Diagnostic Ultrasound - Abdomen and Pelvis
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Diagnostic Ultrasound - Abdomen and Pelvis

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09.07.2019 Views

PART II SECTION 3 Pancreas Introduction and Overview Approach to Pancreatic Sonography 350 Pancreatitis Acute Pancreatitis 354 Pancreatic Pseudocyst 358 Chronic Pancreatitis 362 Simple Cysts and Cystic Neoplasms Mucinous Cystic Pancreatic Tumor 366 Serous Cystadenoma of Pancreas 370 Intraductal Papillary Mucinous Neoplasm (IPMN) 374 Solid-Appearing Pancreatic Neoplams Pancreatic Ductal Carcinoma 380 Pancreatic Neuroendocrine Tumor 384 Solid Pseudopapillary Neoplasm 388

Approach to Pancreatic Sonography Diagnoses: Pancreas Imaging Anatomy The pancreas resides in the anterior pararenal space of the retroperitoneum, which also includes the second-fourth segments of the duodenum and the ascending and descending segments of the colon. The gland is an elongated structure situated in the transverse plane, with the head to the right of the midline, surrounded by the c-loop of the duodenum, and the body/tail extending laterally and slightly cranially to the splenic hilum. The head, neck (isthmus), and body are almost always visible via transabdominal ultrasound; the tail and uncinate are variably obscured by bowel gas. The gland is typically isoechoic or slightly hyperechoic to the liver, often increasing in echogenicity with age, which may in part be secondary to increasing lipomatosis. In patients with good sonographic visualization, the pancreatic duct can be identified as a thin curvilinear structure situated within the center of the gland, oriented along the long axis, although when normal in caliber it may not always be visible. It can be seen as two thin echogenic lines, representing the epithelial walls of the duct, separated by a thin hypoechoic layer of fluid within the duct itself. Other readily visible anatomic landmarks include the superior mesenteric vein between the uncinate and pancreatic neck; in the head, the gastroduodenal artery anteriorly and common bile duct posteriorly; and in the body, the splenic vein along the posterior margin. Anatomy-Based Imaging Issues Frequently, the pancreatic tail, and often parts of the distal body, are not visible secondary to the presence of gas within the stomach, colon, and small bowel. Obesity is another common limitation in scanning of the pancreas. Related fatty infiltration of the liver may alter the relative echogenicity of the pancreas, which may then appear as hypoechoic relative to the steatotic liver, potentially mimicking a pathologic process such as pancreatitis. Pathologic Issues The pancreas can be affected by acute and chronic inflammatory processes, benign and malignant cystic and solid neoplasms, and autoimmune processes. Imaging Protocols Transabdominal ultrasound imaging can be facilitated by fasting prior to the exam, preferentially for at least six hours or overnight, in order to reduce the amount of gas within the stomach and bowel. Imaging is obtained with a curved transducer with the highest possible frequency, typically up to five MHz, although technological advances on modern scanners may allow for imaging at up to nine MHz without loss of acoustic penetration. Tissue harmonic imaging is used to improve image quality, particularly of fluid-filled structures such as cystic lesions, pancreatic duct, and the vasculature system. Compound imaging is used to improve tissue contrast and spatial resolution. Doppler ultrasound is essential to evaluate the vascular structures, as well as internal vascularity of tumors. The gland should be evaluated in both the transverse and longitudinal planes. Imaging in different orientations such as in decubitus or erect positions, or with suspended respiration (inspiration or expiration), may improve visualization of structures not visible in the usual supine position. Graded continual transducer pressure on the abdomen can improve visualization by collapsing and mobilizing bowel; however, this may be limited by focal tenderness depending upon the clinical setting. Although not routinely utilized, a moderate amount (100-300 mL) of degassed water or oral contrast administered prior to imaging can improve visualization of the tail; however, this can also introduce air bubbles leading to additional artifacts. Overdistention of the stomach should be avoided, as it is less compressible and may make the exam uncomfortable for the patient. The spleen can be used as an acoustic window to visualize the pancreatic tail. Contrast-enhanced ultrasound can be obtained using second generation microbubble contrast agents, following a conventional ultrasound in which focal or diffuse pancreatic pathology has been detected. As microbubble contrast remains entirely intravascular, the distinction between solid and cystic masses is improved. Parenchymal enhancement can also be evaluated, which can potentially aid in distinguishing focal pancreatitis from neoplasm. Imaging requires specialized software, most commonly pulse inversion, to suppress background tissues and allow visualization of only vascularized structures. Imaging acquisition occurs immediately after intravenous administration in order to evaluate the arterial inflow to the pancreas and early parenchymal enhancement. Usage is limited in the United States, as there are no contrast agents approved by the Food and Drug Administration for noncardiac use. Clinical Implications The major role of sonography in imaging of the pancreas is in the evaluation of acute pancreatitis and pancreatic malignancy. Acute Pancreatitis Acute pancreatitis is diagnosed by a combination of clinical presentation and laboratory abnormalities, with imaging acquired to evaluate atypical presentations and for complications. Ultrasound is the primary imaging test obtained within the first 48-72 hours in a patient presenting for the first time with classic pancreatitis, in order to assess for the presence of gallstones. Transabdominal ultrasound is limited in evaluating the pancreas in the acute inflammatory phase, and findings may be subtle or absent in mild cases. Grayscale assessment includes evaluation of the pancreatic parenchyma for signs of hemorrhage or necrosis, and peripancreatic tissues for the presence of fluid and fluid collections. The duct is visualized for signs of obstruction, either from stones in the common bile duct or secondary to pancreatic edema. Color Doppler can demonstrate the presence of splenic vein thrombosis. Chronic Pancreatitis Chronic pancreatitis results from progressive destruction of the gland secondary to multiple episodes of mild or even subclinical pancreatitis, with development of fibrosis and atrophy. Ultrasound is not sensitive for the diagnosis; however, the presence of ductal dilatation with ductal and parenchymal calcifications is highly suggestive. The location of stones, i.e., intraductal vs. parenchymal, may be better demonstrated with ultrasound than with CT. Diffuse or focal enlargement of the gland is common, and the appearance can mimic neoplasm, particularly when focal in the pancreatic head. Contrast-enhanced MR and endoscopic ultrasound (EUS) are useful for distinguishing the two. 350

PART II<br />

SECTION 3<br />

Pancreas<br />

Introduction <strong>and</strong> Overview<br />

Approach to Pancreatic Sonography 350<br />

Pancreatitis<br />

Acute Pancreatitis 354<br />

Pancreatic Pseudocyst 358<br />

Chronic Pancreatitis 362<br />

Simple Cysts <strong>and</strong> Cystic Neoplasms<br />

Mucinous Cystic Pancreatic Tumor 366<br />

Serous Cystadenoma of Pancreas 370<br />

Intraductal Papillary Mucinous Neoplasm (IPMN) 374<br />

Solid-Appearing Pancreatic Neoplams<br />

Pancreatic Ductal Carcinoma 380<br />

Pancreatic Neuroendocrine Tumor 384<br />

Solid Pseudopapillary Neoplasm 388

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