The development and widespread availability of helical CT and advanced post-processing techniques has made computed tomography angiography (CTA) a practical alternative for the assessment of the vascular system. CTA has several advantages over other vascular imaging modalities, particulary, that it is a noninvasive method of evaluation. The authors discuss scan protocols, clinical applications, and future developments in CTA.
Dr. MacEneaney is a Fellow and Dr. Dachman is an Associate
Professor in the Department of Radiology, University of Chicago
Hospitals, Chicago, IL.
Computed tomography angio-graphy (CTA) is a noninvasive method
of assessing the vascular system; in some cases it has replaced
conventional angiography. The development and widespread
availability of helical CT and advanced post-processing techniques
has made CTA a practical alternative. The rapid acquisition speed
of helical CT allows a large volume to be imaged during the narrow
temporal window of first pass peak vascular opacification during a
single breath hold. The ongoing development of multidetector
scanners will further increase CTA capabilities. The volumetric
data sets generated are suitable for three-dimensional (3D)
reconstruction. This process is becoming easier and faster with the
advent of more powerful computer processors and software in
commercially available workstations. Despite the rapid progress in
MR angiography, the widespread use of helical scanners ensures the
future of CTA.
CTA has several advantages over the other vascular imaging
modalities. The examination is minimally invasive.
High-rate/high-pressure delivery of contrast is essential but only
venous access is required and the patient is spared the potential
complications of conventional catheter angiography. Unlike MR,
pacemakers and ferromagnetic aneurysm clips do not preclude CTA.
Artifacts induced by turbulent blood flow through stenoses may
cause over estimation of the degree of stenosis on sonography and
MR. This does not influence CTA, although with suboptimal scanning
protocols CTA can also overestimate stenoses. Important nonvascular
pathology may be diagnosed.
Renal impairment and contrast hypersensitivity may limit CTA in
One of the relative disadvantages of spiral CT is poor z-axis
resolution. This is responsible for partial volume effect and
stairstep artifacts encountered on multiplanar and 3D
reconstructions. Since many CTA applications require 3D and
multiplanar reconstructions, optimization of z-axis resolution is
vital. To minimize these problems and maximize vascular
enhancement, careful attention should be paid to CTA scanning and
reconstruction protocols. Performance is based on collimation,
pitch, reconstruction spacing, and timing of scan with respect to
Selection of appropriate collimation and pitch is influenced by
several variables, including patient specific characteristics such
as breath hold capacity, quality of venous access, and the
examination specific characteristics, particularly the size of the
vessel and region of interest. There is a trade-off between
collimation, pitch, and the volume to be scanned. Narrow
collimation and low pitch improve the z-axis resolution, but
increase scan time and tube heating. First, choose a collimation
less than the diameter of the vessel of interest and then increase
the pitch to cover the distance required,
e.g., 3 mm for renal arteries, 7 to 10 mm for the aorta. If the
desired scan is large and it is necessary to increase either the
pitch or collimation, it is preferable to increase the pitch.
Increasing pitch from 1 to 2 increases the effective slice
thickness by one third. A pitch of 2 is the practical upper
Scanning at peak vascular enhancement is vital: as pitch
increases, low-contrast resolution decreases and peak opacification
simplifies 3D reconstructions. Appropriate timing optimizes
contrast and allows either arteries or veins to be scanned.
Scanning the abdominal aorta at 25 seconds after the start of the
injection will usually give satisfactory images. In a healthy
patient, empirical timing may suffice; however, time to peak
enhancement varies particularly in patients with cardiac
There are two methods to ensure correct timing. The first uses a
series of static scans at a single level of interest, i.e., the
aortic arch for cases of suspected dissection and the level of the
renal arteries for assessment of abdominal aortic aneurysms.
Fifteen to 20 mL of contrast is injected at the anticipated
injection rate and a scan performed every 3 seconds, beginning 10
seconds after injection. A region of interest is then placed on the
vessel and the time to peak vascular enhancement is calculated. The
second method is semi-automated. A series of scans is performed at
the start of contrast injection. Image acquisition is triggered
when a preset enhancement threshold is reached within a preselected
region of interest. Visual assessment can be used to override the
semi-automated software and commence scanning earlier.
Contrast is typically injected at 3 to 5 mL/sec. Higher rates of
injection are preferred for identification of smaller vessels.
