MAGNETIC
RESONANCE ANGIOGRAPHY
by
K. T. Fan, M.D.
Introduction:
Magnetic Resonance Angiography ( MRA) techniques have been advanced
at furious pace. Advances in phase contrast, 2D-TOF and dynamic
gadolinium 3D-TOF have led to a reduction in the number of artifacts
that have historically compromised MRA image quality. Coupled with
advances in gradient technology and software improvements, MRA has
become an invaluable noninvasive modality evaluating the vascular
system without risks of arterial catheterization, ionizing radiation,
or contrast nephrotoxicity. The purpose of this article is to examine
MRA new important clinical role in Atherosclerotic Renal Vascular
disease (ASRVD), Aortic Dissection, and Peripheral Vascular disease.
Atherosclerotic
Renal Vascular Disease
Scobel
et al, performed angiography in patients reaching end-stage renal
disease when they had combination of significant hypertension, asymmetric
kidneys, unexplained renal failure and systemic atherosclerosis,
revealing ASRVD as probable cause in 14% of patients aged 50 years
or older (1). At this rate, ASRVD can be estimated to be the cause
of 11,000 new cases of end-stage renal disease in the United States
in 1996 (based on a total incidence of 73,091 patients reaching
end-stage in 1996 reported in the USRDS). In addition, Schreiber
et al showed a 20 % or greater increase in serum creatinine in 54%
of 37 patients with angiographic progression of ASRVD over the course
of 31-74 months, and Dean et al prospectively showed a 25% decline
in isotopic glomerular filtration rate or creatinine clearance in
40% of 30 patients over the course 18-36 months (2, 3). Thus, a
screening noninvasive test to detect renal vascular disease is crucial
step determining renal salvageability.
Traditionally,
nuclear renal scan and ultrasound are the imaging modalities for
evaluating renal vascular disease. However, with recent technical
advances, MRA offers several advantages: 1. Direct visualization
of renal vessels, including accessory vessels (figure 1, 2). 2.
Grading of stenosis (figure 3). 3. Visualization of renal vessel
is possible with gadolinium even with elevated creatinine; there
is no evidence of nephrotoxicity. 4. Pre-operative planning for
operative vascular bypass versus percutaneous PTA or stent (figure
4).
In
addition, MRA sometimes offer diagnoses different from atherosclerotic
renal disease such as fibromuscular dysplasia. (figure 5). MRA also
plays an important role in renal transplant surveillance (figure
6).
Aortic
Dissection
Aortic dissection involves separation of the aortic media by stream
of blood that is associated with a primary intimal tear. The most
common sites of entry are supravalvular ascending aorta and descending
thoracic aorta distal to the left subclavian origin. Blood may dissect
antegrade and/or retrograde. Patient often presents with dull aching
intrascapsular pain with pulse deficits in arms or extremities.
Patient can also present with symptoms due to occlusion of aortic
branches: myocardial ischemia, stroke, paraplegia, intestinal ischemia,
renal failure, extremity ischemia, abdominal pain, low back pain.
While
angiogram is still the gold standard in comparative studies, MRA
offers several advantages: 1. Ability to detect small flaps which
may be missed by angiogram. 2. Ability to detect spontaneous medial
hemorrhage with dissection and intact intima, which compose of 5%
of patient with dissection. 3. No risks of arterial catheterization,
ionizing radiation, contrast nephrotoxicity. 4. Ability to monitor
the extent of dissection noninvasively.
While
TEE also can diagnose aortic dissection, MRA offers two advantages:
1. Ability to delineate dissection flap down to the iliac artery
(Fig 7). 2. Patient comfort.
Peripheral
Vascular Disease
Peripheral vascular disease is a disease of older patient, typically
> 50 years. Arterial bifurcations are frequently involved. Risk
factors include smoking, hyperlipidemia, hypercholesterolemia, diabetes,
hypertension, obesity and genetic predisposition. The disease can
be complicated by ulceration of plagues, subintimal hemorrhage,
thrombosis, and sub-intimal calcification.
Imaging
of the arterial system is necessary in determining the diagnosis
of patient's symptoms of claudication, rest pain or non-healing
foot ulcers. It is also necessary in determining the treatment options
and in providing a road map for reconstructive surgery. The role
of conventional angiography as the primary means for demonstrating
vascular anatomy in these evaluations is increasingly challenged
by the safer, less invasive MR angiography alternatives with no
risks of arterial catheterization, ionizing radiation, and contrast
nephrotoxicity. In addition, the fixed hospital costs for MR angiography
are significant lower than those for conventional angiography. MR
angiography may be performed with a single physician and technician,
whereas conventional studies may require one physician, a nurse,
a technician, and recovery room personnel.
MRA
plays a critical role in treatment of the peripheral vascular disease.
For example, in patients with decreased renal function, MRA reveals
the stenosis in the iliac artery allowing the interventional radiologist
to treat the stenosis with PTA and stent without the contrast toxicity
of initial diagnostic angiogram. Furthermore, MRA also affect the
choice of therapeutic access for the interventional radiologist.
For example, in diabetic patient population where the vast majority
the disease is outflow small vessels, interventional radiologist
may choose antegrade access treating the infrainguinal disease with
MRA clearing any inflow lesion. In addition, MRA also can be very
useful evaluating complicated bypass where access to the arterial
system may be difficult (Fig 8).
The
most exciting recent development in MRA technology is bolus-chasing
MRA of the peripheral vascular system. With this technique, the
whole vascular system from abdominal aorta to the foot can be obtained
in less than 2 minutes. This will undoubtedly change how we diagnose
and treating patient with peripheral vascular disease (Fig 9, 10).
MRA
has changed how we diagnose, evaluate and treat atherosclerotic
renal disease, aortic dissection, and peripheral vascular disease.
MRA avoids the risks of arterial catheterization, ionizing radiation,
and contrast nephrotoxicity. It also lower the hospital costs than
conventional arteriogram. With continuing new development in technology,
MRA's clinical potential is awaiting to be realized.
Bibliography
1. Scobel JE, Maher ER, Hamilton G, Dick R, Sweny P, Moorhead JF.
Atherosclerotic renovascular disease causing renal impairment: a
case for treatment. Clin Nephrol 1989; 31:119-122.
2. Schreiber MJ, Pohl MA, Novick AC. The natural history of atherosclerotic
and fibrous renal artery disease. Uro Clin North Am 1984; 11: 383-392.
3. Dean Rh, Kieffer RW, Smith Bm, et al. Renovascular hypertension:
anatomic and renal function changes during drug therapy. Arch Surg
1981; 116:1408-1415.
I
would like to thank my colleagues in Radiology Associates, Dr. Harvey
Greenberg, Dr. Patrick Elvin, and Dr. Mark Kozlowski for their thoughtful
inputs.
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