CT ANGIOGRAPHY EVALUATION OF THE RENAL VASCULAR PATHOLOGIES: A PICTORIAL REVIEW

and non-invasive method, has become a widely used imaging modality in imaging of renal vascular pathologies (1, 2). The most common renal vascular pathologies include renal artery stenosis, renal artery aneurisms, dissection, vasculitiditis and fibromuscular dysplasia. At renal CT angiography, not only vascular pathologies, but also related secondary parenchymal alterations of kidneys such as infarction and atrophy can be demonstrated. Indications of renal CT angio graphy include imaging work-up of the diagnosis of occlusive diseases of the renal artery and related hypertension, follow up of balloon angioplasty or renal stenting, preoperative assessment of the patients that will undergo partial or total nephrectomy, post-operative followup, preoperative assessment of renal transplant candidate donors and acute onset flank pain in some occasions (3, 4).

CT angiography, which is a rapid and non-invasive method, has become a widely used imaging modality in imaging of renal vascular pathologies (1,2). The most common renal vascular pathologies include renal artery stenosis, renal artery aneurisms, dissection, vasculitiditis and fibromuscular dysplasia. At renal CT angiography, not only vascular pathologies, but also related secondary parenchymal alterations of kidneys such as infarction and atrophy can be demonstrated. Indications of renal CT angiography include imaging work-up of the diagnosis of occlusive diseases of the renal artery and related hypertension, follow up of balloon angioplasty or renal stenting, preoperative assessment of the patients that will undergo partial or total nephrectomy, post-operative followup, preoperative assessment of renal transplant candidate donors and acute onset flank pain in some occasions (3,4).

Image acquisition
The diagnostic accuracy of renal CTA relies on the quality of the raw data acquired. MDCT protocol for the assessment of renal vessels consists of both unenhanced and enhanced CT scans (at arterial and venous phases). Unenhanced scan of kidneys with contiguous sections of 3-mm thickness is necessary for evaluation of vascular calcifications, and renal calculi. The optimal coverage for arterial phase scan should cover the area between the celiac artery and the terminal part of the main iliac artery (Fig. 1), but in patients with ectopic or transplanted kidney the threshold enhancement of 100 HU is reached within the abdominal aorta under bolus tracking. For evaluation renal venous structures and abdominal viscera whole abdomen is scanned with a slice thickness of 5 mm following a delay of 60-80 sec after IV contrast injection. The scanning parameters are summarized in table I.

Renal artery stenosis
Occlusive diseases of the renal artery can result in hypertension, renal dysfunction and renal failure, ultimately. Renal artery stenosis coverage can be increased. 1-1,5 mm thickness slices are obtained after intravenous injection of a 100-mL bolus of 300-400 mg/ml non-ionic iodinated contrast at a rate of 4cc/sec and 70-mL bolus at a rate of 5-6 cc/sec at 16 and 64-Channel MDCT scanners, respectively. Image acquisition is started after a delay of 4-5 and 6-7 seconds at 16 and 64-channel MDCT scanners, respectively after a in up to 30% of patients (7) (Fig. 2).
The pathophysiology of the hypertension is the reduction in the renal perfusion associated with constriction of the renal artery resulting in the activation of the reninangiotensin-aldosteron pathway and the sympathetic nervous system, which ultimately leads to renovascular hypertension, renal dysfunction, even to renal failure (8).
FMD is the second common cause of RAS, which is most commonly seen in females of 2 nd to 5 th decades (9). The distinction of FMD lesions from atherosclerotic disease is typically made by their location which is usually at mid to distal portion of the main renal artery. The disease is generally bilateral (10). In FMD, intima, media and adventitial layers of the vessel walls are all affected and in approximately 90% of cases; however, the media layer is mainly affected. The socalled "rosary beads" , representing (RAS) is responsible for secondary hypertension in 5% of adults and atherosclerosis is the most common etiology among elder population (> 55 years old), whereas fibromuscular dysplasia (FMD) is the most common cause among young individuals (5,6). In atherosclerotic RAS, the stenosis typically occurs due to calcification and atherosclerotic plaque(s) localized to the proximal portion of the renal artery just close to the orifice, which can be bilateral  A B the alternating dilated and stenosed segments, is the typical imaging finding (10,11). Even though there are publications stressing the excellent diagnostic accuracy of CT angiography (12); careful evaluation should be made in every patient and further imaging studies (e.g. catheter angiography) can be needed to con-and volume rendering techniques are both useful and complementary in the assessment of the stenosis at CT angiography. Indeed, axial images are not solely sufficient due to the tortuous nature of arteries, their variable anatomical course and presence of accessory renal arteries (14,16). Moreover, secondary signs of the arterial stenosis such as post-stenotic dilatation (Fig. 4), differences in the parenchymal perfusion and morphological alterations (atrophy, contour changes etc.) can also be accurately assessed on CT (Fig. 5). MIP images provide angiography-like firm the diagnosis, specifically in young individuals and middle aged women (13) (Fig. 3).
CT angiography is a non-invasive and reliable imaging modality in the detection of RAS with sensitivity values close up to 100% in diagnosis of the severe stenosis (> 50%) (14)(15)(16). Maximum intensity projection (MIP)     The endovascular treatment with balloon angioplasty and/or stents is almost always the first therapeutic intervention for RAS. The outcome of endovascular treatment (stent lumen patency) can be accurately assessed at CT angiography by using raw and reconstructed images (Fig. 6).

