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Pancreatic Ultrasonography
THE PANCREAS REVISITED I: DIAGNOSTIC, CHRONIC PANCREATITIS
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PANCREATIC ULTRASONOGRAPHY Genevieve L. Bennett, MD, and Lucy E. Ham, MD
Abdominal ultrasonography (US) is used for initial imaging evaluation of patients with abdominal pain, jaundice, and nonspecific abdominal complaints. As a screening examination, US often provides the first opportunity to diagnose pancreatic abnormalities. Diagnostically, US findings may identify pancreatic abnormalities, differentiate benign pancreatitis from neoplasms, and guide percutaneous interventional procedures and biopsies for definitive diagnosis. Also, findings on US are useful to determine the need and priority of subsequent imaging, such as contrast-enhanced CT, MR imaging, and endoscopic or laparoscopic US, that may be required to assess extent of disease and to stage pancreatic tumors. This article discusses current techniques for pancreatic US, including the role of Doppler imaging and harmonic US. The normal and abnormal appearance of the pancreas are described, with emphasis on characteristic US findings that may aid diagnosis. This section includes the differential diagnosis of pancreatic abnormalities detected by US because there is considerable overlap in imaging appearance of benign and malignant conditions. The role of US-guided biopsy of the pancreas also is included. Newer applications of US for evaluation of pancreatic transplants used for the treatment of diabetes also is discussed. TECHNIQUE
Abdominal US is performed after a minimum fast of 6 hours to allow for distension of the gallbladder. The fasting state also may improve visualization of the pancreas because there is less gaseous distention of the stomach and upper gastrointestinal tract. Scanning is performed with a curved array or
From the Department of Radiology, New York University Medical Center (GLB); the Department of Radiology, Weill Medical College, Cornell University Medical Center (LEH); and the Memorial Sloan-Kettering Cancer Center (LEH), New York, New York
SURGICAL CLINICS OF NORTH AMERICA VOLUME 81 * NUMBER 2 * APRIL 2001
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sector 3.5- to 5.0-MHz transducer. The patient's position is varied to minimize intervening bowel gas, and scans may be obtained in the erect, supine, and shallow left or right decubitus positions. US offers multiplanar imaging of the pancreas; longitudinal and transverse images are included in the standard examination, but the orientation of the images may seem less familiar to the surgeon than CT images that are in the axial plane. This difference in image display is inherent in US, which is limited by artifacts from air interfaces; therefore, views are obtained from various projections using the solid organs, such as the liver, spleen, and kidneys, as an "acoustic window" to deeper structures, such as the pancreas. HOW RELIABLE IS ULTRASONOGRAPHY FOR PANCREATIC DIAGNOSIS?
Some reports have indicated that intervening bowel gas limits US visualization of the pancreas, especially of the pancreatic tail.", 36, 47, 56 Other studies have shown, however, that experienced sonographers and newer equipment increase 15,39, 43, 67 Campbell and the rates of pancreatic visualization to more than W i l s ~ n 'used ~ US to evaluate 51 patients with new pancreatic neoplasms. A technically adequate study was defined as one in which the splenic vein, portal venous confluence, pancreatic head, neck, body, and tail were all seen. The pancreatic tail was visualized from the epigastrium and left flank positions. Oral contrast was not used. All 51 patients had technically adequate pancreatic studies, and 50 of 51 pancreatic masses were detected by US alone. Similar findings were reported by Karlson et al,43who performed pancreatic US on 919 patients; 140 of these patients had pancreatic tumors diagnosed within 1 year of US as reported by the Swedish Death and Cancer Registry. US detected 124 of 140 pancreatic masses, for a sensitivity of 88.6% for all tumors and a sensitivity of 90% (79 of 88 patients) for detection of exocrine pancreatic cancer. 43 highlight that the diagnostic efficiency of pancreatic These two studiesI5* US has improved compared with earlier reports. These differences may be explained, in part, by advances in US equipment that now provide better resolution and tissue contrast, but other factors also should be considered. US is the most operator-dependent cross-sectional imaging technique. Karlson et ale evaluated interobserver variability for diagnostic accuracy of pancreatic abnormalities. The operator's level of US experience was a significant factor in the diagnostic accuracy of pancreatic abnormalities among three sonographers with varying experience. The excellent results of pancreatic US diagnosis reported by Campbell and Wilson15 also may be explained by their meticulous technique and experience. TECHNOLOGIC ADVANCES THAT AID ULTRASONOGRAPHY DIAGNOSIS OF THE PANCREAS Oral Contrast Agents
Gastrointestinal tract air may significantly limit pancreatic visualization caused by artifact on US. To eliminate shadowing from intraluminal air, the stomach can be filled with fluid that acts as an acoustic window through which the pancreas may be seen. Degassed water is most commonly used. Occasionally, air may be ingested with the fluid, further exacerbating air artifact, but little air
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is swallowed if the patient is instructed to drink slowly and a straw is not used. By shifting patient position, the fluid in the stomach may be moved to allow for visualization of each portion of the pancreas.’ Orally administered US contrast agents, including simethicone and methylcellulose, also have been used to im53 These agents absorb gas bubbles, have prove visualization of the pan~reas?~, decreased transit time compared with water, and are superior to water for reduction of gas shadowing. In a phase 2 trial, Lev-Taoff et a149reported that oral contrast provided better images of the pancreatic head in 61% and pancreatic tail in 67% of 99 patients who were scanned before and after the administration of US oral contrast. Optimal results were obtained when imaging was performed immediately after oral contrast ingestion. Oral contrast also increased diagnostic confidence for exclusion of pancreatic dis0rders.4~ Other investigators have used a combination of water and simethicone as an inexpensive alternative for the reduction of gas artifact. This technique was described by Abu-YouseP in a study of 65 patients who had limited visualization of the pancreatic tail on standard US. In 74% of patients, the pancreatic tail was seen in its entirety after the administration of water and simethicone, and in an additional l8%, visualization of the pancreatic tail was significantly improved.’ These US techniques using oral agents to disperse gas may reduce the need for additional CT imaging of the pancreas in selected patients. Harmonic Imaging
Tissue harmonic imaging is a new US technique that provides higher-quality images, decreased artifact, and improved tissue contra~t?~,As the US beam passes through tissues, it generates higher harmonic frequencies that are multiples of the transmitted frequency. These harmonics are similar in principle to the harmonics generated with musical instruments. The harmonic US signal is created within the tissues and is therefore not degraded by artifacts from the body wall. Harmonic US has increased resolution and improved contrast at tissue interfaces. Shapiro et a1@used conventional and harmonic US to evaluate the pancreas in 60 patients. Harmonic US images of the pancreas improved penetration in 45 patients (75%), provided better detail in 54 (go%), and improved total image quality in 50 patients (83%) as judged by three radiologists “blinded to the US technique used to generate the image.@Tissue harmonic imaging is routinely available, and the improvement in image quality has made it the technique of choice for US evaluation of the abdomen, including the pan~reas?~, @
@
NORMAL PANCREATIC ANATOMY ON ULTRASONOGRAPHY
The pancreas usually has uniform texture, and pancreatic echogenicity is slightly greater than, or the same as, that of the liver. Fatty infiltration of the pancreas that occurs with advanced age and obesity may increase pancreatic echogenicity so that it resembles the adjacent retroperitoneal fat. Occasionally, a texture change may be observed in the uncinate and head of the pancreas caused by focal fatty sparing within the ventral p a n c r e a ~ .This ~ , ~normal ~ variant can be differentiated from tumor by the absence of mass effect or vascular or ductal displacement. The pancreas is defined by the adjacent vasculature (Fig. 1).On transverse US images, the body of the pancreas is seen anterior to the splenic vein,
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Figure 1. Normal pancreatic anatomy. Transverse ultrasonography (US) reveals a normal sized pancreas (white arrows), with adjacent peripancreatic vasculature. Aorta and origin of right renal artery (a), inferior vena cava (i), left renal vein (curved arrow), splenic vein, and superior mesenteric vein confluence (s), superior mesenteric artery (black arrow).
with the superior mesenteric vein and superior mesenteric artery visualized posteriorly. The left renal vein courses transversely between the superior mesenteric vein and aorta. On sagittal views, the pancreas contacts the inferior vena cava. This vascular anatomy is well depicted by gray-scale US. When spectral or color Doppler also is used, vascular patency and flow direction within the peripancreatic vessels also may be documented on US. The pancreatic duct is best seen transversely, and it normally is less than 2 mm diameter in the body of the pancreas and 3 mm in the head34(Fig. 2).
Figure 2. Pancreatic duct. In the body of the pancreas, a 2-mm normal-caliber pancreatic duct (arrow and calipers) is well visualized. a = aorta; p = portal vein.