Contrast volumes range from 100 to 180 mL. Non-ionic 300 mg/mL
contrast is used, except for cases of suspected pulmonary embolism
or dissection of the thoracic aorta. In these instances, lower
concentrations of contrast media are used (150 to 240 mg/mL), as
dense contrast medium may obscure low density thrombus or an
intimal flap. Saline bolus chasers of 50 to 70 mL are employed to
force the contrast through the tubing and the venous system to
prolong the plateau phase of enhancement.
Axial resolution is approximately 1 mm, however, longitudinal
resolution is poorer. Two parameters partially compensate for the
relatively low longitudinal resolution, and so reduce partial
volume artifacts and stairstep artifacts on multiplanar
reformatting (MPR) and 3D reconstructions. One is the use of 180°
rather than 360° interpolation algorithms (vide infra), and the
other is reconstruction of overlapping images. Reconstructing 2 to
3 images per rotation is recommended.
A useful rule of thumb is to reconstruct images at one-third the
collimation. Highly overlapped reconstructions are required when
the vessel of interest runs in the axial plane.
Initially, interpretation should be done on the axial source
images. We recommend interactive reading at the console to
facilitate use of overlapping reconstructions and minimize film
waste. By displaying findings in a more familiar format, 3D
reconstruction can help in difficult cases by revealing information
present, but not readily evident on axial images alone.
Postprocessing techniques include MPR, maximum intensity
projection (MIP), shaded-surface display (SSD), and volume
rendering. MPR is the simplest technique and is an adjunct to
viewing axial images for problem solving.
In SSD, a threshold attenuation level is selected by the
radiologist. All voxels in the data set with a Hounsfield value
within this threshold are displayed as a single structure. Relative
depth is provided by shading from a computer-generated light source
(figure 1). This model provides excellent anatomical detail and is
popular among clinicians, but relative attenuation values are lost.
Therefore, calcifications and stents cannot be identified on the
basis of attenuation alone. SSD images must always be reviewed in
conjunction with axial images to identify calcifications to prevent
underestimation of degrees of stenosis due to adjacent mural
calcifications. Selection of the threshold level is important, as
inappropriate selection may artifactually generate or obscure
A threshold level of 160 HU is typical.
MIP views display the highest Hounsfield value voxel (contrast
medium, calcium, bone)--along any plane. In contrast to SSD, MIP
preserves relative attenuation values and lacks depth perception.
This leads to difficulty in separating overlapping vessels in sites
where several vessels are opacified (e.g., pulmonary artery and
thoracic aorta) and in identifying small intravascular hypodense
lesions (e.g., thrombus and intimal flaps). Calcified plaques may
also obscure adjacent pathology. However, MIP views are better
suited to reconstructing smaller vessels than SSD. Editing is
necessary for both SSD and MIP images.
In practice, CTA has replaced conventional catheter angiography
in many diagnostic studies. It is well established in the
assessment of aneurysms of the thoracic and abdominal aorta, of the
pulmonary vasculature in suspected embolism, of the carotid and
renal arteries for stenoses, and in the assessment of potential
Link et al
have demonstrated 100% agreement between CTA and digital
subtraction angiography in cases of carotid stenosis >70%.
Overall agreement between the two techniques was 89% using 2-mm
collimation and a pitch of 1 (using only 100 mL of 270 mg/mL
contrast). The overall agreement between Doppler sonography and
angiography was 75%. While CTA outperformed Doppler sonography in
this study, there was no significant difference between CTA and
sonography in identifying patients whose stenoses required surgery.
This lack of significance most likely reflects the relatively small
study population, only 28 patients and 56 arteries. The addition of
volume rendering to MIP images does not improve performance
Carotid CTA is not used for primary screening at our institution;
its role is limited to those patients who have abnormality
demonstrated and who are unsuitable for MR angiography (MRA) or
conventional catheter angiography.
In the thoracic aorta, CTA is valuable in the diagnosis
aneurysms and suspected dissection. CTA performs as well as
transesophageal echo and MR in assessment of dissection, although
MR can demonstrate the entire length of the aorta (multidetector
row CT also has this capability).
Spiral CT is superior in the assessment of the supra-aortic
branches (figure 2). As axial images have the highest resolution,
these best demonstrate the intimal flap, however, difficulty may
arise if the dissection extends into a horizontally oriented
vessel. MR images may be of assistance in surgical planning.
In cases of suspected aortic dissection, a noncontrast-enhanced CT
examination is performed initially to assess for media-stinal
hematoma, followed by the enhanced scan. In these patients there
may be significant cardiac compromise and a test injection should
be used to calculate timing of scan delay. A protocol of 3-mm
collimation with pitch of up to 2, or 5 mm with a pitch of 1, is
CTA is widely used for the diagnosis of pulmonary emboli (PE),
though there is ongoing debate regarding the precise place in the
hierarchy of investigations for those patients with suspected PE.
