Pancreas: Pancreatitis: Spiral CT Evaluation of Pancreatitis: An Overview

Elliot K. Fishman, M.D.

CT of the Pancreas: The Normal Pancreas

The interpretation of pancreatic pathology rests on the appreciation of the appearance of the normal pancreas. The body of the pancreas is anterior to the superior mesentery artery at that artery's origin from the aorta. The splenic vein often is seen coursing along the posterior aspect of the pancreatic head and tail. The normal anterior-posterior width of the body of the pancreas varies, but 2.0 cm is accepted as the upper limit of normal. The pancreatic head is located just anterior to the junction of the left renal vein and inferior vena cava and usually is well-defined by the duodenal C-loop. The normal upper limit for width of the pancreatic head is 2.5-3 cm. The pancreatic tail is located just anterior to the splenic vein and extends toward the splenic hilum. The top normal width for the pancreatic tail is considered to be approximately 1.5 cm. These of course are at best only a rough guide to pancreatic size, and it is important to correlate the full dimensions of the gland when evaluating for suspected pancreatic enlargement. For example, in a 70-year-old male, a pancreatic head of 3 cm would probably be pathologic if the body measured 1.5 cm and the tail 1 cm. The relative size of the pancreas decreases with advancing age. In our experience, the largest pancreas is seen in the younger population, particularly in the younger female.

Enhancement of the pancreatic gland will vary depending on the rate and volume of contrast injected. The ideal phase for data acquisition for definition of the pancreatic gland is about 50 seconds after injection which is the pancreatogram phase of enhancement. It is the optimal time for detection of most pancreatic tumors as well as for detecting glandular changes due to pancreatitis. This is the ideal phase for detecting glandular necrosis.

Evaluation of Pancreatic Pathology: Potential Pitfalls and Limitations

(1) The administration of an adequate amount of properly timed doses of oral contrast cannot be overemphasized. One of the common errors is the erroneous diagnosis of a pancreatic mass when in reality all that is present is the pancreas interfacing with nonopacified bowel. These pseudo-masses most commonly are seen at the juxtaposition of pancreatic head with C-loop, and of pancreatic tail with jejunum. If one has difficulty in separating pancreas from bowel loops, additional oral contrast is given, and repeat scans performed. We are currently using water as a contrast agent with ever increasing frequency.

(2) In a small percentage of cases, pancreatitis and carcinoma may look identical. In such cases, the clinical history and the secondary roentgenographic signs of disease take on added importance. In select cases follow-up scans may be needed.

(3) Peripancreatic adenopathy may occur in patients with lymphoma or metastatic disease. In the great majority of cases the nodes can be separated from pancreas, allowing proper diagnosis. Occasionally the nodes will so intimately involve the peripancreatic zone that it may seem impossible to distinguish them from a pancreatic mass. In such cases, biopsy may be necessary for a definitive diagnosis. A helpful key differential diagnosis point is that with nodes or lymphoma the common bile duct is typically not dilated.

(4) The pancreatic tail and splenic vein are closely related. Care should be taken to separate these two structures so as to avoid misinterpreting the interface between splenic vein and pancreas or a "dilated pancreatic duct." One helpful hint is that the duct usually is in the anterior one-third of the gland, while the venous "pseudoduct" occurs posteriorly.

Technique of Examination

The ideal examination of the pancreas allows for clear evaluation of the status of the pancreatic gland and the peripancreatic tissues. In the past, we routinely gave positive oral contrast material for opacification of the stomach and small bowel. In most cases we still give oral contrast unless specific evaluation of vascular structures is indicated. In those cases instead of giving positive oral contrast we will give water or water with granules for gastric distention. The ideal examination of the pancreas also requires intravenous contrast material. This is given by a bolus injection at a rate of 2-3 cc/sec typically of 100-120 cc of Omnipaque-350. In cases where positive oral contrast is given the oral contrast is 500 cc of 3% Hypaque approximately 30 minutes prior to examination and an additional 250 cc at the time of the exam.

