Dr. Close is a fellow in vascular and interventional
radiology in the Department of Radiology at the University of
Michigan Medical Center, Ann Arbor, MI.
Dr. Cho is the William Martel Professor of Radiology in the
Department of Radiology at the University of Michigan Medical
School, Ann Arbor, MI.
Magnetic resonance angiography (MRA) was first shown to be
clinically feasible in 1985.
Since that time, MRA has become useful both from a clinical and a
research standpoint in the assessment of the vascular system and
its pathologies. MR imaging offers several advantages over computed
tomography (CT) and conventional angiography. It is inexpensive
relative to invasive angiography and allows cross-sectional imaging
without the risks of ionizing radiation, iodinated contrast
material, or arterial puncture.
MRA facilitates planning for interventions and increases the
number of interventions, such as percutaneous transluminal
angioplasty (PTA) and stent placement. From an interventional
radiologist's standpoint, the initial use of MRA as a screening
modality for vascular disease should increase referrals for
vascular interventional procedures. By knowing in advance the
location and extent of vascular pathology, MRA should help the
interventionalist perform procedures more efficiently. Furthermore,
a preprocedure MRA allows the diagnostic angiogram to be tailored
to a more limited study that reduces the iodinated contrast agent
load and the degree of catheter manipulation required.
At the University of Michigan Medical Center, a 1.5-T
superconducting magnet with a body coil and Software Release 8.3
(Signa Horizon LX; GE Medical Systems, Milwaukee, WI) is used for
MR angiography (figure 1). The use of postprocessing techniques to
improve the image quality of three-dimensional (3D) gadolinium
(Gd)-enhanced MR angiography is done on a computer workstation
MR angiography requires several basic pulse sequences for
imaging the vascular system and associated lesions. These include
T1-weighted spin-echo imaging ("black blood" imaging),
time-of-flight (TOF) imaging ("bright blood" imaging),
phase-contrast (PC) imaging ("bright blood" imaging) and
gadolinium-enhanced 3D MR angiography ("bright blood" imaging). The
T1-weighted spin-echo imaging provides the anatomic information for
organs and large vessels including the aorta, vena cava, and portal
These sequences have limited spatial resolution with complex signal
patterns from the varying velocity and turbulence of flowing blood,
which may be difficult to interpret (figure 3).
The "bright blood" techniques, because of their MRA appearance,
are especially useful to differentiate slow flow from thrombus,
which both appear as high signal on T1- and T2-weighted images. The
time-of-flight imaging sequences rely on the inflow of unsaturated
blood into the imaging volume, so that the vessel appears brighter
than the stationary saturated tissue.
Postprocessing of two-dimensional (2D) or 3D gradient-echo (GRE)
sequences leads to a projective format similar to a conventional
Usually, post-processing is performed using a maximum-intensity
projection (MIP) algorithm. With MIP, the brightest pixels in a
certain direction are used to create the projection image. These
MIP projections are then created to visualize vessels in a 3D
format. The drawback of TOF imaging is in the evaluation of
diseased vessels in which the MR signal is degraded by slow,
turbulent, or in-plane flow. Maximum-intensity projection also has
its drawbacks, which include evaluation of the edges of blood
vessels and small vessels with slow flow. Because these vessels may
have slow flow, they have poor flow contrast and may be obscured by
overlap with brighter stationary tissue. This can then cause false
Phase-contrast imaging is another "bright blood" technique,
which is also flow-sensitive. Slow flow is well depicted in this
It is more sensitive to slow flow than TOF.
Phase-contrast imaging can also be used to determine the direction
of flow. Blood in the direction of the flow encoding axis is
bright, stationary tissue is gray, and flow in the opposite
direction is dark.
With hemodynamically significant stenosis, spin dephasing
(signal dropout) is identified on 3D phase-contrast images (figure
The newest clinically accepted technique is 3D Gd-enhanced MRA.
This largely eliminates motion and flow artifacts because it does
not depend on blood inflow. By imaging during the infusion of Gd,
there is selective visualization of arteries without excessive
venous or background tissue enhancement.
It is considered the state-of-the-art technique for MRI evaluation
of the abdominal vasculature.
Usually, 3D contrast-enhanced MRA uses Gd, which is a paramagnetic
contrast agent. Gd-MRA has been shown to be accurate for evaluation
of the aorta and its major branches.
The principle behind the use of Gd lies in the fact that the
contrast agent makes the T1 relaxation time of blood much shorter
than the surrounding tissues when Gd is administered rapidly.
