[Diagnosis of renal artery stenosis with magnetic resonance angiography and stenosis quantification].

Atherosclerotic disease is the most common pathologic condition of renal artery stenosis, which typically compromises the ostium or the proximal 1-2 cm of renal arteries and is also usually present in the abdominal aorta. Fibromuscular dysplasia is the second most common cause of renal artery stenosis (RAS) which usually involves the distal two-third of the main renal artery with bed-like stenosis alternating with small fusiform or saccular aneurysms. Magnetic Resonance Angiography (MRA) was initially performed without contrast media injection using two- or three-dimensional Time-of-Flight (TOF) or Phase-Contrast (PC) techniques. Sensitivity and specificity of non-enhanced MRA in detection of proximal RAS are comprised between 53%-100% and 47%-97% respectively (table I). Main limitations of non-enhanced MRA are the long acquisition time, i.e. 5-8 min, the short field of view with lack of kidney visualization and major artifacts. Recent improvements allowed a three-dimensional acquisition during a single breath-hold (18-23 sec), associated to a bolus injection of a gadolinium chelate demonstrating a lack of nephrotoxicity. 3D gadolinium-enhanced ultrafast gradient-echo MRA techniques (3D enhanced-MRA) requires a precise technique. Firstly, kidney localization and morphologic imaging is performed before a 3D MRA data acquisition without injection (fig. 1). Secondly two successive 3D MRA sequences are performed synchronized with the gadolinium chelate bolus injection: the first acquisition corresponds to the arterial enhancement (fig. 4) and the second one to the venous enhancement. At last, a three-dimensional phase contrast could also be performed. After data acquisition, image post-processing is performed including image subtraction, maximum intensity projection (MIP) and reformation images of each renal artery, the abdominal aorta and its main branches (fig. 2, 3). The normal findings, pitfalls and anatomic variation are explained in detail. Particularly, when 3D enhanced MR angiography shows a normal artery, it is considered to be normal. It is also important to be aware of the existence of accessory or aberrant renal arteries that are well diagnosed by 3D enhanced MRA in 75% to 100% of the cases (fig. 2). 3D enhanced-MR angiography present several advantages in comparison to nonenhanced MRA: 1) a great field-of-view (30-36 cm) could be used allowing visualization of the abdominal aorta as well as its main branches; 2) the fast acquisition time allows an arterial imaging followed by a venous enhancement; 3) the kidneys are analyzed: kidney length, cortical thickness, corticomedullary differentiation and renal enhancement are well evaluated; 4) an accurate sensitivity and specificity in detection of proximal RAS comprised between 88%-100% and 71%-100% respectively (table II). Because a severe RAS (i.e. degree of stenosis > 50%) may cause renal ischemia leading to a blood pressure elevation that is often difficult to control with medical therapy, imaging has to assess the severity of RAS. MRA assessment of hemodynamic significance of RAS can be further refined by considering additional factors (fig. 4): arterial stop of signal, post stenotic dilatation, delayed renal enhancement and functional changes in the renal parenchyma (i.e. reduced kidney length and parenchymal thickness, loss of corticomedullary differentiation) (fig. 1). Precise evaluation of degree of stenosis requires the development of dedicated software such as MARACAS (MAgnetic Resonance Angiography Computer ASsisted analysis) software (fig. 5). In conclusions, 3D enhanced MRA allows an accurate diagnosis of proximal RAS, mainly due to atherosclerosis, without the risks associated with nephrotoxic contrast agents, ionizing radiation or arterial catheterization.