As MR imaging technology has advanced, so has accurate depiction
of the carotid arteries. With the stronger, faster gradients
typical of echo planar imaging (EPI) MR systems, contrast-enhanced
MR angiography (CE-MRA) can now be performed during the initial
passage of gadolinium through the arterial system, effectively
eliminating venous contamination.
CE-MRA allows a much more accurate depiction of carotid stenosis
and ulceration than unenhanced MRA techniques. Clinical
applications and the technical details behind this new technique
are discussed in this issue of Applied Imaging.William G.
Bradley, Jr., MD, PhD, FACR
Contrast-enhanced MR Angiography of the Carotids
contrast-enhanced MR angiography
(CE-MRA) generally refers to the rapid acquisition of vascular
images while a bolus of gadolinium passes through the arterial
phase following an intravenous injection. Typically, CE-MRA
requires the strong, fast gradients that are used for echo planar
imaging (EPI). Thus, while gadolinium has been used occasionally to
supplement three-dimensional (3D) time-of-flight (TOF) MRA over the
years, eg, for demonstration of very slow flow or the relationship
of an enhancing tumor to blood vessels, CE-MRA has really only come
into its own since EPI units were installed on a widespread basis 4
to 5 years ago.
At the heart of CE-MRA is the effect of saturation on
When fully magnetized, ie, unsaturated, spins enter the first slice
of an imaging volume, and return a strong signal after exposure to
the first radiofrequency (RF) pulse ("flow-related enhancement").
For thin, two-dimensional (2D) TOF slices oriented perpendicular to
the direction of blood flow, spins are unlikely to be excited by
more than one RF pulse. For a thicker 3D TOF slab, the spins may be
exposed to multiple RF pulses and eventually run out of
magnetization (Figure 1). At this point, they are said to be
"saturated." The effect of saturation is magnified when the spins
flow parallel to, ie, within, the slice rather than perpendicular
to it. This is because they are now susceptible to multiple RF
exposures over the greater distance of the field-of-view (FOV), eg,
25 to 35 cm. Gadolinium shortens the T1 of the flowing protons,
allowing them to sustain multiple RF exposures without saturation,
even while traversing the large distances of a typical FOV (Figure
2). Thus, CE-MRA is generally performed in the same (coronal) plane
as the arteries .
CE-MRA typically involves a bolus injection of gadolinium by a
power injector with acquisition of a 3D TOF slab. Inplane spatial
resolution is typically on the order of 1 mm, while slice thickness
is on the order of 1 to 2 mm. Typically, zero interpolation (ZIP)
is applied in the slice direction to minimize "stair-step"
artifacts when the images are subsequently computer-processed by a
maximum intensity projection (MIP) and rotated about a vertical
axis to be viewed from any angle.
Since the base sequence for a 3D TOF application is a T
-weighted gradient echo, subcutaneous fat normally appears bright.
For this reason, we typically acquire a pre-injection mask image
and then subtract it from the image acquired during the arterial
CE-MRA is typically used to image arteries such as the carotids,
which places certain technical demands on the gradients.
Specifically, they need to be able to complete an arterial phase
acquisition before the gadolinium gets to the venous phasea
period of approximately 10 sec (Figure 3). There are two ways to
acquire the data to ensure minimal venous opacification. The first
technique consists of multiple 10-sec acquisitions following the
injection of contrastone of which will predominately cover the
arterial phase. This "multiphase" acquisition typically consists of
a 10-sec mask image (Figure 4A) followed by 4 or 5 separate 10-sec
acquisitions (Figures 4B and C). The arterial-phase image is then
selected, masked, and subjected to the MIP algorithm for viewing
from any projection (Figure 4D).
More recently, a clever scheme for data acquisition has been
introduced that allows higher resolution imaging of arteries while
still suppressing the signal from veins. This technique is known as
"elliptical-centric" CE-MRA, referring to the filling of k-space in
[k-space is a mathematical construct that facilitates
visualization of different fast MR imaging techniques.
Diagrammatically, k-space consists of a matrix of 256 ¥ 256 points
(ie, the number of frequency and phase points), with each phase
value corresponding to a horizontal row and each frequency sampling
point corresponding to a vertical column. Typically, the most
negative values of the phase-encode gradient are at the bottom of
the matrix, those with the weakest phase encoding are in the
center, and the k-space lines with the strongest value of the
phase-encode gradient are at the top. Since weaker phase encoding
leads to less dephasing, most of the signal comes from the center
of k-space. The idea behind elliptical-centric k-space acquisition
is to start acquiring in the center of k-space just as the contrast
hits the arterial phase. The term "centric" k-space coverage refers
to a single-slice technique (eg, 2D-TOF), while the term
"elliptical-centric" refers to a 3D-TOF technique in which there
are two phase directions (ie, traditional phase and slice).]
In order to ensure that the acquisition does not begin until the
contrast begins to fill the arteries, either a timing run must be
performed or automated bolus detection software must be used. Such
software (ie, SmartPrep [GE Medical Systems, Milwaukee, WI]
BolusTrak [Philips Medical Systems, Bothell, WA] and CareBolus
[Siemens Medical Systems, Iselin, NJ]) is particularly useful for
the abdominal aorta and below; however, it is less reliable for
imaging the carotid arteries. Therefore, a timing run is usually
performed, which consists of a 2-mL injection of gadolinium with
image acquisition every second or so at the level of the carotid
bulb. While many schemes have been utilized to optimally determine
the timing delay, we simply wait until the carotid bulb has first
reached its maximum intensity and use that as the timing delay.
