Renal Relevant Radiology: Use of Ultrasound in KidneyDisease and Nephrology Procedures

W. Charles O’Neill

AbstractUltrasound is commonly used in nephrology for diagnostic studies of the kidneys and lower urinary tract and toguide percutaneous procedures, such as insertion of hemodialysis catheters and kidney biopsy. Nephrologistsmust, therefore, have a thorough understanding of renal anatomy and the sonographic appearance of normalkidneys and lower urinary tract, and theymust be able to recognize common abnormalities. Proper interpretationrequires correlation with the clinical scenario. With the advent of affordable, portable scanners, sonography hasbecome a procedure that can be performed by nephrologists, and both training and certification in renalultrasonography are available.

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IntroductionSonography is an essential tool in nephrology for notonly the diagnosis and management of kidney dis-ease, but also for the guidance of invasive procedures.For this reason, it is essential for nephrologists tohave a thorough understanding of sonography and itsuses in nephrology. Technological advances over thepast 15 years have resulted in high-quality scannersthat are both portable and affordable, which hasgreatly expanded the use of point-of-care sonographyby clinicians. Although nephrologists have been laggingin this area, an increasing number are incorporatingsonography into their practice, and training programsare finally starting to meet this need. This review willcover the salient points of renal ultrasonography and itsapplication to the evaluation of kidney disease and theperformance of invasive procedures. For more completecoverage, several articles and books directed towardnephrologists can be found (1–4).

Sonographic Evaluation of the Kidneys andUrinary Tract

Because of their location, architecture, and limitedspectrum of pathology, the kidneys are ideally suited forevaluation by ultrasound. In addition, it is safe, readilyavailable, easily performed at the bedside or in the office,and free of radiation. For these reasons, sonography is thepreferred imaging modality and often the only one re-quired. Evaluation includes assessment of the size andshape, the echogenicity, the urinary space (including thelower urinary tract), the presence of masses, and the vas-culature. Very few findings are specific, and interpreta-tion, therefore, requires clinical correlation, another reasonfor the participation of nephrologists in this procedure.

Size and ShapeSize is a key parameter that should be measured

carefully, since it is the basis for important clinical

decisions (Figure 1). Given its poor precision (5), thismeasurement should be performed several times.Since the poor precision stems mostly from under-measurement, the maximum length is the value thatshould be reported. Measurement of other dimen-sions is even more imprecise and is of no utility.The same applies to the estimation of kidney volume,which correlates poorly with more accurate tech-niques (5). Kidney length in adults should usuallybe between 10–12 cm but varies with body size and,unfortunately, there are no nomograms for normalkidneys based on large population studies. Nomo-grams are available for children (2,6). Cortical thick-ness should be estimated in addition to length and ismeasured from the base of the medullary pyramid tothe edge of the kidney. It generally should be between7 and 10 mm (7–9) but varies within a kidney, beingthicker at the poles.When the medullae are not visible,one has to rely on parenchymal thickness, whichshould be 1.5–2.0 cm, but varies within the kidney.Accentuation of the lobulation is often a sign of cor-tical thinning. Enlargement of the kidney because ofinflammation or infiltration is often accompaniedby a decrease in the aspect ratio, resulting in amore globular shape.

EchogenicityEchogenicity of a structure (acoustic interface) refers

to the amount of sound it reflects back to the probe,which is dependent on the amplitude of incident sound,how much of the sound is absorbed, how much isreflected, and the angle of reflection. These sameproperties also dictate tissue echogenicity, which isthe collective back-scatter from numerous microscopicinterfaces and is determined by the micro-architecture.Increased echogenicity lacks specificity and in histologicstudies has correlated with interstitial fibrosis, tubularatrophy, inflammation, and glomerulosclerosis (10,11).

