AJR 2002; 178:827-831
© American Roentgen Ray Society
Multidetector CT Angiography of Pancreatic Carcinoma
Part I, Evaluation of Arterial Involvement
Karen M. Horton1 and
Elliot K. Fishman
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 August 29, 2001;
accepted after revision October 16, 2001.
Address correspondence to K. M. Horton.
Introduction
The increasing availability of multidetector CT scanners is having a major
impact on imaging. This new technology acquires true volume data sets that can
be easily manipulated with three-dimensional imaging to provide more
information than classic axial displays. Nino-Murcia et al.
[1] recently described the
value of multidetector CT in generating data sets that were ideal for curved
planar reformations of the pancreas and bile duct system. Similarly, these
same volume data sets are ideal for generating CT angiographic maps of the key
vascular structures that are potentially involved with tumor extension. The
presence of tumor involvement of vessels will, in most cases, make the patient
ineligible for curative resection
[2]. The purpose of this
pictorial essay is to show the unique capabilities of this technique in the
evaluation of key arterial structures and its value in imaging patients with
suspected or known pancreatic cancer.
Technique
All studies were performed on a Somatom Volume Zoom scanner (Siemens
Medical Systems, Iselin, NJ) that used an adaptive detector array. Four slices
were obtained during each 500-msec scan rotation. We used the 1-mm detectors
that allowed us to reconstruct the data with a 1.25-mm scan thickness. Images
were reconstructed at 1-mm intervals through the pancreas and liver. The scan
parameters were 120 kVp and 140-180 mAs.
All arterial scans were obtained with a 25- to 30-sec scan delay after
initiation of injection of 120 mL of iohexol-350 (Omnipaque; Nycomed Amersham,
Princeton, NJ), at a rate of 3 mL/sec through a 19- to 20-gauge catheter
placed in an antecubital vein. After reconstruction of the raw data, the scan
data was transferred to the hospital imaging network to a 3D-Virtuoso
workstation (Siemens Medical Systems). All image reconstructions were
performed in real-time by a radiologist using both volume-rendering and
maximum-intensity-projection techniques. Average time for creating the
three-dimensional images was less than 5 min per patient. This pictorial essay
is based on review of more than 100 individual cases and is limited to
assessment of arterial involvement.
Discussion
Criteria for Arterial Involvement
In the absence of obvious liver metastases or local tumor extension, tumor
resectability will depend on the presence of vascular involvement. Involvement
of important arteries (i.e., celiac axis, superior mesenteric artery, or
splenic artery) will make surgical resection impossible. Isolated involvement
of smaller branches such as the gastroduodenal artery will not preclude
surgical resection. A CT grading system of vascular involvement has been
reported by Lu et al. [2].
These authors prospectively graded vessel involvement using a 0- to 4-point
scale based on circumferential contiguity of tumor to vessel and found that
when more than 50% of the vessel circumference (grades 3 and 4) is in contact
with a vessel, the tumor would not be resectable
[2]. This criterion resulted in
a sensitivity and specificity for unresectability of 84% and 98%, respectively
[2]. However, in a more recent
study of vascular encasement in pancreatic cancer by Nakayama et al.
[3], using the same criteria as
Lu et al., they suggested that a different criteria may be necessary when
evaluating arteries and veins. In this study by Nakayama et al., a grade of 3
or 4 for portal venous involvement was also suggestive of unresectable disease
[3]. However, these authors
found that peripancreatic arteries such as the celiac axis, hepatic artery,
and superior mesenteric artery are occasionally surrounded by fibrous tissue
or inflammatory stranding; therefore, the criterion described by Lu et al. was
not as helpful when applied to arteries
[3]. With volume-rendering, we
do not rely solely on the percentage of vessel surrounded by tumor but on
direct visualization of vessel-caliber change and associated soft-tissue tumor
(Figs. 1, and
2).
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Fig. 1. —45-year-old man with pancreatic cancer. Sagittal
three-dimensional volume-rendered multidetector CT scan shows encasement
(solid arrow) and invasion (open arrow) of proximal superior
mesenteric artery.
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Fig. 2. —64-year-old woman with pancreatic cancer. Sagittal
three-dimensional volume-rendered multidetector CT scan shows mass
(arrow) between celiac axis and superior mesenteric artery. Cancer is
causing mass effect on superior mesenteric artery. Note vessel is not
narrowed. This tumor was successfully resected.
