The kidneys are located between the 12th thoracic and the 3rd lumbar vertebrae lying retroperitoneally in the dorsal abdominal cavity. The left kidney usually is slightly more superior in its position when compared to the right. The normal length of an adult kidney is 9–13 cm with a width of 5–7 cm. It should be noted that kidney size normally decreases with increasing age. The kidneys may be joined at their lower poles by an isthmus of tissue which lies anterior to the aorta at the level of the fourth or fifth lumbar vertebrae in more than 90% of cases. This is an uncommon finding, occurring in less than 1% of the population [32].

For the purpose of sonographic evaluation, the kidneys should be considered to have four main areas. The renal hilum which forms the renal sinus receives the renal artery, renal vein, and the ureter. The sinus, which sonographically is brightly echogenic, contains the renal artery and vein and the collecting and lymphatic systems (Fig. 49.2). This acoustic characteristic is the result of the large component of fatty and fibrous tissue in this area of the organ. The renal parenchymal tissue is divided into two areas: the medulla and the cortex. The cortex, which lies just beneath the renal capsule, constitutes the outermost area of the kidney. The 12–18 triangular-shaped renal pyramids, which carry urine from the cortex to the renal pelvis, can be seen within the renal cortex when the organ is imaged in the longitudinal plane. The pyramids generally have lower echogenicity than the cortex and are usually apparent in the normal adult kidney. The columns of Bertin, comprised of cortical tissue, lie between the renal pyramids.

Sonographic localization of the renal arteries is facilitated by attention to surface and internal anatomic landmarks. The transpyloric plane serves as an excellent surface landmark for locating the renal arteries. The transpyloric plane lies midway between the suprasternal notch and the symphysis pubis. This plane cuts through the lower border of the first lumbar vertebrae, the ninth costal cartilages, and the pylorus. The renal arteries lie approximately 2 cm below this plane and can be visualized arising from the lateral or posterolateral wall of the abdominal aorta. From a transverse image of the aorta, it will be noted that the right renal artery arises anterolaterally from the aortic wall and then arches superiorly just beyond its origin (Fig. 49.3). It then assumes a slightly posterior course to go behind the inferior vena cava (IVC) and the right renal vein. It is the only major artery posterior to the IVC. The left renal artery usually originates from the lateral or posterolateral aortic wall slightly more cephalad than the right and courses superiorly and then moves posterior to the splenic artery and left renal vein. It is crossed by the inferior mesenteric vein. It should be noted that both the right and left renal arteries course anterior to the renal pelvis before entering the renal hilum.

Between 12 and 22% of patients have variant anatomy which includes duplication of the main renal arteries or accessory polar renal arteries [33, 34]. These variants occur more commonly on the left side than on the right. Accessory renal arteries commonly arise from the aortic wall below the main renal artery and course to the polar surface of the kidney. In contrast, the main renal artery normally enters the kidney through the renal hilum. Occasionally, accessory renal arteries originate from the common or internal iliac, superior or inferior mesenteric, adrenal, or right hepatic arteries. Sonographic identification of duplicate and small accessory renal arteries may be problematic in some patients [20, 22, 35, 36]. Detection of accessory renal arteries may be enhanced by careful attention to variations in the diameter of the main renal artery and the dimensions of the kidney. Aytac and associates demonstrated that a normal-sized kidney supplied by a renal artery with a diameter that is smaller than usual correlates with the likelihood of an accessory renal artery, thus providing clues to their sonographic detection [37].

At the level of the renal hilum, the main renal artery normally gives rise to two to five segmental branches which supply blood to the upper, mid, and lower poles of the kidney (Fig. 49.4). The segmental arteries course through the renal sinus and divide into the interlobar arteries which course along the sides of the renal pyramids in close proximity to the collecting system. At the level of the corticomedullary junction, the interlobar arteries branch into the arcuate arteries. The arcuate arteries travel across the superior border of the pyramids and give rise to the interlobular arteries. The interlobular arteries course to the surface of the kidney forming the afferent glomerular arterioles. Blood drains from the glomerulus to the efferent arteries which form the vasa recta. This, in turn, forms the network for venous outflow from the kidney.

