As the use of multidetector-row computed tomography (MDCT) becomes more widespread, potential applications continue to develop. In patients with acute abdominal pain, MDCT has the potential to fill the roles currently served by the more invasive and time-consuming conventional digital subtraction angiography (DSA). Automated processing methods may also enable MDCT to image complex structures, such as the bowel in three dimensions, an advance with potential benefits not only in evaluating acute disease of the bowel, but also in overcoming the challenges posed by the large and complex MDCT data sets.
is a third-year Resident in the Department of Radiology, Stanford
University Medical Center, Stanford, CA. He received his MD in
1999 from Baylor College of Medicine, Houston, TX. He plans to
begin a Neuroradiology Fellowship following completion of his
As the use of multidetector-row computed tomography
(MDCT) becomes more widespread, potential applications continue
to develop. In patients with acute abdominal pain, MDCT has the
potential to fill the roles currently served by the more invasive
and time-consuming conventional digital subtraction angiography
(DSA). Automated processing methods may also enable MDCT to image
complex structures, such as the bowel in three dimensions, an
advance with potential benefits not only in evaluating acute
disease of the bowel, but also in overcoming the challenges posed
by the large and complex MDCT data sets.
Technical advances in computed tomography (CT) have changed the
landscape of abdominal imaging. Where digital subtraction
angiography (DSA) once stood as the only diagnostic option for
certain acute abdominal pathologies, such as mesenteric ischemia,
multidetector-row CT (MDCT) has firmly established itself. The
continuing development of this technology enables rapid and safe
diagnosis of a wide variety of acute conditions. Selected
applications that benefit from the particular advantages of MDCT
over single-detector-row helical CT (SDCT) or DSA are described
here, as are problems created by this technology and the potential
solutions to those problems.
Single-detector CT is used routinely in a wide range of clinical
emergencies in the United States and Europe.
The advantages of CT evaluation of acute abdominal pain have been
described at length.
The limitations of SDCT, however, have precluded its routine use
under certain conditions.
These limitations, including the relatively slow speed of scanning
compared with breath-hold times or blood flow, relatively high
contrast dose, and low z-axis resolution, can be overcome by MDCT.
Therefore, MDCT has become an alternative to, or substitute for,
DSA in carotid angiography, lower-extremity angiography, and
intravenous urography, to name a few applications.
The same advantages that enable MDCT to scan within a specified
vascular phase--namely, rapid, high-resolution scanning in a single
breath-hold--have also been used to create thin-collimation images
and virtual three-dimensional (3D) representations of the colon.
These same capabilities of rapid scanning and high z-axis
resolution have great utility in the emergency evaluation of acute,
nontraumatic abdominal pain. At the same time, they require the
development and refinement of automated methods to display large,
complex data sets in a rapid, reliable, and reproducible way.
Before exploring specific potential applications, it is helpful
to briefly review the basic principles underlying MDCT. An
excellent review by Fuchs et al
describes many of the technical details. Briefly, by using a
two-dimensional (2D) array of X-ray beam detectors, multiple data
channels may be obtained at once in a helical fashion. The number
of data channels that can be sampled at once vary according to
manufacturer, but are increasing. Depending on the slice profile
and filter width, one can achieve effective slice thicknesses with
sub-millimeter z-axis resolution.
Several different advantages result. Scan time can be decreased
since a thicker section of the body can be imaged with each gantry
rotation. This means that single breath-hold images of the abdomen
and pelvis are feasible, even at thin collimation. For example, at
our institution, scans from the dome of the diaphragm to the bottom
of the pubic symphisis are routinely performed in approximately 25
seconds on an 8-detector scanner, at 5 mm collimation using a table
speed of 13.5 mm/gantry rotation. Another way of exploiting the
speed advantage of MDCT is in "multiphasic scanning," when the
abdomen may be scanned rapidly in succession to acquire images
during multiple phases of contrast enhancement. Additionally, as
scan times decrease, so does the volume of contrast necessary for
adequate arterial enhancement. Another advantage inherent to MDCT
is the ability to retrospectively reconstruct the image data to
thinner collimations, down to the collimation set for the smallest
element in the detector array. This allows prospective
reconstruction at thicker collimation to enable efficient data
interrogation, but leaves as an option a more detailed,
thinner-collimation examination of the areas of interest. For
example, one may perform a CT angiogram of the abdomen at 5 mm
collimation to look for acute pathology, such as retroperitoneal
hematoma, while thinner, 1.25-mm reconstructions can be fed to a 3D
postprocessing workstation for creation of high-resolution
representations of the abdominal aorta.
