exam, blood analysis and, in special cases, puncture or drainage. hydronephrosis can also be caused by non-obstructive conditions, such as brisk diuresis in patients treated with diuretics, in pregnant women and in children with vesicoureteral reflux. figure 15. end-stage hydronephrosis with cortical thinning. measurement of pelvic dilatation on the us image is illustrated by ‘+’ and a dashed line. figure 16. hydronephrosis with dilated anechoic pelvis and calyces, along with cortical atrophy. the width of a calyx is measured on the us image in the longitudinal scan plane, and illustrated by ‘+’ and a dashed line. figure 17. same patient as in figure 16 with measurement of the pelvis dilation in the transverse scan plane illustrated on the us image with ‘+’ and a dashed line. kidney stone disease even though us has a lower sensitivity and specificity than ct for the detection of kidney stones (urolithiasis), us, if available, is recommended as the initial imaging modality in patients with renal colic and suspected urolithiasis. us has no risk of radiation, is reproducible and inexpensive, and the outcome is not significantly different for patients with suspected urolithiasis undergoing initial us exam compared to patients undergoing initial ct exam. figure 18. renal stone located at the pyeloureteric junction with accompanying hydronephrosis. with us, larger stones (>5–7 mm) within the kidney, i.e., in the calyces, the pelvis and the pyeloureteric junction, can be differentiated, especially in the cases with accompanying hydronephrosis (figure 18 and figure 19). hyperechoic stones are seen with accompanying posterior shadowing. additional twinkling artifacts below the stone can often be seen using doppler us. large stones filling the entire collecting system are called coral stones or staghorn calculi and are easily visualized with us (figure 20). stones in the ureters are usually not visualized with us due to the air-filled intestines obscuring the insonation window. however, ureteral stones near the ostium can be visualized with a scan position over the bladder. an exam of the ureteric orifices and the excretion of urine to the bladder can be performed by inspecting the ureteric jets in the bladder with color doppler us. figure 19. centrally-located stone with posterior shadowing. no hydronephrosis is present. measurement of kidney length on the us image is illustrated by ‘+’ and a dashed line. figure 20. staghorn calculi filling the entire collecting system and creating pronounced shadowing. figure 21. left hydroureter with ureteric jet. no stone is visible. the red color in the color box represents motion towards the transducer as defined by the color bar. chronic kidney disease us is useful for diagnostic and prognostic purposes in chronic kidney disease. whether the underlying pathologic change is glomerular sclerosis, tubular atrophy, interstitial fibrosis or inflammation, the result is often increased echogenicity of the cortex. the echogenicity of the kidney should be related to the echogenicity of either the liver or the spleen (figure 22 and figure 23). moreover, decreased renal size and cortical thinning are also often seen and especially when disease progresses (figure 24 and figure 25). however, kidney size correlates to height, and short persons tend to have small kidneys; thus, kidney size as the only parameter is not reliable. figure 22. chronic renal disease caused by glomerulonephritis with increased echogenicity and reduced cortical thickness. measurement of kidney length on the us image is illustrated by ‘+’ and a dashed line. figure 23. nephrotic syndrome. hyperechoic kidney without demarcation of cortex and medulla. figure 24. chronic pyelonephritis with reduced kidney size and focal cortical thinning. measurement of kidney length on the us image is illustrated by ‘+’ and a dashed line. figure 25. end-stage chronic kidney disease with increased echogenicity, homogenous architecture without visible differentiation between parenchyma and renal sinus and reduced kidney size. measurement of kidney length on the us image is illustrated by ‘+’ and a dashed line. acute renal injury the acute changes in the kidney are often examined with us as the first-line modality, where ct and magnetic resonance imaging (mri) are used for the follow-up examinations and when us fails to demonstrate abnormalities. in evaluation of the acute changes in the kidney, the echogenicity of the renal structures, the delineation of the kidney, the renal vascularity, kidney size and focal abnormalities are observed (figure 26 and figure 27). ct is preferred in renal traumas, but us is used for follow-up, especially in the patients suspected for the formation of urinomas (figure 28). figure 26. acute pyelonephritis with increased cortical echogenicity and blurred delineation of the upper pole. figure 27. postoperative renal failure with increased cortical echogenicity and kidney size. biopsy showed acute tubular necrosis. figure 28. renal trauma with laceration of the lower pole and subcapsular fluid collection below the kidney. us-guided intervention figure 29. (a) percutaneous nephrostomy tube placed through a calyx in the lower pole of a kidney with hydronephrosis. (b) the pigtail catheter is placed in the dilated calyx. the tube in (a) and the pigtail in (b) are marked with white arrows. sonography is the modality of choice for guidance when performing intervention in the kidney, whether it is kidney biopsy, percutaneous nephrostomy or abscess drainage. historically, thermal ablation of renal tumors is performed under ct guidance, as the risk of injuring neighboring intestines during the us-guided procedure was considered too high due to poor identification of the moving bowels. however, recent guidelines for renal interventional us include radiofrequency, microwave and cryoablation with us as the ideal imaging guide. for percutaneous nephrostomy and abscess drainage, either the one-step or the seldinger technique is used. using the seldinger technique, the cavity is punctured with a sharp hollow needle, called a trocar. a round-tipped guidewire is then advanced through the lumen of the trocar, and after withdrawal of the trocar, a catheter or nephrostomy can be inserted over the guidewire to ensure correct placement. the one-step technique is when insertion of the drain or nephrostomy is done without the aid of a guidewire. the interventions are performed under local anesthesia and in a sterile setup. the procedures can be carried out with or without needle guidance according to preference, experience and setup (figure 29). ceus, image fusion and elastography contrast-enhanced ultrasound (ceus) can evaluate microvasculature, which color doppler us is unable to detect. in renal us examination, ceus can be used to differentiate tumor and pseudotumor, such as prominent columns of bertin. pseudotumors enhance as adjacent renal tissue. the use of ceus is recommended in special cases to distinguish between cystic and hypovascularized solid lesions, to characterize complex cysts, abscesses, traumatic lesions and ischemic lesions. figure 30. renal cell carcinoma successfully treated with thermal ablation, as no contrast enhancement is seen. solid malignant tumors in the kidney do not exhibit specific enhancement patterns like some liver lesions, and no valid enhancement criteria between benign and malignant renal lesions have been proposed. however, ceus is used in some patients after ablation of renal cell carcinoma to evaluate contrast uptake in the treated area (figure 30). image fusion of ultrasound with a previously recorded dataset of ct or other modalities is rarely used in renal us. reports on image fusion using ceus or us combined with ct or mri in the examination of renal lesions and in difficult us-guided renal interventions have been published (figure 31). however, no recommendations have been published so far. elastography is a us method to visualize the elasticity of tissue. preliminary reports on us elastography used on transplanted kidneys to evaluate cortical fibrosis have been published showing promising results (figure 32). figure 31. unspecific cortical lesion on ct is confirmed cystic and benign with contrast-enhanced ultrasound (ceus) using image fusion. figure 32. strain elastography of a normal kidney. red depicts soft areas, and blue depicts hard areas relative to the entire elastography image. note that the medulla is softer than the cortex. a color bar is shown to the left of the image, where "s" and "h" denote soft and hard tissue, respectively.