Dr. Gotwald is a Fellow and Dr. Beauchamp is an Assistant
Professor in the Neuroradiology Division, Russel H. Morgan
Department of Radiology, Johns Hopkins Medical Institutions,
Although catheter angiography remains the gold standard,
non-invasive angiographic techniques are increasingly used for
assessment of the neurovasculature. Computed tomographic
angiography (CTA) and magnetic resonance angiography (MRA) are
particularly appealing due to their low morbidity and
comprehensiveness. The main goal of this article is to clarify the
role of CTA and MRA in the evaluation of intracranial vascular
disease by discussing the relative strengths and weaknesses of both
techniques in the context of specific neurovascular disorders.
Imaging and post-processing methods
CT angiography--With modern spiral CT scanners it is possible to
scan with continuing gantry rotation and patient table movement.
High capacity x-ray tubes and faster (subsecond) tube rotations
allow rapid acquisitions of very accurate volume data sets and
enable scan completion during the period of optimal intravascular
In a standard CTA procedure the patient is scanned in the supine
position and axial orientation. In general, 2-mm beam collimation
and 3 mm/s table feed are sufficient, although smaller
reconstruction intervals can be afforded with newer generations of
CT scanners or multidetector equipment. The scanning parameters
depend on the anatomic region being imaged. The scan volume should
be restricted to the region of interest.
Optimization of vascular opacification is an essential
prerequisite for CTA. An attenuation coefficient of approximately
150 HU is considered optimal for evaluation of vessel diameter.
This can be obtained by using a power injector and an injection
rate of 3 mL/sec. The time interval between the start of contrast
infusion and initiation of the scan sequence can either be
determined empirically or with automated techniques (e.g. SMART
Prep [GE Medical Systems, Milwaukee, WI], CARE bolus [Siemens
Medical Systems, Iselin, NJ]). However, all these methods seem to
be equally reliable with the former approach somewhat more time
With these settings, the entire spiral CT study can be accomplished
in <1 minute. The image data can be reconstructed to obtain an
angiographic display. The reconstruction algorithms and display
methods will be discussed later. Fundamentally, CTA is an
'anatomic' study, similar to catheter angiography, in which a
patent vessel will fill with contrast that will be subsequently
incorporated into the vascular reconstruction.
MR angiography--Magnetic resonance angiography is a
"physiologic" study in which generation of intravascular signal is
also impacted by flow velocity, turbulence, and vascular
orientation. The two fundamentally different methods are
time-of-flight (TOF) and phase-contrast (PC) MRA. MRA enables
vascular delineation with either bright (flow increases signal) or
black (flow decreases signal) blood techniques. TOF MRA is used far
more often. Multiple radiofrequency (RF) pulses are applied in a
single acquisition that result in suppression of signal from
stationary tissue. Areas not saturated, i.e., blood flowing into
the plane of imaging, leads to the generation of intravascular
signal differentiable from background tissue. Three-dimensional
(3D) TOF acquisitions with thinner volume slabs afford greater
signal to-noise ratios and spatial resolution than two-dimensional
(2D) TOF. The quality of TOF MRA continues to improve, including
optimization of background suppression using magnetization transfer
(intracranial only), and minimization of turbulence- and
saturation-related artifactual signal suppression using multiple
overlapping thin-slab acquisition and tilted optimized
non-saturated excitation. Finally, although TOF MRA is relatively
insensitive to slow flow due to the RF saturation of the slow flow
spins, paramagnetic contrast agents can be applied to increase the
signal amplitude and, thus, improve vessel display.
The notable limitation of TOF MRA is that in addition to areas
of flow, tissue with very short T1 will not be suppressed during
repetitive stimulation (e.g., methemoglobin or fat) and can be
falsely incorporated into the vascular reconstruction.
In these cases phase-contrast (PC) MRA should be applied. PC MRA
uses phase shift differences induced by velocity of flow to create
an angiographic image. Stationary tissue is subtracted from two
acquisitions with bipolar velocity gradients. Thus, only tissue
that has changed position between the two acquisitions, regardless
of T1 relaxation properties, is included in the vascular
reconstruction. This method affords excellent background
suppression. However, two factors limit the more frequent
application of PC MRA. The addition of the second acquisition (for
subtraction) leads to increased evaluation time. Also, PC imaging
requires predetermination of the range of expected velocities.