There are disadvantages that arise with the use of MDCT. As the
resolution of the study increases, so does the noise. This can be
offset with increased tube current, but at the expense of increased
Another issue is the size of individual studies. CT angiography
(CTA) of the lower extremities, for example, can routinely generate
thousands of images. The time to review each individual axial
section can become prohibitive. Primary evaluation of 3D
representations of the data set is possible. At the moment,
however, physicians or specially trained technologists are required
to construct these 3D representations, often taking hours at a time
for complex cases.
Further, 3D images, by necessity, do not contain all of the
information in the axial source images. For example, a 3D
representation of the surface of the abdominal aorta, may not
demonstrate an embolus within the lumen of the superior mesenteric
artery, which would be evident on an axial image. This inevitably
results in a trade-off between diagnostic accuracy and the time
burden associated with interpretation of huge studies. The
combination of computer automation and computer-assisted diagnosis
that will likely evolve as the technologies mature, in response to
the large datasets and 3D nature of volumetric scanning, may entail
a fundamental change in the way we read and interpret these
Acute, nontraumatic abdominal pain
The causes of acute abdominal pain are numerous and varied.
CT has been well utilized in the diagnosis of many of them, but its
use in the evaluation of vascular causes of abdominal pain has been
One of the particular strengths of MDCT is the ability to perform
CTA. Its use in lower-extremity angiography and aortic angiography
has been described elsewhere.
The application of MDCT to the diagnosis of ischemic bowel in the
acute ill patient is a logical progression.
Mesenteric ischemia can result from either an acute or chronic
reduction in arterial flow, or from venous thrombosis.
CT findings in bowel ischemia are broad, and in many cases
In acute arterial ischemia, the small bowel may demonstrate four
types of enhancement patterns: no enhancement; delayed enhancement
with respect to other bowel loops; strong homogeneous enhancement
with respect to other bowel loops; and a layered, or "target,"
pattern of bowel wall enhancement. Chen et al
found that nonenhancement and "target" enhancement were equally
frequent, followed by strong homogenous enhancement and delayed
enhancement. Secondary findings include bowel wall thickening,
portal venous gas, pneumatosis intestinalis, arterial filling
defects or occlusion, and solid organ parenchymal infarcts (Figure
The causes of bowel ischemia are varied, and include
atherosclerosis, thromboembolic disease, vasculitides, and
hypoperfusion resulting from low-flow states.
Mechanical occlusion as a result of herniation, intussusception,
torsion, or direct invasion or encasement by tumor may also be
Conventional helical CT has been used in the evaluation of
ischemic bowel, and demonstrates certain findings that are typical
Specifically, in addition to the aforementioned enhancement
patterns, bowel wall thickening and bowel dilatation may be seen. A
specific vascular distribution, or later findings (such as
pneumatosis intestinalis and frank obstruction), may suggest the
diagnosis as well.
These findings are relatively nonspecific, and, in the setting of
chronic mesenteric ischemia, the protean clinical symptoms may make
the diagnosis difficult. Multidetector-row CT, however, enables
exquisite visualization of the anatomy of the mesenteric arterial
vasculature. With the aid of postprocessing, one can determine
degree of vessel stenosis, detect mural calcification, and
visualize thrombosis, or embolic occlusion in acute ischemia, and
evaluate for the presence of collateral vessels in chronic ischemia
Venous thrombosis accounts for 15% to 20% of cases of mesenteric
The pathology can be demonstrated easily with venous phase
Although SDCT is sufficient for the evaluation of large-vessel
venous occlusion--at the level of the portal and superior
mesenteric veins, for example--the higher resolution of MDCT
enables depiction of occlusions in more-distal branch vessels
(Figures 3 and 4). CT has particular advantages in the depiction of
the portal venous anatomy, as it is difficult to access the portal
system directly by conventional DSA.