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ULTRASONOGRAPHY IMAGING IN ACUTE AND CHRONIC PANCREATITIS Acute Pancreatitis
US traditionally has a limited role in establishing the diagnosis of acute pancreatitis. Visualization of the pancreas in patients with pancreatitis often is limited because of ileus and superimposed bowel gas. Also, the US appearance of the pancreas may be normal in early pancreatitis. CT is considered to be the imaging modality of choice for the diagnosis and staging of acute pancreatitis because of its superb ability to show early inflammatory change, extrapancreatic fluid collections, and pancreatic necrosis.6,7, CT also has been found to be helpful in predicting the severity of an attack of acute pancreatitis and the development of complications and in establishing prognosis?, US, however, does have a role in the evaluation of patients with acute pancreatitis. It often is the first examination for the evaluation of patients with abdominal pain, and it is the best method for the assessment of the gallbladder and biliary tree. When a patient presents with suspected acute pancreatitis, US is used to evaluate the gallbladder and biliary tree and to exclude other causes of abdominal pain, such as cholecystitis. The finding of gallstones provides a likely etiology for acute pancreatitis, and the identification of choledocholithiasis and biliary obstruction, with obstructive jaundice or cholangitis, affects immediate patient management because these patients may be triaged for emergent endoscopic retrograde cholangiopancreatography (ERCP) and sphincterotomy. Whether the mere presence of gallstones in a patient with acute pancreatitis is an indication for routine presurgical ERCP is contr~versial.'~, 27 The reported sensitivity, specificity, and accuracy of US for the detection of cholelithiasis are more than 95%.19, 37, 44 CT is less sensitive because many stones are composed of calcium, bile pigment, and cholesterol and are isoattenuating to bile? Sensitivity of US for choledocholithiasis ranges from 55% to 85%, with a specificity of 89% to 91%?0,45, Choledocholithiasis is confidently identified when there is an echogenic shadowing focus in the common bile duct (CBD) surrounded by bile (Fig. 3A). The limitations of US for the detection of CBD stones include stones in nondilated ducts, nonshadowing stones, and stones present in the distal CBD, which often is obscured by bowel gas. Helical CT has proven to be a useful adjunctive imaging technique for the detection of choledocholithiasis, with a reported sensitivity and specificity of 88% and 97% relative to ERCP (Fig. 3B). Magnetic resonance cholangiopancreatography (MRCP) also has offered sensitivities in the range of 92% to 93%, and diagnostic accuracy of as much as 97%.'O~~~ US findings related to the pancreas vary depending on the severity of pancreatitis, timing of imaging, and ability to achieve an adequate US window. This latter issue is improved with optimized imaging t e c h n i q ~ e s Scans . ~ ~ obtained in the semiupright and right posterior oblique position can facilitate visualization of the pancreatic head and distal CBD with fluid accumulation in the gastric antrum and proximal duodenum, and displacement of air to the region of the gastric fundus. The left lobe of the liver serves as an acoustic window for evaluation of the pancreatic body and lesser sac, while the spleen can serve as a window for evaluation of the pancreatic tail and anterior pararenal space. In early pancreatitis, the pancreas shows normal size and US echotexture. Eventually, there will be a change in echotexture of the gland, becoming more hypoechoic and heterogeneous relative to the healthy pancreas. Subsequently,
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Figure 3. Choledocholithiasis. A, Common bile duct stone (s)appears as echogenic focus in the distal common bile duct (CBD), with associated posterior acoustic shadowing (arrow). B, Helical CT image reconstructed in the coronal plane shows stone (arrow) in distal
CBD (d).
the gland will enlarge and develop an irregular contour (Fig. 4). The margins of the pancreas become indistinct, secondary to peripancreatic inflammatory changes and edema within the peripancreatic fat. Dilatation of the pancreatic duct may be observed. A focal intrapancreatic abnormality may be secondary to complications such as fluid collection, hemorrhage, or necrosis. US cannot differentiate between necrotic and non-necrotic pancreatic tissue, and CT is considered the study of choice for identifying pancreatic necrosis." 42 Necrosis appears on CT as an area of nonenhancing parenchyma. MR imaging can be used for the evaluation of patients who cannot receive intravenous contrast. If the gland shows significantly altered echotexture on US, or if no normal definable pancreatic tissue is present, CT evaluation should be performed (Fig. 5 ) . Occasionally, pancreatitis may be a focal process, with involvement of one segment of the gland, usually the head. On US, this appears as a focal enlargement with altered echotexture. On occasion, this is difficult to distinguish from a pancreatic Clinical correlation is important, as is a history of pancreatitis. The presence of calcification is reassuring; however, CT or MR evaluation may be necessary. Also, ERCP may be helpful to show changes within the pancreatic duct. In addition to the evaluation of the gland, US can be used to identify extrapancreatic spread of inflammation. Fluid collections are commonly seen within the lesser sac, anterior pararenal spaces, transverse mesocolon, and small
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Figure 4. Acute pancreatitis related to recent endoscopic retrograde cholangiopancreatography (ERCP). A, US reveals diffuse enlargement of the pancreas (p), with overall decreased echogenicity and irregular contour. 6, CT scan performed with intravenous contrast demonstrates enlarged, heterogeneously enhancing pancreas (p). There is contrast present in the distal CBD (d) caused by recent ERCP. g = gallbladder; s = splenic vein.
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Figure 5. Severe acute pancreatitis with pancreatic necrosis. A, Transverse US demonstrates enlarged hypoechoic, heterogeneous pancreas (p). B, CT scan performed with intravenous contrast demonstrates only a small amount of enhancing pancreatic tissue in the region of the head (h). The remainder of the gland is nonenhancing, consistent with necrosis (n).
bowel mesentery and peripancreatic spaces.4O This fluid can have a spectrum of US appearances, from simple anechoic fluid to complex fluid with septations. Other extrapancreatic findings include ascites, bowel wall thickening, and thickening of the gallbladder wall. US also may have an important role in the identification of complications related to pancreatitis. A pseudocyst is a well-defined, walled-off fluid collection that persists on serial examinations for at least 4 weeks after the onset of inflamrnati0n.2~Pseudocysts develop in 10% to 20% of patients with acute pancreatitis and usually require 4 to 6 weeks to form.62Diagnosis can be suggested based on clinical and laboratory parameters but can be confirmed with a variety of imaging techniques. Typically, a pseudocyst appears on US as a welldefined fluid collection that is anechoic with posterior acoustic enhancement (Fig. 6). These may have a more complex appearance if infected or complicated by hemorrhage4*(Fig. 7). Other complications include rupture, biliary tract obstruction, and involvement of the gastrointestinal tract, liver, or spleen. Color and spectral Doppler US have a crucial role in the identification of
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Figure 6. Pseudocyst. A, Pseudocyst appears on US as well-circumscribed anechoic fluid collection (c) anterior to the tail of the pancreas (p). 6, CT with intravenous contrast demonstrates well circumscribed fluid collection (c) anterior to the pancreas.
vascular complications of acute pancreatitis. These include venous thrombosis and pseudoaneurysm formation. Venous thrombosis can involve the portal or splenic veins and can manifest as complete thrombosis with lack of flow or as an echogenic filling defect within the vein. If thrombosis is chronic, cavernous transformation of the portal vein results. Pseudoaneurysms most commonly involve the splenic artery, gastroduodenal arteries, pancreaticoduodenal arcade, or occasionally the hepatic artery and result from severe necrotizing pancreatitis or erosion by a pseudocyst. These are readily identified with color Doppler and show characteristic findings with spectral Doppler. US cannot distinguish an uninfected peripancreatic fluid collection or pseudocyst from an infected one. The presence of echogenic foci corresponding to gas bubbles is suggestive; however, air also may be present secondary to fistula with bowel. If there is any clinical suspicion, US can provide image guidance for fluid aspiration or drainage and for nonsurgical decompression of a pseudocyst!
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Figure 7. Complex pseudocyst (c) with debris and septations. Needle aspiration demonstrated evidence for hemorrhage.
Chronic Pancreatitis
US findings in patients with chronic pancreatitis include alterations in the size and echotexture of the gland, calcifications, pancreatic duct dilatation and irregularity, bile duct dilatation secondary to stricture, and chronic p~eudocysts.~ The gland generally atrophies and becomes heterogeneous in echotexture secondary to fibrosis and calcification. Calcifications appear as hyperechoic foci with associated posterior acoustic shadowing, which may be focal or distributed throughout the gland (Fig. 8). They usually are intraductal, resulting from deposition of calcium carbonate on intraductal protein plugs. The pancreatic duct may be dilated and often is irregular in appearance, with dilatation of side branches. This may be difficult to distinguish from other causes of pancreatic duct dilatation, such as cancer. A history of pancreatitis and presence of calcifications without definite focal mass are reassuring; however, further evaluation with helical CT, MR, or ERCP may be necessary. DIFFERENTIAL DIAGNOSIS OF PANCREATIC ABNORMALITIES
Because US often is the initial imaging study for evaluation of the pancreas, it is useful to differentiate the appearance of pancreatitis on US from neoplasms to appropriately triage patients. Pancreatic Duct Dilation
The intraductal papillary mucinous tumor (IPMT) may mimic pancreatic duct dilation of chronic pan~reatitis?~, 66 This pancreatic cystic neoplasm spreads in the pancreatic ducts and secretes thick mucin that fills the main or branch ducts and causes ductal dilatation (Fig. 9). Many of the radiologic
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Figure 8. Chronic pancreatitis with calcification. A, Prominent pancreatic duct (pd) contains echogenic foci with posterior acoustic shadowing within the pancreas, consistent with calcification (arrows) from pancreatitis. 6, CT scan performed with intravenous contrast demonstrates calcifications within the pancreas (p).
features mimic the main duct dilatation seen in patients with chronic pancreatitis. When IPMT involves the entire length of the duct with associated parenchymal atrophy, differentiation from chronic pancreatitis may be impossible. Markedly dilated branch ducts may mimic pseudocysts. CT, MR, endoscopic US (EUS), and ERCP may allow for further characterization. The presence of mural nodules, much globs, or a solid mass identified by any of these modalities is important in making the correct diagnosis. A virtually pathognomonic finding is dilatation of the major or minor papilla, or both, with bulging into the duodenal lumen. Diagnosis of IPMT with ERCP is certain when much is observed leaking from the papilla. Solid Mass Adenocarcinorna
A focal mass, observed in as many as 40% of patients with chronic pancreatitis, may appear hyper- or hypoechoic and may be difficult to distinguish from
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Figure 9. Dilated pancreatic duct with intraductal papillary tumor. Transverse US image of the body of the pancreas reveals a dilated pancreatic duct (d) and polypoid intraductal mass (arrow). v = superior mesenteric vein.
