Sonographic Mimics of Renal Calculi

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1 Review Article Sonographic Mimics of Renal Calculi Durr-e-Sabih, MBBS, MSc, Ali Nawaz Khan, MBBS, FRCP, FRCR, Marveen Craig, RDMS, Joseph A. Worrall, MD, RDMS Objectives. To review sonographic findings that can mimic renal calculi. Methods. We comment on a number of echoes that can mimic renal calculi. Results. There are a number of sonographic renal artifacts, vascular and nonvascular, that may confound a correct diagnosis. Conclusions. Awareness of these potential artifacts will result in a more specific sonographic examination and will accurately guide the referring physician toward appropriate patient treatment. The importance of other imaging modalities is also emphasized to ensure that a correct diagnosis is obtained whenever the sonographic findings are inconclusive. Key words: renal artifacts; renal calculi; sonography. Abbreviations CT, computed tomography Received November 24, 2003, from the Multan Institute of Nuclear Medicine and Radiotherapy, Nishtar Hospital, Multan, Pakistan (D.-e.-S.); North Manchester General Hospital, Manchester, England (A.N.K.); author/consultant, Tucson Arizona, USA (M.C.); and The Fairbanks Clinic, Fairbanks, Alaska, USA (J.A.W.). Revision requested January 22, Revised manuscript accepted for publication June 1, Many individuals from all over the world helped in the preparation of this review. All are members of a medical imaging ultrasound discussion group, which can be accessed at group/medicalimaging. In particular, we thank the following members: Anatoly Garkusha, MD (Ukraine), for sharing his philosophy on how findings are sometimes created ; Chitra Arun Kamath, MD (India), for suggesting causes for some of the potential artifacts seen on renal sonography; Fazeel Uz Zaman, MD (Pakistan), for suggesting maneuvers to increase the sensitivity of sonographic stone diagnosis; Gunjan Puri, MD (India), for suggesting causes for some of the artifactual echoes on renal sonography; Latha Natarajan, MD (India), for superb knowledge of differential diagnoses in all things related to sonography; Ravi Kadasne, MD (United Arab Emirates), for images, especially the image of the milk of calcium cyst used in this review; and Rochita V. Ramanan, MD (India), for insight into how radiologists approach the clinical question of flank pain. Address correspondence and reprint requests to Marveen Craig, RDMS, N Charoleau Dr, Tucson, AZ, USA. [email protected]. Renal calculous disease is a frequent cause of lumbar pain, but an incorrect diagnosis of renal calculous disease can have important clinical implications for the patient. If lumbar pain is erroneously ascribed to the presence of a renal calculus, the patient may be deprived of appropriate treatment. Proper sonographic technique usually allows visualization of most calculi larger than 5 mm. When imaging smaller renal calculi; however, recognition and diagnostic accuracy are less clear, and such stones may be missed or misdiagnosed because of the presence of many inherently bright intrarenal noncalculous echoes. This review concentrates on those bright echoes that represent sonographic artifacts, normal or common (anatomic) renal structures that may mimic renal calculi. Bright Reflectors Within the Kidney Normal or calcified renal vessels are the most notable and common causes of intrarenal bright echo reflectors. 1 The kidneys are among the most vascular organs within the body; they can even be considered to form physiologic arteriovenous shunts. The renal volume of both kidneys is approximately 300 ml, and they are perfused by the equivalent of 25% of cardiac output, which can be up to 1.5 L. It is not surprising that, with so many vessels, some may present orthogonally to an ultrasound beam and may appear more echogenic than the background parenchyma, and some of these normal vessels, imaged at just the right angle, may appear as small, brightly reflective specks by the American Institute of Ultrasound in Medicine J Ultrasound Med 2004; 23: /04/$3.50

