This article provides a primer for conceptual understanding of parameters that allow optimization of CTA studies.
is currently Chief Resident in Diagnostic Radiology at Johns
Hopkins Hospital. He completed internship at the University of
Hawaii, and graduated from Washington University School of Medicine
in St. Louis, where he was a Howard Hughes Fellow. Dr. Hsu will
begin a Body Imaging fellowship at Stanford in 2003.
Image quality in computed tomographic angiography (CTA)
is critically dependent on contrast injection, data acquisition,
and postprocessing technique. First, injection parameters are
chosen to achieve intense homogeneous enhancement despite
inter-individual hemodynamic variability. Next, CT data
acquisition must obtain adequately thin sections over the
appropriate volume of interest during a limited temporal scanning
window. Finally, reconstruction and rendering techniques must be
rationally selected to demonstrate relevant anatomy and disease
processes in an accurate manner. Advances in these areas and in
scanner technology will enable the development of new
applications. This article provides a primer for conceptual
understanding of parameters that allow optimization of CTA
Computed tomographic angiography (CTA) is a minimally invasive
technique used to study intracranial, thoracic, abdominal, and
peripheral vessels. The utility of a number of applications has
equaled or surpassed catheter angiography. In comparison with
catheter angiography, CTA offers more viewing angles with
simultaneous evaluation of vascular lumen, wall, and surrounding
soft tissue, yet is less expensive and time consuming, with fewer
complications and usually with lower radiation dose. In comparison
with magnetic resonance angiography (MRA), CTA usually offers
better resolution and less artifact, although ionizing radiation
and iodinated contrast are required.
Image quality is critically dependent on contrast injection,
data acquisition, and postprocessing technique. In each of these
areas, conceptual understanding of the effects of various
parameters is necessary for optimization of CTA studies. In
addition, advances in these areas are facilitating improved
diagnostic quality and the development of novel applications.
Image quality starts with the mechanical injection of iodinated
contrast. Injection techniques have been developed to achieve
optimal enhancement and increase the efficiency of contrast
utilization. This, in combination with dramatically faster
scanners, allows the use of much lower contrast doses than just a
The ideal vascular enhancement pattern is homogeneous and
intense, delivered to match the timing of data acquisition.
The goal of contrast injection is to achieve a relatively
homogenous plateau in intravascular enhancement throughout the
duration of data acquisition. This focus on prolonged homogenous
plateau differs somewhat from conventional angiography, where less
inherent image contrast makes a high narrow enhancement peak
preferable to maximize vascular opacification.
Plateau homogeneity can be characterized by "plateau deviation,"
defined as the standard deviation of the time-attenuation curve in
an intravascular region of interest (ROI) during the scanning
Homogeneous enhancement improves diagnostic quality of images and
facilitates image processing. Non-uniform enhancement can result in
artifactual pathology such as filling defects and perceived
stenoses. Inhomogeneity also complicates image processing,
particularly parameter selection for three-dimensional (3D)
The interplay of multiple injection and acquisition parameters
must be understood to overcome intra- and inter-individual
variability in vascular time-attenuation curve. Achievement of a
homogeneous temporal scanning window improves image quality, while
efficient utilization of injected contrast decreases required
Independent injection par-ameters alter intravascular bolus
The effect of various injection and patient parameters on the
ultimate intravascular geometry of a monophasic contrast bolus was
presented by Cademartiri
at the annual meeting of the Radiological Society of North America
(RSNA 2001). Time-enhancement curves were constructed for
monophasic injections, examining the effect of injection volume,
rate, and concentration; use of saline flush; and patient size and
cardiac output. Peak enhancement (PE) and time to peak enhancement
(tPE) were specifically examined, with some consideration of
Increasing volume of injection while maintaining constant rate and
iodine concentration results in increased PE and lengthened tPE,
primarily via summation and potentially by recirculation. The
intravascular curve becomes higher and broader with a delayed peak.
Of course, the maximum injected volume is limited by iodine load. A
practical application of the delayed peak phenomenon is necessary
when a test injection is used to plan scan timing. Time to peak
enhancement measured with the small test injection will
underestimate tPE of the actual CTA injection, and scan timing
should be appropriately delayed.
