Magnetic resonance angiography (MRA) can be an accurate, fast, and safe tool in the workup of patients with lower extremity (LE) ischemia. Setting up an LE-MRA protocol, however, can be complex due to the many proposed sequences, injection protocols, and postprocessing techniques. The author details the LE-MRA protocol developed at the Mayo Clinic, Scottsdale, AZ, which provides a relatively straightforward technique that can be applied to a wide range of patients.
is the Section Head for Body and Musculoskeletal MRI, Department
of Radiology, Mayo Clinic, Scottsdale, AZ.
Many recent publications have demonstrated that magnetic
resonance angiography (MRA) can be an accurate, fast, and safe tool
in the workup of patients with lower extremity ischemia.
These studies have proven that MRA can reliably and quickly depict
significant stenoses of the abdominal aorta and the arteries of the
lower extremities (LE). In some medical centers, MRA has even
replaced conventional X-ray angiography (XRA) as the initial method
of evaluating LE ischemic symptoms.
Setting up an LE-MRA protocol, however, can be a complex task
due to the many proposed sequences, injection protocols, and
postprocessing techniques. To simplify this process, this article
will detail and explain the methods we have developed for use at
our institution for performing LE-MRA. This protocol provides a
relatively straightforward technique that can be applied to a wide
range of patients.
The major challenges faced by technologists and radiologists
attempting to perform MRA of the abdominal aorta and LE include the
need to quickly cover a relatively large region of interest, the
desire to achieve high spatial resolution, and the need to make
multiple measurements and calculations during scan set-up. In
addition, the limitation on the maximum contrast dose and the wide
variability of circulation time have made it difficult to optimize
MRA parameters for the entire region of interest. To meet these
challenges, we have developed a protocol that employs a fast
three-dimensional (3D) contrast-enhanced MRA (CE-MRA) sequence with
a dual-injection modified stepping-table protocol, standardized
fields of view, and an extremity vascular phased array coil.
Choosing the best MRA sequence
The most successful LE-MRA protocols call for the use of a 3D
gadolinium-enhanced, T1-weighted spoiled gradient- echo (SPGR)
sequence. Studies have shown that the enhanced 3D SPGR sequences
outperformed two-dimensional (2D) time-of-flight sequences for both
speed and accuracy.
As is seemingly customary, the major MR vendors have given this
SPGR sequence a variety of unrelated names: enhanced fast
gradient-echo (EFGRE) on General Electric (GE Medical Systems,
Waukesha, WA), fast low angle shot (FLASH) on Siemens (Siemens
Medical Solutions, Iselin, NJ), and T1-weighted fast field echo
(T1W-FFE) on Philips (Philips Medical Systems, Bothell, WA). The
SPGR sequence bears similarities to the more familiar T2*-weighted
gradient-echo sequences, using a low flip angle and gradient
refocusing pulse to create an echo, but differs by the application
of a radiofrequency spoiling pulse prior to every alpha excitation
pulse. This spoiler pulse destroys the residual transverse
magnetization, essentially resulting in a T1-weighted sequence.
Administration of IV gadolinium greatly enhances vascular signal
due to its T1 shortening effects. Using short TR and TE settings (5
msec and 1.8 msec) minimizes scanning time and background signal.
The optimal flip angle for maximizing arterial signal-to-noise
ratio (SNR) is 30š to 60š, depending on the intra-arterial
gadolinium concentration, with 45š providing a good compromise.
The choice of a 3D over a 2D acquisition permits the use of
thinner slices while maintaining good SNR. Since information for
all voxels in a 3D dataset is acquired throughout the entire length
of the sequence, better volume reconstructions can be created.
Minor patient motion results in less prominent artifacts than is
seen with the 2D mode. While the choice of the 3D acquisition mode
increases scan time slightly, a high-resolution 3D CE-MRA sequence
can still be performed in 20 to 30 seconds. These qualities make
this sequence ideal for use in breath-hold and stepping-table
Optimizing vascular SNR with timing
The addition of small bolus timing tests, power-injectors,
automated bolus detection methods, and now MR fluoroscopy has been
found to be very helpful at accurately identifying circulation time
from the injection site to the imaging slab. Circulation time to
the abdominal aorta has been shown to vary from 10 to 60 seconds in
one group of subjects.
If the circulation time of the bolus can be pinpointed, arterial
SNR can be optimized in CE-MRA by timing the acquisition of the
central points of image k-space with this peak enhancement. These
central or low order points of k-space comprise the subset of the
images' raw data that determines image contrast.