Anatomic variants are also noted on the renal venous side of the circulation. It is important to be aware of these variants because the left renal vein serves as an excellent anatomic landmark for locating the renal arteries during the sonographic examination. On each side, the renal vein courses anteriorly from the renal hilum on a path that is superior to the ureter and the renal artery. On the right side, the renal vein has a short course from the renal hilum to the IVC. The left renal vein normally courses anterior to the aorta and posterior to the superior mesenteric artery (Fig. 49.5). In approximately 3% of patients, the left renal vein may assume a retroaortic path, while up to 18% of patients will have a circumaortic (bifid) left renal vein with both a retroaortic branch and another branch coursing anterior to the aorta [38, 39].

Prior to initiating the examination, patients are asked to lie supine on the examination table with their head slightly elevated on a pillow. The table is raised to a height that is ergonomically correct and comfortable for the sonographer and then placed in reverse Trendelenburg so that the patient’s feet are 15°–20° lower than the heart. This allows the viscera to descend into the lower abdomen and pelvis, increasing the likelihood of finding acceptable acoustic windows for imaging the renal arteries and kidneys. With the patient lying in this position, the aorta, celiac trunk, proximal superior mesenteric artery (SMA), renal ostia, and the proximal-to-mid segments of the renal arteries are usually visualized. In patients with a low rib cage, it may be helpful to have them raise their arms over their head to elevate the ribs and allow improved access to the proximal renal arteries. To facilitate examination of the mid-to-distal segments of the renal arteries, renal veins, and the kidneys, the patient is moved to the right or left lateral decubitus position with the arm raised over his/her ear and his/her legs extended to elongate their body. In some cases, the patient may be asked to lie prone with his/her midsection flexed over a pillow or foam wedge to allow visualization of the distal renal arteries and kidneys through intercostal windows.

The examination requires a high-resolution ultrasound system that provides grayscale (B-mode) as well as sensitive spectral, color, and power Doppler. Real-time compound and harmonic imaging modalities improve resolution and help to reduce artifacts. The examination is performed using pulsed Doppler transducers ranging in frequency from 2.25 to 5.0 MHz to allow adequate penetration to the depths of the abdominal aorta and distal renal arteries. While not absolutely required for successful renal vascular examination, color flow and power Doppler imaging greatly facilitate visualization of vessels, confirmation of arterial and venous patency, and identification of regions of flow disturbance. B-mode, spectral, color, and power Doppler must be optimized and integrated throughout the examination because velocities may vary over short focal segments of the arteries and veins due to plaques, tortuosity, dissections, or webs. At initiation of the study, color Doppler settings, including gain, velocity scale (PRF), and wall filter, are adjusted to accurately depict laminar flow in a normal segment of the aorta or renal artery. This practice facilitates rapid recognition of the color aliasing artifact (mosaic color pattern) that occurs as a result of elevated velocities in stenotic segments. Similarly, spectral, color, and power Doppler settings must be optimized for low flow velocity when pre-occlusive stenosis or arterial occlusions are suspected. Sensitivity to low flow is increased when spectral, color, and power Doppler gain, velocity scale, and wall filter are decreased; the width of the color box is reduced; and the size of the Doppler sample volume is increased.

The examination of the abdominal aorta is initiated in a left paramedian scan plane at the level of the diaphragm and extended throughout the aorta and its bifurcation to include the common iliac arteries. Using B-mode imaging and a sagittal image plane, atherosclerotic plaque, aneurysmal dilation, dissection, anatomic anomalies, and other pathologies are noted. The evaluation is complemented with color flow imaging to identify regions of disturbed flow and segments where the arterial lumen is compromised. The aortic peak systolic velocity is documented and retained for later utilization in calculation of the renal-aortic velocity ratio.