2 Sonographic Mimics of Renal Calculi Some of these renal vessels appear in predictable locations and in a recognizable pattern, thus allowing their identification as nonstones. A brief review of renal vascular anatomy shows that each renal artery divides into 5 segmental arteries, with each segmental artery subsequently dividing into interlobar arteries. Segmental arteries often cross the papillae at right angles, with the division frequently occurring very near the medullary pyramids. The interlobar arteries travel along the sides of the renal pyramids and, at the level of the pyramidal base, give off parallel arcuate arteries (Figure 1). The vessels location and configuration suggest their true identity. Bright intrarenal reflectors can be subdivided into vascular and nonvascular causes. Vascular Reflectors Segmental Arteries The division of the segmental arteries into the interlobar arteries can be seen as regularly spaced, tiny echogenicities at the periphery of the renal sinus. Their similarity in size and their regular spacing are often the clues to their correct identification. In addition, bright echogenicities are sometimes noted at the tips of the medullary pyramids. These echo patterns, too, may have vascular origins (Figure 2). Figure 1. Renal arterial system. Note the proximity of the segmental artery divisions and the relationship of the arcuate arteries to the bases of the renal pyramids. Arcuate Arteries Often visible as tiny threadlike echogenicities, the arcuate arteries travel along the base of the medullary pyramids, and their similar sizes and locations suggest their true identity (Figure 3). Sinus Vessels A number of small transversely oriented echogenicities, some with a tramline or railroad track appearance, can be encountered. They most likely represent echo reflections from the sinus vessels. Nonvascular Reflectors: Prominent Papillae The apexes of the medullary pyramids project into the minor calyces and can appear prominent in mild hydronephrosis with calyceal dilatation (Figure 4). These structures can also appear echogenic in conditions such as early medullary nephrocalcinosis (Anderson-Carr progression), 2 papillary necrosis, medullary sponge kidneys, infection with Cytomegalovirus or Candida albicans, and medullary fibrosis. An important cause of a transient increase in medullary echogenicity in neonates is precipitation of Tamm-Horsefall proteins (Figure 5). Reflectors Within the Renal Parenchyma Milk of Calcium Cysts Calcium salts typically collect in simple renal cysts, usually in a calyceal diverticulum, but the salts can accumulate in cysts occurring in polycystic renal disease as well. The milk of calcium is typically seen as an echogenic layer of intracystic material, which reverberates and sometimes gravitates to the dependent portion of the cyst (Figure 6). 3 In some cases, calcified material may completely fill the cysts, making diagnosis difficult. 4,5 However, the presence of reverberation artifacts is diagnostically helpful. Renal Cortical Calcification There are a variety of causes that produce renal cortical calcification, shown as increased cortical echogenicity and shadowing, and that may mimic calculi. Some of these causes include acute tubular necrosis, chronic glomerulonephritis, Alport syndrome, oxalosis, rejected renal transplants, and chronic neoplastic hypercalcemia J Ultrasound Med 2004; 23:

3 Durr-e-Sabih et al Junctional Parenchymal Line Small renal contour notches mark the boundaries of the renunculi that fuse to make up the kidney. They are often associated with the junctional parenchymal line (interrenuncular septum) that extends from the contour reflector up to the hilum. These junctional parenchymal reflectors can appear very echogenic, but their triangular shape and location (anterosuperior and posteroinferior) point to the correct diagnosis (Figure 7). Angiomyolipomas Angiomyolipomas are benign renal tumors containing variable amounts of fat. They are highly vascular and often highly echogenic. Frequently seen within the cortex, the sharp contour and echogenicity of the angiomyolipoma are often similar to those of the renal sinus fat; however, the echogenicity of these tumors is variable depending on the amount of fat within the tumor and the number of tissue interfaces. These findings and the highly vascular nature of these masses are suggestive enough to indicate annual follow-up for confirmation of stability when small tumors of less than 1 cm diameter are found. Fatty attenuation seen within the tumor on computed tomography (CT) is virtually diagnostic of angiomyolipoma (Figure 8). Foreign Bodies Foreign bodies are typically not encountered within the kidneys. However, the presence of nephrostomy catheters should be obvious from their external drainage ends or the presence of a self-retaining balloon, such as a Foley bulb (Figure 9). In some cases, a ureteric stent may have been left inside the patient for weeks or even months before the patient is sent for a sonographic examination. Unless the stent is correctly recognized, it can cause confusion. It is important to remember that a stent in the ureter or renal pelvis will look like a stent anywhere else: a tubular structure with parallel bright walls and a regular lumen , Plain abdominal radiography and sonography should replace intravenous urography; , Intravenous urography should be added to the protocol if radiography and Figure 2. A and B, Coronal sections of the kidney. Arrows mark a segmental artery. C, Renal Doppler image showing the profuse vascular nature of the kidney. A B C Discussion Current imaging algorithms in the developed world use CT as an initial imaging modality for diagnosis of possible renal calculi. However, these algorithms have changed fairly rapidly during the last few years. Examples of the changing recommendations include the following: J Ultrasound Med 2004; 23:

