Standard Imaging Methods in the Pancreatobiliary Region Using Radial EUS

Lesions frequently occur in the head of the pancreas, so you need to observe this area very carefully. In the “long-scope” position, it is not possible to bring the scope tip as close as necessary to the distal side. Thus, it is important to be aware of the fact that you will not be able to observe the entirety of the lower part of the head of the pancreas. Furthermore, if the scope is pushed forcibly in the “long-scope” position, perforation is a very real risk. Be sure to proceed with caution.

To observe the entirety of the lower part of the head of the pancreas, you will also need to use the “short-scope” position. Then image the cross-sections of the aorta and the inferior vena cava. instill lukewarm water to make it easier to recognize the horizontal part of the duodenum (Fig. a), where the head of the pancreas is located on the proximal side of the horizontal part of the duodenum . Angulate the scope tip upward to visualize the superior mesenteric artery and vein. The lower part of the head of the pancreas will be visible between the scope and the mesenteric vessels. Observe this area carefully. If possible, follow the lower part of the head of the pancreas as far as the bifurcation of the superior mesenteric vein (Fig. b).

The uncinate process of the pancreas is a small part of the pancreas. It refers to the hook-like projection rising from the area between the lower part of the head of the pancreas and the left dorsal side of the superior mesenteric vein. An incisure created by the uncinate process of the pancreas projecting from the head of the pancreas is called the pancreatic notch, across which the superior mesenteric artery descends. The size of this hook-like projection varies widely from one individual to another, and may not even be noticeable in some cases. The notch is visualized using the ‘short-scope’ position in the duodenum, using the superior mesenteric artery and vein as landmarks. The uncinate process of the pancreas will be visible between the superior mesenteric artery/vein and the aorta – projecting from the head of the pancreas (Fig. c). To make it even easier to distinguish, rotate the CT image 90 degrees clockwise and compare it with the ultrasound image (Fig. d).

The pancreas is a horizontally long, narrow organ of about 20 cm in length and 2 to 4 cm in width. It is necessary to image it from both the stomach and duodenum. In order to successfully observe the entire pancreatic parenchyma using radial EUS without missing anything, there are a few points you should keep in mind. Ultrasound images are, after all, two-dimensional images. When using ultrasound images, be conscious not only of the screen’s vertical and horizontal dimensions, but also of the “depth” between the front and back. In this way, you can build a three-dimensional image, so to speak, while minimizing misses.

When observing from the stomach, use the long-axis image of the splenic vein as a landmark. Start from the body of the pancreas, which is near the transducer, and proceed to the tail of the pancreas. It is important that you follow the pancreatic parenchyma – not the vessel. Pay special attention to the distal edge of the pancreatic tail because it often veers away from the splenic vein (see “Point ④”).

When you follow the pancreatic parenchyma continuously from the duodenum, employ the “short-scope” position and bring the scope tip around the IDA. Then inflate the balloon and start the observation. Advance the scope tip as closely as possible to the distal side, and be careful not to miss any lesions in the lower part of the head of the pancreas or uncinate process (see “Point ⑥”).

Then, while continuously imaging the pancreatic parenchyma and slowly pulling the scope, the scope catches in the duodenal bulb. This means that you are in the pancreatic head-body junction, which is likely to be missed in radial EUS. To observe throughout the pancreatic head-body junction, pull the scope some more and hook the scope tip to the pylorus until the duodenum invaginates the junction in the ultrasound image. From here, deflate the balloon a little at a time and slowly pull the scope tip back to the stomach to continuously observe the body and tail of the pancreas.

When diagnosing pancreaticobiliary maljunction (PBM), check that the pancreatic and bile ducts join outside the duodenal wall and that the sphincter of Oddi does not regulate the pancreaticobiliary junction. At the same time, confirm that an abnormally long common channel and/or an abnormal union between the pancreatic and bile ducts is evident in direct cholangiography, magnetic resonance cholangiopancreatography (MRCP), or multi-detector-row computed tomography (MDCT).

Radial EUS is helpful in detecting anatomical evidence of PBM (Fig. a) and has excellent diagnostic capability. It is relatively easy to diagnose PBM associated with bile duct dilation with EUS. On the other hand, when PBM is not associated with bile duct dilation, hyperplasia is frequently recognized in the epithelium of the gallbladder. This is often imaged as low-echo thickening inside the cystic wall via EUS and provides an important factor for diagnosis (Fig. b).

With PBM not associated with bile duct dilation, there is a high rate of occurrence of gallbladder cancer as a comorbidity. Hence, evaluation of the thickened cystic wall needs to be performed carefully, while keeping in mind that cancer may be present.

After identifying the low-echo area in the vicinity of the papilla while employing the “short-scope” position in the scanning of the descending part of the duodenum, image the bile duct and the main pancreatic duct (the duct of Wirsung) in that low-echo area (Fig. a). Then image the main pancreatic duct in a long-axis image. Subsequently, rotate the scope slightly counterclockwise to image another tubular structure (accessory pancreatic duct or the duct of Santorini) that points towards the duodenal lumen (on the transducer side) (Fig. b). Visualization of this structure may be difficult when the accessory pancreatic duct is very narrow. It can also be affected by the branching morphology. Moreover, even when the accessory pancreatic duct can be visualized, diagnosis of pancreatic duct dysraphism is often difficult. For best results, MRCP should also be used.

Below we look at the relationship between the scanned ultrasound images and the scope’s direction in the trans-D2 (descending part of the duodenum) approach (“short-scope” position) from the point of view of three scope positions.

(1) Inferior duodenal angle (IDA)

When the scope tip is angulated upward after straightening the scope in the descending part of the duodenum and is inserted as far as the IDA, the transducer points to the patient’s left. The displayed ultrasound image resembles a sagittal section viewed from the left side of the patient. Consequently, the aorta and the superior mesenteric vein are imaged in the long axis, while the lower part of the head of the pancreas and uncinate process, which are sandwiched between the aorta and superior mesenteric vein, can be visualized. The upwardly angulated scope tip is pointing to the proximal side, so the six o’clock direction corresponds to the proximal side.