CJASN ePress. Published on January 23, 2014 as doi: 10.2215/CJN.03170313 Renal Relevant Radiology: Use of Ultrasound in Kidney Disease and Nephrology Procedures W. Charles O’Neill Abstract Ultrasound is commonly used in nephrology for diagnostic studies of the kidneys and lower urinary tract and to guide percutaneous procedures, such as insertion of hemodialysis catheters and kidney biopsy. Nephrologists must, therefore, have a thorough understanding of renal anatomy and the sonographic appearance of normal Renal Division, Department of kidneys and lower urinary tract, and they must be able to recognize common abnormalities. Proper interpretation Medicine, Emory requires correlation with the clinical scenario. With the advent of affordable, portable scanners, sonography has University School of become a procedure that can be performed by nephrologists, and both training and certification in renal Medicine, Atlanta, ultrasonography are available. Georgia Clin J Am Soc Nephrol 9: ccc–ccc, 2014. doi: 10.2215/CJN.03170313 Correspondence: Dr.W.Charles O’Neill, Emory Introduction decisions (Figure 1). Given its poor precision (5), this University School of Sonography is an essential tool in nephrology for not measurement should be performed several times. Medicine, Renal only the diagnosis and management of kidney dis- Since the poor precision stems mostly from under- Division WMB 338, 1639 Pierce Drive, ease, but also for the guidance of invasive procedures. measurement, the maximum length is the value that Atlanta, GA 30322. For this reason, it is essential for nephrologists to should be reported. Measurement of other dimen- Email: woneill@ have a thorough understanding of sonography and its sionsisevenmoreimpreciseandisofnoutility. emory.edu uses in nephrology. Technological advances over the The same applies to the estimation of kidney volume, past 15 years have resulted in high-quality scanners which correlates poorly with more accurate tech- that are both portable and affordable, which has niques (5). Kidney length in adults should usually greatly expanded the use of point-of-care sonography be between 10–12 cm but varies with body size and, by clinicians. Although nephrologists have been lagging unfortunately, there are no nomograms for normal in this area, an increasing number are incorporating kidneys based on large population studies. Nomo- sonography into their practice, and training programs grams are available for children (2,6). Cortical thick- are finally starting to meet this need. This review will ness should be estimated in addition to length and is cover the salient points of renal ultrasonography and its measured from the base of the medullary pyramid to application to the evaluation of kidney disease and the the edge of the kidney. It generally should be between performance of invasive procedures. For more complete 7 and 10 mm (7–9) but varies within a kidney, being coverage, several articles and books directed toward thicker at the poles.When the medullae are not visible, nephrologists can be found (1–4). one has to rely on parenchymal thickness, which should be 1.5–2.0 cm, but varies within the kidney. Sonographic Evaluation of the Kidneys and Accentuation of the lobulation is often a sign of cor- tical thinning. Enlargement of the kidney because of Urinary Tract fl fi Because of their location, architecture, and limited in ammation or in ltration is often accompanied spectrum of pathology, the kidneys are ideally suited for by a decrease in the aspect ratio, resulting in a evaluation by ultrasound. In addition, it is safe, readily more globular shape. available, easily performed at the bedside or in the office, and free of radiation. For these reasons, sonography is the Echogenicity preferred imaging modality and often the only one re- Echogenicity of a structure (acoustic interface) refers quired. Evaluation includes assessment of the size and to the amount of sound it reflects back to the probe, shape, the echogenicity, the urinary space (including the which is dependent on the amplitude of incident sound, lower urinary tract), the presence of masses, and the vas- how much of the sound is absorbed, how much is fi fi culature. Very few ndings are speci c, and interpreta- reflected, and the angle of reflection. These same tion, therefore, requires clinical correlation, another reason properties also dictate tissue echogenicity, which is for the participation of nephrologists in this procedure. the collective back-scatter from numerous microscopic interfaces and is determined by the micro-architecture. Size and Shape Increased echogenicity lacks specificity and in histologic Size is