Magnetic resonance angiography and computed tomography angiography are two highly sensitive, noninvasive imaging modalities that are likely to play a role in the future screening and management of unruptured cerebral aneurysms.
is a third-year Radiology Resident at the Mallinckrodt Institute of
Radiology in St. Louis, MO. He received his medical degree in 1998
from Duke University, Durham, NC. He plans to pursue further
training as a Neuroradiology Fellow at Mallinckrodt beginning in
Cerebral aneurysms represent a significant public health
risk due to their potential for rupture and the life-threatening
sequelae of subarachnoid hemorrhage. Early detection and
treatment of unruptured aneurysms is necessary to reduce the
incidence of aneurysm rupture. Magnetic resonance angiography and
computed tomography angiography are two highly sensitive,
noninvasive imaging modalities that are likely to play a role in
the future screening and management of unruptured cerebral
Based on reports of autopsy series, cerebral aneurysms occur
with an incidence of approximately 5% in the general population.
Thus, approximately 10 to 15 million Americans may be harboring an
aneurysm. These aneurysms are of concern due to the 30% to 40%
mortality rate associated with their rupture, as well as the
approximate 30% incidence of permanent neurologic disability among
While the risk of rupture remains controversial, approximately
27,000 cases of subarachnoid hemorrhage (SAH) due to aneurysm
rupture are seen in the United States every year. Given the
prevalence of cerebral aneurysms and the life-threatening impact of
their rupture, early detection and treatment of unruptured
aneurysms have significant potential to improve public health.
The rate of identification of unruptured aneurysms has increased
dramatically in recent years as a result of the increased use and
improved resolution of computed tomography (CT) and magnetic
resonance imaging (MRI). CT and MRI studies are now commonly used
for a wide variety of neurologic indications, including headache,
dizziness, stroke/transient ischemic attack, trauma, cranial nerve
pathologies, and sinus disease, and the detection of unruptured
cerebral aneurysms is often an incidental finding on such
Conventional CT and MRI techniques are not well suited for the
detection of cerebral aneurysms, however, since standard protocols
do not provide the necessary high-resolution imaging and
reconstruction of vessels. The gold standard for vascular
investigation remains direct catheter cerebral angiography. This
technique traces its roots back to Moniz's first description of a
method to opacify the carotid arteries and vessels of the brain in
Angiographic techniques have been refined to the increasingly safe
and effective procedure known as intra-arterial digital subtraction
angiography (DSA) (Figure 1).
While DSA remains the most accurate modality for vascular
imaging, its disadvantages include a small risk of neurological
complication (approximately 0.07%
), high cost, use of iodinated contrast media, and the fact that it
is a time-consuming and labor-intensive technique. Thus, the search
for other imaging options continues. Computed tomography
angiography (CTA) and magnetic resonance angiography (MRA) are the
two most notable modalities for noninvasive imaging of the cerebral
vasculature. The role of these modalities in the identification and
management of unruptured intracranial aneurysms still needs to be
defined and should be done in terms of decreasing the incidence of
SAH and its associated morbidity and mortality.
CTA is a relatively new technique, made possible by the improved
resolution and rapidly increasing acquisition speeds of newer CT
scanners. In particular, multidetector scanners have made vascular
applications such as pulmonary angiography, aortic aneurysm
modeling, and cerebral angiography viable clinical tools. Current
techniques involve multidetector scanners employing narrow
collimation (down to 1 mm), high kVp technique, bolus tracking for
image timing, and advanced reconstruction algorithms. These
scanners also allow for the use of improved postprocessing
techniques for rapid online manipulation and interpretation of the
three-dimensional (3D) data set (Figure 2).