Centric, elliptical centric, low-high, and spiral k-space
acquisition strategies were all recently applied to MRA to acquire
the image contrast information rapidly at the beginning of the
sequence, so that arterial MRA signal can be optimized and venous
signal minimized. If one of these scan options is available, it
should be used during acquisition of MRA images of the aorta and LE
because of the short time window between the arrival of the bolus
and the appearance of venous contrast. With centric phase-encoding
techniques, venous filling occurs in the later parts of the MRA
sequence, when the peripheral points of k-space are being acquired.
These peripheral points contain the image's spatial resolution
data, so venous filling does not usually cause significant contrast
enhancement on the resulting images.
Optimal enhancement of the abdominal aorta on CE-MRA can be
achieved with as little as 0.1 mmol/kg gadolinium, when injected at
2 mL/sec, in patients with normal body habitus and cardiac output.
However, for stepping-table CE-MRA of the aorta and lower
extremities, a standard dose of 0.3 mmol/kg, up to the regulatory
maximum of 40 mL, has been proposed for use in all patients.
This higher dose is needed to allow for the unavoidable dilution of
the contrast bolus in those patients with lower cardiac output, and
it better opacifies the smaller vessels in the legs.
In stepping-table MRA, the table automatically translates from
one imaging station to another, chasing a contrast bolus as it
travels down the aorta and passes into the legs. In most patients,
the inflow and outflow vessels, from the suprarenal abdominal aorta
down to the pedal arches, can be imaged with three 45-cm imaging
stations overlapped by 5 cm. We term these three regions the
aorto-iliac, femoral-popliteal, and tibial-pedal stations (Figure
1). While many authors have limited the coverage of their MRA exams
to exclude the upper abdominal aorta or the feet, these are regions
that frequently need to be evaluated if surgical bypass grafting is
Most stepping-table MRA protocols have called for a single bolus
injection of 40 mL of gadolinium for imaging the aorto-iliac
station, followed by automated table stepping and scanning of the
femoral-popliteal station, then stepping to the tibial-pedal
station. Bolus-chase MRA studies performed with this
stepping-table, single-injection technique generally provide
excellent images of the aorto-iliac and femoral-popliteal stations,
but have encountered occasional problems with asymmetric contrast
filling in the tibial-pedal arteries.
Depending on the circulation time to the lower legs, and the amount
of time spent scanning the first two stations, the contrast
enhancement peak might not reach the tibial-pedal station arteries
coincident with the timing of the third station MRA
Not surprisingly, these problems with contrast timing in the
lower legs parallel the contrast timing difficulties found in
conventional X-ray angiography runoff studies that use a
bolus-chase, stepping-table technique. If there are proximal
occlusions or low cardiac output, then underfilling of the lower
leg arteries will occur. Selective injections with digital
subtraction can provide better angiograms in these cases.
With distal arterial underfilling on bolus-chase MRA studies,
the tibial-pedal vessels may have underestimated diameters, and
stenoses may be overestimated. On the other extreme, if the imaging
occurs too late after the injection, then the rapid venous return
from the feet will likely obscure some of the ankle and lower leg
vessels. Multiphase imaging at one anatomical station can provide
more consistent results in these cases.
Recently, MRA research has explored methods of improving imaging
of the lower leg arteries. Biphasic contrast injection protocols
have been studied with stepping-table MRA in an attempt to prolong
the duration of peak arterial enhancement with some success. Other
successful solutions used a two- or three-injection stationary
table strategy, using smaller doses of contrast for each station.
Newer, improved MR imaging sequences and bolus timing techniques
have reduced the dose of contrast needed to achieve good arterial
It is possible to divide the contrast bolus into two or three
smaller boluses, with separate injections for the various imaging
stations. Digital subtraction postprocessing is then used to
optimize arterial conspicuity. This technique is analogous to a
conventional angiographer's use of stationary table digital
subtraction angiography (DSA), since the multiple divided bolus MRA
image of the lower legs can use more optimized timing and
multiphase scanning for delayed unilateral arterial filling. Some
authors have termed this an MR-DSA technique.
We have chosen to combine elements of the MRA-DSA and
stepping-table techniques by using a two-injection method with
automated table stepping to image the aorto-iliac station and the
femoral-popliteal station in rapid succession after the first
contrast bolus. After a brief 5-minute delay, a single station
injection is used with a higher resolution technique for optimized
scanning at the tibial-pedal region. A sample case performed with
this technique is shown in Figure 2.
Receiver coil choice and positioning
The patient's legs and pelvis are imaged with a lower extremity
vascular coil, several of which are available commercially. One of
these uses a phased-array coil for the legs and a quadrature coil
for the pelvis (Peripheral Vascular Coil 675GE, IGC-Medical
Advances, Milwaukee, WI). Both the quadrature and phased-array
designs increase SNR compared with the body coil and permit
scanning at higher resolution without sacrificing SNR or scan time.