Single detector or Multidetector spiral CT can be performed although at the present time we are routinely using MDCT for evaluation of any suspected pancreatic pathology (tumor). Our protocol will vary depending on which scanner we use but an ideal protocol might be from the level of the diaphragm caudally with 1-5 mm thick sections, 6-15 mm/sec table speed and reconstruction of data at 1-5 mm intervals. This allows not only accurate evaluation of the pancreas but all potential pathways of pancreatic pathology.

If spiral CT is not available, dynamic CT scanning can be done using 5-8 mm thick sections typically at 1 sec and 250 mAs. Scans are obtained at 5-8 mm intervals. An important technical note is that regardless of whether one is using spiral or dynamic CT scanning that the imaging should extend far enough proximally and distally to make sure the entire extent of pancreatic pathology is covered. It is important to remember that pancreatitis can extend upward into the posterior mediastinum or down into the lower pelvis. Please not that at the current time we are not doing any scanning but spiral CT.

SDCT or MDCT of Pancreatitis

The precise mechanism of acute pancreatitis remains unknown, but common etiologic factors include alcohol abuse, biliary tract disease, drugs, and vital infections. It usually is impossible to determine the cause of acute disease from the CT scan appearance, since the various precipitating factors share a final common pathway. In our experience, all patients with clinical manifestations persisting at least one week, exhibit CT abnormalities of the pancreas and peripancreatic tissues. The CT findings in acute pancreatitis are as follows;

Pancreatic changes

(a) Enlargement - enlargement may be diffuse or focal. Enlargement of the pancreatic head is the most common form of focal change.

(b) Edema - the enlarged pancreas commonly shows edematous changes of the parenchyma, with typical Hounsfield measurements in the range of 5-20 HU below that of the normal gland.

(c) Necrosis - in the most severe cases of edematous or hemorrhagic pancreatitis, necrosis of the gland may occur. This is associated with a significant mortality rate. Air may be seen within the pancreatic tissue and is not uncommon with pancreatic necrosis.

Peripancreatic changes

(a) Blurring - the peripancreatic fat planes appear blurred and may also appear to be thickened. The adjacent fascial planes may also become thickened and more prominent. Peripancreatic fluid collections may be seen.

(b) Pancreatic and extrapancreatic fluid collections - the most common site of extrapancreatic fluid collections is in the lesser sac (which is located directly anterior to the pancreas and posterior to the stomach). The next most common location is the anterior pararenal space, more commonly on the left side. The anterior pararenal space is bounded posteriorly by the anterior layer of Gerota's fascia and contains the pancreas and descending colon. Thickening of Gerota's fascia is common to both acute and chronic pancreatitis. When seen, pancreatic abscess usually occurs between three and five weeks after the initial insult to the pancreas.

Between 3-10% of patients with acute pancreatitis subsequently will develop a pancreatic abscess. Pancreatic gas collections can be seen in pancreatic abscesses through they are not necessary for diagnosis. Intraparenchymal hemorrhage can also occur and can be due to pseudoaneurysms of the splenic, gastric, duodenal or pancreaticoduodenal artery among other causes. On CT scans, hemorrhage will appear as increased CT attenuation, in the range of 90-120 HU.

Chronic Pancreatitis

The CT findings in chronic pancreatitis can be different from those in acute pancreatitis. Most importantly, pancreatic size is very variable in chronic pancreatitis. Patients with repeated insults to the pancreas may actually develop pancreatic atrophy with scarring and inflammation However, the typical CT findings in chronic pancreatitis are listed below.

(a) Mass effect - Focal enlargement of the pancreas due to edema, inflammatory changes or fibrous induration is common. In one series, 36% of the cases of chronic pancreatitis had pancreatic enlargement or localized mass. As noted earlier, a normal-sized gland does not rule out chronic pancreatitis.

(b) Dilated pancreatic duct - With high resolution CT, a normal pancreatic duct can be seen. If the pancreatic duct should measure more than 2-3 mm, it is definitely abnormal. Often, ductal calcifications will be seen as well.

(c) Pancreatic calcification - In our experience nearly 55% of cases of chronic pancreatitis had pancreatic calcifications. The calcifications usually are seen within the pancreatic duct but may also be intraparenchymal. The presence of thickening of Gerota's fascia is a secondary sign suggesting previous pancreatic disease. However, this sign is not specific as thickening of Gerota's fascia may be seen with renal or colonic inflammatory disease as well.

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