Renal Division,Department ofMedicine, EmoryUniversity School ofMedicine, Atlanta,Georgia

Correspondence:Dr. W. CharlesO’Neill, EmoryUniversity School ofMedicine, RenalDivision WMB 338,1639 Pierce Drive,Atlanta, GA 30322.Email: woneill@emory.edu

www.cjasn.org Vol 9 February, 2014 Copyright © 2014 by the American Society of Nephrology 373

Decreased echogenicity usually results from edema. Corticalechogenicity can only be evaluated qualitatively and shouldbe less than the liver or spleen (12). The medulla should haveless echogenicity than the cortex, but the ability to discernthis is dependent on scanning parameters and overlyingstructures. Although the lack of cortico-medullary differentia-tion is frequently mentioned in ultrasound reports, the inabil-ity to see the medullae is common and not abnormal. Rather,prominence of the medullae is usually abnormal, generallyindicating increased echogenicity of the cortex (Figure 1C).

Urinary SpaceThe center of the kidney should be echogenic because of

the presence of sinus fat, and the calyces are not visibleunless dilated (Figure 2). Hydronephrosis appears asbranching, interconnected areas of decreased echogenic-ity that show sonographic evidence of fluid (see below).This is usually easily distinguishable from dilated renalveins but can be confirmed with Doppler ultrasound ifnecessary. In cases of hydronephrosis, attempts should bemade to visualize the ureter, which normally is not visi-ble. The urinary bladder should be imaged in all casesof hydronephrosis. Volume is estimated from cross-sectional dimensions on transverse imaging and the lon-gitudinal dimension in the sagittal plane using the formulafor the volume of an ellipsoid (0.523the three orthogonaldimensions). The distal ureters can be visible at the base

of the bladder, and dilatation indicates a problem at thebladder.

MassesMasses are usually apparent as distortions in the renal

contour or architecture, although this appearance may notbe apparent for masses with echogenicity that matches thesurrounding tissue (Figure 3). Most are benign simplecysts, which are round and surrounded by a smooth,thin wall. The fluid is characterized by the complete lackof echogenicity (i.e., black) and enhancement of the distalwall and deeper structures because of the lack of soundattenuation in fluid. Difficulties arise with complex cysts,which exhibit internal echoes, have some thickening orirregularity of the wall, or contain septations. The vastmajority of these are minimally complex and do not re-quire additional imaging (13) beyond follow-up sonogra-phy. More complex cysts (prominent thickening orirregularity of the wall) should prompt additional imag-ing. Cysts are common and are often associated with CKD(14,15), with acquired cystic disease being the extremeform. Autosomal dominant polycystic kidney disease(ADPKD) is the other common multicystic disease, and itis characterized by severe renal enlargement. Echogenicmasses are usually neoplasms but can also represent hem-orrhagic cysts, and they often require additional imaging.Stones typically appear as echogenic foci within the

Figure 1. | Sonographic appearance of the renal parenchyma. (A) Normal right kidney (longitudinal view). (B) Normal right kidney (transverseview). (C) Echogenic right kidney with prominent medullary pyramids (arrows). (D) Atrophic right kidney (longitudinal view) with thin pa-renchyma and containing mostly sinus fat. L, liver.

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collecting system that cast acoustic shadows, but eitherfinding may be absent. A twinkling artifact (nonvascularcolor signal) on Doppler imaging has been proposed as anadditional method to detect stones, but it has poor sensi-tivity and specificity (16).

VasculatureAlthough blood vessels can be seen with B-mode

ultrasound, they are usually imaged with Doppler ultra-sound. A detailed description of renal vascular ultrasoundis beyond the scope of this review and can be found in arecent review (4). Imaging usually consists of direct visu-alization of the renal arteries or veins with measurement ofblood velocity and of analysis of velocity waveforms (suchas resistive index) in segmental arteries. Examination ofthe renal artery is technically difficult due to poor insona-tion angles (should be less than 70o), overlying bowel, andthe frequent occurrence of multiple arteries, and requiressignificant expertise. It has a high failure rate (up to 30%)even in expert hands (4). While sensitivity and specificitycan be high, they vary considerably between publishedstudies (17) indicating that the results are center-specificand probably reliable only in highly experienced centers.While intra-renal studies are much easier and angle-independent, interpretation of the results is problematic.

Although resistive index is commonly used as an indica-tion of renal vascular resistance, it is also influenced by anumber of extra-renal factors, including heart rate andperipheral vascular resistance, and is not specific for renaldisease (4,18). Resistive index has been reported to provideprognostic information (19,20), but this has been ques-tioned and may be based on extra-renal, rather than renal,factors (21).

