Duplex Ultrasound of Native Renal Vasculature

Introduction

Duplex and color Doppler ultrasound are utilized to evaluate a number of vascular diseases of the kidneys. The location of the renal vessels and the wide variation in the anatomy and physiology of the renal vessels can make the examination challenging and difficult to master. This chapter will provide a thorough review of all aspects of Doppler ultrasound in the assessment of the renal arteries and veins. The anatomy and principles of sonographic examination of the native renal vessels are considered first, followed by a discussion of renal vascular disorders, with special emphasis on renal artery stenosis and occlusion, and the renal vasculature assessment postintervention. Key features in the diagnosis of renal arterial aneurysms, arteriovenous fistulas (AVF), arteriovenous malformations (AVMs), renal vein thrombosis, and renal masses with tumor invasion of the renal veins will also be reviewed.

Anatomy

Each kidney receives its arterial supply from one or more renal arteries. The renal arteries arise from the proximal abdominal aorta just below the origin of the superior mesenteric artery, which serves as a reference point ( Fig. 28.1 ). The right renal artery arises anterolaterally from the aorta and passes posterior to the inferior vena cava (IVC). It is the only major vessel that courses posteriorly to the IVC. The left renal artery generally arises from the lateral or posterolateral aspect of the aorta. Anterior to each renal artery runs a corresponding renal vein, and both vessels course anterior to the renal pelvis before entering the medial aspect of the renal hilum ( Fig. 28.2 ). The left renal vein lies between the superior mesenteric artery and the aorta (as opposed to the splenic vein, which lies anterior to the superior mesenteric artery). The most common and most significant anatomic variants of the renal venous system are a circumaortic left renal vein, when one of the limbs of the left renal vein courses anterior to the aorta and another runs posterior to it; and a retroaortic left renal vein, when the left main renal vein lies posterior to the aorta. No other vessels course behind the aorta, and when a vascular structure is seen behind the aorta, a renal vein anomaly should be considered ( Fig. 28.3 ).

The right kidney is relatively inferior to the left kidney, which explains the downward course of the right renal artery, traversing posterior to the IVC and right renal vein. The left renal artery arises below the level of the right renal artery origin and is more horizontally oriented. It has a direct upward course to the more superiorly positioned left kidney ( Fig. 28.4 ). Each kidney is supplied by a single renal artery in only 70% of the population, with the remaining 30% supplied by multiple vessels. The three most common anatomic variations of the renal arterial circulation are: duplicated main renal arteries, accessory renal arteries, and polar renal arteries. On average, the main renal arteries measure approximately 4.6 mm in anteroposterior (AP) diameter. When more than one renal artery is present, the diameter of the renal arteries is variable, although they are generally smaller than when there is a single main renal artery. Duplicated renal arteries are defined as arteries that arise from the aorta, course toward the renal hilum, and supply similar proportions of the renal parenchyma ( Fig. 28.5 ). Accessory renal arteries also travel to the renal hilum but may arise from the aorta or the iliac arteries and supply smaller distinct segments of the renal parenchyma. Polar renal arteries do not enter the renal hilum but rather course directly to the cortex of one of the renal poles, and similar to accessory renal arteries, they may originate either from the aorta or iliac arteries. Both accessory and polar renal arteries commonly go unrecognized on ultrasound, but even duplicated main renal arteries may be overlooked sonographically. Rarely, renal arteries may arise from the superior mesenteric, inferior mesenteric, celiac, middle colic, or sacral arteries.

The main renal artery usually divides into five segmental arterial branches at the level of the renal hilum: the posterior, apical, upper, middle, and lower segmental renal arteries that feed the corresponding segments. The segmental arteries then course through the renal sinus and divide into the interlobar arteries, which are within the renal parenchyma and in close proximity to the collecting system. The interlobar arteries divide into the arcuate arteries, which course around the medullary pyramids and lead to the interlobular arteries ( Fig. 28.6 ). The interlobular arteries give rise to the afferent arterioles, which feed each glomerulus. Blood flows from the glomerulus to the efferent arterioles followed by the vasa recta, which provides the network for venous drainage of the kidney.

Venous drainage follows the same branching pattern as the arteries. However, unlike the arterial system, multiple communications exist between the renal segments within the venous system.

The normal peak systolic velocity (PSV) in the renal arteries ranges from 74 to 127 cm/s in both adults and children. Children tend to have slightly higher velocities than adults. The kidneys are end organs that require continuous forward flow throughout both systole and diastole, therefore the renal arteries demonstrate a low-resistance waveform pattern with a rapid systolic upstroke and persistent forward flow in diastole ( Fig. 28.7 ). An early systolic compliance peak or notch may be seen in some patients.