3D Gd-enhanced MRA allows extra signal-to-noise ratio over other
bright blood techniques such that rapid 3D volume imaging can image
entire vascular structures, such as the portal vein, easily in a
30-second breath-hold. Other advantages of this technique include
the ability to image other organ parenchyma, such as the kidney and
liver. In addition, Gd exhibits no clinically detectable
nephrotoxicity and thus may be used safely in cases of renal
It has a low incidence of allergic reactions
and therefore can be used in patients with a history of allergic
reaction to iodinated contrast.
The technique of 3D contrast-enhanced MRA consists of imaging in
different phases of the bolus. For arterial phase imaging, bolus
timing is most critical and may be performed by a variety of
methods, including: 1) the operator can inject contrast and then
scan 10 to 15 seconds later (the empirical-timing method); 2) the
operator can use an automated sequence such as SmartPrep (GE
Medical Systems, Milwaukee, WI) (figure 5), Bolustrack (Philips
Medical Systems, Shelton, Conn), or Care Bolus (Siemens Medical
Systems, Iselin, NJ); and
3) MR fluoroscopy may be used after a test bolus.
The use of an MR system is contraindicated for patients with
electrically, magnetically, or mechanically activated implants,
such as cardiac pacemakers, because the magnetic and
electromagnetic fields produced by the MR system may interfere with
the operation of such devices. The magnetic field of the MR system
can cause a ferrous implant such as an aneurysm clip, surgical
clip, or a cochlear implant to move and be displaced. Therefore,
before the MR examination, the procedure should be explained in
detail to the patient. The patient must be informed about the
potential complications that can occur. In particular, the patient
should be screened for any metallic implants. Table 1 presents a
patient questionnaire used at our institution that must be
completed by the patient or referring physician prior to the
A mild sedative is given prior to the examination to relieve
anxiety secondary to claustrophobia. The oral dose of 5 or 10 mg
diazepam (Valium, Roche Pharmaceuticals, Roche Products, Inc.,
Puerto Rico) is usually effective when given approximately a
half-hour prior to scanning.
Clinical applications of MR angiography
MR angiography should be used in the evaluation of the vascular
anatomy and suspected vascular lesions when conventional
angiography is contraindicated. Such contraindications include
hypersensitivity to iodinated contrast material and renal
insufficiency (serum creatinine level >2.0 mg/dL). Even in
patients without such contraindications, MR angiography is used in
the evaluation of the vascular anatomy in a variety of disorders
and suspected arterial lesions. Table 2 lists clinical applications
for the use of MR angiography.
We have found that MR angiography provides excellent assessment
of the thoracic aorta and related lesions, including aneurysm,
dissection, aortitis, atherosclerosis, penetrating ulcers,
coarctation, and occlusive disease of the great vessels. Pulse
sequences include a coronal single-shot fast spin echo (FSE), axial
2D TOF, and 3D contrast MR acquisition in the coronal or sagittal
MR angiography is a very useful diagnostic examination in
patients with suspected dissection (figure 6). MR angiography can
establish the presence of aortic dissection and define the origin
and extent of the dissection. Multiplanar reformations of the 3D
data set can help elucidate whether branch vessels originate from
the true or false lumen. The true lumen is usually identified by
the relative increase in signal intensity caused by the Gd from
faster-flowing blood. The true lumen is also usually identified by
being smaller and more oval, and by being parallel to the inner
curve of the aorta. Patency of the false lumen and entry and
re-entry tears can usually be identified.
However, some physicians still may choose to use helical CT to
evaluate aortic dissection because it is faster, more widely
available, and particularly useful for imaging unstable patients
connected to multiple mechanical devices.
For the evaluation of thoracic aortic aneurysms, the size and
extent of the lumen and its relationship to aortic side branches is
easily obtained. The 3D quality of the technique allows the data
set to be reformatted in any plane.
Postoperatively, 3D contrast MRA is helpful in the evaluation of
aortic grafts for the surgical treatment of aneurysm repair. All of
these grafts are now MR-compatible.
The development of aneurysms or stenoses at graft anastomoses is
well depicted using the multiplanar reformatting capability of 3D
After endovascular interventions, such as fenestration and
stent-graft placement, MR angiography is useful in assessing the
blood flow in the false and true lumen and the status of organ
perfusion. In chronic aortic dissection, MR angiography is used to
follow the progress of the disease with measurement of the size of
We recently examined a patient with multiple penetrating
atherosclerotic ulcers of the descending thoracic aorta and a 5-cm
aneurysm using 3D contrast MR angiography for back pain. She
subsequently underwent percutaneous transfemoral digital
subtraction aortography in the left anterior oblique and lateral
projections, and intravascular ultrasound (IVUS) as preparation for
stent-graft placement. All three images showed penetrating ulcers
well (figure 7).