cancer2,54(Figs. 10 and 11).The presence of calcifications favors the diagnosis of chronic pancreatitis because calcification in cancer is unusual. Correlation with helical CT, with thin-section imaging of the pancreas during the dynamic bolus infusion of intravenous contrast or MR imaging, is recommended to further characterize and assess for spread of disease. These procedures also may be inconclusive, and image-directed biopsy or close follow-up to assess for interval change may be necessary. The utility of endoscopic US for the diagnosis of early changes of chronic pancreatitis and for characterizing focal abnormalities is the subject of current investigation.16 Vascular invasion is an early hallmark of pancreatic cancer. Color Doppler US is useful for determining the patency of peripancreatic vessels and can be used to determine resectability of pancreatic cancer. Although pancreatitis may be associated with venous thrombosis, frank encasement of the celiac axis, superior mesenteric vessels, and splenoportal confluence are more common in pancreatic cancer (Fig. 12). Color Doppler US is useful for determining the patency of peripancreatic vessels and can be used to determine resectability of pancreatic cancer?', 72* 74 Ralls et a161 reported on color Doppler findings in 51 patients with pancreatic cancer, 49 of whom had technically complete examinations with visualization of all index vessels. The lesions of all 30 patients who were considered to have unresectable disease by color Doppler were unresectable at surgery. The lesions of 6 of 15 patients (40%) that were considered resectable on US were unresectable for cure. Atrophy of the gland and pancreatic duct dilation may be present in pancreatitis and pancreatic cancer, but when these findings are present only distal to a mass, the diagnosis of pancreatic cancer is favored. Bile duct obstruction also may be present in pancreatitis and pancreatic cancer, but abrupt cutoff of the duct and the double duct sign are more common in pancreatic cancer than in pancreatitis. Hepatic metastases detected on US may confirm the diagnosis of malignancy. Metastases are more common at presentation of pancreatic body and tail tumors because these usually become symptomatic at a later stage than pancreatic head tumors that cause biliary obstruction.
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Figure 10. Focal chronic pancreatitis. A, Transverse US image of an enlarged head of pancreas (p) with calcifications. 6,MR image correlation demonstrates enlarged head of pancreas (p).
Other Solid Masses Neuroendocrine tumors of the pancreas derived from the amine precursor uptake and decarboxylation (APUD) system may be functioning or nonfunctioning and may be associated with multiple endocrine neoplasia (MEN) syndrome?, 22 Patients with functioning tumors, such as insulinomas and gastrinomas, usually present with clinical symptoms related to excessive insulin, gastrin, or other hormones, but approximately one third of neuroendocrine tumors are nonfunctioning. Ninety percent of insulinomas are benign, but the prevalence of malignancy is higher for gastrinomas and other functioning islet cell tumors. Nonfunctioning tumors are more likely malignant. Helical CT and MR imaging are preferred for presurgical imaging of neuroendocrine tumors.3,52, 68 Using arterial and venous phase helical CT, Van Hoe et a P reported accurate presurgical identification of 9 of 11 pancreatic neuroendocrine tumors, one of which measured only 4 mm in diameter. The rate of transabdominal US detection of islet cell tumors is relatively low, in the range of 60%, because of the small size of these tumors.3,32, 33, 50,52* 63, 68 Neuroendocrine
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Figure 11. Bile-duct obstruction from carcinoma in the head of the pancreas. Longitudinal, left lateral decubitus US image reveals a pancreatic head carcinoma (calipers and curved arrow) obstructing a dilated CBD (bd).
tumors less than 2 cm in diameter usually are hypoechoic or isoechoic to the pancreas. Large islet cell tumors may be seen on US as echogenic, irregular masses with calcification or cystic areas from hemorrhage: Although transabdominal US is of limited value, intraoperative US (IOUS) of the pancreas is extremely accurate for the identification of islet cell tumors. IOUS has a true positive detection rate of 98'3'0, and tumors as small as 3 mm in diameter may be detected by IOUS.31,32 Also, IOUS may be useful to identify unsuspected multiple tumors, which are present in 10% of insulinomas and 20% to 40% of gastrin~mas.~~ Non-Hodgkin's lymphoma may involve the pancreas diffusely or as a focal mass (Fig. 13). It is hypoechoic on US, and the appearance may mimic pancreatitis if the patient presents with abdominal pain. The presence of bulky adenopathy, or adenopathy below the level of the renal veins, may be useful in establishing the diagnosis of lymphoma.51Lack of pancreatic duct dilation and patent vessels on Doppler imaging also are characteristic US findings in pancreatic lymphoma.51 Cystic Neoplasms
Cystic neoplasms are relatively uncommon, representing approximately 10% to 15% of pancreatic cysts and 1% of pancreatic cancers.41US provides excellent characterization of cystic lesions and easily depicts mural nodules, septations, and internal debris. Cystic neoplasms often have discriminating features on US imaging that may be useful for differentiation from pseudocysts of pancreatitis. Serous microcystic adenomas are benign neoplasms with multiple cysts ranging in size from 1 mm to 2 cm. Although microcystic adenomas are cystic neoplasms, they have a variable appearance on US depending on the size of the CyStS.13. 29. 30, 41 Microcystic adenomas with 1- to 2-mm cysts may appear solid because of the numerous interfaces produced by the walls of the small cysts. Microcystic adenomas with larger cysts have anechoic areas on US and, charac-
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Figure 12. Pancreatic adenocarcinoma involving the splenoportal venous confluence. A, Transverse US image at the level of the pancreatic neck reveals an irregular hypoechoic 2-cm mass (calipers and arrow). The mass compromises the splenic vein (SV). B, An oblique sonogram of the portal vein reveals focal nodularity (arrows)that narrows the lumen of the portal vein (pv). Tumor involvement of the portal vein was confirmed by histopathology. A = aorta.
teristically, these cysts are distributed in a peripheral location within the tumor (Fig. 14).Central stellate scars, shown on US as central linear echogenic areas, are present in approximately 13% of cases, and calcification may be present within the scar.41Tumors are well defined on US, with smooth margins. They occur more commonly in the pancreatic head, but because the tumors are relatively soft, there is no obstruction of the pancreatic duct or compromise of the peripancreatic vessels. Doppler imaging of microcystic adenomas aids in diagnosis. The lack of vascular encasement may be useful to differentiate these benign neoplasms from adenocarcinomas that typically involve the peripancreatic vessels. Color or power Doppler also may be used to show internal vascularity within microcystic adenomas, which are very vascular neoplasms that 29 have numerous vessels within the pseudocapsule and ~eptations.'~, In contrast to microcystic adenomas, macrocystic mucinous tumors of the
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Figure 13. Lymphoma of the pancreatic body and tail. A, Transverse sonogram shows an enlarged hypoechoic pancreatic body and tail (arrows). Appearance is consistent with neoplasm or inflammation. B, CT image shows low-attenuation mass (arrows) infiltrating the pancreatic tail. CT-guided biopsy revealed lymphoma. a = aorta; i = inferior vena cava.
pancreas are malignant or potentially malignant. Macrocystic mucinous cystadenoma and cystadenocarcinoma have larger cysts (>2 cm in diameter). Characteristic features of macrocystic tumors are thick septations, cysts with solid mural nodules, or cysts with echogenic f l ~ i d ~(Fig. , ~ l 15).Calcifications may be present, often are coarse, and are peripheral in distribution.4l Johnson et a14' reported that 78% of microcystic tumors and 95% of macrocystic adenomas or adenocarcinomas were correctly classified on US in a retrospective analysis. It is not possible to definitively distinguish between benign and malignant macrocystic tumors, but, in general, as solid components increase, there is a greater risk for 30 malignan~y.~, Another neoplasm that appears cystic on US is solid and papillary epithelial neoplasm (SPEN) of the pancreas. This tumor is uncommon, has low-grade malignant potential, and is seen in young women.I4 SPEN is characterized by areas of internal hemorrhage and cystic degeneration with fluid-debris levels and posterior enhancement on US.14Calcification may be present and is more
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Figure 14. Microcystic adenorna. A, Sagittal sonogram shows a large echogenic pancreatic head mass (rn, open arrows) with small peripheral cystic areas (curved arrows). The mass obstructs the CBD (CD). 6,T-2 weighted MR image shows the cysts within the pancreatic head mass (open arrows). gb = gallbladder; v = superior mesenteric vein.
readily identified on CT. MR imaging may provide specific diagnosis by demonstrating blood products. Mucinous cystic neoplasms may have similar features, but SPEN lacks internal septations and multiple loculations that are present in microcystic and macrocystic adenoma~.'~ OTHER APPLICATIONS OF ULTRASONOGRAPHY Ultrasonography-GuidedPercutaneous Biopsy of the Pancreas
US-guided fine-needle aspiration (FNA) biopsy provides definitive diagnosis of pancreatic abnormalities. Biopsy positive for malignancy has a reported specificity of 23, 57, 73 Di Stasi et alZ3 reported on US-guided percutaneous
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Figure 15. Macrocystic adenoma. A, Transverse sonogram reveals a cystic mass (calipers) with multiple thick septations in the tail of the pancreas. The splenic artery (s)contacts the mass (arrows) but is not involved. B, CT image shows mass (arrows) that contains septations and calcification (open arrow).