A preliminary study examining MDCT in acute ischemia was performed
using 4-detector MDCT at 1.25-mm collimation imaged 25 seconds and
60 seconds after IV contrast infusion at 4 mL/sec. The results
revealed a high sensitivity and specificity for acute arterial
mesenteric ischemia (100% and 92%, respectively) using combined CT
Multidetector-row CT also enables the detection of other, less
common causes of mesenteric vascular injury
The ability to scan the entire abdomen and pelvis at once
enables the depiction of other pathologies, distant from the
mesenteric vascular axis, which may not have been identified in
selective DSA. Also, while DSA is relatively safe, it has been
noted to be susceptible to operator-dependent complications
(reported rates of 1.8% to 14.7%), and the noninvasive nature of CT
imaging enables faster, safer visualization of the vascular
anatomy, with less potential for complication.
These last two points are illustrated nicely in Figure 6, which
demonstrates an unsuspected iatrogenic femoral pseudoaneurysm and
active extravasation from the superficial femoral artery after
MDCT and virtual endoscopy
Thin-collimation, rapid scanning made possible by MDCT has been
used to depict the inner surface of hollow lumens, such as the
colon and tracheo-bronchial tree.
These techniques are most successful when the lumen is relatively
large (tracheo-bronchial tree) or distended (colon). Extensive
research on automated processing methods and computer-aided
diagnosis are likely to be crucial to the successful use of
multidetector-row technology in CT colonography.
Evaluation of the small bowel can also now be performed using
such techniques as CT enteroclysis.
While methylcellulose with barium has been used in traditional
enteroclysis, low attenuation contrast, such as water or
methylcellulose, can provide adequate luminal contrast in
intravenous contrast-enhanced CT.
Using the techniques described by Maglinte et al,
with a 14Fr 155-cm nasointestinal tube, low-attenuation contrast,
such as water, can be used to provide luminal contrast in the small
bowel. Additionally, the same tube can provide for gastric
decompression, which, if used routinely, can allow for therapeutic
decompression and diagnosis without the discomfort and difficulty
of repeated nasogastric intubation.
With adequate small-bowel distension, automated 3D processing
techniques, such as center-path finding, can create simple
representations of the complex anatomy of the bowel, as is done in
In patients with acute abdominal pain, bowel obstruction often
results in loops of distended bowel that transition to
nonobstructed, nondistended bowel. It is possible, using the same
3D reconstructive techniques, to create 3D representations of the
obstructed bowel lumen. With further advances in computer automated
reconstructions, it may also be possible to "lay-out" the bowel in
such a way as to accurately identify the point of obstruction.
MDCT and 3D representation
A common thread throughout this discussion is the necessity for
advances in the representation and manipulation of large MDCT data
sets. When a study comprises hundreds, or even thousands, of axial
images, the efficiency of viewing each individual image is
compromised. Several options besides viewing contiguous axial
images are available for interpretation of MDCT examinations.
Various types of 3D and postprocessed images have been reviewed
at length elsewhere, but will be briefly described here.
The first, in terms of simplicity, is the multiplanar reformat
(MPR). This is simply a reconstruction of the data set into another
orthogonal plane, perpendicular to the original imaging plane. With
thin-collimation examinations, the resolution of a MPR can approach
and equal the resolution of the study in the original acquisition
Curved planar reformations (CPR) are similar to MPR, except that
the imaging plane is not orthogonal. Since few structures in the
body line-up perfectly with the three orthogonal image planes
(transverse, sagittal, and coronal) or single-plane obliques,
typically, the CPR follows the contour or lumen of a blood vessel,
bile duct, loop of bowel, or other structure, as it travels in and
out of the orthogonal plane. The path of the vessel has
traditionally been determined manually, by specifying the center of
the vessel on contiguous axial images on a 3D workstation. Computer
algorithms can reliably, reproducibly, and quickly find the center
path down a blood vessel or other structure with little user input,
and create a CPR.
