The development of the 4-slice multidetector computed tomography (MDCT) scanner, combined with state-of-the-art three-dimensional (3D) reconstruction software, has optimized the resolution of today's computed tomographic angiography (CTA) to the extent that clinicians are increasingly relying on this noninvasive means of evaluating vascular disease rather than conventional arterial angiography.
is Chief Resident in Diagnostic Radiology in the Department of
Radiology and Radiological Sciences; and
is Professor of Radiology and Oncology and Director of Diagnostic
Radiology and Body CT, Johns Hopkins Hospital, Baltimore,
The development of the 4-slice multidetector computed tomography
(MDCT) scanner, combined with state-of-the-art three-dimensional
(3D) reconstruction software, has optimized the resolution of
today's computed tomographic angiography (CTA) to the extent that
clinicians are increasingly relying on this noninvasive means of
evaluating vascular disease rather than conventional arterial
angiography. CT angiography truly applies to whole-body vascular
imaging, ranging from the detection of cerebral aneurysms and
coronary artery disease to the evaluation of peripheral vascular
disease in the lower extremities. For the evaluation of abdominal
pathology, CTA has become a routine diagnostic tool as well as an
integral step in preoperative planning.
The increased scanning speeds provided by MDCT have made
detailed imaging of the vasculature possible. With the old
single-detector CT scanners with rotation speeds of 1 second, only
30 rotations could be made in a 30-second breath hold. Therefore,
scanning parameters were chosen to either cover a small region with
high resolution or a larger volume using thick slices.
Today's MDCT scanners have four rows of detectors simultaneously
acquiring data combined with sub-
second rotation (0.5 to 0.8 sec) of the gantry, increasing scanning
speed by a factor of eight.
As a result, a greater portion of the z-axis can be covered (150
cm) in a shorter amount of time, and ultra-thin slices can be
obtained (1 mm).
These thin sections are then reconstructed at small intervals (1
mm), producing high-resolution 3D images. Since the image is
acquired as a volume, it follows that a very efficient method for
viewing such large amounts of data is via a 3D display. This is
accomplished using a computer workstation and graphics software.
The radiologist processes the image by selecting the optimal 3D
rendering technique, windowing parameters, and viewing cut-plane.
By rotating the data set in any plane on the workstation, the
entire length of the vessel may be displayed and evaluated on one
image (Figure 1).
3D rendering techniques
The three principle techniques for 3D processing of CT datasets
include surface rendering, maximum intensity projection (MIP), and
volume rendering. Surface rendering was one of the first methods
developed, and it relies on the comparison of a voxel intensity to
some defined threshold value, which the computer uses to define an
The MIP algorithm selects out the maximum value in a voxel along a
line from the viewer's eye through the image and displays only that
value in the corresponding pixel.
The limitations of both MIP and surface rendering lie in the
forfeiting of most of the available data in order to increase
Volume rendering, on the other hand, sums the contributions of each
voxel along a line from the viewer's eye through the dataset, such
that all of the information is included in the final image.
Therefore, volume rendering is more accurate and allows the
visualization of multiple tissue types simultaneously. For these
reasons, volume rendering has become the preferred method for 3D
postprocessing. With MDCT, MIP can play a complementary role and is
used in conjunction with volume rendering in our practice.
CTA of the kidney
CT angiography has come to the forefront in the preoperative
evaluation of candidates for renal transplants and partial
nephrectomies. It is also a reliable noninvasive exam for the
detection of renal artery stenosis. A triphasic CT vascular map is
now indicated for noninvasive evaluation of both the donor organ
and selection of the organ to be transplanted, identifying any
aberrant vessels or anatomic variations
(Figure 2). In addition to evaluating the vasculature, CTA has the
advantage of simultaneously excluding the presence of renal cell
carcinoma or renal artery stenosis in renal donors, both of which
may render the patient inoperable. CT angiography has also played a
key role in the presurgical evaluation of patients for laparoscopic
partial nephrectomies and in the postoperative evaluation of
complications including urinoma, pseudoaneurysm, and perinephric
Halpern et al
prospectively studied 56 patients who underwent both CTA and
Doppler ultrasound of the renal arteries for the evaluation of
renal artery stenosis, and compared the results with conventional
angiography. The results indicated that CTA was more accurate than
Doppler ultrasound for the detection of renal artery stenosis.
Kaatee et al
prospectively studied 71 patients who had been examined with CTA
and digital subtraction angiography (DSA) for renal artery renal
artery stenosis. They found that the sensitivity and specificity of
CTA for renal artery stenosis >50% was 96%, which is similar to
that of DSA.