FNA biopsy results in 519 patients. For cytology, histology, and cytology plus histology, retrieval rates were 94%, 967'0, and 97%; sensitivity was 8770, 94%, and 94%; and diagnostic accuracy was 9170, 90%, and %YO,respecti~ely.~~ Brandt et all2 compared US- and CT-guided biopsy of the pancreas and found an accuracy of 95% for US and 86% for CT. The rate of false-negative results, including unsatisfactory specimens, was lower with US biopsies (3 of 58 procedures, 5%) compared with CT (30 of 211 procedures, 12%). Accuracy was higher with masses larger than 3 cm and larger needle size. Results also were better for masses located in the pancreatic body or tail rather than the head. The rate of major complications caused by percutaneous pancreatic biopsy is rare, at less than 1%.l2,65 Pancreatitis is the most common complication of percutaneous biopsy and is unrelated to the method of imaging guidance or type of needle used for the procedure. US-guided biopsies, using 18-gauge needles with automated, spring-loaded sampling devices, have reported sensitivity of 92% to 94%, with no increase in complication rate.", 65 Tumor seeding
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along the needle tract is rare (0.003%-0.009%) but is more common with pancreatic tumors than with other rnalignancie~.~~
Pancreatic Transplant Evaluation
Simultaneous pancreas and kidney transplantation is a therapeutic option for end-stage renal disease in patients with type 1 diabetes mellitus, and various complications may be identified with imagingz6Several techniques may be used for transplantation, and physicians should understand the surgical anatomy when assessing for potential complications. The transplanted pancreas is one of the most difficult organs to evaluate, and findings usually are not evident until there is extreme compromise of the transplant?* Complications include abscess or pseudocyst formation, vascular thrombosis, leak, or compromise of adjacent structures. The role of US is limited because the transplanted pancreas often is obscured by adjacent bowel. If the pancreatic duct is visualized, it can serve as a useful landmark. Also, differentiating between pancreatitis, rejection, and vascular compromise is difficult because these problems may result in a similar sonographic picture, with an enlarged heterogeneous, hypoechoic, edematous gland. US may identify peripancreatic fluid collections, such as abscess or hematoma, although CT usually provides better delineation of size and extent of these abnormalities and evaluation of the gland.21 Vascular complications are the second most common cause of pancreatic graft failure, after graft rejection. The most important role for US is in the evaluation of the transplant vasculature using color and spectral Doppler. Vascular complications, such as graft thrombosis (arterial or venous), stenosis, or pseudoaneurysm formation, can be identified and subsequently confirmed with conventional angiography or MR angiographyZ6The ability to use resistive index elevation as a positive predictor of transplant rejection has not proven useful because many rejecting transplants have a normal resistive index (RI), and elevation of the RI is nonspecific and can be seen with vascular compromise, in a series of 11 patients with venous such as venous t h r o m b ~ s i s ~ however, ~; thrombosis evaluated with duplex US, Foshager et alZ8found that reversal of diastolic flow in pancreatic transplant arteries was highly specific for the detection of graft venous thrombosis during the first 12 days after transplantation. Also, they found that an RI greater than or equal to 1.00 and absence of venous flow, in combination, are highly sensitive and specific for the diagnosis of pancreatic graft venous thrombosis.
SUMMARY
Pancreatic abnormalities usually are detected on US when it is used for screening patients with abdominal pain and for assessment of the gallbladder and bile ducts. Pancreatic visualization is limited by bowel gas, but with experienced sonographers and newer techniques, including harmonic imaging and oral contrast US, diagnosis of pancreatic abnormalities has significantly improved compared with earlier reports. Appropriate initial diagnosis by US can tailor further investigation, and US-guided biopsy may establish definitive diagnosis.
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References 1. Abu-Yousef MM. Improvement in pancreatic tail visualization by sonography using simethicone water and patient rotation. J Ultrasound Med 19(suppl):65,2000 2. Alpem MB, Sandler MA, Kellman GM, et a1 Chronic pancreatitis: Ultrasonic features. Radiology 155:215219, 1985 3. Apestrand F, Kolmannskog F, Jacobsen M: CT, MR imaging and angiography in pancreatic apudomas. Acta Radio1 34:468-473, 1993 4. Atri M, Finnegan PW. The pancreas. In Rumack CM, Wilson SR, Charboneau JW (eds): Diagnostic Ultrasound, ed 2. St. Louis, Mosby, 1998, pp 241-256 5. Atri M, Nazarria S, Mehio A, et a1 Hypoechogenic embryologic ventral aspect of the head and uncinate process of the pancreas: in vitro correlation of US with histopathologic findings. Radiology 190:441-444, 1994 6. Balthazar EJ, Freeny PC, vansonnenberg E: Imaging and intervention in acute pancreatitis. Radiology 193297-306, 1994 7. Balthazar EJ, Robinson DL, Megibow AJ, et al: Acute pancreatitis: Value of CT in establishing prognosis. Radiology 174331-336,1990 8. Balthazar EJ, Ranson JH, Naidich DP, et al: Acute pancreatitis: Prognostic value of CT. Radiology 156:767-772, 1985 9. Barakos JA, Ralls PW, Lapin SA, et al: Cholelithiasis: Evaluation with CT. Radiology 162:415418, 1987 10. Boraschi P, Neri E, Braccini G, et al: Choledocholithiasis: Diagnostic accuracy of MR cholangiopancreatography. Three year experience. Magn Reson Imaging 1712451253, 1999 11. Brambs HJ, Claussen CD: Pancreatic and ampullary carcinoma: Ultrasound, computed tomography, magnetic resonance imaging and angiography. Endoscopy 25:58-68,1993 Stephens DH, et al: CT-and USguided biopsy of the 12. Brandt KR, Charboneau JW, pancreas. Radiology 18799-104,1993 13. Buck JL, Hayes W S From the Archives of the AFIP Moser RP: Microcystic adenoma of the pancreas. Radiographics 10313-322, 1990 14. Buetow PC, Buck JL, Pantongrag-Brown L, et al: Solid and papillary epithelial neoplasm of the pancreas: Imaging-pathologic correlation in 56 cases. Radiology 199:707711,1996 15. Campbell JP, Wilson SR Pancreatic neoplasms: How useful is evaluation with US? Radiology 167341-344,1998 16. Catalan0 MF, Lahoti S, Geenen JE, et al: Prospective evaluation of endoscopic ultrasonography, endoscopic retrograde pancreatography, and secretin test in the diagnosis of chronic pancreatitis. Gastrointest Endosc 48:ll-17, 1998 17. Chang L, Lo S, Stabile BE, et al: Preoperative versus postoperative endoscopic retrograde cholangiopancreatography in mild to moderate gallstone pancreatitis: A prospective randomized trial. Ann Surg 231:82-87,2000 18. Clavien PA, Hauser H, Meyer P, et a1 Value of contrast-enhanced computerized tomography in the early diagnosis and prognosis of acute pancreatitis. Am J Surg 155:457466, 1998 19. Cooperberg PL, Burhenne HJ: Real-time ultrasonography: Diagnostic technique of choice in calculous gallbladder disease. N Engl J Med 3021277-1279, 1980 20. Cronan JJ: US diagnosis of choledocholithiasis: A reappraisal. Radiology 161:133-134, 1986 21. Dachman AH, Newmark GM, Thistlethwaite JR, et a1 Imaging of pancreatic transplantation using portal venous and enteric exocrine drainage. AJR Am J Roentgen01 171:157-163, 1998 22. Davies PF, Shevland JE, Shepherd JJ: Ultrasonography of the pancreas in patients with multiple endocrine neoplasia type I. J Ultrasound Med 2:67-72, 1993 23. Di Stasi M, Lencioni R, Solmi L, et al: Ultrasound-guided fine needle biopsy of pancreatic masses: Results of a multicenter study. Am J Gastroenterol 93:1329-1333, 1998 24. Donovan PJ, Sanders RC, Siegelman SS Collections of fluid after pancreatitis: Evalua-
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tion of computed tomography and ultrasonography. Radiol Clin North Am 20:653665, 1982 Elvin A, Andersson T, Scheibenpflug L, et a1 Biopsy of the pancreas with a biopsy gun. Radiology 176:677-679, 1990 Eschwege P, Rocher L, Rondeau Y, et a1 Complication of pancreas transplantation: Radiological evaluation. Transplant Proc 302795,1998 Folsch U, Nitsche R, Lutdtke R, et al: Early ERCP and papillotomy compared with conservative treatment for acute biliary pancreatitis. N Engl J Med 336237-242, 1997 Foshager MC, Hedlund LJ, Troppmann C, et a1 Venous thrombosis of pancreatic transplants: Diagnosis by duplex sonography. AJR Am J Roentgenol 169:1269-1273, 1997 Friedman AC, Lichenstein JE, Dachman AH: Cystic neoplasms of the pancreas. Radiology 149:45-50,1983 Fugazzola C, Procacci C, Bergamo IA,et al: Cystic tumors of the pancreas: Evaluation by ultrasonography and computed tomography. Gastrointest Radiol 16:53-61, 1991 Gorman B, Charboneau J W Intraoperative ultrasound of the pancreas. In Kane RA (ed): Intraoperative, Laparoscopic and Endoluminal Ultrasound. Philadelphia, Churchill Livingstone, 1999, pp 68-74 James EM, et al: Benign pancreatic insulinoma: PreoperaGorman 8, Charboneau JW, tive and intraoperative sonographic localization. AJR Am J Roentgenol 147:929-934, 1986 Gunther RW, Klose KJ, Ruckert K, et al: Islet-cell tumors: Detection of small lesions with computed tomography and