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There continues to be significant limitations to viewing vessels in the
standard axial plane. Axial images are typically not optimal for visualizing
the course of the peripancreatic arteries that run perpendicular to the axial
plane. In a study by Raptopoulos et al.
[4], the authors found that CT
angiograms were more accurate than axial images alone in revealing
unresectable disease. They found that by adding the CT angiogram to the axial
images alone, the negative predictive value of a resectable tumor was 96%
compared with 70% for axial images alone.
Celiac Artery and Branch Vessels
The celiac artery and its major branches are often involved by pancreatic
cancer, especially in larger tumors and in tumors of the pancreatic body and
tail. Using interactive real-time rendering techniques, the radiologist placed
a cut plane posteriorly and moved it anteriorly to the level at which the
celiac axis and superior mesenteric artery arise off the aorta. We then viewed
the course of the celiac axis, identifying any variations in vascular mapping
from an anterior projection similar to that in a classic angiogram (Fig
3A,3B).
The course of the key branch vessels including the splenic artery, the hepatic
artery, and the gastroduodenal artery was clearly defined. Either
maximum-intensity-projection CT or volume-rendering provides specific
advantages, although on current workstations, it is easy to go back and forth
between the two rendering techniques. Therefore, we can use both techniques in
all patients, with minimal effort. To date, to our knowledge, no publication
has documented any significant difference between volume-rendering and
maximum-intensity-projection CT in determining vessel patency or invasion in
pancreatic cancer, although it has been reported that volume-rendering is
superior to maximum-intensity-projection CT for visualization of the
pancreaticoduodenal arcades and dorsal pancreatic artery
[5]. We prefer volume-rendering
in most situations because it allows us to image both the tumor and the
vascular map (Fig
4A,4B).
Control of the specific imaging plane is optimized with volume-rendering as
opposed to maximum-intensity-projection CT, which is a projection technique.
Maximum-intensity-projection CT does provide a global perspective in cases of
vessel occlusion and subsequent collateralization. Using multidetector CT
volume data with 1.25-mm-thick sections at 1-mm intervals, we can now
routinely define branch vessels like the gastroduodenal artery and the
pancreatic arcade not just as cross-sectioned vessels but along the course of
the vessels in their entirety (Figs.
5 and
6A,6B).
One potential advantage of using a volume display is that we can detect vessel
narrowing and invasion and not just tumor surrounding the vessel. This may be
useful in the more accurate definition of vessel encasement.
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Fig. 3A. —45-year-old man with pancreatic cancer. Volume-rendered
three-dimensional (3D) multidetector CT scan obtained in coronal projection
shows normal anatomy of celiac axis and vessel branches in orientation similar
to that on classic angiography. Note gastroduodenal artery
(arrowhead), splenic artery (curved arrow), left gastric
artery (open arrow), and hepatic artery (solid straight
arrow).
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Fig. 4A. —63-year-old woman with pancreatic cancer. Slightly oblique
axial three-dimensional (3D) volume-rendered multidetector CT scan reveals
mass (arrows) arising from body and tail of pancreas and extending
posteriorly to involve left renal hilum. Mass caused delayed renal function
seen as decreased cortical enhancement in left kidney.
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Fig. 4B. —63-year-old woman with pancreatic cancer. Coronal 3D
volume-rendered multidetector CT scan shows mass (arrows) seen in
A. In this orientation, mass is seen encasing splenic artery
(arrowhead).
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Fig. 5. —62-year-old woman with small periampullary mass. Coronal
maximum-intensity-projection CT scan shows normal gastroduodenal artery
(arrow). Vessel is much easier to appreciate on coronal
three-dimensional images than on axial CT images. No evidence of arterial
encasement is seen.
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Fig. 6A. —73-year-old man with pancreatic cancer. Coronal
three-dimensional (3D) volume-rendered multidetector CT scan shows mass
(arrow) encasing gastroduodenal artery (arrowhead). This
alone would not make patient ineligible for curative resection.
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Fig. 6B. —73-year-old man with pancreatic cancer. Coronal oblique 3D
volume-rendered multidetector CT scan obtained during portal venous phase
shows mass (arrows) encasing superior mesenteric artery and portal
confluence (arrowhead). Venous involvement would make patient
ineligible for curative resection.
