Urogenital ImagingRenal artery stenosis
(
RAS), narrowing of the
lumen of the
renal arterty. The main diseases of the main
renal artery and its major branches are
atherosclerosis, fibrosing conditions, neurofibromatosis, arteritis, thrombosis or embolism, aneurysms and other anomalies. These conditions may interfere with the blood flow to the kidney and cause
hypertension. If severe, they interfere significantly with the excretory function of the kidney.
Atherosclerosis and fibrosing lesions of the walls of the vessels are the most common causes of
RAS;
atherosclerosis being the most frequent cause.
Stenosis of the
renal artery produces a pressure gradient, the magnitude depending on the systemic blood pressure and degree of narrowing and intrarenal
vascular resistance.
Imaging
Hypertensive urography is no longer performed. Features include disparity in the size of the two kidneys; a difference in length of 1.5 cm of normal or more is considered significant. There is delayed appearance of the contrast medium into the calyces on intravenous urography; a difference of 1 minute or more is significant. The urine flow is decreased resulting in a spidery pyelogram. A difference in the concentration of the contrast medium in the pelvicalyceal systems or ureter may be seen, the affected side showing greater or lesser radiodensity than the other side. Ureteric notching may be seen due to extrinsic pressure from collaterals.
On ultrasound, one method of detecting RAS is direct interrogation of the renal artery. On Doppler imaging, when the ratio of the peak renal artery velocity divided by the peak aortic velocity (renal : aortic ratio) is greater than 3.5, this indicates a renal artery stenosis of greater than 60%. Other findings associated with a greater than 60% stenosis are a peak renal artery velocity of greater than 200 cm/sec. Unfortunately, the examinations are technically difficult (bowel gas may obscure the renal arteries) and time consuming, and do not reliably detect accessory renal vessels.
Another sonographic examination involves intrarenal arterial evaluation (which is simpler than renal artery testing) and involves the detection of abnormal waveforms downstream from an area of stenosis. Normally, intrarenal waveforms demonstrate a sharp rise during systole. In RAS, there is a slow rise to peak velocity (pulsus tardus or decreased early systolic acceleration) and the time to peak velocity is lengthened. Essentially, the stenosis decreases the changes between systole and diastole and decreases the pulsatility. An early systolic acceleration of less than 3 m/sec is abnormal and indicates a haemodynamically significant renal artery stenosis.
Angiographic findings include a delayed nephrogram and a stenosis with poststenotic dilatation. Renal vein sampling can detect the increased renin levels which localize to the involved side in the setting of renovascular hypertension (Fig.1) (Fig.2).
Using nuclear imaging, patients are scanned using Tc-MAG3 before and after the administration of captopril (an angiotensin-converting enzyme (ACE) inhibitor). A positive ACE inhibition scintigraphy examination indicates that renovascular hypertension is present and implies the existence of haemodynamically significant renal artery stenosis (greater than 6075% of the lumen). For renal artery stenosis of greater than 50% using Tc-MAG3, the sensitivity is 90%, specificity is 91%, positive predictive value is 70% and negative predictive value is 97%.
Today, CT angiography with MIP and quantitative measurement of stenosis is an accurate noninvasive method for the evaluation of vascular stenosis.
Renal artery MR angiography has now emerged as a safe, accurate approach to renal arteriography (Fig.3). A comprehensive examination, including both three-dimensional (3D) dynamic gadolinium-enhanced and 3D phase contrast MRA techniques, allows evaluation of both the aortorenal and splanchnic arterial anatomy as well as the haemodynamic significance of any stenoses identified. The 3D gadolinium-enhanced MRA technique produces a contrast arteriogram but without risks of iodinated contrast or ionizing radiation. The 3D phase contrast technique is a flow-based technique, which may show dephasing in the presence of haemodynamically significant stenoses. Also, see hypertension, renal.
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