ultrasound. Radiology 148:485488, 1983 Hadidi A: Pancreatic duct diameter: Sonographic measurement in normal subjects. J Clin Ultrasound 11:17-22, 1983 Hann LE, Bach AM, Cramer LD, et al: Hepatic sonography: Comparison of tissue harmonic and standard sonography techniques. AJR Am J Roentgenol 173:201-206, 1999 Hessel SJ, Siegelman SS, Mc Neil BJ, et al: A prospective evaluation of computed tomography and ultrasound of the pancreas. Radiology 143:129-133, 1982 Hessler PC, Hill DS, Detorie FM, et al: High accuracy sonographic recognition of gallstones. AJR Am J Roentgenol 136:517-520, 1981 Jacobs JE, Coleman BG, Arger PH, et al: Pancreatic sparing of focal fatty infiltration. Radiology 190:437439, 1994 Jeffrey RB: Sonography in acute pancreatitis. Radiol Clin North Am 27:5-17, 1989 Jeffrey RB, Laing FC, Wing VW: Extrapancreatic spread of acute pancreatitis: New observations with real-time ultrasound. Radiology 159:707-711, 1986 Johnson CD, Stephens DH, Charboneau JW, et a1 Cystic pancreatic tumors: CT and sonographic assessment. AJR Am J Roentgenol 151:1133-1138, 1988 Johnson CD, Stephens DH, Sarr MG: CT of acute pancreatitis: Correlation between lack of contrast enhancement and pancreatic necrosis. AJR Am J Roentgenol 156:93-95,1991 Karlson BM, Ekbom A, Lundgren PG, et a1 Abdominal US for diagnosis of pancreatic tumor: Prospective cohort analysis. Radiology 213:107-111, 1999 Laing FC, Federle MP, Jeffrey RB, et al: Ultrasonographic evaluation of patients with acute right upper quadrant pain. Radiology 140:449455,1981 Laing FC, Jeffrey RB, Wing VW, et al: Biliary dilatation: Defining the level and cause by real-time US. Radiology 160:3942, 1986 Laing FC, Jeffrey RB: Choledocholithiasis and cystic duct obstruction: Difficult ultrasonographic diagnosis. Radiology 146:475479, 1983 Lawson TL: Sensitivity of pancreatic ultrasonography in the detection of pancreatic disease. Radiology 128:733-736, 1978 Lee CM, Chiang-Chien CS, Lim DY, et al: Real-time ultrasonography of pancreatic pseudocyst: Comparison of infected and uninfected pseudocysts. J Clin Ultrasound 16:393-397, 1988 Lev-Toaff AS, Langer JE, Rubin DL, et a1 Safety and efficacy of a new oral contrast agent for sonography: A phase I1 trial. AJR Am J Roentgenol 173:431436,1999 London JF, Shawker TH, Doppman JL, et a1 Zollinger-Ellison syndrome: Prospective
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assessment of abdominal US in the localization of gastrinomas. Radiology 178:763767, 1991 Merkle EM, Bender GN, Brambs HJ: Imaging findings in pancreatic lymphoma. AJR Am J Roentgenol 174:671475, 2000 Moore NR, Rogers CE, Britton BJ: Magnetic resonance imaging of endocrine tumors of the pancreas. Br J Radiol 68:341-347, 1995 Muradali D, Bums PN, Pron G, et al: Improved retroperitoneal and gastrointestinal sonography using oral contrast agents in a porcine model. AJR Am J Roentgenol 171:475481, 1998 Neff CC, Simeone JF, Wittenberg J, et al: Inflammatory pancreatic masses: Problems in differentiating focal pancreatitis from carcinoma. Radiology 150:3540, 1994 Neitlich JD, Topazian M, Smith RC, et al: Detection of choledocholithiasis: Comparison of unenhanced helical CT and endoscopic retrograde cholangiopancreatography.Radiology 203:753-757, 1997 Passanen P, Partanen K, Pikkarainen P, et al: Diagnostic accuracy of ultrasound, computed tomography and endoscopic retrograde cholangiopancreatography in detection of pancreatic cancer in patients with jaundice or cholestasis. In Vivo 6297-302, 1992 Pilotti S, Rilke F, Claren R, et a1 Conclusive diagnosis of hepatic and pancreatic malignancies by fine needle aspiration. Acta Cytol 3297-38, 1988 Pozniak MA, Propeck PA, Kelcz F, et al: Imaging of pancreas transplants. Radiol Clin of North Am 33:581-594, 1995 Procacci C, Graziani R, Bicego E, et al: Intraductal mucin producing tumors of the pancreas: Imaging findings. Radiology 198:249-257, 1996 Procacci C, Megibow AJ, Carbognin G, et al: Intraductal papillary mucinous tumor of the pancreas: A pictorial essay. Radiographics 19:1447-1463, 1999 Ralls PW, Wren SW, Radin R, et al: Color flow sonography in evaluating the resectability of periampullary and pancreatic tumors. J Ultrasound Med 16131-140, 1997 Rattner DW, Warshaw AL: Surgical intervention in acute pancreatitis. Crit Care Med 16:85-95,1988 Semelka RC, Cumming MJ, Shoenut JP, et al: Islet cell tumors: Comparison of dynamic contrast-enhanced CT and MR imaging with dynamic gadolinium enhancement and fat suppression. Radiology 186:799-802, 1993 Shapiro RS, Wagreich J, Parsons RB, et al: Tissue harmonic imaging sonography: Evaluation of image quality compared with conventional sonography. AJR Am J Roentgenol 171:1203-1206, 1998 Smith E H Complications of percutaneous abdominal fine-needle biopsy. Radiology 178~253-258,1991 Sugiyama M, Atomi Y Intraductal papillary mucinous tumors of the pancreas. Ann Surg 228:685491, 1998 Tanaka S, Kitamra T, Yamamoto K, et a1 Evaluation of routine sonography for early detection of pancreatic cancer. J Clin Oncol 26422427, 1996 Van Hoe L, Gryspeerdt S, Marchal G, et al: Helical CT for the preoperative localization of islet cell tumors of the pancreas: Value of arterial and parenchymal phase images. AJR Am J Roentgenol 165:1437-1439, 1995 Varghese JC, Liddell RP, Farrell MA, et al: The diagnostic accuracy of magnetic resonance cholangiopancreatography and ultrasound compared with direct cholangiography in the detection of choledocholithiasis. Clin Radiol 54604-614, 1999 Vassilopoulou-Sellin R, Ajani J: Islet cell tumors of the pancreas. Endocrinol Metab Clin North Am 23:5345, 1994 Wong JJ, Krebs TL, Klassen DK, et al: Sonographic evaluation of acute pancreatic transplant rejection: Morphology-Doppler analysis versus guided percutaneous biopsy. AJR Am J Roentgenol 166803-807, 1996 Wren SW, Ralls PW, Stain SC, et al: Assessment of resectability of pancreatic head and periampullary tumors by color flow Doppler sonography. Arch Surg 131:812-818, 1996
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73. Yamamoto R, Tatsuta M, Noguchi S, et al: Histocytologic diagnosis of pancreatic cancer by percutaneous aspiration biopsy under sonographic guidance. Am J Clin Pathol 83:409414, 1985 74. Yassa NA, Yang J, Stein S, et al: Gray-scale and color flow sonography of pancreatic ductal carcinoma. J Clin Ultrasound 25:473480, 1997
Address reprint requests to Lucy E. Hann, MD Department of Radiology Memorial Sloan-Kettering Cancer Center 1275 York Avenue New York, NY 10021 e-mail: [email protected]
0039-6109/01 $15.00
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PANCREATIC ULTRASONOGRAPHY Genevieve L. Bennett, MD, and Lucy E. Ham, MD
Abdominal ultrasonography (US) is used for initial imaging evaluation of patients with abdominal pain, jaundice, and nonspecific abdominal complaints. As a screening examination, US often provides the first opportunity to diagnose pancreatic abnormalities. Diagnostically, US findings may identify pancreatic abnormalities, differentiate benign pancreatitis from neoplasms, and guide percutaneous interventional procedures and biopsies for definitive diagnosis. Also, findings on US are useful to determine the need and priority of subsequent imaging, such as contrast-enhanced CT, MR imaging, and endoscopic or laparoscopic US, that may be required to assess extent of disease and to stage pancreatic tumors. This article discusses current techniques for pancreatic US, including the role of Doppler imaging and harmonic US. The normal and abnormal appearance of the pancreas are described, with emphasis on characteristic US findings that may aid diagnosis. This section includes the differential diagnosis of pancreatic abnormalities detected by US because there is considerable overlap in imaging appearance of benign and malignant conditions. The role of US-guided biopsy of the pancreas also is included. Newer applications of US for evaluation of pancreatic transplants used for the treatment of diabetes also is discussed. TECHNIQUE
Abdominal US is performed after a minimum fast of 6 hours to allow for distension of the gallbladder. The fasting state also may improve visualization of the pancreas because there is less gaseous distention of the stomach and upper gastrointestinal tract. Scanning is performed with a curved array or
From the Department of Radiology, New York University Medical Center (GLB); the Department of Radiology, Weill Medical College, Cornell University Medical Center (LEH); and the Memorial Sloan-Kettering Cancer Center (LEH), New York, New York
SURGICAL CLINICS OF NORTH AMERICA VOLUME 81 * NUMBER 2 * APRIL 2001
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sector 3.5- to 5.0-MHz transducer. The patient's position is varied to minimize intervening bowel gas, and scans may be obtained in the erect, supine, and shallow left or right decubitus positions. US offers multiplanar imaging of the pancreas; longitudinal and transverse images are included in the standard examination, but the orientation of the images may seem less familiar to the surgeon than CT images that are in the axial plane. This difference in image display is inherent in US, which is limited by artifacts from air interfaces; therefore, views are obtained from various projections using the solid organs, such as the liver, spleen, and kidneys, as an "acoustic window" to deeper structures, such as the pancreas. HOW RELIABLE IS ULTRASONOGRAPHY FOR PANCREATIC DIAGNOSIS?