There are several advantages of CTA over DSA and MRA. While the
resolution of CTA is not as good as that of DSA, it continues to
improve, primarily due to the narrower collimation available on
newer multidetector scanners. Perhaps the greatest advantages of
CTA are the relatively short imaging times, wide availability, and
logistical ease of imaging critically ill patients. Of note, CTA is
subject to fewer artifacts relative to MRA and offers extraluminal
information not available with DSA. Like MRA, CTA is a less
invasive technique than DSA and has multiplanar reconstruction
capabilities. Finally, CTA has excellent sensitivity for
intracranial hemorrhage, which is critical in the imaging of
There are few disadvantages of CTA, but they are important to
remember. The most significant disadvantage is the use of iodinated
contrast media, which cannot be administered in patients with
documented allergies or in many patients with significant renal
insufficiency. In cases of small aneurysms, the lower resolution of
CTA (compared with DSA) and the lack of real-time imaging
information (which is provided by DSA) is particularly problematic.
Finally, as with DSA, CTA delivers high radiation doses that should
be considered on a case-by-case basis.
Like CTA, MRA is a rapidly evolving modality. The predominant
technique used for MRA is time-of-flight imaging performed on 1.5 T
magnets. Phase-contrast imaging is used much less frequently.
Recent advances include contrast-enhanced 3D MRA imaging (CE MRA),
increased magnet strengths of 3.0 T (Figure 3), and improved
gradients on newer scanners. Contrast enhancement and increased
field strengths increase the signal-to-noise ratio, which may
improve the diagnostic information available. Higher performance
gradients allow thinner slices, resulting in improved resolution in
reconstructed data sets. In addition, faster gradients allow
improved in-plane spatial resolution due to improved frequency
resolution. MRA will continue to improve as it is developed in a
wide variety of clinical applications including head/neck, body,
and peripheral angiography studies.
MRA has two unique advantages over CTA and DSA: the lack of
iodinated contrast material and the lack of radiation exposure. In
particular, patients with a documented contrast allergy may be best
served with MRA imaging. MRA, like CTA, offers a less invasive
technique, extraluminal imaging, and extensive multiplanar imaging
and reconstruction capabilities not available with DSA.
The unique disadvantages of MRA involve artifacts--such as over
estimation of stenosis due to turbulent flow--and the logistical
difficulties of imaging critically ill patients in an MRI suite.
Like CTA, MRA lacks the resolution and real-time imaging of DSA.
Finally, although shorter than those for DSA, MRA imaging times are
longer than those for CTA.
MRA versus CTA for detection of aneurysms
Multiple studies have been published assessing the relative
abilities of CTA and MRA to detect cerebral aneurysms. One such
study is the extensive review by White and Wardlaw
that employed detailed meta-analysis of 38 studies evaluating the
relative ability of CTA and MRA versus DSA to detect aneurysms.
Most of the studies reviewed involved patients undergoing both DSA
and CTA or MRA. Compared with the number of aneurysms detected on
DSA, the overall accuracy rates of CTA and MRA for aneurysm
detection were 89% and 90%, respectively.
The sensitivity for aneurysms <3 mm in size was 61% for CTA and
38% for MRA. Meanwhile, the sensitivity for aneurysms >3 mm was
96% for CTA and 94% for MRA. This high detection rate for aneurysms
>3 mm approaches that of the gold standard of DSA, which is
known to have a small (<5%) but real rate of missed aneurysms.
White and Wardlaw
suggest that the detection rates for CTA and MRA may increase as
MRA techniques improve and the use of workstations for clinical
interpretation becomes more common.
The enthusiasm for these encouraging numbers, however, must be
tempered. White and Wardlaw
point out that some early studies failed to show significantly
improved results with the use of CE-MRA. Also, they note that no
studies have demonstrated that real-time manipulation and
interpretation of data on workstations improves aneurysm detection.
Finally, skewed, high-risk populations have been used throughout
the literature. In the majority of studies, this high pretest
probability of aneurysm presence may induce observer bias and
corresponding falsely elevate the detection rates.
The merits of CTA and MRA for cerebral vascular imaging were
confirmed by a prospective study by White et al
in 2001. For aneurysms >5 mm in size, they reported
sensitivities of 94% and 86%, respectively, for CTA and MRA. Again,
for smaller aneurysms (<5 mm), the sensitivities were much
lower, 57% for CTA and 35% for MRA. Overall, White and Wardlaw
and White et al
have found that CTA and MRA are both highly sensitive techniques
for identifying the majority of aneurysms, but both remain limited
in their ability to detect smaller aneurysms.