Branches of the celiac and superior mesenteric arteries course in close proximity to the renal arteries and may be mistaken for accessory or main renal arteries. To avoid confusion, Doppler spectral waveforms are recorded from the celiac and superior mesenteric arteries with the primary purpose being recognition of the mesenteric flow patterns. A secondary benefit would be detection of unsuspected mesenteric occlusive disease, a common finding in patients where atherosclerotic plaque is present along the aortic walls in the region of the mesenteric arteries.

To localize the renal arteries, the aorta is imaged in a cross-sectional plane at the level of the superior mesenteric artery. The left renal vein can be identified as it courses between the SMA and the anterior aortic wall (Fig. 49.5). As noted previously, care must be taken to identify anomalous venous anatomy. Attention should be given to the vein diameter and flow patterns to rule out thrombosis or extrinsic venous compression due to overlying small bowel or superior mesenteric artery compression syndrome.

The renal arteries normally lie immediately inferior to the left renal vein; their location, however, is variable (Fig. 49.6). The origin of accessory renal arteries is unpredictable, and because they do not enter the renal hilum, they are often not identified sonographically. Color and power Doppler imaging facilitate identification of these arteries and make it easier to follow their course. Recognition of a small-diameter main renal artery supplying a kidney with normal dimensions can serve as an additional clue to their presence.

To identify orifical renal artery stenosis commonly associated with plaque on the aortic wall, the Doppler sample volume is swept slowly from the aortic lumen through the renal ostium into the proximal renal artery. Doppler spectral waveforms should be continuously recorded throughout this maneuver (Fig. 49.7). The transition in spectral waveform patterns from the high-resistance, low diastolic aortic flow to the low-resistance, high diastolic renal artery flow allows recognition of abnormal, high-velocity, turbulent signals reliably present in stenotic ostial disease.

Thereafter, Doppler spectral waveforms are obtained continuously throughout the visualized length of the renal artery. Representative waveforms are then recorded from the proximal and mid-segments of the vessel taking care to use a small Doppler sample volume and an angle of insonation less than 60° with the Doppler cursor parallel to the arterial wall (Fig. 49.8).

To interrogate the mid-to-distal segment of the renal artery and to assess parenchymal flow, the patient may be moved to the right or left lateral decubitus, lateral oblique, or prone position. From the decubitus or oblique positions, the liver and kidneys can be used as acoustic windows to visualize the renal arteries. The right renal artery is relatively easy to follow from the renal hilum to its origin from the aortic wall using either the liver or the kidney as an acoustic window (Fig. 49.9). The left renal artery can usually be interrogated by moving the patient to a right lateral decubitus position and placing the transducer in a left posterolateral plane, using the left kidney as an acoustic window. When the arteries are tortuous, color and/or power Doppler imaging help to define the course of the vessel and facilitate appropriate angle correction. When assessing the mid-to-distal segments of the artery, care must be taken to use the same range of Doppler angles that was employed for the proximal-to-mid segments.

Doppler spectral waveforms are recorded at a 0° angle of insonation throughout the distal renal artery and the segmental and arcuate arteries within the renal sinus, medulla, and cortex (Fig. 49.10). Color flow imaging helps to guide placement of the Doppler sample volume and recognition of disordered or absent flow. Care is taken to note regions with increased signal amplitude which may suggest inflow from multiple renal arteries. The highest peak and end-diastolic velocities are documented from vessels within the upper, mid, and lower poles of the organ. Pathology, including renal cortical thinning, cysts, masses, calculi, hydronephrosis, and/or perinephric fluid collections, is noted (Fig. 49.11).

It has been shown that renal size decreases with progression of renal artery stenosis and that renal atrophy may be a contraindication to revascularization [24, 26, 40, 41]. If renal artery stenosis is detected before renal ischemia develops, it is likely that successful renal revascularization could be achieved. Conversely, revascularization is most often unsuccessful when renal length is less than 9 cm [27]. Given this, the pole-to-pole length of each kidney is measured during each examination (Fig. 49.12). Optimal visualization of the renal margins is achieved using B-mode imaging through a flank approach during maximum inspiration. Three separate measurements are taken, averaged and, if the patient is being monitored for disease progression, compared to the value obtained at the previous duplex examination. In adults, normal renal length is between 9 and 13 cm in the greatest longitudinal plane, and there is usually less than 1 cm difference in length between the two kidneys.

