Renal Arteries

Renal Arteries

The kidneys are paired organs that originate in the pelvis and ascend into the abdominal cavity in a retroperitoneal location. As each kidney travels cephalad it is supplied sequentially by a series of arteries from the aorta that regress spontaneously. Ultimately, each kidney is usually supplied by a single renal artery that originates from the aorta below the superior mesenteric artery (SMA) at roughly the L1-L2 disk space in about two thirds of individuals. The right renal artery orifice is located on the anterolateral wall of the aorta, frequently quite close to the SMA origin. The right renal artery courses posterior to the inferior vena cava (IVC), and assumes a position posterior to the right renal vein in the retroperitoneum ( Fig. 12-1 ). The left renal artery originates in a more lateral location, and courses through the retroperitoneum posterior to the left renal vein. With an understanding of the typical locations of the renal artery orifices, the operator will avoid much frustration during selective angiography.

The renal artery is usually 4-6 cm long and 5-6 mm in diameter. Each renal artery gives rise to a small proximal branch to the adrenal gland (the inferior adrenal arteries) and the renal capsule ( Fig. 12-2 ). In the region of the renal pelvis, the artery bifurcates into anterior and posterior divisions. The anterior division supplies the upper and lower poles and the anterior portion of the mid-kidney. The posterior division supplies primarily the posterior renal parenchyma, with supplemental supply to the upper and lower poles. The divisional arteries divide into segmental arteries (apical, upper, middle, lower, and posterior), which quickly give rise to the lobar and then interlobar arteries. At the corticomedullary junction, the interlobar arteries divide into the arcuate arteries. The terminal branches of the renal artery are the interlobular arteries, which ultimately supply the glomeruli.

Variations in number, location, and branching patterns of the renal arteries are present in more than 30% of people ( Fig. 12-3 ). These vessels can enter the kidney through the hilum or travel directly to a renal pole (termed polar artery ) ( Table 12-1 ). Supernumerary renal arteries can arise from the abdominal aorta and iliac (usually common) arteries. Renal artery origins arising above the SMA origin are extremely rare. Congenital anomalies of renal position and configuration are often associated with aberrant locations of renal artery origins and supernumerary vessels. In particular, horseshoe kidney has a 100% incidence of multiple renal arteries. The fused lower poles of a horseshoe kidney, termed the isthmus , are trapped under the inferior mesenteric artery as the kidneys ascend out of the pelvis ( Fig. 12-4 ). The isthmus can derive arterial blood supply from the distal aorta and iliac arteries.

The renal pelvis and proximal ureters are supplied by small branches of the segmental and distal main renal arteries. The middle portions of the ureters are supplied by the gonadal arteries (see Fig. 10-1 ). The distal ureters are supplied by terminal branches of the internal iliac arteries, most notably the cystic artery.

Adrenal Arteries

The adrenal glands are retroperitoneal organs that receive their blood supply from the renal arteries, directly from the aorta, from the inferior phrenic arteries, and rarely from the celiac artery or SMA. There are usually three adrenal arteries; the inferior, middle, and superior (see Fig. 10-1 ). In many instances these vessels are linked to capsular renal branches, and therefore are potential pathways for collateral blood supply to the kidney.

The inferior adrenal artery arises directly from the proximal renal arteries in two thirds of people. The middle adrenal arteries are small vessels that usually arise directly from the aorta. These arteries are slightly more common on the left than the right. The origins of the middle adrenal arteries may be replaced to the celiac artery or SMA in 2%-5% of patients. The superior adrenal arteries are constant branches of the inferior phrenic arteries. However, the origins of the inferior phrenic arteries are less predictable than their adrenal branches. These arteries arise from the aorta or the celiac artery in two thirds of patients, but may also originate from the renal arteries or the left gastric artery.

Renal Arteries

Ultrasound is an excellent modality for imaging renal parenchyma, and for detection of nephrolithiasis, and hydronephrosis. The renal size should always be noted. A long axis kidney measurement of less than 8 cm in an adult in the setting of renal artery stenosis suggests a hemodynamically significant and longstanding stenosis. Color-flow duplex ultrasound is required to image the renal arteries. Main renal arteries are reliably depicted in 95% of patients, but the detection of small accessory renal arteries is less accurate. The normal renal artery is a low-resistance vessel, with antegrade flow present throughout the cardiac cycle and a peak systolic velocity less than 180 cm/sec (see Fig. 3-2 ). Evaluation of Doppler waveforms and velocities is required to identify stenoses in the main and segmental renal arteries (see Renal Artery Occlusive Disease and Fig. 3-3 ).

The resistive index (RI) has been variably successful in predicting functional outcomes of renal revascularization:

Measured from the arcuate or interlobar arteries, normal values should be less than 0.7; borderline increased resistance is 0.7-0.8; abnormal resistance is greater than or equal to 0.8. In children, RI of greater than 0.7 may be normal up to age 4 years. In adults, a high RI suggests chronic parenchymal disease that will not improve with revascularization. Blood pressure control response to intervention is not predicted by the RI.