One simplified method for calculating the appropriate delay is
to assume a fixed "circulation time" from the center of the bolus
to the time of PE. For the high volume injection, the "circulation
time," calculated from the test injection, is added to the temporal
center of the bolus to predict the time of PE. Scanning is then
centered on the predicted peak.
Increasing only the rate of injection with constant volume and
iodine concentration results in increased PE, decreased tPE, and
shortened plateau. For practical purposes, this means that scanning
must be accomplished earlier, more quickly, and with more precise
timing. One application of these effects is when CTA acquisition is
performed with fixed postinjection delay rather than tailored
delay. In this case, a less rapid injection can be utilized to
achieve a broader, and therefore more forgiving, time-attenuation
Increasing iodine concentration with constant rate and volume
results in increased PE with no change in tPE. This concept may be
employed to increase arterial attenuation without altering the
temporal course of enhancement, subject to limits on iodine load
and FDA-approved concentrations (up to 370 mgI/mL).
Immediate saline flush, though not routinely employed, uses
contrast to better advantage.
Following contrast injection with immediate saline flush uses
injected contrast more efficiently by significantly decreasing
venous pooling in the injected extremity. This results in increased
PE, delayed peak, and prolonged plateau. Use of saline flush can
thus improve the vascular-enhancement profile with a fixed contrast
dose, and this technique can, in fact, be used to decrease contrast
Use of flush has the additional advantage of decreasing streak
artifact, particularly in thoracic CTA, where there can otherwise
be high residual contrast concentration in the subclavian vein
ipsilateral to an antecubital injection. Of course, excess flush
will also dilute the contrast bolus.
Commonly, approximately 10 mL isotonic saline is injected
manually, although the timing and rate of a manual injection is
neither ideal nor reproducible. Alternatively, attempts to layer
saline and contrast in the same syringe have proved tedious and
unreliable. The best way to achieve a rapid and reproducible flush
would be to use a double-barreled power injector. One experimental
protocol using 30 mL flush with such an injector demonsttrated a
20% decrease in contrast material requirement.
Unfortunately, double-barreled injectors used for MRA are of
insufficient volume for CTA, and to our knowledge, a system
adequate for CTA is not currently commercially available in the
Inter-individual differences in intravascular time-attenuation
re-sponse presents further challenges to
the selection of optimal injection and acquisition
Greater dilution of contrast will occur in a patient of greater
size and circulating volume, potentially necessitating a higher
contrast dose to maintain image quality.
Cardiac function and age:
In a patient with high cardiac output, the bolus is both circulated
and diluted more rapidly, resulting in a narrower time-attenuation
curve. With decreasing cardiac output and increasing age, the bolus
is circulated less rapidly and with less initial dilution.
Consequently, scan timing and contrast dosage can be rationally
The ratio of cardiac output to weight has been used to predict
the kinetics of arterial enhancement.
However, noninvasive estimation of cardiac output can be quite
Tracer kinetic theory can be applied to analyze injected bolus
geometry, vascular response function, and resultant intravascular
An "impulse" of tracer is a theoretical infinitesimal test bolus
at the point of injection. The vascular "impulse-response function"
is the resultant intravascular time-attenuation curve in a
particular vessel after an injected impulse. The impulse-response
function of a particular vessel is determined by mathematical
deconvolution, after measuring an actual test injection and the
resultant vascular response.
Knowledge of the impulse-response function of a vessel is useful
in the following manner: At the point of injection, the actual
injected bolus has known geometry that can be represented by a
temporal summation of impulses. Therefore, the actual resultant
intravascular time-attenuation curve can be predicted by a similar
summation of impulse-response functions. Conversely, bolus geometry
may be designed to achieve a desired intravascular time-attenuation
Fleischmann provides a more detailed technical discussion of
tracer kinetics relevant to CTA.
With knowledge of patient impulse-response function, injected
input function can be tailored for optimal intravascular bolus
Several strategies can be employed in tailoring injected input
function to optimize time-attenuation curve in the vessels of
interest. Although full application of tracer kinetic analysis is
not common clinical practice, limited use of these concepts is
valuable for altering injection protocols.
