation, echoendoscope goes toward the right. (B) Transducer is facing posteriorly as aorta along with celiac artery and superior mesenteric is visualized. (C) Advancing Advance echoendoscope, to bring the pancreas body into the center. (D) On anticlockwise rotation from pancreas body, portal vein confluence (formed by SV [splenic vein], SMV [superior mesenteric vein], and IMV [inferior mesenteric vein]) is visualized along with pancreas head. (E) On further anticlockwise rotation, pancreatic genu and SMV are visualized. Echoendoscope needs to be advanced for proper examination. (F) On anticlockwise rotation, dropping pancreatic duct (PD) along with genu and head is visualized.

When the echoendoscope is oriented toward the left side (e.g., in the second part of the duodenum), on clockwise rotation scope goes anteriorly, and on anticlockwise rotation it goes posteriorly ([Fig. 8A, B]). The common bile duct (CBD) is located posterior to the pancreatic duct (PD). When echoendoscope is facing the papilla, clockwise rotation visualizes the PD along with ventral pancreas (head and uncinate process; [Fig. 9A–D]), and further clockwise rotation reveals superior mesenteric vessels ([Fig. 10A–D]). On anticlockwise rotation from the ampulla, the CBD is visualized with the aorta, and with further anticlockwise rotation, the inferior vena cava (IVC) appears posterior to the CBD ([Fig. 11A–D]).

Fig. 8 (A, B) Echoendoscope rotation in vivo when transducer is facing toward the left in duodenum part 2 (kissing ampulla position). (A) When transducer is facing left, on clockwise rotation echoendoscope goes anteriorly and anterior structures are seen. (B) On anticlockwise rotation, echoendoscope goes posteriorly and posterior structures are seen.

Fig. 9 (A–D) Understanding clockwise rotation in duodenum part 2 when transducer is facing ampulla (left side). (A) On clockwise rotation in D2, anterior structures are seen. (B) EUS image showing the pancreatic duct (PD) with part of ventral (hypoechoic) and dorsal pancreas. (C) On clockwise rotation, uncinate process is visualized. (D) On further clockwise rotation, mesenteric vessels (SMV [superior mesenteric vein], SMA [superior mesenteric artery]) are seen. EUS, endoscopic ultrasound.

Fig. 10 (A–D) Understanding anticlockwise rotation in duodenum part 2 when transducer is facing ampulla (left side). (A) On anticlockwise rotation in D2, posterior structures are seen. (B) EUS image showing the pancreatic duct (PD) with part of ventral (hypoechoic) and dorsal pancreas. (C) On anticlockwise rotation, common bile duct (CBD) is visualized along with aorta (posteriorly). (D) On further anticlockwise rotation, inferior vena cava (IVC) is visualized behind the CBD. EUS, endoscopic ultrasound.

Fig. 11 Organ/structure tracing: if the structure is on right extreme of the screen, echoendoscope must be pulled to bring the structure in the center.

Understanding normal anatomy is crucial for grasping the orientation during linear echoendoscope rotation. It also aids in identifying the specific scope maneuvers needed to locate the particular structure, and the ability to interpret EUS images improves the overall learning curve.

 

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Organ/Structure Tracing

Tracing organs and structures is a crucial part of the EUS examination. To trace an organ effectively during a linear EUS examination, in addition to clockwise and anticlockwise movements, pull-and-push movements of the echoendoscope are required. It is vital to understand when to perform these maneuvers.

For any organ or structure tracing, first bring the structure of interest to the center of the screen. If the structure is on the right side of the screen, pull the echoendoscope slightly to bring it to the center of the screen ([Fig. 11]). If the structure is seen on the left side of the screen, push the echoendoscope downward to bring it to the center ([Fig. 12]). However, while pushing the echoendoscope down, the big knob must be released to avoid the resistance. Once the structure is centered on the screen, perform clockwise or anticlockwise rotation to trace the entire structure.

Fig. 12 Organ/structure tracing: if the structure is on left extreme of the screen, echoendoscope must be pushed (advanced) to bring the structure in the center. While pushing, the big knob of echoendoscope is to be released.

 

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Scanning Plane

Linear EUS images can be well correlated with sagittal or coronal sections of computed tomography (CT) images.[9] When the echoendoscope is in a straight position, linear EUS images nearly align with sagittal imaging ([Fig. 13A, B]). Longitudinal structures appear linear or tubular, while horizontal structures appear oval or round shaped ([Fig. 14A, B]). For instance, the descending aorta appears as a longitudinal structure, whereas the arch of the aorta appears round. Similarly, the pancreas and splenic artery appear as oval and round structures, respectively.

Fig. 13 When linear echoendoscope is in straight position, it provides images nearly similar to sagittal section of computed tomography (CT). (A) Illustration showing sagittal image of mediastinum on CT. (B) Corresponding EUS image. DA, descending thoracic aorta; EUS, endoscopic ultrasound; LA, left atria; LMP, left main bronchus; PT, pulmonary trunk; RA, right atria; RPA, right pulmonary artery; RV, right ventricle; RVOT, right ventricular outflow tract.