Some reports have indicated that intervening bowel gas limits US visualization of the pancreas, especially of the pancreatic tail.", 36, 47, 56 Other studies have shown, however, that experienced sonographers and newer equipment increase 15,39, 43, 67 Campbell and the rates of pancreatic visualization to more than W i l s ~ n 'used ~ US to evaluate 51 patients with new pancreatic neoplasms. A technically adequate study was defined as one in which the splenic vein, portal venous confluence, pancreatic head, neck, body, and tail were all seen. The pancreatic tail was visualized from the epigastrium and left flank positions. Oral contrast was not used. All 51 patients had technically adequate pancreatic studies, and 50 of 51 pancreatic masses were detected by US alone. Similar findings were reported by Karlson et al,43who performed pancreatic US on 919 patients; 140 of these patients had pancreatic tumors diagnosed within 1 year of US as reported by the Swedish Death and Cancer Registry. US detected 124 of 140 pancreatic masses, for a sensitivity of 88.6% for all tumors and a sensitivity of 90% (79 of 88 patients) for detection of exocrine pancreatic cancer. 43 highlight that the diagnostic efficiency of pancreatic These two studiesI5* US has improved compared with earlier reports. These differences may be explained, in part, by advances in US equipment that now provide better resolution and tissue contrast, but other factors also should be considered. US is the most operator-dependent cross-sectional imaging technique. Karlson et ale evaluated interobserver variability for diagnostic accuracy of pancreatic abnormalities. The operator's level of US experience was a significant factor in the diagnostic accuracy of pancreatic abnormalities among three sonographers with varying experience. The excellent results of pancreatic US diagnosis reported by Campbell and Wilson15 also may be explained by their meticulous technique and experience. TECHNOLOGIC ADVANCES THAT AID ULTRASONOGRAPHY DIAGNOSIS OF THE PANCREAS Oral Contrast Agents
Gastrointestinal tract air may significantly limit pancreatic visualization caused by artifact on US. To eliminate shadowing from intraluminal air, the stomach can be filled with fluid that acts as an acoustic window through which the pancreas may be seen. Degassed water is most commonly used. Occasionally, air may be ingested with the fluid, further exacerbating air artifact, but little air
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is swallowed if the patient is instructed to drink slowly and a straw is not used. By shifting patient position, the fluid in the stomach may be moved to allow for visualization of each portion of the pancreas.’ Orally administered US contrast agents, including simethicone and methylcellulose, also have been used to im53 These agents absorb gas bubbles, have prove visualization of the pan~reas?~, decreased transit time compared with water, and are superior to water for reduction of gas shadowing. In a phase 2 trial, Lev-Taoff et a149reported that oral contrast provided better images of the pancreatic head in 61% and pancreatic tail in 67% of 99 patients who were scanned before and after the administration of US oral contrast. Optimal results were obtained when imaging was performed immediately after oral contrast ingestion. Oral contrast also increased diagnostic confidence for exclusion of pancreatic dis0rders.4~ Other investigators have used a combination of water and simethicone as an inexpensive alternative for the reduction of gas artifact. This technique was described by Abu-YouseP in a study of 65 patients who had limited visualization of the pancreatic tail on standard US. In 74% of patients, the pancreatic tail was seen in its entirety after the administration of water and simethicone, and in an additional l8%, visualization of the pancreatic tail was significantly improved.’ These US techniques using oral agents to disperse gas may reduce the need for additional CT imaging of the pancreas in selected patients. Harmonic Imaging
Tissue harmonic imaging is a new US technique that provides higher-quality images, decreased artifact, and improved tissue contra~t?~,As the US beam passes through tissues, it generates higher harmonic frequencies that are multiples of the transmitted frequency. These harmonics are similar in principle to the harmonics generated with musical instruments. The harmonic US signal is created within the tissues and is therefore not degraded by artifacts from the body wall. Harmonic US has increased resolution and improved contrast at tissue interfaces. Shapiro et a1@used conventional and harmonic US to evaluate the pancreas in 60 patients. Harmonic US images of the pancreas improved penetration in 45 patients (75%), provided better detail in 54 (go%), and improved total image quality in 50 patients (83%) as judged by three radiologists “blinded to the US technique used to generate the image.@Tissue harmonic imaging is routinely available, and the improvement in image quality has made it the technique of choice for US evaluation of the abdomen, including the pan~reas?~, @
@
NORMAL PANCREATIC ANATOMY ON ULTRASONOGRAPHY
The pancreas usually has uniform texture, and pancreatic echogenicity is slightly greater than, or the same as, that of the liver. Fatty infiltration of the pancreas that occurs with advanced age and obesity may increase pancreatic echogenicity so that it resembles the adjacent retroperitoneal fat. Occasionally, a texture change may be observed in the uncinate and head of the pancreas caused by focal fatty sparing within the ventral p a n c r e a ~ .This ~ , ~normal ~ variant can be differentiated from tumor by the absence of mass effect or vascular or ductal displacement. The pancreas is defined by the adjacent vasculature (Fig. 1).On transverse US images, the body of the pancreas is seen anterior to the splenic vein,
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Figure 1. Normal pancreatic anatomy. Transverse ultrasonography (US) reveals a normal sized pancreas (white arrows), with adjacent peripancreatic vasculature. Aorta and origin of right renal artery (a), inferior vena cava (i), left renal vein (curved arrow), splenic vein, and superior mesenteric vein confluence (s), superior mesenteric artery (black arrow).
with the superior mesenteric vein and superior mesenteric artery visualized posteriorly. The left renal vein courses transversely between the superior mesenteric vein and aorta. On sagittal views, the pancreas contacts the inferior vena cava. This vascular anatomy is well depicted by gray-scale US. When spectral or color Doppler also is used, vascular patency and flow direction within the peripancreatic vessels also may be documented on US. The pancreatic duct is best seen transversely, and it normally is less than 2 mm diameter in the body of the pancreas and 3 mm in the head34(Fig. 2).
Figure 2. Pancreatic duct. In the body of the pancreas, a 2-mm normal-caliber pancreatic duct (arrow and calipers) is well visualized. a = aorta; p = portal vein.
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ULTRASONOGRAPHY IMAGING IN ACUTE AND CHRONIC PANCREATITIS Acute Pancreatitis
US traditionally has a limited role in establishing the diagnosis of acute pancreatitis. Visualization of the pancreas in patients with pancreatitis often is limited because of ileus and superimposed bowel gas. Also, the US appearance of the pancreas may be normal in early pancreatitis. CT is considered to be the imaging modality of choice for the diagnosis and staging of acute pancreatitis because of its superb ability to show early inflammatory change, extrapancreatic fluid collections, and pancreatic necrosis.6,7, CT also has been found to be helpful in predicting the severity of an attack of acute pancreatitis and the development of complications and in establishing prognosis?, US, however, does have a role in the evaluation of patients with acute pancreatitis. It often is the first examination for the evaluation of patients with abdominal pain, and it is the best method for the assessment of the gallbladder and biliary tree. When a patient presents with suspected acute pancreatitis, US is used to evaluate the gallbladder and biliary tree and to exclude other causes of abdominal pain, such as cholecystitis. The finding of gallstones provides a likely etiology for acute pancreatitis, and the identification of choledocholithiasis and biliary obstruction, with obstructive jaundice or cholangitis, affects immediate patient management because these patients may be triaged for emergent endoscopic retrograde cholangiopancreatography (ERCP) and sphincterotomy. Whether the mere presence of gallstones in a patient with acute pancreatitis is an indication for routine presurgical ERCP is contr~versial.'~, 27 The reported sensitivity, specificity, and accuracy of US for the detection of cholelithiasis are more than 95%.19, 37, 44 CT is less sensitive because many stones are composed of calcium, bile pigment, and cholesterol and are isoattenuating to bile? Sensitivity of US for choledocholithiasis ranges from 55% to 85%, with a specificity of 89% to 91%?0,45, Choledocholithiasis is confidently identified when there is an echogenic shadowing focus in the common bile duct (CBD) surrounded by bile (Fig. 3A). The limitations of US for the detection of CBD stones include stones in nondilated ducts, nonshadowing stones, and stones present in the distal CBD, which often is obscured by bowel gas. Helical CT has proven to be a useful adjunctive imaging technique for the detection of choledocholithiasis, with a reported sensitivity and specificity of 88% and 97% relative to ERCP (Fig. 3B). Magnetic resonance cholangiopancreatography (MRCP) also has offered sensitivities in the range of 92% to 93%, and diagnostic accuracy of as much as 97%.'O~~~ US findings related to the pancreas vary depending on the severity of pancreatitis, timing of imaging, and ability to achieve an adequate US window. This latter issue is improved with optimized imaging t e c h n i q ~ e s Scans . ~ ~ obtained in the semiupright and right posterior oblique position can facilitate visualization of the pancreatic head and distal CBD with fluid accumulation in the gastric antrum and proximal duodenum, and displacement of air to the region of the gastric fundus. The left lobe of the liver serves as an acoustic window for evaluation of the pancreatic body and lesser sac, while the spleen can serve as a window for evaluation of the pancreatic tail and anterior pararenal space. In early pancreatitis, the pancreas shows normal size and US echotexture. Eventually, there will be a change in echotexture of the gland, becoming more hypoechoic and heterogeneous relative to the healthy pancreas. Subsequently,
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Figure 3. Choledocholithiasis. A, Common bile duct stone (s)appears as echogenic focus in the distal common bile duct (CBD), with associated posterior acoustic shadowing (arrow). B, Helical CT image reconstructed in the coronal plane shows stone (arrow) in distal
CBD (d).
the gland will enlarge and develop an irregular contour (Fig. 4). The margins of the pancreas become indistinct, secondary to peripancreatic inflammatory changes and edema within the peripancreatic fat. Dilatation of the pancreatic duct may be observed. A focal intrapancreatic abnormality may be secondary to complications such as fluid collection, hemorrhage, or necrosis. US cannot differentiate between necrotic and non-necrotic pancreatic tissue, and CT is considered the study of choice for identifying pancreatic necrosis." 42 Necrosis appears on CT as an area of nonenhancing parenchyma. MR imaging can be used for the evaluation of patients who cannot receive intravenous contrast. If the gland shows significantly altered echotexture on US, or if no normal definable pancreatic tissue is present, CT evaluation should be performed (Fig. 5 ) . Occasionally, pancreatitis may be a focal process, with involvement of one segment of the gland, usually the head. On US, this appears as a focal enlargement with altered echotexture. On occasion, this is difficult to distinguish from a pancreatic Clinical correlation is important, as is a history of pancreatitis. The presence of calcification is reassuring; however, CT or MR evaluation may be necessary. Also, ERCP may be helpful to show changes within the pancreatic duct. In addition to the evaluation of the gland, US can be used to identify extrapancreatic spread of inflammation. Fluid collections are commonly seen within the lesser sac, anterior pararenal spaces, transverse mesocolon, and small
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Figure 4. Acute pancreatitis related to recent endoscopic retrograde cholangiopancreatography (ERCP). A, US reveals diffuse enlargement of the pancreas (p), with overall decreased echogenicity and irregular contour. 6, CT scan performed with intravenous contrast demonstrates enlarged, heterogeneously enhancing pancreas (p). There is contrast present in the distal CBD (d) caused by recent ERCP. g = gallbladder; s = splenic vein.