Diagnostic Applications of Sonography in KidneyDiseaseSonography is useful in many aspects of the diagnosis

and management of kidney disease. Because the findingsare rarely specific, clinical correlation is essential to the inter-pretation, an important reason why nephrologists shouldbe performing these studies.

CKDSonography should be performed in all patients with

CKD primarily to recognize advanced, irreversible kidneydisease that would obviate any additional workup, in-cluding biopsy (11). Signs include small size, thin cortex,and cysts, but one must be cautious in making the diag-nosis based solely on size in the 9- to 10-cm range when

Figure 2. | Sonographic appearance of the upper and lower urinary tract. (A) Hydronephrosis of a left kidney (longitudinal view) with dilatedrenal pelvis and major and minor calyces. (B) Another longitudinal view of the same kidney showing a dilated ureter (arrows) tracking un-derneath the lower pole. (C) Transverse viewof the urinary bladder (B) showing dilated distal ureters (arrows). (D) Transverse viewof the urinarybladder with an enlarged prostate gland (P).

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the kidneys are otherwise normal. Although cortical echo-genicity is often increased in CKD, normal echogenicity isnot unusual, and echogenicity can increase in reversibledisease. Thus, echogenicity alone is not a reliable indicatorof CKD. Sonography can also identify specific causes, suchas urinary obstruction, polycystic kidney disease, refluxnephropathy, and interstitial nephritis.

Acute Renal FailureWhile sonography can be useful in acute renal failure, its

use should be limited to those patients in whom the causeis not apparent or in whom urinary obstruction is possible(22,23). In addition to ruling out urinary obstruction, so-nography is useful in diagnosing unrecognized CKD. Thekidneys often appear normal in acute tubular necrosis(ATN) but can be enlarged and/or echogenic. Assessmentof kidney size in ATN is difficult because the baseline sizeis rarely known and there is often underlying CKD. In astudy of 26 patients with ATN, only 6 had a renal lengthgreater than the normal range (24). The aspect ratio wasincreased in 12 patients, indicative of swelling, but thismeasurement is unreliable and dependent on the planein which the kidney is scanned. An increase in kidneysize can also occur in other causes of acute renal failure.Echogenicity is nonspecific and can increase in other

causes of acute renal failure, including glomerulonephritisand interstitial nephritis, and can be influenced by fluidstatus (12). Increased resistive index is a nonspecific find-ing that lacks sensitivity in diagnosing ATN (25).

Cystic Kidney DiseaseCystic kidney disease is either genetic or acquired.

ADPKD is the most common genetic type and character-ized by renal enlargement in addition to multiple cysts.Hepatic cysts are common but absent in other conditionsthat can mimic ADPKD, such as tuberous sclerosis and vonHippl–Lindau disease. Ultrasound is sufficient to make thediagnosis, and although the measurement of length is im-precise, it suffices for prognosis (26). Criteria for the diag-nosis of ADPKD in subjects at risk have been developedbased on the prevalence of sporadic cysts (27). Acquiredcystic disease is associated with advanced CKD and easilydistinguished from ADPKD on the basis of kidney size.Sonography can also be useful in identifying infected orhemorrhagic cysts.

Pain and HematuriaCT scanning is usually indicated for the workup of pain

and hematuria, but in some cases the diagnosis can be madeby ultrasound and it is not an unreasonable initial study.

Figure 3. | Sonographic appearance of renal masses. (A) Simple cyst in the lower pole of a renal allograft (transverse view). Note the round,smooth, and thinwall and absence of internal echoes. The distal enhancement (arrows) indicates that it is fluid-filled. (B) Complex cyst in the leftmid-kidney containing echogenic material (transverse view). (C) Solid mass in the right mid-kidney (longitudinal view). Note the echogenicitywithin the mass and the lack of distal enhancement. (D) Stone in the lower pole of a left kidney (longitudinal view) casting an acoustic shadow(arrows). There is also a simple cyst (C).