Principles of Examination

Doppler ultrasound evaluation of the renal arteries is one of the more challenging examinations to perform, given the small size of the renal vessels, their depth, and variable anatomy. The examiner is required to have an understanding of the renal anatomy, normal waveform physiology, and image optimization techniques. With time and experience, however, sonographers become adept at this study and are able to perform the examination in a reasonable period of time. Literature reports indicate that as many as 95% of main renal arteries can be adequately examined in adult patients. The key to the renal Doppler examination is accurate demonstration of the vascular anatomy.

There are a number of other imaging modalities that evaluate the renal vessels, each of which has strengths and weaknesses. Catheter angiography is considered the gold standard examination for this type of evaluation but is limited by its invasive nature and the fact that it exposes patients to iodinated contrast material and radiation. Pressure measurements between different segments of the renal arteries may be obtained only with catheter arteriography. Multidetector computed tomographic angiography (CTA) and contrast-enhanced magnetic resonance angiography (MRA) are less invasive alternatives to angiography, and both techniques have proven valuable in the demonstration of renal vascular disease . CTA offers higher resolution than MRA but also requires iodinated contrast material and is contraindicated in patients with low glomerular filtration rate (GFR) and residual renal function. CTA also exposes patients to ionizing radiation. MRA usually requires intravenous contrast material (gadolinium-based agents), which may be contraindicated in patients with renal failure, and is an expensive and time-consuming examination. MRA cannot be performed on claustrophobic patients or on patients with certain types of metallic implants. Generally, CTA and MRA provide superb anatomic information, but are limited in their ability to assess overall renal function. They also have limited utility in those patients who have undergone revascularization with stent placement.

Doppler sonography has significant benefits when compared with these other imaging modalities; it is an inexpensive and noninvasive method for evaluation of the renal vasculature and does not require contrast material. The Doppler examination is superior to other modalities because it not only provides anatomic information but also allows evaluation of physiologic factors. These physiologic changes can determine the hemodynamic significance of a renal vascular lesion and can assess the need for intervention. Doppler examination is often able to clarify uncertain or indeterminate CT or MR imaging (MRI) diagnoses.

There are several key elements to a successful abdominal Doppler examination. Adequate patient preparation is important to reduce the amount of bowel gas, which produces scatter and attenuates the ultrasound beam. We recommend a 12-hour fast before the examination. We prefer to schedule our renal Doppler studies first thing in the morning, before patients have breakfast, to improve visualization of vascular structures. We do not give any medication prior to performing the study. The examination is performed on a modern ultrasound unit, offering high-resolution gray-scale imaging, and sensitive color, power, and pulsed Doppler imaging. We routinely utilize harmonic imaging during our investigations to improve resolution and decrease artifacts . The technical success of each study is also influenced by the degree of operator experience, and we have had tremendous success by training sonographers in the techniques required to perform complete renal Doppler studies in a timely manner. The best examiners share several characteristics: motivation, patience, and a commitment to success. The learning curve is variable and often requires between several months and a year of experience, depending on the volume of cases performed.

Technique

The study is performed using 2- to 5-MHz curved array transducers, which provide the depth of penetration needed to visualize the abdominal aorta and its major branches: celiac, mesenteric, and renal arteries. Color Doppler flow imaging is an integral component of renal artery ultrasound examination and is useful for demonstrating patency of renal arteries and detecting flow disturbances that indicate stenosis. However, when used alone, this modality may give a false impression of renal artery stenosis because atherosclerotic plaques can cause flow disturbances in vessels that are not significantly stenotic. A low pulse repetition frequency (PRF) setting may also produce an aliased color flow signal in an area of normal velocity. Pulsed (spectral) Doppler analysis must be used in conjunction with color flow imaging, as it provides quantitative information through the measurement of blood flow velocities in the renal vessels.

There are a number of technical shortcuts that increase the likelihood of identifying the renal arteries in their entirety and decreasing the time of the examination. The first step is to optimize the gray-scale and color Doppler parameters to improve renal artery visualization as well as the conspicuity of flow-reducing lesions. Adjustment of the color Doppler parameters, including color gain, PRF (color velocity scale), and wall filter, is performed in areas of laminar flow within either the aorta or a normal segment of a renal artery. These adjustments in areas of normal blood flow allow the examiner to “screen” a vessel quickly for stenosis because elevated velocities in stenotic regions produce a color aliasing artifact that is readily apparent. The examiner can then place the Doppler sample volume at the site of blood flow disturbance to determine the highest PSV.