One of the advantages of elliptical-centric k-space coverage is
that a much longer time is available for acquisition, eg, 30 to 60
sec, compared with the 10-sec multiphase acquisition. This allows
higher spatial resolution while still suppressing venous signal,
since it is acquired in the low signal-to-noise periphery of
k-space (Figure 5). Our current technique is spoiled gradient echo
(SPGR) 6.3/1.5/45š (with a 224 ¥ 256 matrix acquired over a 26-cm
FOV. We use a 31.25 kHz bandwidth, a single excitation, and a
partial Fourier acquisition (80%) in the first-phase direction. The
second-phase direction has 50 1.2-mm thick slices. We zero
interpolate in the slice direction and to 512 ¥ 512 in plane.
We typically start a 20-gauge intravenous line in an antecubital
vein and connect it to a power injector. A 2-mL bolus of gadolinium
is injected to determine the timing delay. Then, following
acquisition of a mask image, the injection is begun and, following
the predetermined timing delay, the acquisition of
contrast-enhanced images is performed. Following substraction, the
data is MIPPed and displayed every 15š of rotation about a vertical
Contrast-enhanced MRA is more accurate in quantifying stenoses
than conventional, unenhanced 2D TOF or 3D TOF MRA. While
unenhanced techniques are prone to saturation effects that
overestimate the degree of stenosis, the gadolinium in a CE-MRA
technique effectively eliminates saturation effects, resulting in a
truer rendering of the degree of stenosis. Even "string signs," ie,
99% occlusions (Figure 6), can be detected with CE-MRA. Since the
vertebral arteries are also included in the coronal slab, vertebral
artery stenosis (Figure 7) can also be detected. Carotid bulb
irregularities suggestive of ulceration can be detected easily with
CE-MRA (Figure 8).
Intracranial Vascular Screening
Using conventional volume neck coils, the origins of the carotid
and vertebral arteries are routinely visualized on a CE-MRA
examination (Figures 4D and 5). By expanding the FOV to
approximately 30 cm in the read direction (along the long axis of
the body), the acquisition can be extended through the circle of
Willis without venous contamination (Figure 9). While the
intracranial vessels are thereby displayed with lower resolution
than our usual unenhanced multiple overlapping thin-slab
acquisition (MOTSA) MRA brain study,
it still serves as a good screening study of the proximal vessels
in the Circle of Willis. While CE-MRA can be performed in the
brain, the necessity for a longer acquisition to provide higher
resolution typically results in greater venous contamination;
therefore, gadolinium is rarely used in the brain at present.
Perhaps with venous editing techniques in the future, CE-MRA will
be applied to the brain as well.]
As MR technology continues to advance, so does accurate,
noninvasive depiction of the carotid arteries. With the stronger,
faster gradients present on echo planar MR systems, CE-MRA can now
be performed much more accurately for sizing stenoses than was
possible with previous unenhanced MRA techniques. In many centers,
the combination of a positive contrast-enhanced MRA and a positive
duplex Doppler ultrasound is a sufficient indication for a carotid
endarterectomy, obviating the need for the more expensive, and more
invasive, catheter angiogram.
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Advanced MR Imaging Techniques.
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9. Melhem ER, Poon EK, Weinreich DM, et al. Comparison of 2D-
and 3DFT multiple overlapping thin-slab acquisition TOF MR
angiography in carotid disease.
Clinical Quiz: True or False
1. Contrast-enhanced MRA (CE-MRA) should be performed in every
patient with stroke or TIA.
2. CE-MRA requires an arterial injection.
3. CE-MRA can be performed on any modern MR system.
4. CE-MRA can only be performed on high-field systems.
5. CE-MRA can be performed in more than 60 seconds and still
minimize the signal coming from veins.
1. False. Patients with cardiac pacemakers and ferromagnetic
intracranial aneurysm clips cannot undergo MRA.
2. False. Contrast-enhanced MRA is performed with an intravenous
injection that catches the gadolinium bolus during the arterial
3. False. Only those systems with high-performance gradients,
eg, echo planar systems and cardiovascular systems, can perform CE
4. False. Low-field systems with fast, strong gradients can
perform CE MRA as well.
5. True. Using elliptical centric techniques, the examination
can be extended to 60 seconds, improving the spatial resolution
while minimizing the signal from enhancing veins. This is
accomplished by filling the high signal-to-noise center of k-space
during the arterial phase and the low signal-to-noise periphery of
k-space during the venous phase.
Note: No contrast agents are approved by the U.S. Food and
Drug Administration for use in magnetic resonance
William G. Bradley, Jr., MD, PhD, FACR * Editor-in-Chief
O. Oliver Anderson * Publisher
Elizabeth A. McDonald * Editor
Beverly Harris * Assistant Editor
Felice Ponger-Shaloum * Art Director/Production Manager
Applied Imaging is published by Anderson Publishing, Ltd. It is
supported by a grant from Amersham Health. The views and opinions
expressed in this publication are those of the authors and do not
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