Pulmonary 3D contrast MR angiography using multiple-bolus
infusions can image the pulmonary arterial and venous anatomy
(figure 8). Schoenberg et al
have shown that segmental arteries or veins could reliably be
assessed for patency. Others have shown that lobar or segmental
arteries were visualized in 100% of cases and subsegmental
pulmonary arteries in 81% of cases.
There are many clinical applications, including suspected
thromboemboli in the pulmonary veins and preoperative resection of
pulmonary arteriovenous malformation or carcinoma.
Radionuclide scanning (V/Q scanning) and spiral CT are usually
the initial imaging examinations in patients with suspected
pulmonary embolism (PE). Pulmonary angiography continues to be the
gold standard for the diagnosis of PE. A recent study from our
institution showed that 3D contrast MR angiography is a safe and
reliable technique for the detection of PE with a diagnostic
accuracy of 75% to 100% sensitivity and 95% to 100% specificity.
Presently, the resolution of MRA is not good enough to replace
helical CT or contrast angiography in patients suspected of having
PE. However, MRA is useful as an alternative imaging technique when
the use of iodinated contrast material or radiation is
has shown that Gd-enhanced MR abdominal aortography had an 88%
sensitivity and a 97% specificity for detection of stenoses or
occlusions, as well as a 100% sensitivity and a 100% specificity
for detection of aortic or iliac artery aneurysms (figure 9).
Abdominal aortic aneurysm dimensions are well evaluated with the
MIP images from 3D MRA. The large field of view allows easy
visualization of the extent of the aneurysm, especially into the
iliac arteries. Assessment in the coronal plane illustrates the
relationship to the renal arteries and important venous and renal
anomalies. Post-contrast T1-weighted spin-echo or gradient-echo
image analysis is crucial in order to assess regions of thrombosis
and the aortic wall. For instance, a thick circumferential rind of
enhancement of the aortic wall or periaortic tissues may indicate
an inflammatory process, such as retroperitoneal fibrosis or an
inflammatory aneurysm. All of these findings are taken into
consideration with either open surgical repair or the newer
endovascular management of abdominal aortic aneurysms using aortic
Before and after an iliac artery intervention, MRA may be used
to detect significant (>50%) stenoses and evaluate patency.
Percutaneous transluminal angioplasty has been shown to be a widely
recognized therapeutic modality for treatment of iliac narrowing or
Because the iliac arteries are difficult or impossible to image by
color duplex ultrasound, evaluation is performed by other
noninvasive vascular laboratory tests, conventional angiography, or
MRA. Johnston et al
reported that MRA before PTA of the iliac arteries was 95%
sensitive and 97% specific in assessing the site and degree of
stenosis and in determining balloon size. Unfortunately, MRA
usually cannot be employed as a post-stent imaging modality, as
most stents create MR imaging artifacts, precluding adequate
evaluation of the lumen. The most MR-compatible stents are composed
of nitinol, which is totally nonmagnetic.
At our institution, our complete imaging sequence to evaluate
for renal artery stenosis consists of a T1-weighted spin-echo
localizer, a single-shot fast spin-echo localizer, an axial 2D
T2-weighted sequence with fat saturation (for characterization of
masses), a 3D Gd-enhanced MRA, and a 3D phase-contrast sequence.
Postprocessing is performed by the radiologist.
Applications for MRA of the renovascular system include
evaluating the extent of tumor thrombus from renal cell carcinoma
in the renal vein and inferior vena cava, and whether it extends to
the hepatic vein or above the diaphragm (figure 10). MR imaging has
been shown to have 100% sensitivity for detection of tumor thrombus
beyond the distal renal vein.
The renal vein and inferior vena cava can be evaluated by 3D
Gd-enhanced MRA during the venous and equilibrium phases (figure
Other applications include the evaluation for suspected renal
artery stenosis. This clinical scenario affects a small percentage
of patients with hypertension. It is usually a manifestation of
systemic atherosclerosis with involvement of the cerebral,
coronary, and peripheral vessels, but atherosclerotic renal artery
stenosis is isolated in 15% to 20% of cases.
It is important to detect because PTA/stenting or surgical
revascularization can reduce the blood pressure. This may also help
to reduce the loss of renal parenchyma and subsequent progression
to renal failure. The MRA techniques of 2D and 3D TOF and PC
imaging have shown benefit in diagnosing renal artery stenosis.
