Improving Interpretation of MRA and CTA in Patients with Suspected Renal Artery Stenosis
- Volume - Issue - February 2011
- Posted on: 2/2/11
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Honglei Zhang, MD, PhD, FACR and Martin R. Prince, MD, PhD
ABSTRACT:Contrast-enhanced computed tomographic angiography (CTA) and magnetic resonance angiography (MRA) are widely used in patients with suspected renal artery stenosis as screening techniques. Both are accurate in depicting anatomical changes of renal vasculature. However, when interpreting MRA/CTA, additional functional information including asymmetrical size, post-stenotic dilatation and asymmetric enhancement may help determine the hemodynamic significance of renal artery stenoses, thus improving the interpretation and accuracy of renal MRA/CTA. Exciting advances in MR including new contrast agents and techniques to characterize borderline lesions are now becoming available.
VASCULAR DISEASE MANAGEMENT 2011;8:E34–E37
Key words: renal artery stenosis; MRA; CT
Renal artery stenosis causes renal ischemia and hypertension. When bilateral, progressive ischemic nephropathy may lead to renal failure and dialysis.1 Angioplasty and stent therapy for renal artery stenosis is controversial since several trials have shown similar benefits with medical therapy alone. The CORAL (Cardiovascular Outcomes in Renal Atherosclerotic Lesions) study is a prospective, multicenter study randomizing patients with systolic hypertension and severe renal artery stenosis to either medical therapy or medical therapy with renal artery stenting. Patients with atherosclerotic renal artery stenosis are randomized into either medical therapy or balloon angioplasty/stent and are followed for up to 10 years to determine which therapy has the best outcomes.
To avoid the devastating consequences of uncontrolled renovascular hypertension and renal failure, it is critical to diagnose renal artery stenosis as early as possible. Renal artery stenosis may be identified on a variety of imaging modalities even when it is unrelated to the patient’s hypertension. As a result, there is an emerging trend toward using advanced imaging techniques to evaluate not just the renal artery caliber, but to also look for signs indicating that the stenosis is hemodynamically significant and thus likely to be the cause of hypertension.
Conventional angiography with measurement of pressures proximal and distal to the stenosis is the gold standard for diagnosing hemodynamically significant renal artery stenosis,2 but it is not an ideal screening examination due to the invasiveness, radiation exposure and nephrotoxic iodinated contrast media. Less invasive techniques have been developed to detect and assess renal arterial disease.3–5 Doppler ultrasonography and captopril renography have focused on detecting the hemodynamic effects of a functionally significant renal artery stenosis, whereas computed tomography (CT) and magnetic resonance (MR) angiography are more effective in detecting renal artery morphological changes using a subjective impression of stenosis severity to predict functional significance. In the CORAL study, several noninvasive imaging techniques are utilized as randomization pathways which include duplex ultrasound, CTA and MRA. Here we review the utility and technical aspects of renal artery CTA and MRA.
Advances in CT technology that allow spiral multidetector acquisitions with slice thickness typically 1.5 mm or thinner can provide accurate anatomic images of even small renal arteries during the arterial phase of a fast iodinated contrast agent bolus. The sensitivity and specificity for diagnosing renal artery stenosis range from 67–100% and 77–98%, respectively.6
Compared to conventional catheter angiography, CTA is less invasive with faster acquisition, better soft tissue visualization, and it allows multiplanar imaging of the renal arteries in any obliquity. CTA has the advantage over MRA of being technically easier to perform with accuracy comparable to MRA.7–9 However, CTA has the risks of ionizing radiation and nephrotoxicity from iodinated contrast agents. Currently, CTA is not utilized in patients with azotemia or kidney transplant unless alternative techniques that do not use iodinated contrast have been inadequate or are not available. When there is severe calcification in the renal arteries, the luminal narrowing may be obscured and/or overestimated.
A major limitation of CTA is that the technique is only capable of providing an anatomic assessment with minimal physiological information about the stenosis. This emphasizes the shortcoming of using the widely accepted anatomic criterion of a 75% decrease in cross-sectional area for diagnosing severe and significant stenosis to predict the functional significance of the stenosis without considering the influence of renal blood flow. A morphologically severe stenosis might not induce a pressure gradient if the artery has slow flow due to renal parenchymal damage. There is no benefit from dilating a severe renal artery stenosis when the ischemic nephropathy is already end-stage.