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Everything you need to know about Computed Tomography (CT) & CT Scanning


Pancreas: Tumors: CT Angiography of Pancreatic Cancer

Introduction

CT Angiography now represents the state of the art in oncologic imaging for the detection and staging of disease. The coupling of the fast scanning capabilities of Multidetector Row CT (MDCT) with the ability to obtain thin sections and close interscan spacing provides the ability to better detect the presence of disease and to accurately stage disease. The use of MDCT with CT Angiography is ideally suited for the evaluation of the patient with suspected pancreatic cancer and is the subject of this presentation.

Study Protocols

The protocols for MDCT and CT angiography of the pancreas include dual phase imaging with data obtained 25 and 60 seconds after the injection of 120 cc of Omnipaque -350 (Nycomed Amersham, Princeton, NJ) at a rate of 3 cc/second. Scan parameters for MDCT are 1.25 mm scan width reconstructed at 1 mm intervals and 3 mm scan width at 1mm intervals for single detector CT. We currently use a Siemens SOMATOM Volume Zoom scanner which uses an adaptive array detector and creates 4 images per scan rotation (500 milliseconds). After data acquisition the images are sent over the hospital-imaging network to the workstation (3D Virtuoso Workstation, Siemans Medical Systems) and select 3D images are generated from both phases of acquisition. The 3D images are routinely reconstructed with both volume rendering (VRT) and maximum intensity projection (MIP) techniques. This is often supplemented by minimum intensity projection techniques and thin slab MPR images.

Study Analysis and Interpretation

The classic description for a pancreatic tumor was a mass within the pancreas commonly ranging in size from 3-6 cm. Detection of the mass was typically based on size parameters with less attention paid to the differential enhancement of normal and abnormal pancreatic tissue. In part this was due to the slow injection rates as well as longer scan times on early generation CT scans. Additional parameters commonly used include the presence of pancreatic or common bile duct dilatation, as well as changes in the pancreatic contour. Most pancreatic masses are best seen on the portal venous phase images, as they are hypovascular . Other tumors including islet cell tumors and metastases are hypervascular and are best seen on arterial phase images. Graf et al. found that tumor conspicuity of pancreatic adenocarcinoma was better in the portal phase where the tumor to pancreas contrast difference was 54+/-31 H than in the arterial phase when it was 31 +/- 29H . Regardless of the tumor type, the use of thin section CT with close interscan spacing allows smaller tumor detection when changes in the gland enhancement patterns are optimized. Although axial imaging and review of these images alone has been the standard mode of CT review, it would not remain the gold standard, as newer technologies became available. Bonaldi et al. found that reviewing the images on a workstation with a cine display provided better definition of tumors as well as vascular anatomy and ductal anatomy. The use of 3D imaging can be very helpful in select cases by defining the mass or suspected mass in multiple viewing planes. In many cases this may distinguish a true pancreatic mass from adjacent duodenal or small bowel tumor or peripancreatic adenopathy. Reconstruction of the common bile duct and/or the pancreatic duct may also prove useful by defining the transition point and helping identify the cause of obstruction.

Vascular Mapping

The area of greatest challenge in pancreatic imaging has been the ability of CT to accurately determine the presence of vascular invasion. The accuracy of CT to define arterial or venous invasion has been the subject of numerous articles often focusing on its accuracy when compared to either catheter based angiography or to surgical findings. Several articles even in the pre-spiral CT era has clearly shown the equivalence of carefully performed CT angiography and catheter angiography for vascular encasement .

Yet, with spiral and then multidetector CT our capabilities have gone far beyond looking at vessels in the axial plane. There are significant limitations when looking at vessels in the axial plane. Partial averaging of data, questions as to whether a tumor is adjacent to rather than encasing a vessel as well as the lack of an ideal display are problems with axial CT alone. The use of image reconstruction especially with a 3D vascular map has obvious advantages especially to the referring surgeon who is more comfortable with a volumetric display.

Arterial Imaging

The accurate display of arterial anatomy (SMA, celiac axis) is the major focus of arterial phase 3D mapping in the evaluation of the pancreas. The goal of preoperative vascular mapping in patients who are potential candidates for a Whipple procedure is to clearly define the angiographic map with accuracy equal to or exceeding a classic catheter angiogram. With 3D CT angiography this goal can be obtained. Anatomic variation such as a common celiac and SMA trunk or the presence and location of a right hepatic artery arising off the SMA are all easily detailed on the 3D display. The viewing of this data with a stereoscopic display may add further information in cases of complicated vascular anatomy.