Renal vein thrombosis is an underdiagnosed vascular disorder that is often associated with significant clinical complications including pulmonary embolism, nephrotic syndrome, hematuria, and/or renal failure. Confirmation of renal vein thrombosis is dependent on both specific and nonspecific findings. In cases of acute renal vein occlusion, parenchymal edema results in enlargement of the kidney and increased parenchymal echogenicity. Venous flow may be detected within the renal parenchyma as the result of the rapid development of large hilar collaterals. The renal vein is invariably enlarged (Fig. 49.13), and the absence of flow can be demonstrated using spectral and color Doppler optimized for low flow (decreased PRF, gain, wall filter). Direct identification of thrombus may be technically challenging because acute clot is hypoechoic and may be anechoic. Application of color flow imaging facilitates detection of partial thrombosis of the vein and highlights regions for Doppler sampling. Low-velocity, continuous, nonphasic Doppler signals are usually noted in the small channels surrounding the clot. The diameter of the renal vein may be smaller than usual when it is chronically thrombosed. Intraluminal echoes may be detected with real-time compound and/or harmonic grayscale imaging to enhance resolution and increase conspicuity. In the absence of venous recanalization or collateralization, no flow will be present within the renal vein. In many cases, the renal artery exhibits a Doppler spectral waveform with retrograde, blunted diastolic flow as a result of the obstruction to venous outflow. Tumor invasion of the renal veins is common in cases of renal cell carcinoma with involvement of the main renal vein occurring in 21–35% of patients with large tumors with propagation into the IVC also present in 5–10% of cases [42, 43].

Color duplex scanning of the renal arteries can be technically challenging and is influenced by both operator expertise and patient factors, such as overlying bowel gas, patient body habitus, and inability to optimally position the patient due to lack of patient cooperation [23]. Contrast enhancement to improve imaging and increase diagnostic accuracy is prevalent in radiologic techniques such as angiography, CTA, and MRI. Ultrasound contrast agents are stabilized gas microbubbles that enhance ultrasound echoes due to the difference between them and the surrounding blood in terms of compressibility and density. They reflect transmitted ultrasound waves strongly, enhancing echogenicity at both fundamental and harmonic frequencies, which provides improved vascular imaging and tissue differentiation [44, 45]. Intravenous ultrasound contrast agents have been found to improve ultrasound diagnostic accuracy and are approved in the United States for use in echocardiography. “Off-label” studies have shown the utility of contrast agents in the liver, mesenteric, renal, and peripheral vasculature [46–50].

Blebea et al. performed a prospective study using perflutren contrast agent (Definity, DuPont) to improve imaging of the renal arteries [49]. The results of the ultrasound examination with and without contrast enhancement were compared with intra-arterial angiography. Both duplex alone and duplex with contrast demonstrated excellent identification of the renal arteries. Overall visualization of the renal arteries was 85% for standard duplex and 94% following infusion of contrast. Contrast infusion was particularly helpful in identifying accessory renal arteries and visualization of flow-limiting stenosis in technically challenging studies. In addition, significantly longer lengths of the renal arteries were visualized in continuity when contrast was infused (3.9 cm as compared to 3.3 cm [P = 0.001]).

There were no complications associated with the use of the intravenous contrast. The patients experienced no changes in blood pressure or heart rates during the contrast infusion and no deterioration of renal function as measured by blood urea nitrogen or serum creatinine levels, consistent with the experience of other investigators [47, 50]. Insonated Doppler velocities, however, were increased following contrast administration by an average of 10% in normal or minimally diseased vessels and by 12% in stenotic vessels. Although these differences were statistically significant, they did not lead to a change in category of stenosis. This artifactual increase in measured Doppler velocities has been noted in both human and in vitro studies [48, 51]. House et al. used contrast enhancement (Levovist, Schering, Berlin, Germany) for technically unsuccessful studies or to improve diagnostic confidence in sonographic detection of recurrent stenosis in stented renal arteries [52]. Use of contrast enhancement improved the technical success rate from 89 to 95% and also increased diagnostic confidence in a significant number of examinations.

