Pancreas: Pancreatitis: Spiral CT Evaluation of Pancreatitis 2001-2002
Elliot K. Fishman, M.D.
Introduction
Spiral CT technology (single detector and multidetector row) now represents the state of the art in evaluation of known or suspected pancreatic pathology. Spiral CT capitalizes on those basic principles which are essential for optimal pancreatic imaging, including 1-5 mm thin-section scans at narrow interscan increments and scanning during the phase of optimal vascular opacification (arterial and venous phases) by iodinated intravenous contrast material. Furthermore, for both neoplastic and inflammatory diseases of the pancreas, spiral CT offers many unique advantages over conventional dynamic CT scanning.
In contrast to conventional dynamic CT scanning, in which a slice is obtained, followed by table incrementation (with a 4-8 second interscan delay) and then another slice is obtained, spiral CT is a continuous acquisition of data typically lasting between 20 and 30 seconds as the patient travels through the gantry. This capability of obtaining a series of contiguous scans in a matter of a few seconds provides for the following distinct advantages: 1) scanning during the phase of maximum vascular enhancement (either with single or dual phase arterial and/or portal phase imaging) , with improved visualization of the superior mesenteric and gastroduodenal arteries, and the superior mesenteric, splenic, and portal veins; 2) multiple thin sections through the area of interest during this phase of peak enhancement; 3) lack of misregistration of data since a volume data set is rapidly acquired during a single breath-hold (which typically eliminates patient motion); and 4) capability to view this data as axial slices, as a multiplanar (coronal and sagittal) display or as three dimensional images. (1-10)
Technique
Patients should receive adequate oral contrast (e.g. 750 cc of 3% Hypaque, Nycomed Amersham Inc., Princeton, New Jersey) over a 30-minute period prior to their examination . In cases where three-dimensional imaging with vascular mapping is contemplated a positive oral contrast agent is not given and gastric and bowel distention is achieved by giving the patients an equal amount of water. After a topogram, a single unenhanced scan is obtained to center the image field. A total of 100-120 cc of IV contrast material (e.g. Omnipaque 300 or 350) is administered at a rate of 2-3 cc/sec. For purposes of pancreatic evaluation, contrast is generally administered via a peripheral venous access route using a power injector. Data acquisition typically begins 25-30 seconds after the initiation of injection of contrast material for arterial phase imaging, and at 50 seconds for portal phase imaging.
Scanning parameters will vary on the scanner used but on a single detector system such as a Siemens Plus-4 system (Siemens Medical Systems, Iselin, New Jersey) we use .75 sec scan times, 120 kVp, and 280 mAs with a slice thickness of 3 or 5 mm. On a multidetector scanner we use a Siemens Volume Zoom scanner with 500 ms scan time, 120 kVp and 150 mAs with a slice thickness of 1-3 mm. We routinely hyperventilate the patient just prior to scanning and then have them hold their breath throughout the entire study. The total scanning time is in the 24-40 range depending on the specific case and questions to be answered. The spiral protocol uses either a table incrementation of 1-5 mm depending on whether or not three dimensional imaging is done.
While early spiral scanners had a problem with adequate coverage due to limitations in pitch selection (i.e. pitch of only 1 available) and length of the spiral acquisition (only 24 seconds) current scanners allow the pitch to be varied , usually up to a pitch of two on a single detector scanner and up to 8 on a Multidetector row CT scanner. The increased pitch coupled with subsecond scanning (.500 to .75 per second gantry rotation of 80 cycles) and its one third increase in coverage now allows routine imaging of the entire liver and pancreas without compromising evaluation of either organ.
After acquisition, scans are reconstructed in about 2 seconds per image, and may be sent via Ethernet to a freestanding workstation for further analysis. Three-dimensional display and interactive review of the data are then possible. (6, 7)
Applications / Pancreatic Inflammatory Disease
Contrast CT has greatly contributed to the current understanding of the morphologic abnormalities in pancreatic inflammatory disease. CT remains the modality of choice for the imaging diagnosis of pancreatitis and its complications. The clinical severity of pancreatitis correlates well with the extent of pancreatic necrosis evident on CT. Thus, CT can often predict expected clinical outcome. Early recognition of the complications of pancreatitis is essential for prompt, appropriate intervention.
Acute Pancreatitis
In up to one-third of patients with acute pancreatitis, CT demonstrates a normal pancreas. More often, CT findings in acute pancreatitis will include glandular enlargement due to interstitial parenchymal edema. This glandular enlargement is most commonly diffuse, but when focal, findings may be indistinguishable from a neoplasm. In acute pancreatitis, the pancreatic contour may be irregular with focal hypodense regions likely representing focal necrosis or edema. The peripancreatic tissues will usually demonstrate inflammatory changes resulting in an increase in density of the peripancreatic fat. One notably exception is the preservation of a rim of normal fat around the SMA and SMV. This finding, although not inviolate, may help to distinguish pancreatitis from a neoplastic process. Acute edematous pancreatitis may progress to a necrotizing pancreatitis with extensive phlegmon formation, peripancreatic fluid, hemorrhage and extraglandular fat necrosis. Serial CT scans are essential to evaluate serial disease progression and diagnose complications early.
