The majority of high blood pressure or hypertension has unknown cause, but can be treated effectively with a variety of medications. However, in 1-5% of cases, hypertension will not respond to medical treatment and can be caused by a narrowing of the artery to a kidney (renal artery stenosis), or by production of hormones by tumors of the adrenal gland (pheochromocytoma, aldosteronoma). These adrenal tumors can be screened for, or suggested by blood and urine tests. Identification of these causes for high blood pressure is critical since they can be cured.
Detecting renal artery stenosis is challenging. There are three common methods to screen. Nuclear scintigraphy with ACE inhibitors has the advantage of reflecting physiologic process. This method has been accepted for many years. Sensitivity varies from 51-96% (mean 82%) (1). This variability likely results from the fact that patients with renal failure, obstruction, bilateral renal artery stenosis can reduce accuracy. Nevertheless, a negative ACE inhibitor renogram indicates a low probability of renal vascular hypertension, in some studies as low as 10%(2).
Ultrasound techniques include methods to detect narrowing through velocity accelerations near the aorta, or post stenotic doppler waveforms within the smaller vessels inside the kidney. Both ultrasound methods are plagued by technical difficulty and lack of the ability to reproduce these results in multiple studies (3).
A recent study compared these three methods and found that MR angiography was the best of these choices (6). MR angiography had a 98% negative predictive value and a 58% positive preditctive value relative to captopril renography having a 76% negative predictive value, and a 49% positive predictive value (6).
Another study demonstrated that renal MRA improved the diagnostic confidence by clinicians by 35% and changed their initial clinical impressions in 40 of patients. This resulted in a change in anticipated clinical management in 67% of patients (7).
MRA allows direct visualization of the renal vessels and any narrowings/stenosis through a simple I.V. injection of a safe contrast agent (gadolinium). On the 1.5 Tesla state of the art Sieimens Symphony MR scanner operated by Turville Bay MR Center at St. Mary’s Hospital, a MR fluoroscopically triggered acquisition produces reproducible high quality images sufficient for diagnosis and surgical planning.
CT angiography with state of the art multidetector row scanners can offer images with similar quality to MRA.
Accuracy rates are excellent (sensitivity >95%, specificity >90%) and have been reproduced in numerous studies(4,5). MR angiography can overestimate stenoses and miss small accessory renal arteries, but this is less of a problem with newer techniques. Accuracy is best in middle aged and elderly individuals since atherosclerotic stenosis is the most common cause (90% of renal artery stenosis) and usually affects the proximal vessels well demonstrated with MRA.
In younger patients, fibromuscular dysplasia is of concern. Because this process often affects smaller vessels closer to, or within the kidney , nuclear scintigraphy may be better suited here. However, improvements in the speed and resolution of the MRA techniques have allowed the detection of renal artery stenosis due to fibromuscular dyslplasia (see case 1). Therefore, for fibromuscular dysplasia, the recommended screening test is debatable.
Several examples below were acquired at St. Mary’s Hospital and are typical:
- Prigent A. The diagnosis of renovasacular hypertension: the role of captopril renal scintigraphy and related issues. Eur J Nucl Med 1993; 20:625-44
- Taylor AT, Fletcher JW, Nally JV, et al. Procedure guidelines for diagnosis of renovascular hypertension. J Nucl Med 1998; 39:1297-1302
- Soulez G, Oliva V, Turpin S, et al. Imaging of renovascular hypertension: respective values of renal scintigraphy, renal doppler US, and MR angiography. Radiographics 2000; 20:1355-1368
- Bakker J, Beek FJ, BeutlerJJ, et al. Renal artery stenosis and accessory renal arteries: accuracy of detection and visualization with gadolinium enhanced breath hold MR angiography. Radiology 199; 207: 497-505
- Hany TF, Debatin JF, Leung DA, Pfammatter T. Evaluation of the aortoiliac and renal arteries: comparison of breath hold contrast enhanced, three dimensional MR angiography with conventional catheter angiography. Radiology 1997 ; 204:357-62
- Qanadi S, et al. Detection of renal artery stenosis: Prospective comparison of captopril doppler, captopril scintigraphy and mr angiography. AJR 2001; 177:1123-1127.
- Omary RA, et al. Effect of MR angiography on the diagnosis and treatment of patients with suspected renovascular disease. JVIR 2001;12:1179-1183