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Stones are usually visible, but up to 50% can be missed (28),particularly when small or within the ureter. Thus, com-puted tomography scanning is more appropriate for acuterenal colic. Sonography is not indicated in uncomplicatedpyelonephritis, which usually appears as hypoechoic en-largement of a single lobule caused by edema. Renal infarc-tion may have a similar appearance but with absent bloodflow on Doppler imaging, although the latter may be diffi-cult to demonstrate.

Screening for CarcinomaCertain individuals are at increased risk of renal malig-

nancies, most notably those individuals with prior tumorsand patients with ESRD, even with functioning transplants.With the exception of ESRD, screening should ideally beperformed with the more sensitive modalities of computedtomography or magnetic resonance imaging. Whether ornot and how to screen in ESRD remains controversial basedlargely on the limited lifespan, and it may be appropriateonly before and after transplantation, although it is notcurrently recommended (29). There are no large prospec-tive studies comparing ultrasound with other modalitiesfor this purpose. Although sonography may be less sensi-tive, it is less expensive and does not involve radiationexposure. Because kidneys with acquired cystic diseaseare at particular risk (30), a reasonable strategy may bethe initial use of sonography to screen for cysts.

Transplant NephrologySonography is indicated in most cases of acute renal

failure because of the single functioning kidney and thefrequency of urologic complications. The routine perioper-ative use of ureteral stents has reduced ureteral obstruction,but bladder dysfunction remains common. Because ofproximity to the probe and other factors, the urinary spaceis often visible in the absence of obstruction, and someexperience is necessary in determining when it is clinicallysignificant. Sonography is not useful in diagnosing acuterejection unless it is severe, in which case the allograft isswollen and echogenic. However, this finding can also beseen in acute tubular necrosis and nephritis. Doppler ultra-sound is useful in diagnosing vascular causes of transplantfailure, but these are rare and occur almost exclusively in theimmediate postoperative period. It can be useful in thediagnosis of transplant artery stenosis as a cause of hyper-tension. The measurement of resistive indices is of no utilityin differentiating causes of transplant failure and should notroutinely be performed (25).

Procedural Applications of Ultrasound in NephrologyBecause of its ease of use and availability at the bedside,

sonography has revolutionized the performance of percu-taneous procedures. The enhanced success rates and re-duced complication rates with ultrasound guidance havemade it the standard of care for many of these procedures,including those procedures performed by nephrologists.However, like any tool, success depends on proper use andan understanding of the limitations. There are two basicapproaches. The entry site, angle, and depth can be de-termined with ultrasound, after which the needle is placedwithout direct ultrasound guidance (ultrasound marking), or

ultrasound can be used during the needle insertion (real-timeguidance).

Marking versus Real-Time GuidanceAdvantages and disadvantages exist to each approach,

and in experienced hands, the success and complicationrates are probably similar. No prospective, randomizedcomparisons have been performed. Procedures performedwithout real-time guidance depend on careful marking ofthe entry site and, for deeper targets, adherence to aperpendicular angle during the ultrasound marking andneedle insertion. The principal advantage is having bothhands available for the actual procedure. The major dis-advantage is that sonographic depth is often less than thetrue depth, which can result in the needle not being placeddeep enough. Also, matching insonation and insertionangles is sometimes difficult, and the target may move.While real-time guidance would seemingly be superior,

needles are difficult to detect by ultrasound. The straight,smooth surface reflects sound at the angle of incidence andwill only return sound to the probe when maintainedperpendicular to the beam, which is difficult to achieve(Figure 4A). Although the ends of needles are often rough-ened so that sound reflects at multiple angles, the increase invisibility is marginal. Thus, localization of the needle usuallydepends on movement of adjacent tissue. However, thisonly indicates the presence of the needle, it does not neces-sarily localize the end, which could be outside the soundbeam and, usually, deeper (Figure 4B). Thus, the probemust also be moved to be certain of where the needle endsand to ensure that the needle is not being advanced too far.