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Figure 5. Severe acute pancreatitis with pancreatic necrosis. A, Transverse US demonstrates enlarged hypoechoic, heterogeneous pancreas (p). B, CT scan performed with intravenous contrast demonstrates only a small amount of enhancing pancreatic tissue in the region of the head (h). The remainder of the gland is nonenhancing, consistent with necrosis (n).
bowel mesentery and peripancreatic spaces.4O This fluid can have a spectrum of US appearances, from simple anechoic fluid to complex fluid with septations. Other extrapancreatic findings include ascites, bowel wall thickening, and thickening of the gallbladder wall. US also may have an important role in the identification of complications related to pancreatitis. A pseudocyst is a well-defined, walled-off fluid collection that persists on serial examinations for at least 4 weeks after the onset of inflamrnati0n.2~Pseudocysts develop in 10% to 20% of patients with acute pancreatitis and usually require 4 to 6 weeks to form.62Diagnosis can be suggested based on clinical and laboratory parameters but can be confirmed with a variety of imaging techniques. Typically, a pseudocyst appears on US as a welldefined fluid collection that is anechoic with posterior acoustic enhancement (Fig. 6). These may have a more complex appearance if infected or complicated by hemorrhage4*(Fig. 7). Other complications include rupture, biliary tract obstruction, and involvement of the gastrointestinal tract, liver, or spleen. Color and spectral Doppler US have a crucial role in the identification of
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Figure 6. Pseudocyst. A, Pseudocyst appears on US as well-circumscribed anechoic fluid collection (c) anterior to the tail of the pancreas (p). 6, CT with intravenous contrast demonstrates well circumscribed fluid collection (c) anterior to the pancreas.
vascular complications of acute pancreatitis. These include venous thrombosis and pseudoaneurysm formation. Venous thrombosis can involve the portal or splenic veins and can manifest as complete thrombosis with lack of flow or as an echogenic filling defect within the vein. If thrombosis is chronic, cavernous transformation of the portal vein results. Pseudoaneurysms most commonly involve the splenic artery, gastroduodenal arteries, pancreaticoduodenal arcade, or occasionally the hepatic artery and result from severe necrotizing pancreatitis or erosion by a pseudocyst. These are readily identified with color Doppler and show characteristic findings with spectral Doppler. US cannot distinguish an uninfected peripancreatic fluid collection or pseudocyst from an infected one. The presence of echogenic foci corresponding to gas bubbles is suggestive; however, air also may be present secondary to fistula with bowel. If there is any clinical suspicion, US can provide image guidance for fluid aspiration or drainage and for nonsurgical decompression of a pseudocyst!
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Figure 7. Complex pseudocyst (c) with debris and septations. Needle aspiration demonstrated evidence for hemorrhage.
Chronic Pancreatitis
US findings in patients with chronic pancreatitis include alterations in the size and echotexture of the gland, calcifications, pancreatic duct dilatation and irregularity, bile duct dilatation secondary to stricture, and chronic p~eudocysts.~ The gland generally atrophies and becomes heterogeneous in echotexture secondary to fibrosis and calcification. Calcifications appear as hyperechoic foci with associated posterior acoustic shadowing, which may be focal or distributed throughout the gland (Fig. 8). They usually are intraductal, resulting from deposition of calcium carbonate on intraductal protein plugs. The pancreatic duct may be dilated and often is irregular in appearance, with dilatation of side branches. This may be difficult to distinguish from other causes of pancreatic duct dilatation, such as cancer. A history of pancreatitis and presence of calcifications without definite focal mass are reassuring; however, further evaluation with helical CT, MR, or ERCP may be necessary. DIFFERENTIAL DIAGNOSIS OF PANCREATIC ABNORMALITIES
Because US often is the initial imaging study for evaluation of the pancreas, it is useful to differentiate the appearance of pancreatitis on US from neoplasms to appropriately triage patients. Pancreatic Duct Dilation
The intraductal papillary mucinous tumor (IPMT) may mimic pancreatic duct dilation of chronic pan~reatitis?~, 66 This pancreatic cystic neoplasm spreads in the pancreatic ducts and secretes thick mucin that fills the main or branch ducts and causes ductal dilatation (Fig. 9). Many of the radiologic
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Figure 8. Chronic pancreatitis with calcification. A, Prominent pancreatic duct (pd) contains echogenic foci with posterior acoustic shadowing within the pancreas, consistent with calcification (arrows) from pancreatitis. 6, CT scan performed with intravenous contrast demonstrates calcifications within the pancreas (p).
features mimic the main duct dilatation seen in patients with chronic pancreatitis. When IPMT involves the entire length of the duct with associated parenchymal atrophy, differentiation from chronic pancreatitis may be impossible. Markedly dilated branch ducts may mimic pseudocysts. CT, MR, endoscopic US (EUS), and ERCP may allow for further characterization. The presence of mural nodules, much globs, or a solid mass identified by any of these modalities is important in making the correct diagnosis. A virtually pathognomonic finding is dilatation of the major or minor papilla, or both, with bulging into the duodenal lumen. Diagnosis of IPMT with ERCP is certain when much is observed leaking from the papilla. Solid Mass Adenocarcinorna
A focal mass, observed in as many as 40% of patients with chronic pancreatitis, may appear hyper- or hypoechoic and may be difficult to distinguish from
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Figure 9. Dilated pancreatic duct with intraductal papillary tumor. Transverse US image of the body of the pancreas reveals a dilated pancreatic duct (d) and polypoid intraductal mass (arrow). v = superior mesenteric vein.
cancer2,54(Figs. 10 and 11).The presence of calcifications favors the diagnosis of chronic pancreatitis because calcification in cancer is unusual. Correlation with helical CT, with thin-section imaging of the pancreas during the dynamic bolus infusion of intravenous contrast or MR imaging, is recommended to further characterize and assess for spread of disease. These procedures also may be inconclusive, and image-directed biopsy or close follow-up to assess for interval change may be necessary. The utility of endoscopic US for the diagnosis of early changes of chronic pancreatitis and for characterizing focal abnormalities is the subject of current investigation.16 Vascular invasion is an early hallmark of pancreatic cancer. Color Doppler US is useful for determining the patency of peripancreatic vessels and can be used to determine resectability of pancreatic cancer. Although pancreatitis may be associated with venous thrombosis, frank encasement of the celiac axis, superior mesenteric vessels, and splenoportal confluence are more common in pancreatic cancer (Fig. 12). Color Doppler US is useful for determining the patency of peripancreatic vessels and can be used to determine resectability of pancreatic cancer?', 72* 74 Ralls et a161 reported on color Doppler findings in 51 patients with pancreatic cancer, 49 of whom had technically complete examinations with visualization of all index vessels. The lesions of all 30 patients who were considered to have unresectable disease by color Doppler were unresectable at surgery. The lesions of 6 of 15 patients (40%) that were considered resectable on US were unresectable for cure. Atrophy of the gland and pancreatic duct dilation may be present in pancreatitis and pancreatic cancer, but when these findings are present only distal to a mass, the diagnosis of pancreatic cancer is favored. Bile duct obstruction also may be present in pancreatitis and pancreatic cancer, but abrupt cutoff of the duct and the double duct sign are more common in pancreatic cancer than in pancreatitis. Hepatic metastases detected on US may confirm the diagnosis of malignancy. Metastases are more common at presentation of pancreatic body and tail tumors because these usually become symptomatic at a later stage than pancreatic head tumors that cause biliary obstruction.
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Figure 10. Focal chronic pancreatitis. A, Transverse US image of an enlarged head of pancreas (p) with calcifications. 6,MR image correlation demonstrates enlarged head of pancreas (p).