Acute occlusion of renal artery
Acute renal infarction (ARI) is a cause of acute flank pain. The most common cause is thromboem-demonstrates absence of enhancement in the affected renal tissue, which typically appears as wedgeshaped low attenuation areas (17). An additional CT finding of ARI is the cortical rim sign which represents preserved perfusion in the outer renal cortex which is supplied by renal capsular arteries (18).
bolism. The most common clinical manifestation is acute flank or back pain. Hematuria, proteinuria, fever, leukocytosis and an elevated serum lactate dehydrogenase may also accompany. Parenchymal changes in the kidney depend on the size of the embolus, the location of the arterial occlusion, and its age. MDCT easily

Renal artery aneurysms
Renal artery aneurysms (RAA) are detected approximately in the 0.1% of the patients undergoing imaging studies, mostly in the 4 th and 5 th decades of the life (11,19). The most common causes of RAA are atherosclerosis, polyarteritis nodosa, FMD and trauma (11). Morphologically, they can be saccular or fusiform in shape; a rim calcification can be also seen in 18% of them (19,20) (Fig. 7). Less than 10% of the atherosclerotic aneurysms are symptomatic and again less than 5% of them undergo rupture, however the risk of rupture is higher in ones with a diameter of greater than 2 cm. Non-calcified RAAs and RAAs in pregnant patients are believed to be more prone to rupture (21,22).

Renal arterial dissections
Renal artery dissection (RAD) is generally stenotic and/or occlusive in nature. Individuals with hypertension with an underlying atherosclerosis or FMD are generally more prone to have RAD. Aortic dissections may also cause constriction or occlusion by extending towards renal arteries. Acute dissections can occur as a complication of diagnostic and therapeutic catheter angiography; whereas chronic dissections may either be spontaneous or secondary to renovascular hypertension (23) (Fig. 8).

Vasculitis
Renal CT angiography is used as the first line renal vascular imaging modality in the diagnosis and followup of vasculitic conditions such as Takayasu disease, polyarteritis nodosa (PAN). In Takayasu disease, the thickening of the vessel wall and the luminal stenosis can be accurately assessed at CT angiography and detection of the contrast enhancement within the vessel wall can be postulated as the indicator of an active disease (Fig. 9). In PAN, microaneurysms at the interlobar or arcuate artery bifurcations, and infarct areas at different ages can be seen (24).

Nutcracker syndrome and pelvic congestion syndrome
Nutcracker syndrome occurs secondary to occlusion of the left renal vein between the aorticomesenteric fork (Fig. 10). The disease usually presents with asymptomatic hematuria due to increased peri-glomerular or peri-ureteral capillary pressure. In males, the reflux to the testicular vein might cause the varicosity of the pampiniform venous plexus, that may lead to secondary infertility; whereas, in females the retrograde venous flow into the ovarian veins may result in the varicose dilatation in the ovarian and parametrial venous plexuses (25).
Pelvic congestion syndrome is characterized by the dilatation pelvic venous structures secondary to retrograde blood flow from the left renal vein. It may be the cause of pelvic pain which is more prominent in erect or semi-erect postures (26) (Fig. 11).

Conclusion
MDCT angiography is currently the main imaging method in assessment of the renal vasculature. It provides highly valuable and concluding data in patients with vascular pathologies involving the renal vessels non-invasively.