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Pictorial Essay |
1 Both authors: Russell H. Morgan Department of Radiology and Radiological Sciences, Johns Hopkins Medical Institutions, 601 N. Caroline St., Rm. 3253, Baltimore, MD 21287.
Received October 15, 2001;
accepted after revision November 16, 2001.
Address correspondence to K. M. Horton.
Introduction
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Using the three-dimensional (3D) software, the radiologist can manipulate the volume set, using different orientations and cut planes to best show the pancreas and peripancreatic vessels. This ability to manipulate the volume set is a distinct advantage over traditional axial imaging. In addition to the use of cut-planes, the radiologist can change the opacity, brightness, and window width and level settings. This flexibility allows the radiologist to accentuate the vessels or soft tissues, as needed. This pictorial essay is based on the review of more than 100 individual cases and is limited to assessment of venous involvement.
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The CT grading system described by Lu et al. [1] can be applied to the evaluation of both arteries and veins. In that study, 25 patients with pancreatic adenocarcinoma underwent CT scanning in the pancreatic phase (40- to 70-sec delay) before surgery. These researchers prospectively graded arterial and venous involvement using a 0- to 4-point scale based on contiguity of tumor with the adjacent vessel. When the tumor was in contact with more than 50% of the vessel circumference (grades 3 and 4), surgical resection was not possible [1]. A similar study by O'Malley et al. [2] confirmed the results of Lu et al. In addition to assessing tumor contiguity with vessels, we have found that changes in caliber of the vein or occlusion of key vascular structures is a helpful sign when assessing tumor invasion.
Overall, the reported accuracy of CT for determining the presence of vascular involvement in patients with pancreatic cancer varies in the literature, largely because of differences in techniques and equipment. Early studies reported sensitivities for detecting vascular invasion, ranging from 36% to 64% [3,4,5]. However, more recent studies report higher sensitivities. For example, in a study by Diehl et al. [6], who used helical CT and dual phase imaging, vascular invasion was correctly identified in 35 (88%) of 40 cases. A study by Gmeinwieser et al. [7] produced similar results.
Portal Vein
The portal vein runs in a course perpendicular to the axial plane;
therefore, axial images alone are typically not adequate. The flexibility
possible with interactive 3D volume-rendering allows optimal display of the
portal vein in each patient and, therefore, improves evaluation of this
vessel. In our own practice, we have found that the use of 3D imaging has
greatly improved our evaluation of portal venous involvement because the
angiographic-style vascular maps that can be created often provide more
information than the sum of the axial images. Similarly, research by
Raptopoulos et al. [8] found
that the addition of 3D images improved the accuracy of detecting vascular
involvement when compared with the use of axial images alone
[8].
For optimal depiction of the portal vein, we typically image the volume from a coronal oblique projection. This view allows visualization of the entire course of the extrahepatic portal vein and its junction with the splenic vein and mesenteric veins (Fig. 1). Settings can be optimized to image either the soft-tissue mass or its effect on the portal vein. Tumor involvement will appear as circumferential encasement and narrowing of the vessels, focal invasion, or complete occlusion (Figs. 2,3,4,5).
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Splenic Vein
The splenic vein is often involved by tumors originating in the pancreatic
body or tail. The coronal oblique projection is typically the most useful for
displaying splenic vein anatomy. When tumor results in significant occlusion
or thrombosis of the splenic vein, gastroepiploic collaterals will be present
(Figs.
6A,6B
and
7A,7B).
The presence of these collateral vessels is a helpful secondary sign
indicating the presence of splenic vein involvement. We have found that the
collaterals are well visualized with maximum-intensity-projection CT (Fig.
7A,7B).
If limited isolated splenic involvement is present, surgical resection along
with splenectomy may be attempted at some centers. However, isolated splenic
vein involvement is uncommon.
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Superior Mesenteric Vein
Superior mesenteric vein involvement by pancreatic cancer will deem the
patient ineligible for curative resection (Figs.
8, and
9). Tumors in the uncinate
process, head, or neck can easily involve the proximal portion of the superior
mesenteric vein, often also involving its confluence with the portal vein
(Fig. 3). Significant
encasement of the superior mesenteric vein can result in ischemic changes in
small-bowel loops, depending of the adequacy of collateral vessels.
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For optimal visualization of the superior mesenteric vein, a coronal oblique projection is helpful. Maximum-intensity-projection CT can be used in select cases to improve visualization of distal branches, if necessary.
In conclusion, accurate evaluation of the portal, splenic, and superior mesenteric veins is essential when staging patients with pancreatic cancer because venous encasement by tumor will make the patient ineligible for curative resection. Three-dimensional CT angiography allows optimal visualization of the venous structures and, therefore, can aid the radiologist in CT staging of pancreatic cancer.
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