This refers to a constant injection rate for the duration of the
injection. Of course, injection rate, duration, and concentration
can be varied between injections, as discussed above. For practical
reasons, this technique is most commonly employed, with an
injection rate of 3 to 4 mL/sec. However, with constant injection
rate, the arterial time-attenuation response progressively
increases and reaches peak just after completion of the injection,
followed by relatively rapid decrease in intravascular attenuation.
In other words, monophasic injections result in a non-uniform
vascular enhancement "hump" during image acquisition, without a
true plateau phase.
This technique was developed to achieve more favorable plateau
characteristics. While monophasic injection results in a "hump"
profile, well-designed biphasic injections generally result in a
gentle "saddle" profile. Usual biphasic schemes utilize an initial
rapid phase followed by a slower phase of injection, resulting in a
less dramatic peak, often with a second peak and a more prolonged
and homogeneous plateau, which includes the gentle inter-peak
reported the use of biphasic injections, individually tailored
based on Fourier analysis of a test injection. This approach
resulted in significantly decreased standard deviation of
attenuation during scanning and moderately decreased
inter-individual variability. Most commercially available CTA power
injectors are capable of biphasic injections, programmable with
This technique further tailors the injected input function to
achieve a more prolonged and homogeneous plateau. Bae
designed a physiologically based pharmacokinetic model that
predicts that a multiphasic injection with exponentially decreasing
rate provides the most uniform vascular enhancement. Optimal decay
coefficients were found to be proportional to cardiac output per
body weight. Using proprietary software and injectors, uniform
prolonged enhancement was demonstrated using an animal model,
and similar results in human subjects were presented at RSNA 2001
with quantitatively better time-attenuation curves than monophasic
and biphasic injections.
In practice, unreliable estimation of cardiac output may complicate
selection of decay coefficient, therefore, the addition of a test
injection could be beneficial.
Use of properly selected biphasic and multiphasic injections
ultimately results in more uniform and prolonged enhancement
plateau and more efficient use of iodinated contrast, with the
potential for improved image quality and decreased contrast dose
requirement. With technical improvements in power injectors
allowing more precisely controlled and predictable injection
profiles, the benefits of tracer kinetic analysis will become more
fully realized, particularly as more demanding applications evolve.
In practice, further studies are needed to demonstrate clinically
significant improvement in image quality for specific CTA
applications and to streamline the process of bolus tailoring.
CT data acquisition
The development of spiral CT, resulting in volumetric
acquisition and increased speed, was a prerequisite for CT
angiography. Continued advances over the past decade include faster
gantry rotation, complex interpolation algorithms, and X-ray tube
design with high heat capacity and high photon output, enabling
both prolonged and faster scanning. The recent introduction of
multidetector-row CT (MDCT) has further increased scanning speed,
allowing shorter acquisition times, greater volume coverage, finer
z-axis resolution, and decreased contrast requirement.
For specific applications, various interrelated factors must be
considered in optimizing data acquisition. These include timing,
scanning range, collimation, and pitch.
Scanning should be performed during intravascular enhancement
plateau, but before significant venous and tissue
For arterial CTA, scanning should be performed during the
first-pass arterial phase enhancement plateau, before venous
contamination and significant tissue enhancement. Wide
inter-individual variation can make this a challenging proposition,
particularly for highly vascular organs where parenchymal and
venous enhancement occurs relatively early. In one series, the mean
time between an aortic enhancement of 50 HU and the same
enhancement in the pancreas was <14 seconds.
Strategies for timing the commencement of scanning include fixed
post-injection delay, preliminary test injection, and real-time
radiographic bolus tracking.
Scanning after a fixed post-injection delay is the most convenient
and widely used timing method, with adequate results in most cases.
The delay may be tailored based on anatomic ROI, desired vascular
phase, and patient factors. For example, at our institution,
arterial CTA of the chest and abdomen is performed after a delay of
25 to 30 seconds, with a delay of 60 to 70 seconds used for venous
imaging, and an additional 10 seconds or more added for older
patients and those with decreased cardiac output.