In cases where there is potential vascular invasion the 3D display will define the course of the vessel and its relationship to the pancreatic mass. The ability to determine whether a mass actually encases or just abuts a vessel will be clearly shown in most cases. The ability to view images in any plane or perspective is an important tool supplementing the information from the axial CT and in replacing angiography. Although no large series has yet to be published, our personal experience has been suggestive of a near one to one correlation with surgical findings.

With axial images alone, numerous articles tried to develop strategies for defining resectability. Lu et al. graded vessel invasion on a 0-4 scale based on circumferential contiguity of tumor to vessel. That is, a grade 0 was no contiguity of tumor to vessel, grade 1 was tumor contiguous with less than one quarter circumference, grade 2 between on quarter and one half circumference, grade 3 between one half and three quarters circumference and grade 4 greater than three quarters circumference or any vessel constriction. Involvement of more than one half the circumference or grade 3 was highly specific for unresectable tumor. Yet, even half of the grade 2 cases were proven to be unresectable and 12.5% of grade 3 were resectable. We believe these studies highlight some of the disadvantages of viewing vessels with an axial plane only. This is especially true when it comes to vessel narrowing or constriction. The use of MDCT increases the number of slices through the pancreas over even the best single detector spiral protocol by a factor of 2 or 3. This increased data sampling at 1-1.25 mm slice thickness and 1 mm interscan intervals coupled with the higher resolution of MDCT should prove better at defining the key arterial vessels even if only the axial images are reviewed. However, the ability to view the vessels in multiple orientations provides a more complete and accurate display where even subtle vessel invasion can be detected. The extent of vessel encasement is optimally defined with this visualization.

Venous Imaging

Numerous articles including one by Vedantham et al. have shown that helical CT performed in the portal venous phase at 50-70 seconds after injection of contrast is ideal for defining peripancreatic venous anatomy for determining tumor invasion. However as noted previously the use of axial CT alone is not ideal for pancreatic imaging and that a more volumetric display is required. With the development of single detector and now multidetector CT we believe that a volume display is the most accurate technique for evaluating venous invasion. Even with the use of single detector CT Graf et al. were able to create accurate CT venograms of the mesenteric veins that were able to equal angiography for defining variations in vascular anatomy. However careful analysis of the images would suggest that their detail was not adequate to define early vessel invasion as in pancreatic cancer. Novick et al. did show that using advanced 3D techniques like volume rendering those vascular maps could not only define the venous anatomy but also accurately predict vascular invasion. Raptopoulos et al. had similar results with CT Angiography from single detector spiral CT data sets. They found that by adding the CT angiogram to the axial images alone the negative predictive value of a resectable tumor was 96% compared to 70% for axial images alone. The studies were especially valuable indetermining unresectability.

We have recently shown that the use of MDCT coupled with three-dimensional imaging provides an even better way to image vessel patency . By acquiring data sets of narrow collimation with short acquisitions we are able to obtain a true volume data set for evaluation of the venous system. Using the 3D display we can define vessel patency as well as determine early vessel encasement or invasion. The use of these display tools decreases the potential for false positive studies as well as indeterminate studies. Areas where the 3D display is especially helpful are at the confluence of the portal and superior mesenteric vein as well as the more distal portions of the portal vein. As surgeon become more aggressive in putting vascular grafts when only limited invasion is present the use of these 3D angiographic maps will become even more valuable.

The 3D angiographic maps also clearly define the presence and extent of collaterals which prove to be a strong secondary sign of vascular invasion.

Metastatic Disease

The presence of liver metastases, nodes distant to the tumor bed or carcinomatosis will make the patient with pancreatic cancer unresectable for cure. MDCT with dual phase imaging is ideal for detection of small nodes as well as the presence of small (

Conclusion

MDCT with 3D CT Angiography is the ideal study for the detection and staging of pancreatic cancer . Careful attention to both the acquisition of the CT dataset as well as to the post study image processing are needed to optimize the efficacy of this new imaging technique.

Select References

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