The current resolution of CT allows detection of emboli to fourth
order (segmental) vessels. The significance of emboli in more
distal vessels is uncertain. The sensitivity and specificity of CT
for central emboli has been reported as 91% and 78% respectively.
MPR is used as a problem-solving tool for exclusion of PE when the
axial images are inconclusive.
A negative study effectively excludes pulmonary thromboembolism.
The examination is performed caudocranially to minimize respiratory
motion artifact. Generally, 3 mm collimation and a pitch of 1.5 to
2 are used. Lower density contrast medium (150 to 240 mg/mL) helps
ensure that small emboli are not obscured. In an animal study, CTA
was more sensitive than 3D gadolinium-enhanced MRA for the
detection of PE.
CTA is an excellent screening tool of the renal arteries. Up to
10% of patients with hypertension have a renovascular etiology. CTA
reliably identifies stenoses of the main renal arteries. As the
renal arteries run axially, scan protocol is particularly
important. Sensitivity of 100% has been reported for stenoses
>50% using axial images with MPR and MIP reformats.
Ostial and truncal stenoses can be differentiated (figure 3). A
normal CTA virtually excludes the presence of a stenosis.
CTA with volume rendering has been reported faster and more
accurate than CTA with MIP.
CT is superior to MR and sonography in identification of accessory
CTA has been proposed as the method of choice for investigating
potential kidney donors.
Its vascular capabilities include identification of accessory
vessels, early branching of the renal artery, and aberrant venous
anatomy with an overall accuracy of 95% compared with surgical
Axial images alone may be sufficient for evaluation, although 3D
reconstructions (MIP and SSD) facilitate identification of
important accessory renal vessels. The scan range extends to pelvic
inlet to ensure identification of accessory arteries. Typical
parameters: 3 mm collimation, pitch of 1.2 to 2, and 100 to 150 mL
of contrast at >3 mL/sec. The enhanced scout/topographic image
demonstrates calcifications and a delayed scout/topographic image
after the CTA adequately depicts the collecting system and ureters
and so obviates the need for an intravenous urogram.
CTA is ideal in the preoperative evaluation of abdominal aortic
aneurysms, as it demonstrates their position, extent, and
relationships to the renal and iliac arteries.
Seven millimeter collimation may be required to cover the necessary
volume; however, this runs the risk of missing renal artery
stenoses and small accessory renal arteries. Narrower collimation
should be used where possible. CTA is useful in assessment of
aortic stent grafts after placement. MIP images are favored in this
scenario, as the metallic component of the graft can be identified
CTA has been employed to identify stenoses and occlusions of the
splanchnic vessels. Its main role in this anatomic region is in
assessing resectability of pancreatic tumors. Compared with axial
images alone, CTA is more accurate in identifying unresectable
tumors on the basis of vascular encasement.
A multiphasic scan is performed: a standard nonenhanced scan to
localize the pancreas, a second scan in the arterial phase (25 to
30 seconds after injection) of contrast enhancement (3-mm
collimation, pitch 1 to 1. 5), and then a third scan in the portal
venous phase (60 to 70 seconds after injection) of enhancement to
include the abdomen. In most cases, only axial images are
The (near) future
The development of multidetector row CT has been described as an
evolutionary advance perhaps as important as helical CT itself.
The primary limitation of CTA has been tube loading or heating,
which limited potential scan volume. Multidetector row scanners
have a wider beam and use a higher proportion of emitted photons
than single-detector row CT scanners. The collimation of each
detector row is unaltered. Technically, this implies that a greater
volume of tissue can be scanned: it is possible to scan the entire
aorta and femoral vessels in a single spiral scan (figure 4).
One hundred eighty degree linear interpolation algorithms have
been used in helical CT. These reduce slice sensitivity profiles
(and volume average artifacts); however, the images are relatively
noisy. With multidetector row CT it is possible to use 360°
algorithms by "combining" 180° data from two detector rows. This
reduces noise without increasing slice sensitivity profile. The net
result is improved z-axis resolution.
These technologic advances have implications for scan protocols,
postprocessing (dedicated technologist), interpretation time, and
patient radiation dose. With parallel software developments, there
will be an expansion of the clinical applications of CTA,
especially in intracranial and coronary circulations. The current
reimbursement structure needs to be addressed to take these
developments into account, as the additional physician time
requirements of CTA are not acknowledged. AR