Radial vs. Linear EUS
The scope used for EUS is similar to a regular endoscope with the added component of an ultrasound transducer. Endoscopic ultrasonography is the most sensitive imaging tool for the diagnosis of chronic pancreatitis, and has been proven to be more accurate than the CT scan. Endoscopic ultrasound is a highly technical, low-risk diagnostic procedure that utilizes high-frequency ultrasound during endoscopy to evaluate and diagnose digestive tract disorders. EUS allows imaging of the pancreas at close proximity with high resolution. Hence, it may detect changes consistent with chronic pancreatitis in the patient in whom ERCP and other tests are normal. An EUS scope is advanced within the gastrointestinal tract against, or in close proximity to, the pancreas. From a position in the stomach or duodenum, the endoscope allows visualization of the pancreas and adjacent structures.
There are two types of EUS scopes: radial scanning and linear array. The radial type scans in a plane perpendicular to the axis of the scope to produce 360˚ images similar to a CT “slice.” The transducer appears as a “bull’s-eye” within the image. The linear array type scans in a plane parallel to the axis of the scope. It is therapeutic in nature. It has the advantage of allowing visualization of a needle while performing a procedure.
Radial EUS
Radial EUS is used for staging only because it doesn’t have a working channel. One of the classic EUS images is esophageal cancer and its relationship with the esophageal wall. In the radial images, the esophageal wall is seen in cross section as a hypoechoic ring. Radial scopes produce an image similar to a CT scan, which is easier for most physicians to read because they are familiar with the imaging. Optimal imaging of the gastrointestinal wall is essential to determine the wall layer(s) involved by the disease process. To achieve optimal imaging and limit artifact, the sonographic plane must be placed as close as possible to being perpendicular to the intestinal wall.
Ultrasound images of organs contiguous to the gastrointestinal tract each have their own characteristic appearance. This includes large vessels and lymph nodes. Malignant lymph nodes will generally appear hypoechoic and round, with sharp margins. Benign lymph nodes appear irregular in shape, have indistinct margins and have mixed internal echoic features.
Linear EUS
In the linear array images, one wall of the esophagus is seen and the hypoechoic linear structure is parallel to the axis of the scope.
A needle can be passed through the accessory channel of the endoscope and visualized in real time as it is directed through the esophageal wall and into lesions.
The choice between radial and linear scopes tends to be personal, based on the doctor’s training. For some time, radial scopes were the only option so most doctors trained with them, although they may switch to a linear scope if intervention of FNA is needed. Linear scopes were introduced later, to allow therapeutic devices, so physicians who trained on linear scopes tend to start with them.
Next Page: Fine Needle Aspiration