The results from these and other studies indicate that contrast-enhanced duplex imaging of the renal arteries is safe but not routinely required when an experienced sonographer performs the study. However, it can increase vessel visualization in technically challenging examinations and thus allow more specific placement of the Doppler sample volume. This may increase overall accuracy of the study in patients of large body habitus and in detection of accessory renal arteries. Unfortunately, the Food and Drug Administration has not yet given approval for use of ultrasound contrast agents for renal Doppler studies, but it is hoped that it will be forthcoming in the near future.

It has been estimated that multiple renal arteries are present unilaterally in 30% and bilaterally in 10% of the population [34, 38]. Duplicate main renal arteries usually arise from the lateral wall of the aorta and enter the renal hilum to supply the segmental renal artery branches (Fig. 49.14). Accessory polar renal arteries most often arise from the aorta but may also arise from the iliac arteries and course to the upper or lower poles of the kidney. For an unknown reason, more accessory renal arteries are seen on the left as compared to the right side. When only a single accessory renal artery is present, it is usually supplying the lower pole of the kidney. It has been noted that when an accessory vessel is present, the main renal artery is most often of normal caliber, while the accessory artery has a smaller diameter [37]. None of the vessels may be dominant when multiple renal arteries occur, thus reducing confidence that all vessels have been visualized and interrogated with Doppler.

It is well recognized that flow-limiting lesions in duplicated main renal arteries can cause hypertension and lead to renal insufficiency. In contrast, it is thought that hypertension and renal insufficiency are rarely caused by stenoses in small-diameter accessory polar renal arteries [53–55]. The lack of clinical suspicion for their presence and the small diameter of the polar vessels have contributed to the limited success in identifying them even by those with extensive renal duplex experience. Color duplex imaging has shown some value in the detection of these small vessels, but it is important to remember that color encoding of Doppler-shifted frequencies is angle dependent and accessory renal arteries frequently arise from the aortic wall at angles between 70° and 90°. Power Doppler imaging, which relies on the amplitude of the returned Doppler signal, is not as angle dependent as color Doppler imaging. Given this advantage, this modality should be applied diligently in cases where there is no evidence of stenosis in the main renal artery of a hypertensive patient. Care must be taken to differentiate these arteries from branches of the main renal artery by tracing the vessel back to its origin.

There are several scanning maneuvers that can be used to detect supernumerary renal arteries. It is helpful to increase the size of the Doppler sample volume and scan in the para-aortic region, slowly moving the sample volume from the level of the main renal artery origin to the aortic bifurcation and into the common iliac arteries to detect additional low-resistance signals that could imply renal arterial flow. Lumbar arteries can be confused with accessory polar renal arteries, but lumbar arteries commonly have a more posterior origin from the aorta. Keeping in mind the anatomic course of the right renal artery, it is often helpful to create a sagittal image of the IVC which may reveal multiple right renal arteries beneath the vessel (Fig. 49.15). Using color and/or power Doppler imaging, the origins of the renal arteries can quite often be revealed from a coronal view of the aorta (the “banana peel” approach) (Fig. 49.16). This is not only an excellent imaging approach for use in adult patients, but this view also facilitates detection of fetal supernumerary renal arteries during second or third trimester in utero evaluations.