Curved planar reformations have pitfalls, however. For example,
they can produce artifacts that either obscure or give the
appearance of a stenosis or occlusion. If, for example, during the
creation of a CPR, the path through the lumen of the vessel is
off-center, some fraction of the caliber of the vessel will be
projected, rather than the true diameter, resulting in an
artificial stenosis. Automated algorithms for the creation of CPRs
can prevent the creation of "false-postive" stenosis in this
manner. To avoid missing a stenosis, two CPRs are usually created
for any structure, at perpendicular planes. Automated techniques
now enable dynamic, rotating CPRs to be visualized at any
conceivable angle to the center path, in real time on distributed,
On many 3D postprocessing platforms, using either MPR or CPR, a
thickness of the imaged slab can be defined. The question then
becomes, how to represent a 2D image of a 3D volume? Several
options are available, including the maximum intensity projection
(MIP); average intensity, or ray sum, projection (aIP); and minimum
intensity projection (MinIP). For a slice of a particular
thickness, the MIP will display the highest-attenuation pixel,
whereas the average intensity or minimum intensity projections will
display the average attenuation value, or the lowest-attenuation
Maximum intensity projections are typically used for
contrast-enhanced vessels, and MinIPs for hollow-lumen, air- or
water-containing structures (Figures 3C, 3D, and 7). Each can be
affected by artifacts from either very-high- or low-attenuation
structures outside the region of interest (such as bone or bowel
gas) that obscure the relevant anatomy, but this problem can
usually be overcome by specifying appropriate slab thicknesses or
"cut-planes" to exclude irrelevant structures from the imaging
plane. Raman et al
have developed algorithms that create a curved planar slab, with
the slab thickness automatically adjusted along its length to
include the caliber of the vessels.
These postprocessing techniques are not exclusive; for example,
to lay out the biliary tree, one may combine a CPR (to accommodate
the complex, 3D structure) with a MinIP (as the biliary tree is
typically lower in atten-uation that the surrounding liver
parenchyma), and select a thin slab, or plane thickness of 10 mm,
to prevent overlying bowel gas from obscuring the bile ducts in the
MinIP calculations. The resulting image would be a curved-planar,
thin-slab MinIP (Figure 7).
The preceding are all 2D representations of 3D structures.
Volume rendering (VR) and shaded-surface display (SSD) are two
types of 3D representations. Of the two, SSDs are less
computationally intensive. In SSDs, a threshold value is specified
by the user, applied, and used to define a surface that is then
rendered with an imaginary source of illumination. This 3D
structure can provide great detail of the surface anatomy of a
specific structure, but one loses a large amount of information, as
everything outside the threshold attenuation is thrown away.
Volume rendering is more complex and takes more computing power,
but the 3D images it produces are superior. With VR, each
particular 3D data point, or voxel, in the image is assigned an
opacity level. This level is usually defined by attenuation so, for
example, one could display the skeleton by assigning zero opacity
or total transparency to attenuation values less than those of
bone. The skeleton itself may be made completely opaque, resulting
in an image equivalent to an SSD of the skeleton, or of varying
transparency, enabling visualization of the internal architecture.
More than a single opacity function can be used, enabling the
display of vessels with one function, bone with another, and soft
tissue with yet another. Each can be color-coded as well, and with
an artificial external light source, depict stunning
representations of complex anatomy (Figures 2A, 5D, and 6C);VR can
be subject to artifacts, however. Opacity functions are based on
setting threshold attenuation values, which can, for example,
overestimate the degree of stenosis in a vessel (Figure 5D), or
make small vessels translucent or invisible.
Combinations of these techniques, with automated processing
methods, have successfully been used to create highly detailed,
reproducible, and accurate angiograms of the lower extremities
(Figure 6A through C). As these methods are further developed and
available to the radiologist at the time of interpretation, primary
evaluation of these complex and large examinations can be performed
using postprocessed reconstructions.
As MDCT becomes increasingly widespread, it will likely fill
more and more roles that currently belong to diagnostic DSA. It has
many potential applications in acutely ill patients, including
techniques for the improved diagnosis of conditions that,
heretofore, have required invasive and time-consuming examinations.
The delineation of vascular abnormalities that result in bowel
ischemia is just one example.
Other potential nonvascular applications of MDCT are being
developed. The speed and higher resolution of MDCT enable the
creation of 3D representations of complex data sets. As automated
processing techniques mature, it may become routine to create a 3D
representation of the bowel with a few mouse clicks; with a few
more mouse clicks, a rotating image of the skeleton; and with a few
more, a map of the abdominal vasculature. Although hurdles remain,
MDCT will give the radiologist new tools to help quickly and safely
diagnose a wide variety of disease, including acute abdominal