David M. Einstein, ~ Anne A. Singer, I David M. Paushter, ~ A. Nasif, 2 and Joseph V. Nally, Jr. 2 ... Address offprint requests to: David M. Einstein, M.D., Depart-.
Urol Radiol 13:162-165 (1992)
Urologic Radiology ©Springer-VerlagNewYorkInc.1992
Hypoechoic Renal Pyramids: Sonographic Visualization in Older Children and Young Adults D a v i d M. Einstein, ~ A n n e A. Singer, I D a v i d M. Paushter, ~ A. Nasif, 2 and Joseph V. Nally, Jr. 2 Departments of tDiagnostic Radiology and 2Hypertension and Nephrology, Cleveland Clinic Foundation, Cleveland, Ohio, USA
T h e frequency a n d degree o f visualization o f m e d u l l a r y p y r a m i d s in a n o r m a l population, aged 10-29 years, was analyzed. H y p o e c h o i c pyra m i d s were visualized in 42% o f right kidneys in subjects aged 10-18 years a n d in 27% o f subjects aged 19-29 years. P r o m i n e n t l y hypoechoic pyramids, m i m i c k i n g the a p p e a r a n c e o f neonatal kidneys, were seen in an additional 34% o f subjects aged 10-18 years a n d in 16% aged 19-29 years. P r o m i n e n t p y r a m i d s were present in 50% o f subjects with renal cortical echogenicity (RCE) equal to liver, but also in 21% o f subjects with R C E less than liver. O u r study expands the age at which p r o m i n e n t l y h y p o e c h o i c m e d u l l a r y p y r a m i d s can be considered a n o r m a l finding. T h i s m a y relate to recent i m p r o v e m e n t s in ultrasound technology.
Kidney, m e d u l l a r y p y r a m i d s -Kidney, ultrasound -- K i d n e y a n a t o m y -- Renal cortical echogenicity.
O v e r the past two decades, s o n o g r a p h y has a s s u m e d a p r i m a r y role in the evaluation o f a wide variety o f renal disorders. This is particularly true in the pediatric a n d young adult populations in which it is highly desirable to limit the use o f ionizing radiation a n d iodinated contrast material. It is well
David M. Einstein, M.D., Department of Diagnostic Radiology, Cleveland Clinic Foundation, 9500 Euclid Avenue--A21, Cleveland, OH 44195, USA
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k n o w n that m e d u l l a r y p y r a m i d s can be p r o m i n e n t a n d h y p o e c h o i c in infants a n d y o u n g children. T h e a p p e a r a n c e o f the p y r a m i d s in older age groups has not been as thoroughly analyzed and, specifically, p r o m i n e n t p y r a m i d s o f the neonatal type h a v e not been d o c u m e n t e d . W e retrospectively evaluated the renal s o n o g r a m s o f 68 subjects aged 10-29 years without clinical or sonographic evidence o f renal disease in order to d e t e r m i n e the frequency a n d degree o f visualization o f m e d u l l a r y p y r a m i d s in a n o r m a l p o p u l a t i o n o f children a n d young adults.
Subjects and Methods The clinical records and sonographie images of all patients, aged 10-29 years undergoing renal sonography during a 1-year period, were retrospectively reviewed. Patients with a history of renal disease including pyelonephritis, vesicoureteral reflux, calculi, neurogenic bladder, and long-standing diabetes mellitus or hypertension were excluded from the study as were any patients with an abnormal urinalysis or serum BUN/creatinine. Uncomplicated cystitis without evidence of reflux or ascending infection was not considered grounds for exclusion. Any structural abnormality or significant pathologic process evident on the sonographic exam also led to exclusion. These methods yielded a study population of 68 subjects, which was then subdivided into 38 subjects aged 10-18 years (older children) and 30 subjects aged 19-29 years (young adults). Sonograms were performed using several commercially available real-time units (Acuson 128 and ATL Ultramark 8 and 9). Phased-array and annular-array-type transducers were used. Transducer frequencies were optimized for the subject's body habitus and properly focused to the anatomic region of interest. Scanning was routinely performed in the supine or lateral decubitus positions utilizing sagittal, transverse, and coronal imaging planes. The images were retrospectively and independently reviewed by three radiologists experienced in ultrasound. Both the right and left kidneys were evaluated for renal cortical echogenici-
D.M. Einstein et al.: Hypoechoic Renal Pyramids
Fig. 1. Medullary pyramid categories. A Not present. B Visualized: Two faintly hypoechoic pyramids (arrows) are present in the fight upper pole. C, D Prominent: Prominent h ~ h o i c pyramids (arrows) are distributed around the right kidney and are present anteriorly in the left kidney in this 23-year-old woman.