4 Sonographic Mimics of Renal Calculi Figure 3. Arcuate artery (arrow). Note its orthogonal relationship to the ultrasound beam and its double-channel appearance. Figure 4. Renal pyramids. Echogenic tips of the renal pyramids as seen in mild hydronephrosis are shown. Figure 5. Transient pyramidal echogenicity. These images of an 8-day-old neonate show the characteristic appearance of transient pyramidal echogenicity due to the precipitation of Tamm-Horsefall proteins. These findings disappeared by 6 weeks of age. diagnostic sonography are inconclusive; and , Unenhanced helical CT should replace urography. 6 There are limits to the size of stones that can be sonographically detected, depending on resolution and probe frequency, 7 as well as the type of stone, its location, patient habitus, and hydration. Whenever a nondiagnostic study is performed, a useful tactic is to repeat the sonographic examination after overhydrating the patient to produce mild splitting of the renal sinus and to make the diagnosis easier. 8 Expecting to be able to detect submillimeter calculi on sonography is fraught with danger, and many false-positive diagnoses may be made. Five millimeters appears to be the cutoff size at which we can confidently expect to see renal calculi. Under experimental conditions, some authors have reported sensitivity of 2 mm, 7 but most consider 5 to 7 mm to be the smallest visible size on sonography. 8,9 For detection of smaller stones, diagnostic confidence levels vary with operator expertise, machine sophistication, stone location, and patient anatomy. The sensitivity of sonography for diagnosing small renal stones varies from 24% to 96%. Helical CT has consistently been shown to have superior sensitivity Muscular pain due to mechanical injury is a very common cause of flank pain and affects approximately 85% of Americans during their lifetime. 16 Patients with acute lumbar pain might have a renal stone, but renal stone disease is not invariably present (Table 1). In one series, only 67% of patients had a renal stonerelated cause of acute lumbar pain, and, of these, only 66% had a correct diagnosis by sonography. 17 Helical CT has nearly perfect sensitivity for detecting small stones 18 and is excellent for diagnosing nonstone causes of flank pain. 19 However, helical CT is not universally available, has a considerable radiation burden, and uses iodinated contrast agents. A combination of history, sonography, 20 and plain and contrastenhanced radiology might have to suffice in this imperfect situation. 21 The diagnostic approach to renal colic has recently been changed because of the introduction of new noninvasive radiologic procedures such as Doppler sonography, non contrastenhanced CT and magnetic resonance imag J Ultrasound Med 2004; 23:

5 Durr-e-Sabih et al A Figure 6. A, Milk of calcium cyst. Note the echogenic material that has settled to the dependent portion of the cyst after the patient was moved to a decubitus position. B, Milk of calcium cyst with reverberation echoes. Image courtesy of Ravi Kadasne, MD (Emirates International Hospital, Al Ain, United Arab Emirates). B ing. 22 However, sonography remains the initial imaging procedure of choice in many cases because it is widely available, inexpensive, and noninvasive and does not rely on ionizing radiation or contrast agents. Figure 7. A, Junctional parenchymal defect (arrow). The defect appears as an echogenic area associated with a cortical contour defect. B, Junctional parenchymal line. A B J Ultrasound Med 2004; 23:

6 Sonographic Mimics of Renal Calculi Figure 8. Angiomyolipoma. Note the echogenic area in the upper pole of the right renal cortex. Table 1. Partial List of Causes of Acute Lumbar Pain Abdominal aortic aneurysm Acute nephritis Acute renal infarction Blood clots Cholecystitis Ectopic pregnancy Endometriosis Fungal bezoars Glomerulonephritis Herpes zoster Kidney tumors Muscle pain Oral contraceptive induced loin pain Ovarian cyst rupture or torsion Ovarian vein syndrome Papillary necrosis Pelvic inflammatory disease Pleuritis Pregnancy Pyelonephritis Radiculitis Renal abscess Renal vein thrombosis Retrocaval ureter Retroperitoneal fibrosis Rib pain Splenic infarction Ureteral strictures Ureteropelvic junction obstruction Figure 9. Foreign body. A Foley catheter bulb, placed during nephrostomy, is shown within the right renal pelvis. References 1. Gould RJ, Rochester D, Panella JS. Sonographic demonstration of renal arterial calcification simulating multiple renal calculi. Urology 1985; 25: Patriquin H, Robitaille P. Renal calcium deposition in children: sonographic demonstration of the Anderson-Carr progression. AJR Am J Roentgenol 1986; 146: Patriquin H, Lafortune M, Filiatrault D. Urinary milk of calcium in children and adults: use of gravitydependent sonography. AJR Am J Roentgenol 1985; 144: Melekos MD, Kosti PN, Zarakovitis IE, Dimopoulos PA. Milk of calcium cysts masquerading as renal calculi. Eur J Radiol 1998; 28: Ohyabu Y, Sameshima H, Matuoka K, Noda S, Eto K. Milk of calcium renal stone: a case report. Hinyokika Kiyo 1989; 35: Dalla Palma L, Pozzi-Mucelli R, Stacul F. Present-day imaging of patients with renal colic. Eur Radiol 2002; 12: Kimme-Smith C, Perrella RR, Kaveggia LP, Cochran S, Grant EG. Detection of renal stones with real-time sonography: effect of transducers and scanning parameters. AJR Am J Roentgenol 1991; 157: Chau WK, Chan SC. Improved sonographic visualization by fluid challenge method of renal lithiasis in the nondilated collecting system: experience in seven cases. Clin Imaging 1997; 21: J Ultrasound Med 2004; 23:

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