CTA of the pancreas
The accurate identification of vascular invasion is crucial in
the staging and evaluation for resectability in patients with
pancreatic cancer. In evaluating patients who are potential
candidates for a Whipple procedure, the superior mesenteric artery
(SMA) and the superior mesenteric vein (SMV) must be clearly
defined in relation to the pancreatic mass, and a decision must be
made as to whether the mass invades or just abuts the vasculature
(Figure 3). It has been shown in the literature that while
evaluation of vascular invasion with axial images alone has a poor
correlation with surgical findings, 3D CTA detection has a near
This is in part due to the multiplanar capabilities of MDCT
scanning, but is also a result of enhanced resolution provided by
CTA of the mesentery
Timed administration of the intravenous contrast bolus for both
arterial and venous phases provides a dynamic evaluation of the
mesenteric vasculature that may aid in the diagnosis of bowel
ischemia and inflammatory bowel disease. The CT evaluation of
patients with suspected bowel ischemia has previously relied on the
detection of pneumatosis, pneumoperitoneum, and bowel wall
thickening. However, bowel wall edema is a relatively nonspecific
inflammatory finding and benign causes of pneumatosis have been
reported due to steroid use, pulmonary disease, and collagen
Therefore, a more specific dynamic evaluation of the mesenteric
vasculature is desired for the detection of low-flow states or
embolic vessel occlusion, which may precipitate ischemia to the
bowel. CT angiography of affected bowel loops may demonstrate
absence of enhancement, delay in enhancement, or persistent
enhancement when compared with unaffected loops
(Figure 4). Visualization of the entire data set in one image
improves the detection of collateral vessels between the celiac,
SMA, and inferior mesenteric artery (IMA), which may suggest
chronic ischemia in selected segments of bowel.
Improved resolution with MDCT allows the detailed visualization of
small branching vessels that may contain tiny emboli.
In addition, 3D postprocessing using cut-planes removes
superimposed vessels, isolating the vessel of interest.
Hypervascularity and engorgement of the bowel wall has also been
observed in patients presenting with an acute flare of inflammatory
CTA of the abdominal aorta
CT angiography plays an important role in the preoperative and
postoperative evaluation of abdominal aortic graft placement,
aneurysms, and dissections. The multiplanar capabilities of 3D CTA
are extremely useful in obtaining accurate measurements of the
extent and maximum diameter
of tortuous aortic aneurysms with extensive calcified mural plaque.
Postoperatively, CTA is a low-cost and noninvasive means of
surveillance of aortic stents (Figure 6). Complications after graft
placement including supragraft aneurysms, distal anastomotic
aneurysms, graft infections, perigraft fluid collections, and graft
thrombus are also optimally detected.
Imaging the entire aorta in one plane is ideal for the evaluation
of aortic dissections. CT angiography is used to depict the
relative patency of the true and false lumen and the origin of the
renal arteries can be clearly defined (Figure 7).
CTA of the liver
CT angiography evaluation of the liver is very useful for
preoperative vascular mapping in the evaluation of candidates for
liver transplant as well as detecting vascular complications of
cirrhosis and early identification of hepatocellular carcinoma. The
"typical" hepatic arterial anatomy occurs in only 55% of the
population, and at least 10 variations exist.
In liver transplant patients, CTA identifies aberrant anatomy
pre-operatively, and simultaneously evaluates liver volume
(Figure 8). Another benefit of CTA is the full visualization of the
extent of varices and collateral vascularization in cirrhotic
patients on a single image (Figures 9 and 10). CT angiography is
also particularly useful for the detection of small hepatomas in
patients with hepatitis or cirrhosis. When contrast was
administered directly via canalization of the hepatic artery,
Kanematsu et al
found that the conspicuity of tumors and tumor-feeding arteries was
significantly better on MIP CTA than on conventional
Intra-arterial chemotherapy or che-moembolization for recurrent
hepatocellular carcinoma requires superselective catheterization of
small branches of the hepatic artery under fluoroscopic guidance.
Often, the identification of the main arterial feeding vessels
supplying the tumor mass is made in real time during the
interventional angiogram by analysis of the vascular blush.
However, Sze et al
found that 3D CTA with selective hepatic arterial contrast
administration identified additional hepatic tumors undetected by
conventional angiography in 30% of patients. Also, the vascular map
provided by 3D CTA altered the decision as to which arterial
branches were optimal for catheterization. Thus, it was concluded
that the use of 3D CTA prior to selective chemoembolization
frequently alters treatment plans.
The increase in imaging speed and resolution provided by MDCT,
combined with improved 3D processing software, has brought CTA to
the forefront as an important diagnostic tool for the evaluation of
abdominal pathology. Three-dimensional CTA ideally depicts the
vasculature because the entire length of the vessel may be
visualized on a single image and vascular patency is accurately
defined even in small branching vessels. CT angiography is
increasingly being implemented in the pre-operative evaluation of
patients undergoing hepatic chemoembolization procedures and
transplants of both the liver and kidney. It is also an excellent
noninvasive means of surveillance in patients with cirrhosis for
the evaluation of hepatoma and for stent placement in patients with
aortic grafts. The newest MDCT scanners will be able to acquire 16
slices simultaneously, further increasing scan speed and the
resolution capabilities for optimal 3D imaging of the abdominal
viscera and its vascular supply.