Fig. 14 (A, B) Sagittal section of pancreas on computed tomography (CT) correlated with EUS images. When echoendoscope is in straight position, longitudinal images appear as linear or tubular, and horizontal images appear as round or oval. (A) Illustration showing sagittal section of pancreas on CT. (B) Corresponding EUS images of the pancreas. CA, celiac artery; EUS, endoscopic ultrasound; LRV, left renal vein; P body, pancreas body; SA, splenic artery; SMA, superior mesenteric artery; SV, splenic vein.

Similar to the cross-sectional imaging, different structures are visible at different scanning planes. For instance, when the echoendoscope is positioned at the lower esophagus at the level of gastroesophageal (GE) junction, the scanning axis passes through the ostia of all hepatic veins ([Fig. 15A–F]). Therefore, during clockwise rotation from the left lobe of the liver at the level of GE junction (lower esophagus), all hepatic veins are visualized along with the descending thoracic aorta. When scope is pushed down minimally, the axis of scanning plane goes through the left portal vein, diaphragm, and the descending abdominal aorta ([Fig. 16A–F]).

Fig. 15 (A–F) Understanding plane of scanning: imaging from the lower esophagus. (A) Illustration showing echoendoscope position in lower esophagus. The plane of scanning goes through all hepatic veins and descending thoracic aorta. (B) Corresponding computed tomography image. Structures visualized on clockwise rotation from the lower esophagus. (C) Left hepatic vein (LHV) along with segment II, III, and along with base of heart, is visualized. (D) On further clockwise rotation, middle hepatic vein is visualized along with segment I, IVa, and VIII. The left portal vein (LPV) can also be visualized. (E) On further clockwise rotation, infrahepatic inferior vena cava (IVC) along with right hepatic vein is seen. (F) On further clockwise rotation, descending thoracic aorta is visualized. LV, ligamentum venosum.

Fig. 16 (A–F) Imaging from the cardia esophagus. At this juncture, plane of scanning goes through the left portal vein and abdominal aorta. (A) Echoendoscope should be pushed down into the cardia. (B) Corresponding computed tomography image. Structures visualized on clockwise rotation from the cardia. (C) The left lobe of liver is visualized in neutral position. (D) On clockwise rotation, umbilical portion of left portal vein (UPLPV) is seen along with ligamentum (LT) teres and ligamentum venosum (LV). (E) On further clockwise rotation, liver hilum is visualized. (F) On further clockwise rotation, descending abdominal aorta along with diaphragm is seen. On minimal pushing down echoendoscope from here, celiac artery can also be visualized.

The crus of the diaphragm serves as a landmark dividing the mediastinal and abdominal cavity. Once the crus is identified, further advancement of the echoendoscope leads into the intra-abdominal esophagus (cardia), where the scanning plane intersects through the celiac artery, pancreas, and spleen.

The intra-abdominal part of the esophagus curves towards the left side and joins the stomach. Therefore, in a neutral position, echoendoscope is oriented anteriorly and to the left, allowing visualization of the left lobe of liver. Clockwise rotation from this position helps to locate the aorta, and followed by the pancreas.

Anatomical variations between the individual can alter the images in different scanning planes. In smaller and thinner individuals, multiple structures may be visualized with minimal echoendoscope maneuvering in a single plane.

 

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Station-Wise Approach

For a complete organ examination, a station-wise approach is recommended ([Fig. 17]).[1] [4] [10] [11] [12] [13]

Fig. 17 Station-wise approach is required for complete assessment. For mediastinum, examination of subcarinal window and aortopulmonary window is to be performed by following inferior vena cava from lower esophagus /gastroesophageal (GE) junction. For complete pancreato-biliary system study, examination from the stomach, duodenal bulb, and duodenum part 2 is to be performed.

In mediastinal anatomy, examination of the subcarinal window and AP window should be carried out. Follow the IVC from the GE junction, rotate the scope in anticlockwise direction, and gradually begin withdrawing the echoendoscope.[8] The subcarinal window is seen between the left atria and right pulmonary artery. From the subcarinal space, the AP window can be visualized on clockwise as well as anticlockwise rotation ([Figs. 3A–F] and [4 A–F]). After mastering this examination, gradually learn about other mediastinal lymph node stations for lung cancer staging.

For pancreaticobiliary evaluation, the examination should be conducted from the stomach, duodenal bulb, and descending duodenum. Each station has its home base; follow these home bases to identify the particular organ. These home bases are usually major vessels, such as aorta, porta vein, and IVC.[1] [7]

 

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Check List

It is important to follow a check list for every EUS examination ([Fig. 18]).[14] This list represents the minimum quality indicators required for a standard EUS procedure. Each of these structures should be reviewed and confirmed during every EUS session. This systematic approach helps prevent missing any critical anatomical details.

Fig. 18 Check lists to be followed during linear EUS examination. EUS, endoscopic ultrasound.