If loss of signal (dephasing) on the 3D PC acquisitions is
identified in the region of renal stenosis, the narrowing is
considered hemodynamically significant and the dephasing represents
turbulent flow (figure 4). Severe spin dephasing usually
corresponds to a stenosis of at least 75%.
Atherosclerotic narrowing is usually present in the aorta and
typically narrows the ostium or the proximal 1 to 2 cm of the renal
arteries. Atherosclerosis is rarely isolated to the distal renal
artery or its branches. Other findings associated with a
hemodynamically significant stenosis include poststenotic
dilatation as well as functional changes in the renal parenchyma,
such as loss of corticomedullary differentiation and reduction in
parenchymal thickness, delayed renal enhancement, and asymmetric
concentration of Gd in the collecting systems. At our institution,
patients with suspected renal artery stenosis are referred to MRA.
In most instances, all the necessary vascular information is
obtained by MRA, and therefore a "tailored" angiogram is performed
with either carbon dioxide or Gd before stenting to reduce the risk
of contrast nephropathy (figure 12).
Techniques used to evaluate renal artery stenosis have included
TOF, PC, and 3D contrast-enhanced MRA.
3D tilted optimized nonsaturating excitation (TONE) sequence
evaluation of the renal arteries showed a detection of severe
stenosis (>= 60%) with a sensitivity of 100% and a specificity
MRA performed with a high dose of Gd has been shown in other
studies to detect renal artery stenosis with sensitivities and
specificities exceeding 90%.
A current limitation of renal MRA is in the detection of
supernumerary renal arteries,
which may occur in an estimated 30% of patients.
These aberrant renal arteries usually arise from the aorta but,
rarely, may originate from a common iliac artery. Accessory renal
arteries are known to be especially common in patients with a
horseshoe kidney. The spatial resolution of the 3D TOF techniques
is too low to resolve all accessory arteries,
but this limitation will probably disappear as resolution improves.
The literature has shown that MRA has been successful in depicting
accessory renal arteries in 86% to 100% of patients with a normal
aorta or aortic occlusive disease.
In addition, renal MRA depiction of the distal portions of the
renal arteries is a known limitation of the technique.
Fibromuscular dysplasia (FMD) is the second most common cause of
renal artery stenosis. It affects medium-sized and small arteries
and usually is diagnosed in the renal and carotid arteries. Other
less common locations include the subclavian, axillary, mesenteric,
hepatic, splenic, and iliac arteries. It is classically found in
young (<40 years of age) female patients, usually bilaterally
with the medial fibroplasia subtype manifesting as a
'string-of-beads' appearance in the distal two-thirds of the main
renal artery, with web-like stenoses interspersed with small
fusiform or saccular aneurysms. Since MRA cannot always demonstrate
abnormalities in the distal main renal arteries, the accuracy of
MRA for diagnosing FMD is not established.
Therefore, we currently recommend angiography in all patients with
suspected FMD because percutaneous renal catheterization is needed
for both diagnosis and intervention.
Other applications of renal MRA include planning for renal
revascularization or for repair of abdominal aortic aneurysm,
assessing renal bypass grafts (including anastomoses to renal
transplants), and evaluating renal malignancy and its relationship
to vascular structures.
3D contrast MR angiography has sufficient resolution to evaluate
the origins of the splanchnic vessels. Because reformations may be
made from the 3D data set, visceral anatomy may be evaluated, even
if it is complex in nature. MRA of the mesenteric circulation is
usually only applied to questions of chronic mesenteric ischemia
(figure 13), as acute mesenteric ischemia is usually considered a
surgical emergency. Other clinical applications include the
evaluation of visceral artery aneurysms, tumor encasement, and pre-
and post-liver transplantation.
The MR evaluation of chronic mesenteric ischemia usually
consists of a data set that is collected before, during, and after
the administration of intravenous Gd. The images obtained during
the administration of intravenous Gd are excellent for evaluating
the proximal portions of the celiac axis, the superior mesenteric
artery, and the inferior mesenteric artery, as well as their
branches. The delayed data set can be used to evaluate patency of
the portal vein and hepatic veins. In order to evaluate for more
distal, segmental occlusions, the small and large bowel should be
evaluated for the normal enhancement characteristics following
Early diagnosis of mesenteric ischemia is important because
surgical revascularization or PTA and/or stenting offers an
excellent treatment option. Contrast-enhanced 3D MRA can clearly
evaluate for stenosis of the celiac trunk and the superior
mesenteric artery, with sensitivities of 100% and specificities of
92% to 95%.
MRA of the portal vein is complicated by the fact that flow may
not uncommonly be multidirectional. As a result of this complexity,
sequences must image in both coronal and axial planes.
