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Archived Issues of Radiology Rounds
MGH Department of Radiology Website
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Renal and Lower Extremity Contrast-Enhanced MR Angiography
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| Note: Updated information available (June 2007) |
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- When the clinical suspicion for renal vascular disease is high, renal/abdominal MR angiography
(MRA) is a sensitive method for detecting stenoses in renal arteries as well as vascular
abnormalities in other abdominal arteries
- When the clinical suspicion for peripheral vascular disease in the lower extremities is high, run-off
MRA is a sensitive method for detecting stenoses and aneurysms in the iliac, femoral, popliteal, and
crural arteries
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In
patients who have a high clinical index of suspicion for vascular
disease, MR angiography (MRA) is the preferred imaging method for
assessing the patency of renal and lower extremity arteries. MRA images
can be easier to interpret than CT angiography (CTA) at times because
calcified plaques may obscure the lumen in some CTA. Artifacts from
calcified plaques are usually minor in MRA images. In addition, MRA has
no exposure to ionizing radiation, the contrast agents have low
nephrotoxicity, and MRA is also non-invasive. However, MRA is not
suitable for patients with pacemakers and may be contraindicated by the
presence of some metallic implants.
The sensitivity of
MRA for detecting stenoses is very high but the specificity is lower.
Therefore, the positive predictive value is high only when the clinical
index of suspicion for vascular disease is high. For this reason, it is
good medical practice to arrange a thorough clinical examination by a
physician knowledgeable in the field of vascular medicine before a
patient is sent for an MRA in order to find evidence that the likely
cause of symptoms is vascular disease.
Renal MRA
Renal vascular disease is characterized by symptoms of hypertension
that respond poorly to medication, elevated BUN, and absence of
proteinurea or active urine sediment, which may be associated with
evidence of peripheral vascular disease. In addition, some patients may
be recognized when they develop poor renal function in response to ACE
inhibitors. Renal vascular disease may cause the affected kidney to
shrink, which can be determined by an ultrasound scan to detect
asymmetric renal size. If one kidney is >2 cm smaller than the
other, it is likely that the smaller kidney has lost function
permanently and the patient will not be helped by revascularization.
If the clinical index of suspicion for renal vascular disease is high
or moderate, renal/abdominal MRA is warranted. In this examination, a
bolus of gadolinium contrast agent is administered intravenously and
images are acquired in a series of four to six 20 second breath-holds
as the contrast agent flows through the arteries, into the kidneys, and
in the uretal pelvis, and ureters through renal excretion. The total
time for the examination is less than 45 |
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Renal MRA showing healthy vasculature and early renal artery bifurcation.
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minutes, during which time the patient must lie supine and motionless.
The MRA data are post-processed into 3-D maximum intensity projection
(MIP) images, which show the abdominal aorta and arteries from the
diaphragm to the ileac arteries, as well as contrast agent enhancement
of the kidneys, uretal pelvis, and ureters.
Renal/abdominal MRA can detect renal artery stenoses with 94%
sensitivity and 92% specificity. Although there is more inter-observer
variability in the accuracy of MRA compared to angiography for
detecting stenoses with ≥ 50% occlusion, renal/abdominal MRA can also
detect stenoses and/or aneurysms in other arteries in the abdomen and
pelvis, including the aorta. On the other hand, MRA may miss stenoses
caused by fibromuscular dysplasia, which have a characteristic beaded
appearance in CTA or angiograms. Renal/abdominal MRA is also used to
examine potential kidney donors in order to image the vascular anatomy
prior to surgery, which may detect the presence of accessory arteries
or early branching of the renal artery. However, if an accessory artery
is very small, there is a risk that it may be missed by MRA.
Renal/abdominal MRA may also show evidence of loss of renal function,
which is indicated by asymmetry in renal enhancement or in the time of
arrival of contrast agent into the ureters. In addition, MRA can detect
tumors, if present.