Other Solid Masses Neuroendocrine tumors of the pancreas derived from the amine precursor uptake and decarboxylation (APUD) system may be functioning or nonfunctioning and may be associated with multiple endocrine neoplasia (MEN) syndrome?, 22 Patients with functioning tumors, such as insulinomas and gastrinomas, usually present with clinical symptoms related to excessive insulin, gastrin, or other hormones, but approximately one third of neuroendocrine tumors are nonfunctioning. Ninety percent of insulinomas are benign, but the prevalence of malignancy is higher for gastrinomas and other functioning islet cell tumors. Nonfunctioning tumors are more likely malignant. Helical CT and MR imaging are preferred for presurgical imaging of neuroendocrine tumors.3,52, 68 Using arterial and venous phase helical CT, Van Hoe et a P reported accurate presurgical identification of 9 of 11 pancreatic neuroendocrine tumors, one of which measured only 4 mm in diameter. The rate of transabdominal US detection of islet cell tumors is relatively low, in the range of 60%, because of the small size of these tumors.3,32, 33, 50,52* 63, 68 Neuroendocrine
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Figure 11. Bile-duct obstruction from carcinoma in the head of the pancreas. Longitudinal, left lateral decubitus US image reveals a pancreatic head carcinoma (calipers and curved arrow) obstructing a dilated CBD (bd).
tumors less than 2 cm in diameter usually are hypoechoic or isoechoic to the pancreas. Large islet cell tumors may be seen on US as echogenic, irregular masses with calcification or cystic areas from hemorrhage: Although transabdominal US is of limited value, intraoperative US (IOUS) of the pancreas is extremely accurate for the identification of islet cell tumors. IOUS has a true positive detection rate of 98'3'0, and tumors as small as 3 mm in diameter may be detected by IOUS.31,32 Also, IOUS may be useful to identify unsuspected multiple tumors, which are present in 10% of insulinomas and 20% to 40% of gastrin~mas.~~ Non-Hodgkin's lymphoma may involve the pancreas diffusely or as a focal mass (Fig. 13). It is hypoechoic on US, and the appearance may mimic pancreatitis if the patient presents with abdominal pain. The presence of bulky adenopathy, or adenopathy below the level of the renal veins, may be useful in establishing the diagnosis of lymphoma.51Lack of pancreatic duct dilation and patent vessels on Doppler imaging also are characteristic US findings in pancreatic lymphoma.51 Cystic Neoplasms
Cystic neoplasms are relatively uncommon, representing approximately 10% to 15% of pancreatic cysts and 1% of pancreatic cancers.41US provides excellent characterization of cystic lesions and easily depicts mural nodules, septations, and internal debris. Cystic neoplasms often have discriminating features on US imaging that may be useful for differentiation from pseudocysts of pancreatitis. Serous microcystic adenomas are benign neoplasms with multiple cysts ranging in size from 1 mm to 2 cm. Although microcystic adenomas are cystic neoplasms, they have a variable appearance on US depending on the size of the CyStS.13. 29. 30, 41 Microcystic adenomas with 1- to 2-mm cysts may appear solid because of the numerous interfaces produced by the walls of the small cysts. Microcystic adenomas with larger cysts have anechoic areas on US and, charac-
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Figure 12. Pancreatic adenocarcinoma involving the splenoportal venous confluence. A, Transverse US image at the level of the pancreatic neck reveals an irregular hypoechoic 2-cm mass (calipers and arrow). The mass compromises the splenic vein (SV). B, An oblique sonogram of the portal vein reveals focal nodularity (arrows)that narrows the lumen of the portal vein (pv). Tumor involvement of the portal vein was confirmed by histopathology. A = aorta.
teristically, these cysts are distributed in a peripheral location within the tumor (Fig. 14).Central stellate scars, shown on US as central linear echogenic areas, are present in approximately 13% of cases, and calcification may be present within the scar.41Tumors are well defined on US, with smooth margins. They occur more commonly in the pancreatic head, but because the tumors are relatively soft, there is no obstruction of the pancreatic duct or compromise of the peripancreatic vessels. Doppler imaging of microcystic adenomas aids in diagnosis. The lack of vascular encasement may be useful to differentiate these benign neoplasms from adenocarcinomas that typically involve the peripancreatic vessels. Color or power Doppler also may be used to show internal vascularity within microcystic adenomas, which are very vascular neoplasms that 29 have numerous vessels within the pseudocapsule and ~eptations.'~, In contrast to microcystic adenomas, macrocystic mucinous tumors of the
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Figure 13. Lymphoma of the pancreatic body and tail. A, Transverse sonogram shows an enlarged hypoechoic pancreatic body and tail (arrows). Appearance is consistent with neoplasm or inflammation. B, CT image shows low-attenuation mass (arrows) infiltrating the pancreatic tail. CT-guided biopsy revealed lymphoma. a = aorta; i = inferior vena cava.
pancreas are malignant or potentially malignant. Macrocystic mucinous cystadenoma and cystadenocarcinoma have larger cysts (>2 cm in diameter). Characteristic features of macrocystic tumors are thick septations, cysts with solid mural nodules, or cysts with echogenic f l ~ i d ~(Fig. , ~ l 15).Calcifications may be present, often are coarse, and are peripheral in distribution.4l Johnson et a14' reported that 78% of microcystic tumors and 95% of macrocystic adenomas or adenocarcinomas were correctly classified on US in a retrospective analysis. It is not possible to definitively distinguish between benign and malignant macrocystic tumors, but, in general, as solid components increase, there is a greater risk for 30 malignan~y.~, Another neoplasm that appears cystic on US is solid and papillary epithelial neoplasm (SPEN) of the pancreas. This tumor is uncommon, has low-grade malignant potential, and is seen in young women.I4 SPEN is characterized by areas of internal hemorrhage and cystic degeneration with fluid-debris levels and posterior enhancement on US.14Calcification may be present and is more
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Figure 14. Microcystic adenorna. A, Sagittal sonogram shows a large echogenic pancreatic head mass (rn, open arrows) with small peripheral cystic areas (curved arrows). The mass obstructs the CBD (CD). 6,T-2 weighted MR image shows the cysts within the pancreatic head mass (open arrows). gb = gallbladder; v = superior mesenteric vein.
readily identified on CT. MR imaging may provide specific diagnosis by demonstrating blood products. Mucinous cystic neoplasms may have similar features, but SPEN lacks internal septations and multiple loculations that are present in microcystic and macrocystic adenoma~.'~ OTHER APPLICATIONS OF ULTRASONOGRAPHY Ultrasonography-GuidedPercutaneous Biopsy of the Pancreas
US-guided fine-needle aspiration (FNA) biopsy provides definitive diagnosis of pancreatic abnormalities. Biopsy positive for malignancy has a reported specificity of 23, 57, 73 Di Stasi et alZ3 reported on US-guided percutaneous
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Figure 15. Macrocystic adenoma. A, Transverse sonogram reveals a cystic mass (calipers) with multiple thick septations in the tail of the pancreas. The splenic artery (s)contacts the mass (arrows) but is not involved. B, CT image shows mass (arrows) that contains septations and calcification (open arrow).
FNA biopsy results in 519 patients. For cytology, histology, and cytology plus histology, retrieval rates were 94%, 967'0, and 97%; sensitivity was 8770, 94%, and 94%; and diagnostic accuracy was 9170, 90%, and %YO,respecti~ely.~~ Brandt et all2 compared US- and CT-guided biopsy of the pancreas and found an accuracy of 95% for US and 86% for CT. The rate of false-negative results, including unsatisfactory specimens, was lower with US biopsies (3 of 58 procedures, 5%) compared with CT (30 of 211 procedures, 12%). Accuracy was higher with masses larger than 3 cm and larger needle size. Results also were better for masses located in the pancreatic body or tail rather than the head. The rate of major complications caused by percutaneous pancreatic biopsy is rare, at less than 1%.l2,65 Pancreatitis is the most common complication of percutaneous biopsy and is unrelated to the method of imaging guidance or type of needle used for the procedure. US-guided biopsies, using 18-gauge needles with automated, spring-loaded sampling devices, have reported sensitivity of 92% to 94%, with no increase in complication rate.", 65 Tumor seeding
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along the needle tract is rare (0.003%-0.009%) but is more common with pancreatic tumors than with other rnalignancie~.~~
Pancreatic Transplant Evaluation
Simultaneous pancreas and kidney transplantation is a therapeutic option for end-stage renal disease in patients with type 1 diabetes mellitus, and various complications may be identified with imagingz6Several techniques may be used for transplantation, and physicians should understand the surgical anatomy when assessing for potential complications. The transplanted pancreas is one of the most difficult organs to evaluate, and findings usually are not evident until there is extreme compromise of the transplant?* Complications include abscess or pseudocyst formation, vascular thrombosis, leak, or compromise of adjacent structures. The role of US is limited because the transplanted pancreas often is obscured by adjacent bowel. If the pancreatic duct is visualized, it can serve as a useful landmark. Also, differentiating between pancreatitis, rejection, and vascular compromise is difficult because these problems may result in a similar sonographic picture, with an enlarged heterogeneous, hypoechoic, edematous gland. US may identify peripancreatic fluid collections, such as abscess or hematoma, although CT usually provides better delineation of size and extent of these abnormalities and evaluation of the gland.21 Vascular complications are the second most common cause of pancreatic graft failure, after graft rejection. The most important role for US is in the evaluation of the transplant vasculature using color and spectral Doppler. Vascular complications, such as graft thrombosis (arterial or venous), stenosis, or pseudoaneurysm formation, can be identified and subsequently confirmed with conventional angiography or MR angiographyZ6The ability to use resistive index elevation as a positive predictor of transplant rejection has not proven useful because many rejecting transplants have a normal resistive index (RI), and elevation of the RI is nonspecific and can be seen with vascular compromise, in a series of 11 patients with venous such as venous t h r o m b ~ s i s ~ however, ~; thrombosis evaluated with duplex US, Foshager et alZ8found that reversal of diastolic flow in pancreatic transplant arteries was highly specific for the detection of graft venous thrombosis during the first 12 days after transplantation. Also, they found that an RI greater than or equal to 1.00 and absence of venous flow, in combination, are highly sensitive and specific for the diagnosis of pancreatic graft venous thrombosis.
SUMMARY
Pancreatic abnormalities usually are detected on US when it is used for screening patients with abdominal pain and for assessment of the gallbladder and bile ducts. Pancreatic visualization is limited by bowel gas, but with experienced sonographers and newer techniques, including harmonic imaging and oral contrast US, diagnosis of pancreatic abnormalities has significantly improved compared with earlier reports. Appropriate initial diagnosis by US can tailor further investigation, and US-guided biopsy may establish definitive diagnosis.
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Address reprint requests to Lucy E. Hann, MD Department of Radiology Memorial Sloan-Kettering Cancer Center 1275 York Avenue New York, NY 10021 e-mail: [email protected]