Nuclear medicine studies can provide functional information and infer the presence of occlusive vascular lesions. Occasionally, nuclear medicine studies are obtained in patients with renal masses or nonocclusive vascular lesions to assess renal function. A number of imaging agents are available, including technetium-99m mercaptoacetyltriglycine (MAG3), and diethylenetriaminepentaacetate (DTPA). An abnormal study with any of these agents after administration of an angiotensin-converting enzyme inhibitor such as captopril is highly suggestive of proximal renal artery stenosis.

Multidetector computed tomography angiogram (CTA) has a very high sensitivity and specificity for identification of renal arteries and detection of vascular pathology (see Fig. 12-3 ). A noncontrast scan should be obtained first to evaluate for nephrolithiasis, calcified renal artery lesions, and hyperdense renal masses such as hemorrhagic cysts. A thin-collimation breath-hold contrast-enhanced scan from the diaphragm to the common femoral arteries is required for a complete evaluation of the renal arteries. A good contrast bolus is essential. The celiac artery should be included for patients with suspected renovascular disease as hepatorenal or splenorenal bypasses are potential surgical options for treatment of proximal renal artery occlusive disease. The aorta is evaluated for concomitant occlusive or aneurysmal disease. The iliac arteries are included to detect accessory renal arteries and evaluate for occlusive disease that could impact treatment decisions. The renal parenchyma is inspected carefully for mass lesions. The presence of an adrenal mass in a hypertensive patient should raise the question of a pheochromocytoma or aldosteronoma (Conn disease). When evaluating a patient as a potential donor for renal transplantation, a delayed scan is obtained for venous and collecting system anatomy. Careful postprocessing of the three-dimensional (3-D) volumes improves sensitivity for small accessory vessels

Magnetic resonance imaging (MRI) of patients with renal vascular disease involves both anatomic and flow sequences. In the future, physiologic information will also be obtained during MRI of the kidney. With current techniques, anatomic sequences provide information about renal size and the presence of masses. Contrast-enhanced 3-D sequences provide excellent delineation of the renal vasculature ( Fig. 12-6 ). However, patients with renal insufficiency are at increased risk for nephrogenic systemic sclerosis from the gadolinium (see Chapter 2 ). Several noncontrast MRA sequences have been developed particularly for patients with renal insufficiency (see Fig. 3-14 ). MRA is very sensitive and specific for ostial and proximal renal artery pathology (>90% sensitivity and specificity for good quality studies). Subtle abnormalities in peripheral arteries, such as mild fibromuscular dysplasia (FMD), can be difficult to image with MRA.

Conventional angiography is usually performed following noninvasive imaging of some type with the specific goal of intervention. Previous studies should be reviewed before the procedure. Conventional angiographic evaluation of the renal arteries begins with an aortogram. The number and location of renal arteries, and the condition of the ostia and proximal renal arteries, can be determined from this study. To perform an aortogram for renal arteries, a pigtail or other catheter designed for high-volume aortic injection is positioned with its tip just at or slightly above the renal arteries ( Fig. 12-7 ). The image intensifier should be positioned in a slight left anterior oblique projection to best visualize the renal artery origins. The most advantageous angle can be determined from review of prior cross-sectional imaging studies. Sufficient contrast injection (15-25 mL/sec for 2 seconds) and rapid filming (3-6 frames/sec) are necessary to obtain optimal images of the proximal renal arteries. The right renal artery can arise in a very anterior position, so that a steep left anterior oblique, or even a lateral view, may be necessary to visualize this ostium. Selective angiography is required to fully evaluate the branches of the renal artery and renal parenchymal masses. When multiple renal arteries are present, the renal parenchyma in the watershed areas between arteries typically has an irregular, indistinct contour on selective angiography ( Fig. 12-8 ).

A wide variety of selective catheters can be used for renal angiography, but the basic choices are a curved selective catheter such as a Cobra-2, Rösch Celiac, or a recurved catheter such as a Sos selective. The curved catheter shape is chosen when the renal artery arises at close to a 90-degree angle from the aorta, whereas a recurved catheter can be used to pull down into a renal artery that arises at an acute angle (see Figure 2-10, Figure 2-18, Figure 2-19, Figure 2-20 ). A radial or brachial approach is used to select a renal artery when there is a steep angle of origin, severe infrarenal aortic tortuosity, or aortoiliac occlusion. Injection rates of 5-6 mL/sec for 2-3 seconds provide excellent opacification of intrarenal branches. Rapid filming (3-6 frames/sec) is necessary. Oblique and craniocaudad oblique views are frequently required to optimally display the renal vasculature, but studies must be tailored to each patient. In general, an ipsilateral anterior oblique view presents the kidney enface. Administration of glucagon to reduce artifact from bowel motion on DSA may be helpful in some patients.

Adrenal Arteries

In most cases, the adrenal glands can be evaluated for mass lesions using ultrasound, CT, or MRI. The need to visualize the arterial supply of the adrenals is rare, but the only way is with conventional angiography. The adrenal arteries are too small and variable in location to be reliably evaluated with noninvasive techniques. For the same reasons, selective angiography of the adrenal gland is also difficult. Flush injections of the abdominal aorta, and selective renal and phrenic arterial injections, will aid in identification of the inferior and superior adrenal arteries. The middle adrenal artery, which arises directly from the aorta, can be selected with a recurved catheter. Hand injections of contrast should be used, because rupture of the artery and adrenal infarction can occur. The normal adrenal gland appears as a dense wedged-shaped suprarenal stain.