Pancreas: Pancreatitis: Principles and Overview: Spiral CT of the Pancreas

Elliot K. Fishman, M.D.

Technique

Diagnostic evaluation of the pancreas, whether with Spiral single or Multidetector (MDCT) CT, requires careful attention to study technique and protocols. Spiral CT, with the ability to optimally time contrast injection to data acquisition, presents new opportunities for the radiologist to better detect and define the presence and extent of pancreatic disease. The availability of dual-phase Spiral CT provides the ability to obtain images during both arterial and portal venous phases, which has several specific advantages in the pancreas.

Although the specific scan parameters used will vary from scanner system to scanner system, several basic concepts are constant. With a single detector scanner, slice collimation should be between 3-5mm with a reconstruction interval between 2-5mm, depending on whether or not three-dimensional reconstruction or vascular mapping is required. With a MDCT scanner we routinely use 1.25 mm slice thickness, reconstruction at 1 mm intervals for suspected pancreatic tumors and 3-5 mm slice thickness and 3-5 mm reconstruction interval for suspected pancreatitis. Scans in the arterial phase typically begin around 30sec after the start of contrast injection, and at approximately 50-60sec following initiation of contrast enhancement when portal-phase imaging is required. Dual-phase studies will combine both of these acquisitions. In our experience, although transaxial images do provide most of the information necessary for evaluation of the pancreas, the use of multiplanar and 3D images provides additional information, particularly in terms of staging and subsequent patient management.

The injection rate we routinely use for evaluation of a suspected pancreatic mass is 2-3cc/sec with the higher injection rate mandatory for vascular and 3D mapping. Higher injection rates of up to 5cc/sec have been discussed in the literature but are of little additional value except for perhaps in the detection of vascular masses such as insulinoma. We routinely use nonionic contrast with our agent of choice being Omnipaque-350 (Nycomed Inc, Princeton, NJ). Between 100-120cc of contrast are used in most cases.

Pancreatic Mass

Choi et al. compared the value of arterial- with late-phase Spiral CT scans for the evaluation of pancreatic adenocarcinoma. Not surprisingly the authors found that the arterial phase images were superior to late-phase imaging (180sec) for lesion detection. Late-phase imaging was equivalent to conventional CT scanning in lesion detection. The authors specifically note that in smaller tumors which do not distort the shape or borders of the pancreas that often only on early-phase images can a lesion be detected. It is at this phase that there is optimal enhancement of the normal pancreas and maximum differentiation between tumor and normal tissue. Others have found that the portal venous phase is better for lesion detection as it provides increased differences in contrast enhancement between the tumor and normal pancreatic tissue.

Although this statement refers to pancreatic adenocarcinoma, which represents nearly 95% of pancreatic cancers, it is also true that arterial-phase images are ideal for detection of vascular pancreatic masses such as insulinomas. In fact it is only in arterial-phase imaging that these lesions can typically be detected as they are small and typically become isodense by the time portal-phase acquisition is acquired.

Occasionally, other pancreatic masses can be vascular although in our experience they are often metastatic lesions such as from renal cancer or carcinoid tumors. In these cases multiple lesions or diffuse glandular infiltration can be seen.

Pancreatic Cancer -- Vascular Invasion

Kaneko et al. compared helical CT with standard catheter angiography for determining arterial invasion of pancreatic tumors. The authors found that for the major arterial vessels, including the celiac artery, splenic artery, superior mesenteric artery, and gastroduodenal artery, that both studies were equivalent for diagnosing arterial invasion. The authors found however that angiography was more successful in looking at minor arteries, including the anterior superior pancreaticoduodenal (APDA) artery and the posterior superior pancreaticoduodenal (PPDA) artery. However, the authors do note that encasement or minor arterial invasion of the APDA or PPDA is not a contraindication for pancreatic resection, so demonstration of their involvement is not critical in decision making regarding tumor resectability. The advent of MDCT with 3D rendering optimizes vascular staging with CT scanning.

Monitoring Response to Therapy

Pancreatic resection with a Whipple's procedure still remains the only real possibility to cure the patient with pancreatic cancer. Although only a small percent of patients will be potentially resectable, Spiral CT has made selection of this group a bit more accurate. Following surgery many institutions routinely recommend radiation therapy for those patients who have been successfully resected. The complications of radiation therapy to the tumor bed are well known and in the patient with pancreatic cancer can include thickening of the stomach and small bowel within the therapy field. Inflammatory changes of the peripancreatic tissues including increased density and induration of the surgical bed. Spiral CT can be successfully used in these patients to monitor the response to therapy and detect any potential recurrence even before the patient is symptomatic. The evaluation of the postoperative and postradiated patient can be difficult as post-therapy change can simulate tumor recurrence. Nevertheless, careful attention to technique as well as an understanding of some of the pitfalls and potential limitations will help one be successful in this evaluation.

Typical patterns of recurrence include tumor around the SMA and SMV as well as metastatic disease to the liver. We routinely will obtain thin sections in the range of 3-5mm at 3-5mm intervals to correctly evaluate these patients. Our images are obtained during the postal phase of enhancement as a single-phase study. In cases where findings are indeterminate biopsy can be obtained although this is not commonly done. Indeterminate cases are followed up with CT scans in six to eight weeks at which time one is likely to make a specific diagnosis.