This added resolution proves especially helpful when attempting to
distinguish severely stenotic vessels from complete occlusions. The
length of the coil allows one to acquire high-SNR images from the
pelvis to the feet without stopping to reposition the patient or
To image the abdominal aorta and include the renal artery
origins, however, we need to use the body coil. The peripheral
vascular coil does not reach above the pelvis, and the large
calibers of the aorta and iliac arteries do not require the extra
SNR afforded by the peripheral vascular coil.
If a dedicated peripheral vascular coil is not available for
imaging the legs, the body coil can also be used to image the
femoral-popliteal station during the first injection bolus, and the
torso phased-array coil used for imaging the tibial-pedal station.
If the coil is rotated to orient its long axis in the craniocaudal
direction, then the field of view can reach 40 to 45 cm in the
The ankles are partially plantarflexed and elevated with
approximately 4 inches of padding to bring the lower leg vessels
parallel to the imaging table. These maneuvers decrease the number
of slices needed for MRA coverage of the lower leg and midfoot
Scout series prescription
Because aorto-bifemoral or aorta-iliac bypass procedures
sometimes involve reimplantation of the renal arteries, we attempt
to image the aorta from the renal artery origins to the deep
plantar arches of the feet. Distally, visualizing the plantar
arches is important because delivery of blood to these arches is
essential for adequate perfusion of the forefeet and toes.
By starting the scan set-up at the feet, we can ensure that the
deep plantar arches will be included in all examinations.
Standardized fields of view and coil placement help to simplify the
sometimes tedious set-up of the multiple imaging stations. The
inferior margin of the vascular coil is positioned at the MTP
joints of the feet in order to include the deep plantar arches.
A mark placed on our coil at a level 22.5 cm cephalad to its
inferior margin is used to "landmark," or "zero," the position of
the imaging table. Therefore, a 45-cm field of view coronal slab
centered at this zero level will reach from the deep plantar arch
to the popliteal arteries in most patients. The femoral-popliteal
and aorto-iliac station 3D slabs, which are also imaged with 45-cm
FOV, will be centered 400 and 800 cm cephalad to the first landmark
to insure that the three slabs each overlap by 5 cm (Figure 1). The
use of these standardized imaging stations will ensure easy set-up
for imaging up to the renal arteries in all but the tallest
A coronal T1-weighted spin-echo scout series of the abdomen and
pelvis is performed to obtain a black-blood image of the abdominal
aorta. These images will be needed to align the timing test
sequence or bolus-tracking scan through the often-tortuous aorta.
Axial bright-blood, gradient-echo scouts are then obtained through
all three imaging stations, and the anterior and posterior
locations of the pertinent arteries determined.
Contrast bolus timing
We use a sagittal or axial multiphase, spoiled gradient-echo
timing test with a gadolinium mini-bolus to determine circulation
time from the antecubital vein to the distal abdominal aorta. A 1
mL bolus of gadolinium is injected at 1 mL/sec, followed by 20 mL
of saline flush at the same rate. MR-fluoroscopy techniques, now
commercially available, can also be used to provide accurate,
real-time visualization of the arrival of the contrast bolus in the
For each 3D coronal slab, 30 to 36 slices are prescribed with a
maximum thickness of 3.0 mm. Zero-fill interpolation is applied in
the slice dimension to create a second stack of 3.0 mm thick
slices, offset from the original stack by 1.5 mm in the slice
dimension. This interpolation effectively increases the resolution
of MIP images reconstructed in a perpendicular plane. Scan matrix
is 256 x 160 in the aorto-iliac and femoral-popliteal stations and
256 x 192 at the tibial-pedal station. Zero-fill interpolation is
also applied in the matrix dimension to double the effective
in-plane resolution of the source images, with no added time and
only a small loss of SNR. Field of view is 45 * 36 cm for the two
upper stations and 45 * 27 cm for the lower station. Receiver
bandwidth is set to 62 kHz, which strikes a good balance between
imaging speed and SNR.
At the aorto-iliac and the femoral-popliteal stations, mask
precontrast series are obtained and reviewed to confirm appropriate
slab prescription. The mask series of the second station will be
used during postprocessing for image subtraction. We instruct the
patient to remain motionless to decrease subtraction artifacts.
Fortunately, the securely fastened peripheral vascular coil serves
the added function of immobilizing the patient's legs. The first
20-mL gadolinium bolus is injected at 1 mL/sec, followed by a 20 mL
saline flush at 1 mL/sec. At a predetermined delay time
(circulation time minus 5 seconds) the patient is instructed to
hold his or her breath. The arterial phase is scanned, followed
immediately by automated table stepping to the second station.