In addition to optimizing the color Doppler parameters, the experienced sonographer utilizes all available acoustic windows to obtain velocity information from the renal arteries. The renal arteries can be visualized from an anterior abdominal approach in the supine position through the abdominal wall ( Fig. 28.8A ), in a decubitus position through the liver, spleen, and either kidney, and prone position with a posterior (translumbar) approach through the patient's back (see Fig. 28.8B ). The anterior abdominal approach is useful for evaluating the renal ostia, providing optimal orientation of the renal vessels to facilitate accurate sampling with spectral Doppler (see Fig. 28.8C and D ). A common limitation of this approach is the presence of overlying bowel gas that limits visualization of the renal arteries. This may be overcome by continuous gradual application of transducer pressure that displaces the gas-containing bowel loops away from the vessels of interest. When the anterior abdominal approach is not feasible because of persistent artifacts and attenuation from bowel gas or obesity, other alternative acoustic windows are utilized. In particular, the deep abdominal vessels are frequently seen in the decubitus and prone positions, using the liver and kidneys as acoustic windows to visualize the renal arteries. These windows allow sonographers to obtain all the necessary color flow views and spectral Doppler samples from the renal arteries in a timely manner. Another method of identifying the main renal arteries, particularly their ostia, has been termed the “banana peel” view. This view is obtained by turning the patient to the opposite decubitus position from the vessel being examined, maintaining longitudinal orientation of the transducer. Once the aorta is located, visualization of the origins of the main renal arteries is achieved by moving the transducer in an anterior-to-posterior direction ( Fig. 28.9 ).

The spectral Doppler examination is performed with a small sample volume so as to obtain flow information from only the vessel of interest and minimize contamination from adjacent vessels. Doppler sampling is performed with angles of 60 degrees or less, as angles of greater than 60 degrees will spuriously increase PSV measurements. As with all Doppler studies requiring angle correction, interrogation at a constant angle allows for reproducibility of PSVs between follow-up studies and maintains a similar range of error in PSV measurements. The spectral Doppler PRF is adjusted so that the waveforms are large and easy to interpret without producing aliasing.

Protocol

Our protocol for the renal artery Doppler examination includes not only the evaluation of the renal vasculature but also an examination of the kidneys. Generally, kidney size is an important factor in the decision-making process for consideration of revascularization. The normal adult kidney usually measures greater than 8 cm in longitudinal size, with an average length of 10 to 12 cm. The kidneys are generally symmetric in size with a normal allowable variation in size of 2 cm between the contralateral sides. The kidneys are best examined with the patient in a decubitus position, with the left decubitus utilized for assessment of the right kidney and the right decubitus position used for assessment of the left kidney. In addition to kidney length, we note renal parenchymal echogenicity and thickness. The kidneys are assessed for atrophy, scarring, hydronephrosis, calculi, and masses. Incidental discovery of occult renal cell carcinomas is not uncommon in our practice, with several cases diagnosed each year during routine renal Doppler examinations.

Once the renal examination is complete, we perform a survey of the abdominal aorta from the celiac artery to the iliac bifurcation and evaluate for atherosclerotic plaque. This is done with both gray-scale and color flow Doppler examination of the abdominal aorta in a longitudinal plane. Gray-scale evaluation is important to assess for irregular plaque and ostial lesions (i.e., at the origin of the aortic branches), which may be obscured by color flow blooming. ( Color flow blooming artifact refers to the tendency of the color flow Doppler signal to extend beyond the vascular lumen, obscuring adjacent structures, including atherosclerotic plaque and the vessel wall.)

The presence of significant aortic plaque should increase suspicion for ostial renal artery disease, particularly in older adults and diabetic patients. Conversely, the absence of plaque in the aorta decreases the likelihood of atherosclerotic renal artery stenosis. We also look for flow abnormalities at the origins of the celiac and superior mesenteric arteries, findings that may correlate with significant stenosis. The size and location of abdominal aortic aneurysms are noted, and their relationship to the renal arteries is reported. Finally, angle-corrected PSV measurements are obtained in the abdominal aorta at the level of the renal arteries. These aortic velocity measurements are used to determine the renal artery to aorta velocity ratio (RAR), which will be discussed later.

After the survey of the abdominal aorta, we direct our attention to the renal arteries. Our protocol includes the direct examination of both main renal arteries and sampling of the segmental branches in both renal hila. When possible, we locate the origins of the renal arteries on transverse images of the aorta using an anterior transducer approach. The celiac and superior mesenteric arteries serve as landmarks for localization of the renal artery ostia. Once the mesenteric arteries are located, the transducer is moved inferiorly along the aorta until the origin of each renal artery is seen. The right renal artery is often easier to identify than the left with this approach and is relatively easy to follow to the renal hilum (see Fig. 28.2A ). The left renal artery is harder to follow all the way to the kidney from an anterior approach, and may be better seen by positioning the patient in a right lateral decubitus position and scanning from a left posterolateral transducer approach. The left kidney can be used as an acoustic window (see Fig. 28.4 ). An analogous approach can be utilized to visualize the distal right renal artery and its branches, with the patient in a left lateral decubitus position and using the liver as an acoustic window. In children, both renal arteries can sometimes be viewed simultaneously from a coronal approach through the left kidney. Transverse and sagittal sweeps of the abdominal aorta and kidneys are performed to identify variants in renal artery anatomy. As mentioned earlier, these variants may arise from the inferior aorta or iliac arteries and can be followed to the renal hilum or either pole of the kidney (see Fig. 28.5 ).