a key parameter that should be measured studies has correlated with interstitial fibrosis, tubular carefully, since it is the basis for important clinical atrophy, inflammation, and glomerulosclerosis (10,11). www.cjasn.org Vol 9 February, 2014 Copyright © 2014 by the American Society of Nephrology 1 2 Clinical Journal of the American Society of Nephrology Figure 1. | Sonographic appearance of the renal parenchyma. (A) Normal right kidney (longitudinal view). (B) Normal right kidney (transverse view). (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. Decreased echogenicity usually results from edema. Cortical of the bladder, and dilatation indicates a problem at the echogenicity can only be evaluated qualitatively and should bladder. be less than the liver or spleen (12). The medulla should have less echogenicity than the cortex, but the ability to discern Masses this is dependent on scanning parameters and overlying Masses are usually apparent as distortions in the renal structures. Although the lack of cortico-medullary differentia- contour or architecture, although this appearance may not tion is frequently mentioned in ultrasound reports, the inabil- be apparent for masses with echogenicity that matches the ity to see the medullae is common and not abnormal. Rather, surrounding tissue (Figure 3). Most are benign simple prominence of the medullae is usually abnormal, generally cysts, which are round and surrounded by a smooth, indicating increased echogenicity of the cortex (Figure 1C). thin wall. The fluid is characterized by the complete lack of echogenicity (i.e., black) and enhancement of the distal Urinary Space wall and deeper structures because of the lack of sound The center of the kidney should be echogenic because of attenuation in fluid. Difficulties arise with complex cysts, the presence of sinus fat, and the calyces are not visible which exhibit internal echoes, have some thickening or unless dilated (Figure 2). Hydronephrosis appears as irregularity of the wall, or contain septations. The vast branching, interconnected areas of decreased echogenic- majority of these are minimally complex and do not re- ity that show sonographic evidence of fluid (see below). quire additional imaging (13) beyond follow-up sonogra- This is usually easily distinguishable from dilated renal phy. More complex cysts (prominent thickening or veins but can be confirmed with Doppler ultrasound if irregularity of the wall) should prompt additional imag- necessary. In cases of hydronephrosis, attempts should be ing. Cysts are common and are often associated with CKD made to visualize the ureter, which normally is not visi- (14,15), with acquired cystic disease being the extreme ble. The urinary bladder should be imaged in all cases form. Autosomal dominant polycystic kidney disease of hydronephrosis. Volume is estimated from cross- (ADPKD) is the other common multicystic disease, and it sectional dimensions on transverse imaging and the lon- is characterized by severe renal enlargement. Echogenic gitudinal dimension in the sagittal plane using the formula masses are usually neoplasms but can also represent hem- for the volume of an ellipsoid (0.523the three orthogonal orrhagic cysts, and they often require additional imaging. dimensions). The distal ureters can be visible at the base Stones typically appear as echogenic foci within the Clin J Am Soc Nephrol 9: ccc–ccc, February, 2014 Renal Ultrasonography, O’Neill 3 Figure 2. | Sonographic appearance of the upper and lower urinary tract. (A) Hydronephrosis of a left kidney (longitudinal view) with dilated renal 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 view of the urinary bladder (B) showing dilated distal ureters (arrows). (D) Transverse view of the urinary bladder with an enlarged prostate gland (P). collecting system that cast acoustic shadows, but either Although resistive index is commonly used as an indica- finding may be absent. A twinkling artifact (nonvascular tion of renal vascular resistance, it is also influenced by a color signal) on Doppler imaging has been proposed as an number of extra-renal factors, including heart rate and additional method to detect stones, but it has poor sensi- peripheral vascular resistance, and is not specificforrenal tivity and specificity (16). disease (4,18). Resistive index has been reported to provide prognostic information (19,20), but this has been ques- Vasculature tioned and may be based on extra-renal, rather than renal, Although blood vessels can be seen with B-mode factors (21). ultrasound, they are usually imaged with Doppler ultra- sound. A detailed description of renal vascular ultrasound is