Recurrent Pancreatic Adenocarcinoma: CT Findings

Following Whipple procedure spiral CT with contrast enhancement is necessary for the evaluation of potential recurrence. There can be substantial overlap between the appearance of normal postoperative change and a subtle recurrence. Therefore it is important to recognize that scans in the immediate postoperative period may have inflammatory change that could simulate residual tumor or recurrence. The patterns of recurrence in pancreatic cancer are typically locally in the pancreatic bed or distally to areas such as the liver, lymph nodes and mesentery. In a recent review of our experience at Hopkins we found the most frequent site of recurrence to be in the pancreatic bed. These recurrences typically were soft-tissue masses that either had appeared since the prior study or were still present over time. It is important to recognize that there are many pitfalls in the evaluation of these patients in terms of masses in the region of the pancreas. Often these patients will be unable to drink sufficient contrast material for adequate stomach and bowel distention. In these cases the potential for over- or under-reading obviously exists.

Pancreatic Tumor Resectability

The acquisition of volumetric data with optimization of timing of data acquisition with contrast enhancement provides an increased incidence of lesion detection. We are now able to detect lesions that are smaller than those previously detected, including lesions in the 1-2cm range. With smaller tumors the challenge is to make sure patients are indeed surgically resectable before a complicated procedure with a definite mortality rate is performed.

CT was typically quoted as having accuracy of over 90% for the diagnosis and staging of pancreatic adenocarcinoma. However, within these numbers lie only part of the truth as many of these tumors were larger with only around 10% of patients having potentially resectable tumor at the time of diagnosis. When tumors can be detected earlier resectability can provide survivals of up to 57% as opposed to the typical overall survival rate of less than 5%.

In order to determine whether or not Spiral CT could impact on resectability of small pancreatic cancers we correlated the Spiral CT scans of 64 patients with surgical assessment of tumor and pathologic findings. In 57 of 64 cases (89%) a mass was detected with Spiral CT. Of the 64 patients, 24 carcinomas were resectable at surgery and 40 were not. The average size of resectable tumors was 3.1cm. The overall accuracy of Spiral CT for assessing resectability was 70%. Of resected tumors 14 were hypoattenuating compared with remaining pancreas, and 10 were isoattenuating. Eleven tumors showed neoproliferation arterioles at histologic examination. Though these results are encouraging one can initially be disappointed by the overall success. However, what is important to recognize is that this study was done with older Spiral CT scanners with a limited scan time of 24 seconds and 165 mAs. Collimation was typically in the range of 8mm and reconstruction at 4mm intervals. These studies were also obtained with a single-phase portal study. With newer Spiral scanners dual-phase imaging and narrower collimation can surely increase on the accuracy. An important point of the study was that no patient was overstaged and thus prevented from potentially beneficial surgery (positive predictive value, 100%).

Islet Cell Tumors of the Pancreas

In vascular invasion by pancreatic tumors are the common route of spreads the celiac artery and SMA with involvement of the hepatic artery or several small peripancreatic vessels not uncommon. In cases where tumor invasion is obvious with collateralization or narrowing of the vessels the diagnosis is fairly straightforward. In fact, published results in the pre-Spiral CT era have shown that CT and angiography have a nearly 1:1 correlation in detecting vascular encasement. Freeny el al. recommended that only CT scanning be done because of its accuracy with little additional information gained from standard angiography. One potential limitation of determining vascular encasement in more subtle cases (i.e. such as when the pancreatic mass is adjacent to or abuts a vessel but there is no clear encasement on transaxial images). In this scenario one is often left with a confusing picture of whether there is tumor encasement or simply adjacency. Spiral CT with three-dimensional reconstruction can help in many of these cases. The display of vessels in three dimensions allows one to determine the true shape and contour of the vessel and determine whether or not there is simply immediacy or invasion. This is particularly helpful at the level of the confluence of the portal vein and SMV. It is also valuable in tumors that extend near the hilum of the liver.

Islet cell tumors of the pancreas are classically hypervascular and often quickly become isodense to normal pancreatic tissue. These lesions are small and often multiple in occurrence. CT scanning has classically been a poor examination for the detection of these lesions. The use of dual-phase Spiral CT should prove successful in these patients as well. In a series by VanHoe, 9 of 11 islet cell tumors in 10 patients could be detected with dual-phase helical CT, including a 4mm gastrinoma. The two lesions that were missed were less than 5mm in size. Tumor conspicuity was better on arterial-phase images in two of the patients. This study suggests that thin-section dual-phase imaging is successful for detecting vascular pancreatic neoplasms, especially insulinomas.

Conclusion

Spiral CT is currently the study of choice for the detection of pancreatic cancer and for staging extent of disease. CT provides a highly accurate roadmap for the surgeon in those patients where surgery is the treatment of choice. The role of MRI in evaluation of pancreatic cancer is still reserved as a secondary study to be used in patients who are allergic to IV contrast.

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