It can be difficult to confirm occlusion of the main renal artery sonographically. Careful attention must be given to optimization of color flow and spectral Doppler to ensure detection of low-velocity signals. If the severity of renal artery stenosis is pre-occlusive, flow velocity within the vessel may be below the threshold for spectral, color, or power Doppler display. The problem can be compounded by excessive overlying abdominal gas that may compromise visibility of the renal artery. Quite often, the diagnosis is dependent on secondary indications of renal artery occlusion. Renal atrophy is a common finding when the renal artery is chronically obstructed. In such cases, kidney length is usually less than 8 cm, and the organ is commonly smaller in length by more than 3 cm compared to the contralateral organ (Fig. 49.17). Flow from adrenal and ureteral collaterals may be detected within the renal parenchyma. Cortical velocities are generally less than 10 cm/s with low amplitude and delayed systolic acceleration. In instances where a normal renal artery is acutely occluded, e.g., due to trauma, renal length will likely remain normal within the short term, and minimal or no flow will be detected within the organ as the need for renal perfusion via developed collaterals would not be necessary in the absence of flow-limiting renal artery disease (Fig. 49.18). To enhance diagnostic accuracy, it is important to confirm the presence of flow in the contralateral kidney and renal artery and in vessels at similar depth. Several studies have shown an overall accuracy rate of 93% for diagnosis of renal artery occlusion when attention is given to optimization of color and spectral Doppler parameters [20, 21, 56].

Horseshoe kidneys are usually detected in patients who present for investigation of a pulsatile abdominal mass or they are noted as incidental findings during an abdominal sonographic examination. This anatomic anomaly is uncommon, occurring in less than 1% of autopsies. In the majority of patients, the organs are joined at their lower poles with approximately 10% of kidneys fused at the upper pole. The organs are connected by an isthmus of tissue that usually lies anterior to the aorta at the level of the fourth or fifth lumbar vertebrae (Fig. 49.19). The connection of the poles creates a large, horseshoe-shaped organ which is supplied by multiple renal arteries and drained by multiple renal veins at unpredictable locations [32, 38]. Exclusion of renal artery stenosis is technically difficult because multiple arteries may originate from the aorta or the iliac arteries and enter the body of the kidney posteriorly. Accurate diagnosis of renal artery stenosis is best accomplished with digital subtraction or CT angiography.

The diameter of the adult renal artery is variable and appears to be gender dependent [57]. The mean diameter is 4.5–5 mm at the level of the renal ostium. Renal artery aneurysms are rare, occurring in less than 0.1% of the population, and are most often found incidentally during renal or abdominal sonography. The majority of aneurysms are saccular and noncalcified with a dilation less than 2 cm in diameter. Aneurysmal dilation of the renal artery usually involves the main renal artery or its first-order branches [58]. These extrarenal aneurysms are caused by atherosclerosis and fibromuscular dysplasia. Dilation of the intrarenal branches occurs in approximately 10% of cases. These parenchymal aneurysms are generally small and multiple, ranging in size from 1 to 12 mm.

Sonographic identification of extrarenal aneurysms is relatively easy given their size compared to the diameter of the native renal artery. Rarely are they detected during grayscale examination of the artery and kidney but are most easily identified with color flow imaging which also facilitates definition of the residual lumen (Fig. 49.20).

The therapeutic approach is based primarily on aneurysm size, location, patient’s age and gender, and the severity of associated hypertension. Although rupture of aneurysms less than 2 cm in diameter has been reported, intervention is generally reserved for aneurysms that exceed that diameter [59].

Renal fibromuscular dysplasia (FMD ) , a nonatherosclerotic disease entity, has been classified into three histologic forms dependent on the layer of the arterial wall that is affected [10, 12, 13]. Medial fibromuscular dysplasia occurs more frequently than intimal or adventitial/periarterial types and represents the second most common cause of renal artery stenosis, accounting for up to one-third of adult cases of renovascular hypertension. FMD has been found in arteries throughout the body, but in approximately 65% of cases, the disease involves the renal, carotid, and vertebral arteries [60]. The preponderance of disease is found in hypertensive females in the third or fourth decade of life and, in the vast majority of cases, involves the mid-to-distal segments of the renal artery, frequently extending into the parenchymal branches. The disease process results in web-like stenotic segments which, angiographically, create a “string of beads” appearance (Fig. 49.21). Sonographically, the beading is best visualized using color or power Doppler to highlight the irregular interface between the vessel wall and lumen, but the classic appearance is not always apparent (Fig. 49.22). The regions of concentric arterial narrowing and dilation often produce superimposed high- and low-velocity signals on the Doppler spectral waveform display.