ty (RCE) and visualization of medullary pyramids. RCE was categorized as either less than, equal to, or greater than the adjacent liver or spleen; it was occasionally not possible to compare the left kidney to the spleen, particularly in older subjects; therefore, only the right RCE was used for data analysis. The medullary pyramids were classified as either not present, visualized, or prominent. Visualized pyramids were defined as being only faintly hypoechoic and/or seen in only one segment of the kidney, whereas prominent pyramids were quite clearly hypoechoic compared to the cortex, mimicking the neonatal pattern, and were radially arrayed around the entire kidney (Fig. 1). When a difference in interpretation arose, a mean value of the three readings was used. The data were analyzed with regard to the frequency and degree of visualization of medullary pyramids in the two age groups (ages 10-18 years and 19-29 years). RCE was correlated with the degree of visualization of the medullary pyramids in both age groups. Finally, an analysis of the degree of visualization of medullary pyramids of the right versus the left kidneys in individual subjects was performed.
RCE was less than that of the adjacent hepatic parenchyma in 84% of the subjects aged 10-18 years and 80% of the subjects aged 19-29 years. The re-
maining subjects in each age group had RCE equal to that of the liver; RCE greater than that of the liver was not found in our normal population. Pyramids in the right kidney were visualized in 42% and prominent in 34% of the subjects aged 10-18 years. In the 19- to 29-year-old group, the pyramids were visualized in 27% and prominent in 16%. The incidence of visualized and prominent pyramids was less in both age groups when the left kidney was analyzed, but the differences compared to the right kidney were not statistically significant (sign test, P > 0.10). The greater degree of visualization in the younger group compared to the older was statistically significant on the right (P = 0.02, chi-square test) but not on the left (P = 0.07). The oldest subject displaying prominent pyramids was 29 years old. When the degree of visualization of medullary pyramids was correlated with RCE (Tables I and 2), it became evident that prominent pyramids were seen with greater frequency in patients whose RCE was equal to that of the liver. However, the incidence of prominent pyramids in subjects with RCE
D.M. Einstein et al.: Hypoechoic Renal Pyramids
Table 1. Prominence of medullary pyramids correlated with renal cortical echogenicity (RCE), aged 10-18 years
Table 2. Prominence of medullary pyramids correlated with renal cortical echogenicity (RCE), aged 19-29 years
RCE compared to hepatic echogenicity
RCE compared to hepatic echogenicity
Not visualized Visualized Prominent Total
8 (25) 15 (47) 9 (28) 32 (100)
1 (17) 1 (17) 4 (66) 6 (100)
9 17 13 38
Not visualized Visualized Prominent
15 (62) 6 (25) 3 (13)
2 (33) 2 (33) 2 (33)
17 8 5
Percentages in parentheses.