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Comparison of Conventional Angiography, MRA, and Duplex Doppler Ultrasound
for Renal and Lower Extremity Vascular Imaging
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Advantages |
Limitations |
| Conventional Angiography |
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| Risks from catheterization |
| Requires iodinated contrast agent* |
| Exposure to ionizing radiation |
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| MRA |
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Noninvasive
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| Low toxicity of gadolinium contrast agents |
| High sensitivity and specificity for stenoses |
| May detect other vascular problems |
| Associated anatomic views can detect tumors and other causes of disease |
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Contraindicated for patients with pacemakers and some metal implants
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| May miss stenoses due to fibromuscular dysplasia |
| May miss accessory renal arteries |
| May miss distal renal artery stenoses |
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| Duplex Doppler Ultrasound |
| Screening for asymmetry in kidney size and blood flow |
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Sensitivity and specificity lower than MRA
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| Time-consuming (up to 2 hours for lower extremity imaging and 45 minutes for renal imaging) |
| Technically difficult and highly operator-dependent |
| Limited by body habitus |
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* Radiology Rounds, October 2003, http://www.massgeneralimaging.org/newsletter/october_2003
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Lower
extremity MRA showing severe stenoses in left superficial femoral
artery and smaller stenoses in right superficial femoral artery.
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Lower
extremity MRA showing severe stenoses in right posterior tibial artery
and several stenoses in left posterior tibial artery.
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Lower Extremity MRA
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Scheduling
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Peripheral
vascular disease is characterized by symptoms of pain on walking in the
hips, buttocks, or calves, diminished pulse in the affected limb,
bruit, evidence of poor capillary refill, and/or ulceration. Another
important clinical indication of peripheral vascular disease is the
finding of an abnormal ankle-brachial index, with or without exercise,
although this test may falsely be negative in diabetic patients with
poor circulation.
If the clinical index of suspicion
for lower extremity vascular disease is high or moderate, a run-off MRA
is warranted. In this examination, a bolus of gadolinium contrast agent
is administered intravenously and images are acquired from the pelvis
to the foot as the contrast agent flows through the arteries. The
images may be viewed in multiple projections or using 3D displays.
Significant stenoses in the pelvis and abdomen are detected with 94%
sensitivity and 97% specificity. Run-off MRA can also detect aneurysms
and assess the morphology of arterial by-pass grafts with findings that
are 100% concordant with conventional angiography. A duplex Doppler
ultrasound scan is an alternative examination that can be a good
diagnostic test. However, this examination is time consuming (taking up
to 2 hours) and, because it is user dependent, requires an experienced
ultrasonographer.
If patients have evidence of renal and lower
extremity vascular disease, a lower extremity MRA should
be performed first. The renal arteries can be seen in this scan
although the image quality may not be adequate. If there are still
questions about the renal arteries, they can be examined in a
follow-up abdominal/pelvic MRA. The wealth of data obtained from these
two examinations is helpful in detecting incipient disease and in
decision making about the suitability of therapy.
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After thorough clinical examination for vascular disease, a renal/abdominal or run-off MRA can be ordered by completing the MGH Radiology - Cardiovascular Intervention Procedure Request Form
and faxing it to 617-726-8472. If both renal/abdominal and run-off MRAs
are needed, they must be ordered as two separate examinations because
these two examinations require different kinds of MR coil in order to
acquire high quality MRA images. The two MRA studies may be ordered in
back-to-back appointments or at separate times. If you have any
questions about scheduling, please call 617-726-8314.
Further Information
For further questions on pelvic/renal and run-off MRA examinations, please contact Suhny Abbara, M.D.
, Radiologist, Director of Non-Invasive CardioVascular Imaging, 617-726-0796.
In addition to Dr. Abbara, we would like to thank David Steel, M.D.,
Kenneth Rosenfield, M.D., and Alan Greenfield, M.D., for their advice
and assistance in writing this article.
This article provided useful information about the appropriate use of imaging studies:
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References
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Goyen, M and Debatin, JF. (2004) Gadopentetate dimeglumine-enhanced three-dimensional MR-angiography: dosing, safety, and efficacy. J Magn Reson Imaging 19: 261-73
Mittal, TK, Evans, C, Perkins, T and Wood, AM. (2001) Renal
arteriography using gadolinium enhanced 3D MR angiography--clinical
experience with the technique, its limitations and pitfalls. Br J Radiol 74: 495-502
Vasbinder, GB, Nelemans, PJ, Kessels, AG, Kroon, AA, et al. (2001) Diagnostic tests for renal artery stenosis in patients suspected of having renovascular hypertension: a meta-analysis. Ann Intern Med 135: 401-11
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Vasbinder, GB, Nelemans, PJ, Kessels, AG, Kroon, AA, et al. (2004) Accuracy of computed tomographic angiography and magnetic resonance angiography for diagnosing renal artery stenosis. Ann Intern Med 141: 674-82; discussion 682
Vosshenrich, R and Fischer, U. (2002) Contrast-enhanced MR angiography of abdominal vessels: is there still a role for angiography? Eur Radiol 12: 218-30.
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