Hemodialysis CathetersAlthough ultrasound guidance is not essential for cath-

eterization of central veins, it is now considered thestandard of care, markedly increasing success rates andlowering complication rates (31,32). The site should be im-aged before the procedure to ensure normal anatomy andadequate visualization of the target (33,34). The internaljugular vein is lateral to the carotid artery while the fem-oral vein is medial to the femoral artery. Sites of previouscatheterizations or attempts should be examined carefullyfor venous thrombosis or arterial pseudoaneurysms, andthe internal jugular vein should be imaged down to thesubclavian vein to confirm patency. The vein should beeasily compressible (Figure 5A), and, in the case of theinternal jugular vein, venous pulsations should be easilyvisible. If only one vessel is visible, it is far more likely tobe the artery. The insertion path should not include theartery, which, even when distal to the vein, can be easilyentered even during real-time guidance. It is critical thatthe insertion angle and probe placement be adjusted sothat the needle enters the vein within the sound beam(Figure 5B). Care must be used not to compress the veinduring real-time guidance. It can be avoided by guidingthe needle to the surface of the vein and then puncturingthe vein without real-time guidance.

Percutaneous Renal BiopsyIt is unfortunate that few nephrologists outside of

academic centers perform renal biopsies, despite the fact

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that nephrologists can perform ultrasound-guided biopsiessafely and with high yield (35). In fact, the yield of glo-meruli in this study was higher than in other publishedstudies, indicating that biopsies performed by nephrolo-gists result in higher quality specimens. The great majorityof biopsies not performed by nephrologists are per-formed by CT-guidance, which results in unnecessary ra-diation exposure and expense since the success andcomplication rates are similar for CT and ultrasoundguidance (35,36). Sonography should be performed be-fore scheduling a biopsy to ensure that the kidneys canbe adequately visualized and rule out anatomic abnor-malities (such as lower pole cysts) or advanced CKDthat would preclude biopsy. Even if the kidneys can bevisualized, a depth greater than 10 cm can be problematicdue to the length of the biopsy needles. Since obesity ismore ventral than dorsal, ultrasound guidance is feasiblein many obese patients. Ultrasound guidance should notbe attempted when the kidneys are not well visualized.There are no randomized studies comparing the out-comes with ultrasound marking and real-time guidance,but they are probably similar in experienced hands (36).The choice of technique should be based on individual orcenter preference, experience, and outcomes as well aspatient characteristics.The angle of insertion and timing with respirations are

critically important for ultrasound marking. For nativekidneys, the patient is prone with back as flat as possible toassist in maintaining a perpendicular angle. Timing of thebiopsy is dictated by the presence of overlying ribs andmovement of the kidney with respirations. End expirationis preferable, because it is more consistent; also, the kidneytends to move deeper with inspiration. The lower pole ofthe kidney is localized in both the longitudinal and trans-verse planes with attention to maintaining a perpendicular

angle (Figure 6), and the depth is noted. The sonographicdepth is usually 1.5–2 cm less than the actual biopsy depthbecause of pressure from the ultrasound probe. In our in-stitution, the true depth is determined with a spinal needlethat is also used to deliver deep anesthesia. Initial passesmay not be successful, usually because the needle is notdeep enough or the patient’s position or respiratory move-ment has changed.Real-time guidance avoids the issues of angle and timing,

thus increasing the accuracy. However, this can also resultin a false sense of security for inexperienced operators.Attention must be focused on the location of the end of thebiopsy needle, which, if not within the sound beam, canappear shallower than it is. Reliable localization requiresmovement of both the needle and the probe. The tendencyto go too deep is compounded by the fact that a secondhand is not available to hold the biopsy device at the skinand prevent the further advancement that can easily occurduring activation. This advancement can be avoided byabandoning the probe during the activation.The low complication rate of ultrasound-guided biopsies

has been well documented (36) and this procedure is ex-tremely safe in knowledgeable and experienced hands.However, major complications do occur, but are rarelyreported in the literature. The author is aware of severaldeaths and a number of serious complications from nativerenal biopsies, even ones performed under real-timeguidance.Ultrasound guidance is similar for biopsy of trans-

planted kidneys, except that the patient is in the supineposition. One should avoid entering the peritoneum,which can be immediately adjacent to the allograft andoften overlie it; it is recognizable by the peristalsis.Although it is unavoidable in intraperitoneal allografts,bowel loops should clearly be avoided. The allograft is

Figure 4. | Ultrasound principles of real-time visualization. (A) Needles are specular reflectors from which sounds reflect at the angle ofincidence.Only the sound that strikes at a perpendicular angle returns to the probe.Other soundsmay not reach the probe and those portions ofthe needlewill not be detected. (B)Needles (left) notmaintainedwithin thewidth of the sound beamwill appear shallower than (right) their truedepth on the sonogram.