Immediately, the femoral-popliteal station is scanned; breath
holding is not necessary at this point. In case there is delayed
filling of aneurysmal or collateral vessels, these scans are
repeated automatically at both imaging stations to acquire venous
An interval of 10 minutes between the first and second contrast
injections is imposed to allow for clearing of the gadolinium bolus
from the vascular bed and muscles before imaging the final
tibial-pedal station. During this waiting period the multiphase
timing test is repeated in the coronal plane, centered at the
popliteal arteries, using a 2-mL mini bolus for the smaller
Since no breath-holding is required in the lower leg station,
scan speed is not as important and can be sacrificed for the
purpose of increased spatial resolution. We choose to prescribe an
increased number of thinner slices than we used in the more
proximal stations, in order to increase spatial resolution of the
smaller distal vessels. Subtraction of a mask scan series from the
arterial phase scan will eliminate any persistent venous or
soft-tissue enhancement. For the single station imaging of the
lower legs, the second 20-mL gadolinium bolus can be injected at 2
mL/sec, for a higher peak vascular contrast level.
If an automated stepping table MRA is not available, then a
three-station, three-injection protocol can be performed with a few
small changes to this protocol. The dose for each contrast bolus
will need to be decreased to 0.1 mmol/L for each of the three bolus
injections and delay periods will have to be planned between each
station for contrast clearance.
MRA postprocessing software is now more user friendly and
faster. From a 3D MRA data set, our technologists routinely perform
image set subtraction and the creation of maximum intensity
projection (MIP) images in approximately 10 minutes.
Subtraction of a precontrast mask series from the arterial phase
source images improves visualization of small caliber vessels
(Figure 3) and can also reduce the dose of contrast needed to
obtain good arterial SNR.
If the patient's breath-holding is consistent from the mask series
to the arterial phase images, mask subtraction may be helpful in
the aorto-iliac station to eliminate soft tissues, any bright bowel
contents, and aliasing (phase wraparound artifact).
In the femoral-popliteal and tibial-pedal stations, we routinely
perform mask subtraction in all cases to decrease soft-tissue and
venous signal. If a phased-array coil is used in these lower
stations, then subtraction becomes essential for reduction of high
near-field signal at the skin and subcutaneous fat, where fat
saturation prepulses are often not completely effective. This
bright signal can greatly decrease image quality by obscuring
underlying vessels on the reconstructed MIP images.
MIP reconstructions have become the most accepted mode for
presentation of 3D MRA data sets. With the routine use of mask
subtraction and subvolume coronal MIP reconstructions, there is
little background signal on our routine MIP images. We have found
that there is virtually no need for tedious manual tracing or
cutting of unwanted tissues from the 3D MRA datasets since image
subtraction is so effective at removing background signal.
Our technologists now perform a standardized postprocessing
routine at the scanner console, almost entirely obviating the need
for physician postprocessing. For each arterial phase 3D data set,
whole-volume rotating MIPs are reconstructed at 15š intervals
around a 180š arc, and a set of subvolume coronal MIP
reconstructions is prescribed with 10-mm slice thickness,
overlapping at 5-mm intervals. These whole-volume and overlapping
subvolume MIPs are viewed on a workstation in a cine-mode to take
advantage of the 3D properties of the data set. The use of a 10-mm
subvolume MIP slab thickness removes any overlapping soft tissues
while being thick enough to display moderately long segments of
vessels (Figures 4 and 5).
Because any overlapping structure with moderate or high signal
intensity can result in a false narrowing of an artery on a MIP
reconstruction, the radiologist should be acutely aware of this
possibility for creation of a pseudostenosis. As stated above,
subtraction removes many of these overlapping artifacts, but
examination of subvolume MIPs and source images is necessary when a
stenosis or occlusion is suspected in order to exclude such
In addition, metal prostheses, stents, and surgical clips can
create signal void artifacts that can cause false stenoses on MIP
images. Review of previous angiograms, plain X-rays, and the MRI
source images should reveal these metal devices as the sources of
the signal loss.
Recent advances in commercially available MR hardware and
software have made peripheral MRA faster and more automated, but
the many variables that must be determined by the operator can make
setting up the procedure a complicated task. When one takes into
account the anatomic and disease variability from patient to
patient, it becomes obvious that a single, robust strategy is
needed if the exam is to run efficiently. The MRA protocol
presented here uses generous fields of view and relatively large
contrast doses, in order to provide allowances for many of these
variables without sacrificing image quality. With practice a
complete MRA study of the abdominal aorta and lower extremities can
be performed in approximately 30 minutes using this protocol, with
an additional ten minutes allowed for postprocessing time.
Thanks to Joel Brower, RT, RMR for assistance with creating and
optimizing this protocol.