While it is fairly well established that CTA and MRA are
effective imaging tools for all but small aneurysms (<5 mm), the
more important question is how to use such tools for screening to
help prevent SAH due to aneurysm rupture. Guidelines must be
developed for when, how, and for whom screening should be employed.
The data required to establish such screening guidelines include
identification of those at high-risk for aneurysm development, the
risk of rupture for each aneurysm, and the risks of potential
A substantial amount of data suggests how to determine which
individuals are most at risk for the development of cerebral
aneurysms. Genetic predisposition to the development of aneurysms
has been established in a number of connective tissue disorders
including Ehlers-Danlos Type IV, Marfan's syndrome,
neurofibromatosis Type I, and autosomal dominant polycystic kidney
Smoking, female gender, and a family history of cerebral aneurysms
are all risk factors for aneurysm formation. Finally, cerebral
aneurysms are most prevalent in middle-aged patients. This list of
known risk factors allows for selection of individuals who would
benefit most from screening for the presence of an aneurysm.
There is conflicting evidence in the literature with regard to
the risk of rupture of any given cerebral aneurysm. Historically,
it has been reported that the risk of aneurysm rupture is
approximately 1% per year per aneurysm (not including giant
aneurysms, which are known to have a 6% per year rate of rupture).
However, this notion was brought into question by the International
Study of Unruptured Intracranial Aneurysms (ISUIA) Investigators
The ISUIA stated that rates of rupture were as low as 0.05% per
year per aneurysm (for aneurysms <10 mm in size in patients with
no prior history of SAH). Thus, some have suggested that screening
with MRA and/or CTA may be acceptable as both modalities are highly
effective at identifying aneurysms >10 mm.
A number of criticisms have been leveled at the ISUIA study,
however. It was a retrospective study and subject to selection
bias, since only untreated aneurysms were followed. In addition, if
the 0.05% rate were accurate, the prevalence of aneurysms in people
of 30 years of age would have to be much higher than 5%, given the
annual rates of subarachnoid hemorrhage. Finally, the majority of
ruptured aneurysms treated today are in fact <10 mm in size. As
an example, Forget et al
recently published a report showing that 85.6% of all ruptured
aneurysms seen in their clinical setting were <10 mm in size.
Some have argued that this discrepancy is due to larger aneurysms
contracting after rupturing, thus appearing smaller at the time of
diagnosis. However, this theory has not been proven.
Also, risk factors other than aneurysm size must be investigated
further, including the type of aneurysm (eg, saccular or fusiform),
morphology, location, presence/absence of calcification or clot,
and underlying blood pressure.
These confusing and incomplete data on the risks of aneurysm
rupture provide no basis for screening and management guidelines
for the majority of unruptured cerebral aneurysms. Therefore, the
need to detect aneurysms (other than giant aneurysms) is of
questionable utility for patient management. Further large-scale,
multicenter prospective studies are needed to produce the necessary
data on risks for and causes of aneurysm rupture. Also, some
attempt should be made to quantify observer bias by performing
screening on a portion of the general population at low-risk. Such
studies are challenging given the reliance on DSA for aneurysm
identification. CTA and MRA may facilitate future research of
cerebral aneurysms by allowing safer, easier imaging for following
unruptured aneurysms prospectively. As CTA and MRA become more
widely available and logistically easier, performing a large
multicenter screening study will become less complicated. Thus, CTA
and MRA can be expected to play an increasing role in the future
detection and management of cerebral aneurysms.
MRA and CTA both have been proven to be highly effective in the
detection of cerebral aneurysms larger than 3 to 5 mm in size. This
fact, combined with the increasing availability of MR and CT
imaging modalities, easier imaging protocols, and continually
improving technique, will make both CTA and MRA important tools in
the effort to detect and manage unruptured cerebral aneurysms. More
work is necessary to reduce the incidence of SAH and its associated
devastating morbidity and mortality.
The author wishes to thank Dr. Colin Derdeyn for providing the
pictures used in this manuscript and for his excellent expert
advice and Dr. Robert McKinstry for his expert knowledge and