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often very shallow and easily accessed, but it can be verydeep in obese patients. Due to the difficulty in matchingneedle and insonation angles on the abdomen and thepossibility of overlying bowel, real-time guidance isrecommended in these patients and patients with intraperi-toneal allografts.Sonography also has a role in postbiopsy management.

Routine imaging immediately after a biopsy is not un-reasonable but generally not helpful, because small peri-renal hematomas and pseudoaneurysms can be seen inotherwise stable patients. In a recent study of ultrasound-guided biopsy in native kidneys, small hematomas werenoted in 13% of patients. However, the absence of thesefindings several hours after a biopsy indicates a very lowrate of subsequent complications and may allow for earlydischarge of patients (37). Although sonography should beperformed in cases of pain, hypotension, or laboratory ev-idence of blood loss, other imaging is often required. Peri-renal and subcapsular hematomas are easily seen, butretroperitoneal hematomas may be difficult to discern;also, their extent cannot be reliably determined. Bleedingwithin the urinary space may only be apparent as calycealand ureteral dilatation.

Other Applications of Ultrasound in NephrologySpace considerations prevent a detailed discussion of

additional applications of sonography relevant to thepractice of nephrology and potentially performable bynephrologists. Foremost among these is in hemodialysisvascular access, particularly arteriovenous fistulae, for bothplanning and evaluation of dysfunction. Others includeinsertion of peritoneal catheters (38), diagnosis of tunnelinfections in peritoneal dialysis catheters (39), assessmentof intravascular volume status through examination of theinferior vena cava (40), and detection of lung water (41).

Ultrasound as a Nephrology ProcedureSonography is essential to the practice of nephrology; it is

inexpensive and relatively easy to learn, making it an idealtool to be incorporated into the practice of nephrology.With appropriate training, nephrologists can be competentat both performing and interpreting sonograms (42), andcan provide the clinical correlation that is usually requiredfor interpretation. However, nephrology has the dubi-ous distinction of being one of the few specialties orsubspecialties that has not embraced ultrasound and

Figure 5. | Sonographic guidance of central venous cannulation. (A) Transverse view of the anterior neck. The internal jugular vein (V) isunderneath the sternocleidomastoidmuscle (SCM). The carotid artery (arrow) is posterior andmedial to the vein, and it is positioned outside thepath of a needle into the center of the vein. Right panel shows the collapse of the vein, but not the artery, with compression. (B) The needleshould be offset from the probe and at an angle such that (right) it enters the vein within the sound beam.

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incorporated it into its training and practice. Although thisvoid has been filled to some extent by the American Soci-ety of Diagnostic and Interventional Nephrology in theform of training guidelines and certification (43), fewfellowship programs offer training that meets theseguidelines (44). However, comprehensive training isavailable (45) along with certification (46), and an in-creasing number of nephrology practices are incorporat-ing this procedure. It is hoped that renal ultrasonographywill eventually become an integral component of nephrologytraining.

DisclosuresW.C.O. has consultancy agreements with Baxter Healthcare,

Astellas Pharmaceuticals, Synageva BioPharma, Novo Nordisk,and BioMarin Pharmaceutical; received research funding fromBaxter Healthcare, Genzyme, Amgen, and Synageva BioPharma;

received honoraria from Baxter Healthcare, Astellas Pharmaceuti-cals, andAmgen;haspatents and inventionswithBaxterHealthcare;is a scientific advisor to, or has a membership with, Amgen, KidneyInternational, and American Journal of Kidney Disease; and hasother interests/relationships with Emory University Office ofContinuing Medical Education.

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Published online ahead of print. Publication date available at www.cjasn.org.

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