Pancreas: Tumors: Multidetector CT Angiography in the Evaluation of Pancreatic Carcinoma: Preliminary Observations

Introduction

The critical goals in imaging suspected pancreatic pathology include the early detection of the primary tumor and accurate staging. Classically, involvement of any of the key arterial (i.e. celiac axis or SMA) or venous structures (i.e. portal vein, splenic vein or SMV) will make the patient unresectable. Although changes in CT over the past few years have undoubtedly decreased the number of patients (previously reported in up to 30% of patients) going to surgery expecting to be resectable and found not to be, this still remains a difficult problem in cancer management (1). Discussion over the years has always focused on the need for classic catheter angiography in these cases. Published articles have shown similarity in results between CT and angiography, but surgeons still often get angiography to confirm/exclude vessel invasion and to provide a vascular roadmap (2-3). Although several articles over the past few years have shown that CT can be used to create angiographic style vascular maps (4), its acceptance has been limited in part by the image quality and the lack of any large scale study confirming its accuracy.

Most recently, multidetector CT has been introduced and provides unparalleled capabilities for fast data acquisition and narrow collimation. The limitations of earlier CT angiography disappear and the quality of the CT angiogram in terms of vessel detail and definition more than ever rival angiography. Additionally, unlike classic angiography, CT is not limited by plane or perspective and often the " optimal" view is only determined in retrospect. This pictorial essay reviews our early experience and details the advantages of MDCT with CT angiography for evaluation of suspected pancreatic cancer.

Technique

We are currently using a Siemens Somatom Plus 4 Volume Zoom scanner (Siemens Medical Systems, Iselin , N.J.) with 8 detectors. For imaging the pancreas and to create vascular maps we use the high resolution central 4 detectors which are set at 1mm thickness (4x1). From these detectors we select a 1.25 mm slice thickness and reconstruct the data at 1mm intervals (.25 mm overlap). The other parameters are a 0.5 sec spiral rotation time, 120 kVp and 180-200 mAs. The study protocol is a dual phase acquisition consisting of arterial phase images beginning 25 seconds after the start of contrast injection and the portal phase beginning at about 55 seconds after injection. We inject 120cc of Omnipaque-350 (Nycomed Amersham, Princeton, N.J.) through a peripheral line at 3.0 cc/sec. Image data is reconstructed with the body soft tissue algorithm. All scan data is then transferred to a free standing Silicon Graphics Onyx Infinite Reality workstation (Silicon Graphics, Mountain View, California) running 3D Virtuoso software (Siemens Medical Systems, Iselin, N.J.). All images are reviewed using both the real time viewing capabilities and stereoscopic display as well as both 3D volume rendering technique and maximum intensity projection (MIP). Select 3D images are then printed for subsequent review and to send to the referring clinicians.

Discussion

Specific representative cases to show the value of MDCT angiography in the evaluation and staging of pancreatic cancer are shown below. We will not attempt to compare the accuracy of MDCT with axial images although articles documenting these results will undoubtedly soon follow from multiple institutions.

Superior Mesenteric Artery (SMA)

Invasion of the SMA is a contraindication to attempted curative resection in patients with pancreatic cancer. Vessel involvement can be defined as either occlusion or narrowing of the vessel, usually with an associated soft tissue mass surrounding the area of vessel involvement (5-6). Collateral vessels are also good secondary sign of invasion. With MDCT the course and caliber of the SMA are better defined because of the use of 1-1.25 mm collimation and data reconstruction every 1 mm . Although 1mm incrementation has always been possible, it was not practical previously because of the need for adequate coverage as well as the problem of increased heat units causing interscan delay when dual phase imaging is needed.

The better detail of arterial branching with MDCT allows for detection of earlier invasion with increased certainty because of less volume averaging when looking at the vessels . Involvement of the more distal portions of the SMA is also possible . Better vessel opacification is also a result of the faster scan time and the shorter volume acquisitions.

Celiac Axis and its Branches

Involvement of the celiac axis at its origin or involvement of the common hepatic artery or splenic artery is not uncommon, especially in tumors of the body of the pancreas (7). Our experience has been that these vessels tend to be encased rather than invaded . Branches of the common hepatic artery like the gastroduodenal artery and its tributaries (anterior and posterior pancreaticoduodenal arteries) can be clearly visualized . Definition of the origin of the hepatic artery whether it be classic configuration arising off the celiac artery or one of it s variations (i.e. off the SMA) is also a classic reason why preoperative angiography was obtained. The location and origin of the hepatic arterial supply is routinely visualized on classic spiral CT but to better detail on MDCT .

Portal Venous System including SMV and Splenic Vein

Involvement of either the portal vein, splenic vein or the SMV is classically a contraindication to surgery (8). However, limited involvement of the portal vein may not be an absolute contraindication to all surgeons, as resection with graft placement may be performed. Accurate assessment of these vessels requires a portal phase study as part of a dual phase study in order to allow for vascular opacification of these slower filling venous structures. 3D vascular mapping is ideal for defining these vessels as well as adjacent tumor invasion. Axial images alone may be of limited value in differentiating a mass abutting a vessel and a mass invading a vessel. The use of detailed MDCT images will avoid these problems. Our experience with MDCT has been that the vessel detail, patency and branching pattern is better defined . The presence of collaterals, a secondary sign of vascular invasion is also better seen . Smaller vessels in the venous arcade around the pancreas including the gastrocolic vein, the anterior superior pancreaticoduodenal vein and the posterior superior pancreaticoduodenal vein are best defined with narrow collimation and close interscan spacing provided by MDCT. In cases where invasion of segments of the portal vein are present the 3D map may be useful to help plan the extent of resection.

Conclusions

In our experience, MDCT improves the quality of 3D vascular maps of the mesenteric vaculature obtained in patients with pancreatic cancer, thereby aiding in the detection of vascular encasement/invasion. Although this promises to improve CT accuracy in staging pancreatic neoplasms, prospective studies are necessary.

References

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