A slightly slower injection may also be employed to achieve a
longer but lower plateau, which is more forgiving of hemodynamic
variability. Older patients and those with a lower cardiac output
also tend to have a longer intravascular enhancement plateau,
although there is inherently more inter-individual variability in
A preliminary small-volume test injection of 10 to 15 mL can be
administered to examine vascular response. The test injection is
administered at the same rate as the anticipated injection, with
sequential scanning at a single axial level to determine the
time-attenuation response in the vessel of interest. As previously
discussed, the scanning delay for the actual injection can then be
planned. The delay must factor in the higher and more delayed peak
and prolonged plateau expected based on the larger volume and
duration of subsequent injection.
As detailed above, Fourier analysis of vascular response to a
test injection can also be used to tailor injected bolus
Rather than using a test injection, the actual large volume
injection can be administered and monitored in real time, and
scanning triggered "on the fly" when intravascular attenuation at a
sentinel axial level reaches a pre-determined threshold. A
relatively low threshold, such as 50 HU units above baseline, is
usually chosen to factor in the several second delay before the CTA
acquisition can commence. A higher threshold may be chosen for
patients with lower cardiac output, who are expected to have a less
rapid upslope and delayed plateau. A safety mechanism should be in
place to trigger scanning if bolus tracking fails, perhaps due to
patient motion or failure to reach threshold.
The test-injection and bolus-tracking techniques do involve
additional radiation. These preliminary images can be obtained with
reduced tube current, thus contributing a very small percentage of
patient radiation dose. These techniques allow more accurate and
precise timing, and therefore require a smaller "scanning window."
Clinically relevant improvements in image quality for specific
applications remain to be demonstrated.
Limited scanning window requires efficient use.
The scanning window refers to the time available for scanning.
This can be limited by duration of enhancement plateau, hemodynamic
considerations, and breath holding. Time required to achieve a
particular scanning protocol depends on the desired volume of
coverage, as well as collimation, pitch, and scanner
Limiting the scanning range to the volume of interest decreases
scanning time, allowing thinner collimation, less motion, and more
homogeneity in enhancement. Conversely, requisite thinner
collimation for some applications, such as CTA of the circle of
Willis, may limit volume coverage. Prolonged scanning time can
result in image degradation due to finite intravascular contrast
plateau, appearance of tissue and venous enhancement, and
The development of 4- and 8-row MDCT in combination with faster
tube rotation has dramatically increased scanning speed, allowing
greater volume coverage and thinner collimation. For example, MDCT
has enabled complete lower extremity inflow and runoff studies with
a single injection, as well as thin-section CTA covering the
entirety of the carotid arteries and circle of Willis.
Sixteen-row MDCT may increase scanning speed even further and/or
allow thinner sections, potentially improving image quality by the
mechanisms discussed above, and facilitating the development of
novel applications, such as coronary CTA.
Thin collimation improves resolution but increases noise and
Collimation of the X-ray beam is related directly to slice
thickness, although effective slice thickness will be slightly
wider, depending on specific parameters of helical acquisition and
Maximizing Z-axis resolution would appear to be an important
objective, to decrease partial volume averaging and reduce 3D
anisotropy, an important source of artifact in 3D rendering.
However, thinner collimation requires increased scanning time to
cover a given Z-axis range. In addition, thin collimation increases
quantum noise, which may degrade image quality, particularly in
In practice, collimation is chosen for each application based on
Z-axis resolution requirement, volume of interest, and temporal
scanning window. Thin collimation is most important when evaluating
small vessels and vessels with a significant oblique or in-plane
component, such as renal arteries and the circle of Willis.
Tailored use of thicker collimation may be chosen for obese
patients, and those with decreased breath-holding capability.
Higher pitch improves scanning speed more than it degrades Z-axis
Pitch is defined as table feed per rotation divided by collimation.
Increasing pitch means that the table will move further during a
single rotation of the X-ray tube, therefore the slice-sensitivity
profile is widened, and effective section thickness is increased.