Pancreatic exudate and phlegmon tends to collect most often in the anterior pararenal space, the lesser sac, and eventually, in the greater peritoneal cavity. Spread of inflammation typically occurs along the transverse mesocolon and root of the mesentery, as well as the gastrosplenic, gastrohepatic and gastrocolic ligaments. Pancreatic phlegmons typically have higher attenuation values than pancreatic fluid collections and may appear heterogeneous due to blood, fat and necrotic tissue components. Peripancreatic fluid collections may demonstrate regions of increased attenuation due to hemorrhage within the gland itself or to erosion of a nearby vascular structure.
Spiral CT offers several unique advantages in the evaluation of patients with acute pancreatitis. It can readily establish the presence of necrosis or hemorrhage within the gland. The diagnosis of pancreatic necrosis is of major clinical importance, as it directly affects patient prognosis. Areas of pancreatic necrosis are avascular with contrast enhanced spiral CT. In patients with necrosis of greater than 50% of the pancreas, Balthazar et al. demonstrated that the morbidity and mortality increase substantially . Patients with significant pancreatic necrosis should be followed at close intervals with spiral CT to detect early abscess formation. CT guided needle aspiration may be helpful in selected patients to document infected peripancreatic fluid.
Spiral CT is also valuable in diagnosing vascular complications of pancreatitis such as unsuspected pseudoaneurysm (most commonly of the gastroduodenal artery or splenic artery). Rupture of a peripancreatic pseudoaneurysm may lead to life-threatening hemorrhage. Inflammatory exudate encasing the splenic vein or portal vein with resultant venous thrombosis and collateral vessel formation is also well-demonstrated on spiral CT.
Pancreatic abscess
Pancreatic abscesses may develop in the setting of superinfection of a pancreatic phlegmon or of a pancreatic/peripancreatic fluid collection. This dreaded complication of acute pancreatitis contributes significantly to mortality with this disease. Attenuations of these collections tend to be higher than those of sterile exudates. Gas within a collection is very suggestive of abscess formation, although it may not be present. Furthermore, this finding may also indicate communication with the bowel lumen, either from surgical drainage or from spontaneous fistula.
Pseudocysts
Approximately 30-50% of acute pancreatic fluid collections become encapsulated and evolve into true pseudocysts . In contrast to the dynamic fluid collections of acute pancreatitis, pseudocysts have defined fibrous pseudocapsules and typically occur in the setting of subacute or chronic pancreatitis. The fluid in pseudocysts is usually of homogeneously low attenuation, near that of water. When pseudocyst contents exhibit heterogeneously increased attenuation, this raises the possibility of superimposed hemorrhage or infection.
Spiral CT with multiplanar imaging or 3D volume rendering can clearly define the location and extent of pancreatic pseudocysts. This information can then be used for planning surgical management or percutaneous drainage. Pseudocysts may displace or encase vascular structures adjacent to the pancreas, such as the portal or splenic veins . This may lead to extrinsic venous compression or even obstruction. Venous thrombosis in this setting may result in collateral varices or mesenteric thrombosis. Pseudocysts may erode into pancreatic or peripancreatic arteries with resultant acute hemorrhage. Pseudoaneurysms may develop in the fibrous wall of the pseudocyst or involve adjacent vessels such as the splenic or gastroduodenal artery. Potentially life-threatening pseudoaneurysms are often first suspected on the basis of a spiral CT scan. Other complications include obstruction of the biliary or gastrointestinal tracts, or spontaneous rupture into the peritoneum, retroperitoneum, or bowel lumen. Spiral CT is clearly a very sensitive means for detecting the presence of such complications.
Chronic Pancreatitis
Chronic pancreatitis is a distinct clinical and pathologic entity resulting from repeated bouts of acute pancreatitis. A variety of CT features may be evident in patients with chronic pancreatitis, including a atrophic gland possibly with fatty replacement, dilatation of the pancreatic duct, pancreatic calculi, biliary ductal dilatation, pseudocyst formation, and focal pancreatic enlargement.
An important diagnostic advantage of contrast enhanced spiral CT is the increased conspicuity of the low attenuation pancreatic and biliary ducts against the markedly enhanced parenchyma. Certain characteristics of the pancreatic duct suggest a benign versus malignant process. With chronic pancreatitis, the dilated pancreatic duct will appear beaded and irregular (i.e. "chain of lakes"), while a smooth pattern of dilatation tends to suggest carcinoma. The presence of intraductal calculi clearly favors the diagnosis of chronic pancreatitis. In addition, a ratio of pancreatic duct diameter to pancreatic gland greater than 0.5 favors carcinoma, while a ratio of less than 0.5 favors chronic pancreatitis. Not infrequently, the above described changes of acute pancreatitis will be superimposed on underlying chronic pancreatitis.
One remaining problematic area is the patient with a focal pancreatic mass secondary to chronic pancreatitis. Biopsy, ERCP, and/or close serial follow-up with spiral CT are often required to help differentiate focal pancreatitis from carcinoma.
References
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