For Gd-MRA of the portal vein, the operator must give a triple dose
of Gd (0.3 mmol/kg) because of dilution and extraction of contrast
that occurs en route to the portal circulation, at 2 cc/sec.
Phase-contrast MRA can determine the direction of portal venous
blood flow. Advantages over the sonographic evaluation of the
portal vein include its large field of view.
MRA applications to the hepatic/ portal venous system include
the pre-liver transplant patient. It is crucial to know in advance
of a liver transplant if there is celiac stenosis, as this can lead
to transplant ischemia. MRA is also useful in planning the liver
transplant surgery and in excluding certain patients from the
transplant list, such as those with portal venous thrombosis.
Before placing a patient on the liver transplant list, it is
necessary to demonstrate a patent portal vein. If ultrasound is
unable to adequately visualize the portal vein, 3D contrast MRA is
an excellent option.
MRA is also useful after a liver transplant. In this case, the
surgeon or interventionalist is frequently searching for the
not-uncommon complication of hepatic artery stenosis. The classic
workup initially consists of ultrasound. This is frequently
followed by MRA, since MRA can easily visualize hepatic artery
thrombosis, portal vein stenosis or thrombosis, or IVC stenosis.
Gd-MRA can depict these accurately, with accuracy equivalent to a
Infarct, abscess, or biloma are also within the differential of the
failing transplant and can be seen with the source images of the MR
Other applications of MRA in the liver include the pre-TIPS
(transjugular intrahepatic portosystemic shunt) evaluation in order
to determine the patency of the hepatic veins and the portal vein
(figure 14). MR angiography can identify spontaneous splenorenal
shunts as well as assess the degree of varix formation.
MR angiography has an advantage over ultrasound in the
evaluation of surgical shunts, especially in evaluation of the
distal splenorenal shunt, because bowel gas tends to obscure
Tumor encasement with narrowing of the splenic vein, superior
mesenteric vein, or portal vein from pancreatic carcinoma has been
assessed with MRA. This type of vascular narrowing has been caused
by cholangiocarcinoma as well. However, abnormalities of the
mesenteric veins are nonspecific, because they can also be caused
by inflammatory etiologies.
Hepatic vein occlusion/Budd-Chiari Syndrome is well evaluated by
MRA, usually by TOF and GRE sequences.
Budd-Chiari Syndrome is characterized by heterogeneous enhancement
of the liver with a feathery appearance to hepatic veins (figure
Conventional contrast-enhanced arteriography has been the
standard method of evaluating the peripheral vascular system before
interventional therapy. However, this method of imaging is
associated with certain risks, such as contrast reaction in almost
5% of patients,
as well as other minor and major complications well known to
interventionalists. Additionally, conventional arteriography may
not demonstrate suitable target vessels for surgical bypass in up
to 70% of patients with severe peripheral vascular occlusive
In such cases, MRA of the lower legs can be performed and
occasionally will demonstrate suitable target vessels for bypass
surgery. This scenerio in which vessels are identified by MRA, but
not by conventional arteriography, occurs most often in the
Dynamic contrast-enhanced MR angiography has been shown to have
100% sensitivity and 69% specificity for distinction of vessels in
the lower extremities with >50% stenosis from normal or mildly
In this study, results were evaluated both above and below the
knee, demonstrating the diagnostic potential for infrapopliteal
diagnosis. The authors did acknowledge, however, that results were
somewhat better above than below the knee, possibly due to a larger
vessel size and higher intravascular signal above the knee. A
serious limitation of MRA is in the assessment of vascular bypass
grafts with adjacent surgical clips. This may lead to clip
artifacts that can obscure clinically important underlying disease.
In a 1993 study, Owen
concluded, "two-dimensional TOF MR angiography is an excellent
technique for evaluation of patients with peripheral vascular
disease." Their group concluded that it is a viable alternative to
conventional arteriography for the preoperative assessment of
peripheral vascular occlusive disease.
Now that 3D contrast MRA has been developed, this can augment 2D
TOF techniques in certain areas. The advantages of 3D contrast MRA
over 2D TOF methods include more accurate determination of the
length of an occluded segment and better evaluation of the pelvic
arterial system (due to 3D-contrast MRA being flow-insensitive)
One of the disadvantages of MRA is the fact that MRA tends to
exaggerate stenoses in the peripheral vascular system.
MR angiography has had exponential growth in recent years and
has changed the workup of vascular disease. It is commonly used for
the preoperative evaluation of a variety of disorders and for
follow-up studies after surgical or endovascular interventions.
Because of the safety of gadolinium contrast and the lack of
ionizing radiation, MRA will be used increasingly as a screening