However, reconstruction using of 180š rather than 360š
interpolation limits Z-axis broadening, enabling good image quality
with a pitch as high as 2 with single-detector-row CT (SDCT)
scanners. This obviously reduces scanning time, but also increases
effective section thickness, albeit to a lesser degree.
When pitch is doubled from 1 to 2, scanning speed is doubled, but
effective section thickness only increases by a factor of 1.3. It
is therefore advantageous to use higher pitch with narrower
collimation, to achieve similarly thin sections and still realize a
gain in speed and scanning range. Pitch can be increased even
further with MDCT because interpolation can be performed across
helical channels. However, as pitch increases, peripheral
undersampling artifacts occur, and for SDCT a pitch >2 is rarely
used. By the same token, for MDCT pitch is usually limited to (2 *
N), where N is the number of detector rows.
In the special case of MDCT with adaptive array detectors and
adaptive interpolation algorithm, within limits, effective slice
thickness does not significantly change with increasing pitch.
Brief overview of image reconstruction and
Advanced rendering techniques, particularly those that offer a
3D perspective, facilitate efficient visualization and analysis of
an inherently volumetric dataset. These generated images are also
helpful tools in communication with clinicians, as they can be made
to emulate familiar perspectives, such as catheter angiograms or
surgical approaches. In one author's vision, 3D images are
"oriented to anatomy rather than the CT table."
Image reconstruction using 180š interpolation decreases effective
section thickness, thereby increasing Z-axis resolution. By
decreasing peripheral undersampling artifacts, it also enables the
use of higher pitch, with all of the benefits discussed above, as
well as lower radiation dose.
Reconstruction interval can be arbitrarily varied in spiral CT.
An overlap of approximately 50% is adequate for most CTA
applications. Overlapping reconstruction improves Z-axis resolution
and is most important for imaging small vessels and those oriented
in the axial plane, such as renal arteries. Due to anisotropic
data, CTA is more accurate in assessing luminal diameter of vessels
running perpendicular to the axial plane, such as the aorta or
Artifacts more commonly arise in CTA of vessels not perpendicular
to the axial plane.
The primary disadvantage of highly overlapping reconstruction
interval is the time-consuming generation of larger datasets, which
may tax capacities for storage and manipulation for 3D
reconstructions. Alternatively, selected portions of a study may be
reconstructed with more overlap. For this purpose, raw data should
be stored until initial images are reviewed and desired modified
Initial CT reconstructions can be performed only in the axial
plane. These axial images are traditionally the primary diagnostic
tool in clinical CT. These images also serve as the source data for
a variety of additional reconstruction and rendering techniques.
These techniques each have specific advantages in various imaging
scenarios, and should be used in a complementary fashion.
Images can be reconstructed easily in coronal and sagittal planes
on most scanners and workstations. These additional orthogonal
planes can be used to more easily define and depict the
longitudinal extent of a structure or process. Sagittal and coronal
reconstructions are particularly useful for vessels running in the
axial plane, such as renal arteries. Of course, as long as the
source data are anisotropic, Z-axis resolution will remain inferior
to X- and Y-axis resolution in the reformatted planes.
Curved planar reformations:
This is an additional planar technique in which a curved plane is
defined through the center of a curved structure, such as a
tortuous vessel. For example, to examine the aorta in a curved
coronal plane, the vessel is first viewed in the sagittal plane and
a curved plane is manually defined by tracing a curve through the
center of the vessel. A curved coronal plane is then generated
along the curved line. This process can be performed in any plane.
Once the curved plane is generated, some software packages also
allow scrolling in that plane, as well as adjustment of slice
This technique is commonly used in CTA of the aorta, iliac
arteries, renal arteries, and carotid arteries, and will be
indispensable in coronary CTA. A current limitation of this
technique is its dependence on manually placed center points to
trace the curved plane. Misaligned points often lead to simulated
vascular stenoses in the curved planar reformation. Several
manufacturers will soon market automated systems for vessel
Shaded surface display (SSD):
The first 3D rendering technique to be used clinically, SSD employs
a HU level threshold or range to depict the surface of an object. A
3D effect is accomplished by displaying the generated surface with
shading to emulate the effect of a virtual light source. If the
threshold level is well chosen, the surface of the structure can be
well defined, with all irrelevant structures excluded. SSD can be
constructed with high spatial resolution, and is also the 3D
technique that generally presents the clearest depth cues. Because
of the inherent data compression, SSD images require relatively
little processing power to generate and manipulate.
