AJR 2002; 179:115-117
© American Roentgen Ray Society
Contrast-Enhanced MR Angiography of the Aorta and Lower Extremities with Routine Inclusion of the Feet
Christopher J. Konkus1,
Julianna M. Czum and
John T. Jacobacci
1 All authors: The Heart and Vascular Institute, 111 Madison Ave., 4th Floor,
Morristown, NJ 07960.
Received November 13, 2001;
accepted after revision January 2, 2002.
J. M. Czum recieves scientific support from Philips Medical Systems, Best,
The Netherlands.
Address correspondence to J. M. Czum.
Introduction
Patent vasculature in the pedal arch is associated with distal bypass
patency [1]. Therefore,
visualization of the paramalleolar vasculature may be a favorable indication
of durable patency. Such visualization is an important component of strategic
angiography performed before a distal bypass graft, influencing patient
selection and prognosis [2].
Since its introduction, three-dimensional (3D) moving-table contrast-enhanced
MR angiography has revolutionized peripheral vascular imaging
[3]. However, the overall field
of view of a multistation contrast-enhanced MR angiogram typically extends
from the abdominal aorta to the ankles
[4]. Dedicated imaging of the
arterial vasculature of the distal lower extremity has been performed using
both two-dimensional (2D) and 3D contrast-enhanced MR angiography techniques
[5,
6]. Incorporation of distal
arterial imaging into aortobifemoral moving-table MR angiography could provide
comprehensive depiction of the lower extremity arteries seamlessly with a
single infusion of contrast medium. We designed a modification of the standard
three-station contrast-enhanced MR angiography to provide a four-station study
that includes the feet for comprehensive clinical imaging of the lower
extremity vasculature.
Subjects and Methods
Equipment
All imaging is performed on a 1.5-T MR scanner (Gyroscan Intera CV, version
7.1.2; Philips Medical Systems, Best, The Netherlands) equipped with
high-performance 30 mT/m gradients, a maximal slew rate of 150 mT/m per msec,
and a quadrature body coil. An MR-compatible power injector (Spectris; Medrad,
Indianola, PA) is used for all IV contrast medium and saline
administration.
Patient Positioning and Contrast Medium Administration
We have the patient lie on the table positioned so that the feet will enter
the scanner first. The calves and feet are placed in a lower extremity
immobilizing device and are secured with straps
(Fig. 1). All patients receive
40 mL of gadodiamide (Omniscan; Nycomed-Amerhsam, Princeton, NJ), administered
IV via a 22-gauge IV catheter, in the antecubital fossa. One mL of gadolinium
chelate contrast medium injected IV at 1.0 mL/sec is used for the timing run.
For the 3D MR angiography sequence, 15 mL of contrast medium is administered
at 1.0 mL/sec, followed by 24 mL of contrast medium administered at 0.3
mL/sec, and then by a 15 mL saline flush also delivered at 0.3 mL/sec.
Patients are instructed to first hyperventilate and then breath-hold during
both the unenhanced and contrast-enhanced 3D scanning of the
abdominal—pelvic station.
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Fig. 1. —Photograph shows healthy female volunteer demonstrating
position used in modified three-dimensional MR angiographic protocol. Patient
is supine with legs placed in leg-immobilizing device and straps secured
across top of feet. Two adjustable cushioned supports are positioned under
patient's legs, one placed under distal thighs just above popliteal fossae to
avoid vascular compression and other placed under lower calves below bellies
of gastrocnemius muscles but above malleoli.
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Imaging Sequences
We first perform axial 2D time-of-flight imaging (using a moving table) of
the four stations—the abdomen—pelvis, thighs, calves, and
feet—moving from the feet to the lower chest. The acquisition of each
station takes 45 sec, for a total imaging time of 3 min. The 3D volume for the
subsequent unenhanced and contrast-enhanced multistation sequence is
prescribed on the sagittal and coronal maximum-intensity-projection images
from this time-of-flight localizer sequence
(Fig. 2A).
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Fig. 2A. —79-year-old man with intermittent claudication.
Three-dimensional (3D) volume image of all four stations
(abdomen—pelvis, thighs, calves, and feet) is made from two-dimensional
multistation time-of-flight localizer sagittal (shown) and coronal (not shown)
images. Boxed areas denote boundaries and angulation of each station. For most
caudal station (feet), 3D volume is applied parallel to long axis (soles) of
feet by manually toggling stack alignment parameter to "off"
position for that station only and then adjusting corresponding 3D volume
box.
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Using 1 mL of gadolinium chelate contrast medium injected IV at 1.0 mL/sec,
we then obtain an axial 2D single-slice multiphase gradient-echo image at the
level of the abdominal aorta to determine the arrival time for the contrast
bolus. The image acquisition rate is approximately one image per second.