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Routine visualization of the origin of the celiac axis is also helpful in
detecting vessel stenosis due to atherosclerotic plaque, which can be treated
at the time of the Whipple procedure (Fig.
7).
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Fig. 7. —63-year-old man with pancreatic cancer. Axial oblique
three-dimensional volume-rendered multidetector CT scan imaged from above
shows calcified atherosclerotic plaque narrowing origin of celiac axis
(arrow). No evidence of vascular invasion by tumor was present.
Vascular bypass was performed at time of pancreatic resection.
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Superior Mesenteric Artery
The superior mesenteric artery is the most common arterial vessel involved
by carcinoma of the pancreas (Fig.
8A,8B).
Its critical relationship to the head and body of the pancreas and the typical
patterns of spread of the disease make it especially vulnerable to invasion
and encasement (Fig. 9).
Although many cases of vascular invasion are easy to diagnose, relying only on
the axial display makes others more difficult. The displays that are ideal for
the superior mesenteric artery (and celiac artery) are volume-rendering
interactive displays and volume-rendering in the sagittal projection
[6,
7]. An important advantage for
CT angiography is that by creating an angiographic road map, anatomic
variations can be recognized (Fig.
10A,10B).
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Fig. 8A. —62-year-old man with pancreatic cancer. Sagittal
three-dimensional (3D) volume-rendered multidetector CT scan shows normal
appearance of superior mesenteric artery (straight arrow) and celiac
axis (curved arrow).
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Fig. 8B. —62-year-old man with pancreatic cancer. Coronal 3D
volume-rendered multidetector CT scan shows normal branching pattern of
superior mesenteric artery. These small vessels are almost impossible to
confidently identify on axial scans.
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Fig. 9. —73-year-old woman with pancreatic cancer. Coronal
three-dimensional volume-rendered multidetector CT scan shows pancreatic mass
(arrows) encasing mid portion of superior mesenteric artery
(arrowhead). Narrowed vessel is compatible with tumor invasion. This
patient was not eligible for curative resection.
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Fig. 10A. —53-year-old man with pancreatic cancer. Axial
three-dimensional (3D) volume-rendered multidetector CT scan shows tumor
invasion (straight solid arrows) of superior mesenteric artery
(curved arrow) and hepatic artery (open arrow).
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Fig. 10B. —53-year-old man with pancreatic cancer. Axial oblique 3D
volume-rendered multidetector CT scan shows tumor encasing common hepatic
artery (arrowhead) arising off celiac axis. Note encasement of
replaced right hepatic artery (arrow) off superior mesenteric
artery.
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As previously noted, both volume-rendered images and
maximum-intensity-projection CT—rendered images are valuable in the
evaluation of the superior mesenteric artery and its branch vessels. With
maximum-intensity-projection CT, vessels can be shown further along their
course, although it has not been shown that there is any difference in
accuracy between the two techniques. Because maximum-intensity-projection CT
is based on projection of the brightest pixels along an array, it requires
more editing than the volume display because overlapping vessels and bone
cause obscuring of detail. For example, with volume-rendering, the aorta and
spine do not necessarily need to be edited from the image volume. However,
with maximum-intensity-projection CT, the aorta should be edited from the
volume, or the proximal branching of the superior mesenteric artery and the
celiac axis will be obscured (Fig.
11A,11B).
As noted, in our experience we interactively alternate between the two
displays with the volume-rendered display favored because one can define not
only the vessel map but also the tumor itself.
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Fig. 11A. —52-year-old woman with abdominal pain. Three-dimensional (3D)
volume-rendered multidetector CT scan shows that aorta and spine have not been
removed. Superior mesenteric artery can be identified even though it is
overlying aorta.
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Fig. 11B. —52-year-old woman with abdominal pain.
Maximum-intensity-projection CT scan is in same orientation as A.
Because maximum-intensity-projection is 3D technique that displays brightest
pixel along array, aorta and spine will obscure mesenteric vessels, unless
extensive editing is performed.
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In conclusion, three-dimensional volume display of CT angiographic data
sets provides a comprehensive display of the key arterial anatomy needed to
properly determine resectability of pancreatic cancer. With the continued
development of multidetector CT including the introduction of scanners with
more detector rows allowing faster scanning and isotrophic data sets, the role
and accuracy of CT angiography will continue to increase. The challenge will
be to handle these large volume data sets in an interactive fashion. This
should be possible as more powerful computer hardware is introduced to the
medical market.
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