Although easily done when flow is laminar, vascular tortuosity,
numerous flow dividers, and the presence of stenosis limit the
utility of PC for routine application.
Rendering and display techniques
Surface shaded display, maximum intensity projection, and volume
rendering techniques are now feasible from a time and cost
perspective due to continued advances in computer hardware,
software, and display technology. They can be used for both CTA and
Surface shaded display (SSD)--SSD was the first rendering
technique applied to medical data sets. To segment the structures
of interest for SSD a simple thresholding can be used. The
user-specified range of attenuation values determines the intensity
of each voxel within the data set. Advantages of SSD are superior
speed as well as very accurate surface display. Multiple surfaces
can be displayed when partial opacities are implemented. However,
SSD only uses only approximately 10% of the available data. As a
result, organ structures that do not have naturally differentiated
surfaces are not well visualized.
Maximum intensity projection (MIP)MIP is widely used for
angiographic display of MR and CT data. The highest voxel value
from a projection through the dataset is displayed. This allows
more accurate evaluation of the vessel diameter accuracy compared
with SSD. The MIP technique is very useful for MRA and CTA display.
However, high-density material (i.e., calcifications) will obscure
valuable information from intravascular contrast material. For this
reason, MIP often requires extensive editing to eliminate unwanted
data to create useful images.
Volume rendering (VR)--Neither SSD nor MIP displays vessel,
parenchyma, and bone simultaneously. This only has been afforded by
VR. This interactive technique allows for data processing and
display in real time.
VR sums-up attenuation values of each voxel along parallel or
divergent (i.e., perspective VR) lines through the data set for
each pixel of the display. 3D VR has some advantages over the other
rendering techniques. Most importantly, it incorporates the entire
volume data set into a 3D image. Therefore, the dynamic range of
the image is preserved, enabling simultaneous display of vessels,
soft tissue, viscera, and bone. Window and center settings can be
adjusted and clip planes can be applied to visualize structures in
the volume that would otherwise be obscured. It is important to
state that the superior editing capabilities of VR also present the
risk for misrepresentation of vascular disease. Therefore, careful
adjustment of the rendering parameters is critical. VR can be used
to post-process MRA data sets. However, as noted above, with MRA
the background voxel signals are suppressed for adequate vessel
conspicuity. Bone and soft tissue surrounding the vessels is
therefore poorly delineated. Thus the unique benefit of VR, on the
fly evaluation of vessels and vascular relationships to skull base
and brain tissue, is not taken advantage of with MRA.
Specific applications of CTA and MRA
Stroke--Stroke is the third most common cause of mortality in
the United States. Efficacious therapy with thrombolytic agents is
now available, but safe treatment hinges on the use of
neuroimaging. Non-enhanced CT is valuable in excluding patients
with extensive infarction or hemorrhage and has enabled treatment
of patients presenting within 3 hours of stroke onset. In
attempting to extend the window of treatment beyond 3 hours,
emphasis has been placed on increasing the sensitivity and
specificity in detecting vascular occlusion and areas of
Vascular occlusion can be identified expeditiously with CTA or MRA
with equal sensitivity.
(figure 1) These techniques can also be supplemented with CT
perfusion or MR perfusion imaging to delineate areas of
hypoperfusion or tissue at risk for infarction. Although both CT-
and MRI-based techniques are valuable, they have unique strengths.
Specifically, CT is more readily available, less expensive, and
less time consuming. The relatively wide gan-try opening and lack
of magnetic field increases the ease of monitoring critically ill
patients. Conversely, MRI is more sensitive in the detection of
posterior fossa stroke and does not require the administration of
MR diffusion imaging enables early detection of areas of
infarction minutes after the ictus. MR perfusion is also valuable
to discriminate hypoperfused but still viable regions from
non-salvable infarction zones. CT perfusion may afford similar
delineation of reversible injury. CT perfusion is currently limited
to one single slice. However, with multi-detector CT technology,
whole brain coverage similar to MR perfusion should be attainable
in the future.