The 3D moving-table multistation fast field-echo imaging sequence is
designed to image the patient from the abdomen to the feet. Images of the four
imaging stations, with a 10- to 30-mm overlap between stations, are obtained
both before and during the IV administration of the contrast bolus. The 3D
volume prescription for the first three stations is coronal. For the feet
station, the 3D volume is oblique to parallel the soles of the feet and is
achieved by toggling the stack alignment key to the "off" position
(Fig. 2B). The unenhanced run
is performed in the feet-to-abdomen direction, and then the contrast-enhanced
run is performed in the abdomen-to-feet direction. The acquisition of each
station takes approximately 35 sec, for a total imaging time of approximately
4 min 30 sec. The total examination time is approximately 20 min.
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Fig. 2B. —79-year-old man with intermittent claudication. Composite
coronal maximum-intensity-projection image depicts four-station MR angiogram
of abdominal aorta and lower extremity vasculature, including feet.
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Imaging parameters for both the unenhanced and contrast-enhanced 3D MR
angiography sequences are TR/TE, 4.8/1; flip angle, 35°; acquisition
matrix, 304 x 280 (reconstructed to 512 x 512); 32 slices
(interpolated to 64 slices); slice thickness, 3.0-3.6 mm (interpolated to
1.5-1.8 mm); and rectangular field of view, 380-420 mm x 285-357 mm
(75-85%). The k-space profile order is reverse-centric for the
abdominal—pelvic station and centric for the thighs, calves, and feet
stations. For the contrast-enhanced run, the scanning delay time is calculated
by subtracting one half the single-station acquisition time from the actual
arrival time of the contrast bolus.
Postprocessing Procedure
The mask unenhanced 3D sequence is automatically subtracted from the 3D
contrast-enhanced MR angiography sequence when the autosubtraction feature is
selected during image reconstruction. All postprocessing is performed on the
operator console of the scanner. A set of 12 maximum-intensity-projection
images obtained in 15° increments rotated on a superior-to-inferior axis
is generated for each of the following regions: abdomen—pelvis, right
thigh, left thigh, right calf, left calf, right foot, and left foot.
Additional sets of 12 maximum-intensity-projection images, also in 15°
increments but rotated on an anterior-to-posterior axis, are generated for
each foot (Fig. 2C).
Reformatted MR images orthogonal to the subtracted coronal source images
(axial and sagittal) are also obtained to help to assess the dimensions of the
abdominal aorta and the patency of the visceral vessel origins.
Discussion
Over a 5-month period, we examined 45 patients using this consistently
reproducible method. No patients have required re-imaging because of technical
failure or images that were not diagnostic quality. The ability to make an
adjustment in the plane of the 3D imaging volume for the feet relative to that
of the other stations is the fundamental key to the success of this technique.
Another factor is the biphasic injection of contrast medium, which permits
preferential arterial enhancement over an extended period of imaging.
Certainly, similar contrast-injection techniques and modifications of the
scanning parameters may also provide these results and should be explored.
To date, one patient has been imaged under a protocol in which the 3D
moving-table contrast-enhanced MR angiography using the quadrature body coil
is followed immediately by a separate 3D contrast-enhanced MR angiography of
the feet using a small surface coil (C1 coil; Philips Medical Systems)
(Fig. 3). The actual and
interpolated voxel dimensions achieved with this method are 1 x 1
x 2 mm and 0.6 x 0.6 x 1.0 mm, respectively. The trade-off
of this refinement is that repositioning the patient delays transition to the
separate 3D imaging of the feet. In the future, a dedicated full-length
peripheral vascular coil may further improve signal and spatial resolution,
but coil coverage for taller patients or coil coverage for the feet may be
limited.
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Fig. 3. —Six of 12 maximum-intensity-projection images of left foot
obtained in 15° increments in 59-year-old man with intermittent
claudication as separate acquisition (with small surface C1 coil; Philips
Medical Systems, Best, The Netherlands]) immediately after performance of
three-station contrast-enhanced MR angiography (with body coil) from abdomen
to ankles.
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Our methodology can be readily applied on newer MR scanners with the
capability to perform 3D moving-table contrast-enhanced MR angiography and
bolus tracking. Vendor-specific features of software and hardware may require
additional adjustments. With the increased acceptance and use of
contrast-enhanced MR angiography for vascular diagnosis and preoperative
mapping, as well as for postoperative evaluation and surveillance, readily
available refinements to existing technology can only serve to further bolster
MR angiography as an equivalent, if not a superior, alternative to
conventional catheter-based angiography.
Acknowledgments
We thank Jennifer Matyola for modeling the lower extremity immobilization
unit in Figure 1.
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Introduction
Subjects and Methods