Each renal artery should be examined with color flow imaging from its point of origin to the hilum of the kidney, with the main hilar branches included in the assessment. Look for areas of high-velocity flow, indicated by color shifts or aliasing and turbulence-related flow disturbances, as these may be related to underlying stenosis ( Fig. 28.10 ). If areas of abnormal flow are identified, they should be interrogated with spectral Doppler, and corresponding PSVs should be recorded. Our renal Doppler protocol includes PSV measurements obtained from the origin, proximal, mid, and distal segments of each renal artery. Please note that this is similar to the Native Renal Artery Duplex Practice Parameter developed in conjunction with the American Institute of Ultrasound in Medicine, American College of Radiology, Society of Radiologists in Ultrasound, and Society of Pediatric Radiology that recommends Doppler insonation of the origin/proximal, mid, and distal/hilum segments. We obtain at least four PSV measurements from each renal artery to maximize detection of renal artery stenosis and improve visualization of blood flow changes that infer the presence of flow-limiting lesions. To minimize errors when measuring PSVs, a small sample volume (1.5 to 2.0 mm) should be placed in the center of the arterial lumen and at an angle insonation of 60 degrees or less. Finally, Doppler waveforms are also obtained from the segmental arteries in the upper, mid, and lower poles of each kidney. Thus at least seven waveforms are captured from each side.

It is important to obtain clean, crisp waveforms with well-defined borders for analysis, which can be accomplished by adjusting the spectral display so that the waveforms are large and easily measured. This allows the examiner to readily determine the PSVs throughout the renal arteries and their branches, and accurately determine acceleration time, acceleration index, and the resistivity index (RI) in the segmental renal arteries. Fast sweep speed and velocity scale optimization are helpful to improve waveform interpretation. Angle correction is required for segmental artery duplex evaluation when acceleration index measurements are used for assessment of stenosis. The RI can be calculated by subtracting the end-diastolic velocity from the PSV and dividing that number by the PSV. We have not found RI analysis to be valuable in the evaluation for renal artery stenosis as the RI may be elevated in numerous conditions, including parenchymal renal disease, acute tubular necrosis, pyelonephritis, renal vein thrombosis, and urinary tract obstruction.

Our philosophy when performing a renal artery duplex ultrasound examination is quite pragmatic. We limit the amount of time allotted for our renal Doppler studies. In our experience, a complete renal artery Doppler examination can be performed in as little as 20 minutes, and we never exceed 60 minutes. Experienced examiners can assess a patient quickly and determine if the study can be completed in a timely manner. Studies on difficult patients who cannot cooperate or who are not “sonogenic” are aborted promptly, and an alternative study is recommended for further evaluation. Although considered an “off-label” application by the Food and Drug Administration at this time, ultrasound contrast agents may facilitate visualization of the renal arteries and “rescue” difficult examinations (see Chapter 35 ). It is also important to recognize that atherosclerotic renal artery disease is far and away the most common etiology of significant renal artery stenosis, and these lesions occur at the origin and proximal segments of the renal artery. We pay close attention to these segments in our older adult patients who are apt to have atherosclerotic obstructive lesions. It is more important to visualize the entire renal artery in younger adults as these patients are more likely to have fibromuscular dysplasia (FMD), which can affect the mid and distal renal artery or the segmental branches.

As mentioned earlier, ultrasound contrast (echo-enhancing) agents allow for increased visibility of blood vessels and improved visualization of renal arterial blood flow patterns. Ultrasound contrast may be particularly helpful in visualizing the renal arteries in obese patients. In addition to reducing examination time, the use of these agents may enhance ultrasound visualization of multiple renal arteries and hilar branches.

Vascular Disorders

There are a variety of disorders that affect the renal vasculature. These conditions may result in damage to the renal parenchyma, renal failure, and/or hypertension. The most common vascular conditions affecting the renal arteries are renal artery stenosis (due to atherosclerosis or FMD), renal artery occlusion, and renal artery aneurysm. Renal vein thrombosis can be seen with hypercoagulable states, malignancy (tumor thrombus), or with propagation of clot from the IVC. Other renal vascular pathologies include AVF, dissection, vasculitis, and pseudoaneurysm formation.