PancreasModalities
The plain radiograph
The plain radiograph is of only limited value in the diagnosis of disorders of the pancreas. The pancreas itself is not seen on the plain radiograph, because the organ does not normally contain contrast-forming elements and its surrounding fat planes are inconspicuous. Areas of calcification, however, are well seen, and this finding is of diagnostic value: characteristic types of calcification may be seen in the pancreatic duct following chronic pancreatitis, in the walls of calcified pseudocysts or in the pancreatic parenchyma in hereditary pancreatitis (Fig. 40).
In acute pancreatitis non-specific secondary changes may be seen, e.g. dilatation of loops of bowel in the upper abdomen or an increase in the distance between the distended stomach and the transverse colon owing to the presence of mesenteric oedema. Basal pulmonary infiltrates and pleural exudates, most commonly left-sided, are frequently seen in acute pancreatitis, but these findings are also non-specific. When a large pancreatic cyst or abscess is present, upward displacement of the dilated stomach or downward displacement of the transverse colon may be seen. Gas formation in the pancreas is a pathognomonic sign of a pancreatic abscess. Absence of the right psoas shadow might suggest the presence of a retroperitoneal fluid collection.
Duodenography
Enlargement of the head of the pancreas, e.g. from a pancreatic cancer or cyst, may distort or distend the duodenal loop (Frostberg's sign, or the "inverted 3" sign). This may be seen on a single or double-contrast barium study. The sign is indirect and only seen with marked expansion of the pancreatic head. Barium duodenography has decreased in importance with the greater use of cross-sectional imaging techniques such as ultrasound and CT.
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Figure 40. Calcification of the pancreatic parenchyma (arrows) seen in a case of hereditary pancreatitis. The calcification is visible on the plain film and is so extensive it defines the shape of the organ. |
Ultrasonography
The primary method for studying pancreatic disease is ultrasonography (US). The pancreas may be visualized by ultrasonography in approximately 85 % of subjects, but in the remainder the organ is partly or wholly obscured by bowel gas or other bowel contents; this problem is particularly evident in acute disease associated with secondary dilatation of gas containing bowel loops. The echogenicity of the pancreas is normally somewhat greater than that of the liver. In most individuals, using good equipment, the normal pancreatic duct is seen as a narrow streak in the pancreatic parenchyma and the caudal portion of the common bile duct is also seen as it enters the head of the pancreas. Important landmarks in the vicinity of the pancreas include the inferior vena cava, the superior mesenteric artery and vein, the splenic artery and vein, the hepatic artery and the portal vein (Fig. 41). It is important to assess the calibre of the pancreatic and choledochal ducts which are normally 1-3 mm and approximately 5 mm, respectively. Dilated pancreatic ducts indicate either obstruction or duct ectasia in chronic pancreatitis. Pancreatic tumours are generally solid lesions and are usually of lower echogenicity than the surrounding parenchyma. The smallest tumours that can be detected by US are approximately 1 cm in diameter. Cysts are echofree and show distal acoustic enhancement. Doppler-US (particularly colour Doppler US) is useful to assess the peripancreatic blood vessels which may be involved in acute and chronic pancreatitis or by pancreatic neoplasms. Intra-pancreatic vessels may also be visualized and changes in normal
vascularity may be useful in
tumour diagnosis.
Ultrasound and
CT are complementary techniques in the diagnosis of pancreatic disease.
Computed tomography
The importance of US in the diagnosis of pancreatic disorders has already been emphasized but CT provides important information that cannot be obtained by US alone and, together, the two techniques are the most important imaging methods for the organ. The retro-peritoneal fat that surrounds the pancreas in many patients affords good delineation of the organ on CT (Fig. 41), even in the presence of dilated bowel loops and oedema, circumstances which considerably diminish the diagnostic efficiency of ultrasound. Image quality may, however, be adversely affected by patient movement, e.g. in a restless subject with abdominal pain. Both cysts and areas of calcification are image d with great clarity and the use of intravenous contrast medium enhances the detection of pathological changes in many situations. Tumours, for instance, show slower contrast enhancement than normal pancreatic parenchyma, but the CT -study has to be performed when the contrast difference between the tumour and parenchyma is maximal, i.e. within two minutes of a bolus injection of contrast medium. Cysts do not enhance with contrast medium. In addition to the pancreas itself, neighbouring organs are better seen on CT than US, e.g. the biliary ducts, kidneys, spleen, bowel and mesentery, and this allows for the precise grading of pancreatic disease by CT. Opacification of the bowel with oral contrast medium is important, in order to differentiate between bowel loops and some type of pancreatic pathology such as tumours or cysts (though not acute pancreatitis). Fast CT -scanners will further improve the diagnostic capabilities of computed tomography in the pancreas.