Pediatric FMD occurs more frequently in males and is more likely to be associated with intimal fibroplasia than the medial form of the disease. Because of the similarity of presenting symptoms, it may be mistakenly diagnosed as a systemic necrotizing vasculitis [61, 62]. Additionally, in contrast to adult FMD, stenotic lesions may be found involving the aorta, further compounding the difficult diagnosis. In both adult and pediatric cases, careful attention must be given to location of lesions and grayscale and color flow images in order to differentiate stenotic lesions caused by FMD from arterial luminal compromise resulting from atherosclerosis and vasculitis. Atherosclerotic lesions occur most commonly at the renal ostium and within the proximal third of the artery, whereas medial FMD affects the mid-to-distal arterial segments. Unlike FMD, vasculitis is an inflammatory process involving the arterial wall. Sonographically, the wall appears concentrically thickened and edematous over variable lengths of the vessel. The affected segments are not confined to any specific segment of the artery.

It is important to note that the severity of FMD cannot be determined by applying the validated criteria used for classification of atherosclerotic renal artery stenosis. Given that the regions of narrowing are tandem and occur over a variable length of the renal artery, it is not possible to accurately determine a percentage severity of diameter reduction. The sonographic diagnosis is based on the finding of elevated velocities and post-stenotic turbulence in the mid-to-distal segments of the renal artery. Because fibromuscular dysplasia is frequently found bilaterally, it is important to search for evidence of the disease in the contralateral renal artery.

Flow-limiting atherosclerotic renal artery stenosis is most often treated with surgical bypass grafting, percutaneous transluminal angioplasty, and/or endovascular stent placement to improve hypertension and preserve renal function. Renal artery bypass grafts commonly originate from the native aorta, a prosthetic aortic graft, or the iliac arteries. Other less common inflow vessels include the hepatic or gastroduodenal arteries on the right side and the superior mesenteric or splenic arteries on the left side. Prior to initiating a duplex evaluation of the graft, the sonographer should know specific details of the surgical procedure including the origin of the graft, whether autologous saphenous vein or prosthetic material was used as the conduit, the location of the graft, and the type of distal anastomosis. An operative drawing indicating the donor artery, the course of the graft, and the recipient artery facilitates the duplex examination.

To recognize stenosis that could lead to graft occlusion, renal artery bypass grafts are usually followed postoperatively in a manner similar to the surveillance employed for peripheral artery bypasses. The proximal and distal anastomotic regions must be thoroughly interrogated with B-mode, color, and spectral Doppler to detect intimal hyperplasia and stenotic lesions that commonly occur at the anastomoses. If the conduit was created with the saphenous vein, careful attention is given to the possibility of retained valves or stenosis developing at the site of valve leaflets. The examination is extended into the donor and recipient arteries to detect progression of atherosclerotic disease. Critical stenosis or occlusion of the graft should be suspected if the Doppler spectral waveforms from the segmental or interlobar arteries of the kidney demonstrate delayed systolic acceleration and decreased velocity.

The use of percutaneous renal artery stenting has paralleled the evolutionary development of high-resolution abdominal duplex sonography. In modern endovascular practice, renal artery stenting has been shown to have value as a treatment option with a high technical success rate. Long-term patency of the stented arteries has, however, been problematic with in-stent restenosis rates of 6–20% being reported [63]. Because in-stent renal artery restenosis threatens renal artery patency and renal function, detection of these lesions is important. At present, angiographic techniques (digital subtraction, CTA, and MRA) and duplex sonography are used for evaluation of renal artery stents. In addition to other contraindications and limitations, contrast angiography, CTA, and MRA should be avoided in patients with renal insufficiency due to the nephrotoxic effects of iodinated contrast agents. Color duplex scanning has shown promise as a noninvasive tool for initial post-procedure evaluation and follow-up.