However, optimal threshold level may be difficult to select, and
it may not be possible to simultaneously optimize all structures or
segments of interest on a single image. Outside of vascular
applications, some structures may simply not have well-defined
surfaces. In addition, SSD discards a great deal of data,
effectively reducing each voxel to 1 bit of data, ie, whether it is
above or below the threshold level. For example, vascular
calcifications and enhanced intravascular blood will both be above
threshold for SSD, thus leading to underestimation of calcified
In comparison to maximum intensity projection (discussed below),
SSD better depicts complex branching patterns and spatial
relationships. However, many of the advantages of SSD are also
captured by volume rendering (see below), a technique which
preserves the dynamic range of the source data, and has replaced
SSD for most CTA applications.
Maximum intensity projection (MIP):
This widely available technique projects a volume of interest onto
a specified plane, mapping the maximum intensity in the volume
along each ray perpendicular to that plane. Maximum intensity
projection was originally designed for CT and MRA, and continues to
be used extensively for this purpose.
Inherent to CTA MIP algorithms is the need for data editing, as
overlapping dense structures such as bone can exceed the HU values
in enhanced vessels. Similarly, because the highest intensity
voxels are chosen along each ray, background mean intensity is
raised, and noise can also be accentuated. Use of thin volumes of
interest helps to reduce reconstruction artifacts. A single MIP
image conveys no depth information, and MIP is particularly limited
in the definition of overlapping structures. The angle of the
projected plane must be rotated to define complex spatial
relationships. Use of a cine loop is helpful for this purpose.
Although the concept of MIP bears a slight semblance to SSD in
that it is, in essence, a threshold technique, MIP does preserve
the attenuation value of the displayed voxel. Therefore, with
appropriate windowing, some differentiation between elements of the
displayed structure is possible, such as between mural
calcification and enhancing blood. Despite its limitations, MIP is
generally considered superior to SSD for CTA.
Volume rendering (VR):
Unlike threshold techniques such as SSD and MIP, volume rendering
preserves all of the data from the source images, and real-time
interactive VR has become less cumbersome and more widely available
as computer processing power has increased. The concept of VR is to
preserve the 3D volumetric dataset acquired via spiral CT and
display it as a true volume of data rather than as arbitrary
This is accomplished by selecting a volume to be analyzed, then
interactively using various display algorithms to select and weight
voxels to achieve a display that highlights tissues and
relationships of interest. Transfer functions are used to map
properties such as opacity, brightness, color, and windowing to the
voxels in the volume of interest, with all voxels in the volume
potentially contributing to the final image. In real time, the
displayed image can be cut and rotated, and transfer functions can
be altered. Calhoun
presents a historical perspective of volume rendering and technical
discussion of transfer functions.
Real-time interaction with a volume of data offers many
potential advantages over threshold techniques, including better
delineation of 3D relationships, simultaneous depiction of vascular
and soft-tissue structures, and more display flexibility. However,
for many applications, there continues to be controversy in the
radiology literature as to the relative efficacy of various
rendering techniques. For example, a recent report asserts the
practical observation that MIP is superior for small vessels and
yet recent experimental data suggests that VR is more accurate for
At our institution, VR is the preferred rendering technique for
CTA, with MIP used as an adjunct for smaller vessels within an
CTA rendering techniques should be used in a complementary fashion
based on the strengths and limitations of each, though clearly user
experience will play a significant role.
Computed tomographic angiography is an established area of
diagnostic radiology with rapid improvement and growth driven by
technological advances. Conceptual understanding of injection,
acquisition, and rendering parameters is critical for the
optimization of CTA studies. Much work remains to be done regarding
clinically relevant comparison of technical parameter variations
for a wide range of specific applications.
The author would like to thank Dr. Elliot Fishman for insight
and inspiration, and Dr. Leo Lawler for manuscript review.