ERCP
At endoscopic retrograde cholangiopancreatography (ERCP) the papilla of Vater is cannulated under direct visual control through an endoscope which has been introduced via the oesophagus and stomach into the duodenum, and water soluble contrast medium is injected into the pancreatic duct (Fig. 42). The study provides information about the ductal system, but not about the pancreatic parenchyma. Pancreatitis is a recognized complication of the technique which can result from either the manipulation required for duct catheterization or the action of the
 | Figure 42.Normal endoscopic retrograde pancreatogram. The pancreatic duct of Wirsung is seen. The main duct is filled, but the side branches only minimally. The diameter of the duct in the pancreatic head, body and tai!, respectively, is indicated in millimetres, and the diameter of the endoscope is also shown for comparison. |
contrast medium on the pancreas. Care has to be taken, therefore, not to overfill the ductal system and dilute,
non-ionic, contrast medium should be used. Images of the ductal system are taken in various projections under fluoroscopic control.
ERCP shows changes such as distortion or obstruction of the main ducts or their branches as may occur in cancer, or may reveal communications between the ducts and pancreatic cysts. The main value of ERCP is in the mapping and grading of changes in chronic pancreatitis and in fully delineating the pancreatic duct before pancreatic resection. In the evaluation of changes in the head of the pancreas it is also important to visualize the common bile duct, the distal portion of which traverses this region.
Angiography used to be the definitive method for the diagnosis of pancreatic tumours. Since the introduction of DS, CT, MRI and ERCP as direct imaging methods, however, the role of angiography has been reduced to the diagnosis and preoperative localization of endocrine tumours and, occasionally, the provision of further information on the potential operability of a pancreatic cancer in particular cases. Pancreatic angiography is performed by studies of the coeliac and superior mesenteric arteries, from which multiple smaller arterial branches supply the pancreas. Super selective studies of the pancreatic vessels are frequently necessary, particularly in the search for endocrine tumours and during embolization procedures (see below under Interventional procedures).
Endocrine tumours are often very vascular and appear as enhancing structures on angiography which means that the technique can often detect lesions too small to be identified on US or CT (0.5-1 cm).
Percutaneous transhepatic portography
Another method for localizing endocrine tumours is the fractionated sampling of venous blood through a catheter that is introduced percutaneously through the liver into the portal system. The vascular anatomy is displayed by means of percutaneous transhepatic portography (PTP) and samples are collected from various veins draining the pancreas. The tumour is localized to that part of the pancreas showing the highest concentration of hormone in its draining veins. The accuracy of the technique can be enhanced by the use of arterial and other techniques for stimulating hormone production and the presence of unsuspected hepatic metastatic deposits may be revealed by the analysis of simultaneous hepatic vein samples.
Radionuclide white cells, though they do not localise in uncomplicated pancreatitis, may be useful for diagnosing a pancreatic abscess or an infected pseudocyst. Insulinomas expressing somatostatic receptors (about 50%) may be localised with the somatostatic receptor analogue, In-III pentetreotide (octreotide).
Magnetic resonance imaging
The changes found in pancreatitis and pancreatic tumours are better demonstrated at present by CT and US than by magnetic resonance imaging and MRI is not, therefore, widely used in the diagnosis of pancreatic diseases; bowel contents and bowel movement, vascular motion and movements of the patient may all degrade the MRI study. The free choice of MR imaging planes is, however, an advantage of the technique and shorter imaging times with fast sequences and the development of new oral and intravenous contrast agents will certainly ensure continued research into the proper role of MRI in pancreatic disease (see Fig. 41).
David J. Allison and Carl-Gustaf Standertskjold-Nordenstam