 

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Image Optimization

This is one of the most neglected aspects of EUS examination. Image optimization is a separate topic and falls outside the scope of this article. However, for better imaging, one should know basic image control parameters, including frequency, gain, contrast, focus, range, and Doppler settings.[15] The labelling and settings (range) of these control buttons may vary among different machines ([Fig. 19]).

Fig. 19 (A, B) Image optimization is important part of the EUS. Know the knobology for optimum resolution and quality examination. EUS, endoscopic ultrasound.

Frequency refers to the rate at which piezoelectric crystals oscillate to generate sound wave, playing a vital role in ultrasound imaging. Echoendoscopes are high-frequency probe that operate at frequencies ranging from 5 to 12 MHz.[16] A miniprobe operates at a frequency of 12 to 20 MHz. For routine imaging, a frequency of 6 to 7.5 MHz is commonly used. Higher frequency provides greater resolution but results in reduced penetration. Therefore, high-frequency settings are preferred for superficial lesions (e.g., subepithelial lesions or early malignancies), whereas lower frequencies are preferred for deeper structure evaluation.

Gain controls the amplification of returning sound waves. Higher gain results in brighter images, while lower gain produces darker images. Therefore, gain must be adjusted to optimize the image quality. In ME 2/3 processor (Olympus, Japan), gain ranges from 1 to 19, and it is usually kept between 10 and 14.

Contrast (dynamic range) enhances the distinction between dark and bright structure. Unlike gain, which brightens or darkens the entire image, contrast adjusts the differential between light and dark areas. In ME 2/3 processor, contrast level ranges from 1 to 8, with a typical setting around 4. In some machines, contrast is termed as dynamic range.

Focus (indicated by a small green arrowhead on the right side of the image) is the point where all sound waves converge. Either a single focus or dual focus can be used, based on the area being examined. For lesions near the transducer, a single focus is used and typically set 1 to 2 cm below the organ of interest for better resolution. For deeper or centrally located lesions, dual focus may be applied. Some systems have smart focus, which automatically optimizes the focal zone without manual adjustments.

The range or zoom function allows magnification of the image or specific details. Using zoomed views is recommended, as it enhances resolution and provides a more detailed visualization. Panzoom can be used to zoom in on a specific area of the image.

Tissue harmonic imaging (THI) uses fixed lower frequency to improve image quality. Two THI modes are available—THI -P (penetration) and THI-R (resolution). THI mode is commonly used for its ability to produce high-resolution images while reducing artefacts.

The Doppler helps in evaluating blood flow in vessels by detecting frequency shifts from moving red blood cells. It assesses the presence, direction, velocity, and turbulence of the flow.

There are four types of Doppler flow: color, power, high-flow, and pulse-wave Doppler. The color Doppler uses color to denote the direction of the flow (blue: away from the transducer; red: towards the transducer). If the Doppler range is set too high, Doppler artefacts may interfere with imaging and if set too low, slow-flow vessels may be missed. Therefore, it should be appropriately adjusted by keeping any vessels or the aorta in the center. It is also angle-dependent, if any vessel is not displaying the Doppler flow, perform to-and-fro movements or clockwise and anticlockwise movements before ruling out vascularity. The sensitivity of the color Doppler is lower for vessels with slow flow. In contrast, the power Doppler is highly sensitive to low-velocity or slow-flow vessels. It uses monochromatic orange hue, and provides information on the intensity of the flow. Light orange suggests a low-velocity flow and the brown suggests a high-intensity flow in the vessels. The power Doppler is not angle-dependent and can pick up small vessels; however, it does not provide information on the direction of flow and is prone to motion artefacts caused by cardiac or breathing movements.

The high-flow Doppler uses purple hue, similar to color Doppler, to visualize rapid blood flow. It identifies fast moving blood. It helps differentiate between high- versus low-flow lesions. It is not a separate modality, but rather a parameter adjustment within the color Doppler.

Pulsed-wave (spectral) Doppler provides a graphical waveform representation of blood flow at a specific point. It measures peak systolic and end diastolic velocities, which help differentiate arteries, vein, and the portal vein. To use it, align the pulse wave bar (gate) at the center of vessel, and press Enter (Update), and observe the waveform. The length of bar should be adjusted as per the diameter of the vessel. These waveforms help in detecting portal hypertension (e.g., reversed or monophasic flow in portal vein) and also characterizing flow in shunts, thrombosis, or stenosis.

 

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Summary

Linear EUS is widely used for diagnostic and therapeutic purposes. However, mastering linear EUS presents a significant learning curve, primarily due to the complexity of anatomical orientation and image interpretation. A thorough understanding of spatial orientation and regional anatomy is the cornerstone of proficiency in linear EUS.

By systematically applying the key principles outlined in this article, one can enhance the quality of image acquisition and also build procedural confidence. Integrating these structured steps into routine practice can improve the overall quality of EUS procedure, which ultimately can lead to better patient outcomes.

 

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Conflict of Interest

None declared.

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Publication History

Article published online:
12 June 2025