The optimal approach in cases of stroke varies with the clinical
situation and the availability of MRI technology. When MRI is
immediately available, it is increasingly seen as the test of
choice. If any delay in MRI evaluation is expected, a comprehensive
CT evaluation with CT perfusion and CTA should be performed.
Dural sinus thrombosis--Although relatively uncommon, dural
sinus thrombosis is a potentially life threatening condition.
Predisposing factors are dehydration, hypercoaguable states,
pregnancy, and intracranial infections. CTA and MRA are of value
because the conventional CT and MRI findings (i.e., empty delta
sign in CT) can be insensitive and nonspecific. MR venography is
the technique of choice.
It enables delineation of venous sinuses as well as sensitive
detection of secondary parenchymal changes and other pathologic
processes that may mimic sinus thrombosis symptoms.
CT venography is an alternative technique that enables excellent
vascular delineation. In addition to being the study of choice when
MR venography is not possible, it is also an excellent technique to
assess interval change, response to treatment, and may be the only
test required when clarifying a question of thrombus raised on a
Intracranial aneurysms--Aneurysm rupture is associated with high
morbidity and mortality. Increasing emphasis is being placed on the
detection of aneurysms prior to rupture because elective aneurysm
clipping and coiling can now be performed with extremely low
patient risk. An ideal screening examination should have adequate
sensitivity, minimal risk, and be "acceptable" to patients.
MRA and CTA are ideally suited for this purpose and both tests have
equivalent sensitivity, i.e., detection of aneurysm >= 3 mm.
The advantage of MRA in such cases is the ease of post-processing
given the inherent background suppression. However, this is
becoming less of an issue for CTA as volume rendering workstations
equipped with clip planes enable physicians to interactively remove
obscuring bone and adjacent vessels. Real-time 2D and 3D display
with VR enables expedient evaluation for the presence of aneurysms.
In addition to screening, CTA is uniquely valuable as a
supplement to MRA and catheter angiography. For example, areas of
turbulent flow, such as may be seen in giant aneurysms, result in
loss of intravascular signal secondary to intra-voxel phase
dispersion. CTA is "anatomic" and as such is relatively unaffected
by complex flow patterns. (figure 2) Similarly with catheter
angiography, overlapping vessels can obscure evaluation of aneurysm
neck and dome morphology as well as its relationships to adjacent
vessels. CTA enables a 3D delineation of vascular relationships to
skull base and brain parenchyma not possible with either MRA or
catheter angiography. This is an important issue in surgical
planning and in determining the feasibility of endovascular
Increasingly, non-invasive techniques are used in the evaluation
of patients with suspected aneurysms. For initial screening, MRA
and CTA are essentially equivalent. In pretreatment planning, CTA
is clearly superior due to its unique editing and display
capabilities. Currently, it remains the standard of care to
evaluate all aneurysm patients before surgery with catheter
angiography so that aneurysms < 3 min are detectable. However,
it can be expected that with continued technological advancements
(such as the CT multi-detector array
), non-invasive techniques will offer sensitivity similar to
Arteriovenous malformations (AVMs)-- Pial AVMs are congenital
lesions with an estimated 2 to 4% risk of intraparenchymal
hemorrhage. Hemorrhage is related to associated aneurysms, outflow
restriction, or pure deep venous drainage.
The goal of diagnostic evaluation is delineation of the nidus, the
venous drainage patterns, associated aneurysms, and venous
constrictions. Central to this differentiation is temporal
resolution sufficient to determine whether a vessel is arterial or
venous. CTA/venography is limited due to its low temporal
resolution, as MRA/ venography is limited due to turbulent flow
associated with these high-flow lesions. Thus, the role of
non-invasive imaging is restricted
to certain instances. Specifically, CTA/ venography can be of
valuable to define the morphology of the aneurysms, venous
constrictions, and vascular relationships for surgical planning.
(figure 3) Conventional MR imaging in combination with MRA/
venography is useful for assessing response to radiosurgery.
CTA and MRA techniques are highly valuable tools in many
applications in the evaluation of vasculature in neuroradiology. It
is hoped that this article will facilitate the use of non-invasive
tests whenever appropriate. AR
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