Percentages in parentheses.
less than that of the liver is not insignificant, and many examples of markedly prominent pyramids were seen in subjects from this subgroup, (see Fig. 1C). The relatively small numbers of patients whose RCE was equal to that of the liver did not permit a meaningful statistical analysis of these findings. Table 3 summarizes our findings relating to the degree of visualization of pyramids comparing the right and left kidneys of individual subjects. In the population as a whole, most (72%) displayed equally prominent pyramids when the two sides were compared. When the results were divergent in a given subject, pyramids were more prominently visualized on the right than on the left (22% versus 6%) in a statistically significant distribution (P < 0.025 by chi-square analysis).
faces in the neonatal renal cortex resulting in accentuated echogenicity. The authors speculated that the hypoechoic medullary pyramids might be a relative phenomenon seen as a secondary effect of the increased cortical echogenicity. Other authors studying older populations of normal children found hypoechoic pyramids in progressively older age groups. Han's series of normal pediatric subjects reported prominent pyramids up to age 1 and RCE equal to liver up to age 3 , whereas Vade made similar observations up to 13 and 14 years, respectively . In the latter study, medullary pyramids were visualized in 13% of children between the ages of 10 and 15 years. The appearance of the pyramids in older age groups has not been as thoroughly analyzed. Early reports dealing with normal sonographic renal anatomy described visualization of the medulla in nearly 50% of patients. While different ages were not systematically evaluated, the pyramids were best seen in children and thin adults. The images displayed pyramids similar to our visualized group, but none were prominently seen [6, 7]. Several basic ultrasound textbooks state that normal pyramids should be visible; but, again, all images demonstrate only faintly visualized pyramids [8-10]. Prominent neonatal-type pyramids have not previously been described as a normal finding in older age groups. They have, however, been suggested as a finding in acute and chronic medical renal disease, due either to in-
Discussion The outer renal cortex, containing the glomeruli and convoluted tubules, surrounds the triangular medullary pyramids which contain the loops of Henle and the collecting ducts. The apices of several pyramids coalesce into a renal papilla which projects into the minor calyx. This structural anatomy of the kidney is frequently visible on renal sonograms. Early reports [1, 2] stressed that increased RCE and hypoechoic pyramids are a normal finding in neonates and infants up to the age of 6 months. Knowledge of this pattern is important to avoid mistaking normal anatomic structures for a pathologic entity, such as renal cystic disease or hydronephrosis. A sonographic anatomic correlative study of the neonatal kidney  postulated that the increased cortical echogenicity of the neonatal kidney is due to the following factors present during this period: (1) the glomeruli occupying a larger volume of the cortex, (2) a higher cellular component of the glomerular tuff, and (3) the presence of some loops of Henle within the renal cortex. These anatomic findings presumably caused an increased number of inter-
Prominence of medullary pyramids: comparison of
right and left kidney
R> L R=L L> R Total
8 (21) 29 (76) 1 (3) 38 (100)
7 (23) 20 (67) 3 (10) 30 (100)
15 (22) 49 (72) 4 (6) 68 (100)
L, left; R, right. Percentages in parentheses.
D.M. Einstein et al.: Hypoechoic Renal Pyramids creased cortical echogenicity or congestion, e d e m a , a n d increased m e d u l l a r y b l o o d flow [9, 1 1-13]. O u r data e x p a n d the age range in which these sonographic findings can be seen in n o r m a l subjects. T h e p y r a m i d s were p r o m i n e n t in 34% o f o u r subjects aged 10-18 years a n d in 16°/0 aged 19-29 years. T h e difference in degree o f p y r a m i d visualization between right a n d left kidneys in the p o p u l a t i o n as a whole, a n d in individual subjects, is consistent with the typically better visualization o f the right kidney at sonography, likely due to the acoustic wind o w p r o v i d e d b y the liver. By identifying R C E equal to that o f liver in 18% o f o u r subjects, we h a v e c o n f i r m e d the report o f Platt et al. [ 14] that this is a n o r m a l finding a n d is not a reliable indicator o f renal disease. Although the incidence o f p r o m i n e n t h y p o e c h o i c p y r a m i d s is greater in this subset o f patients, we disagree with p r e v i o u s reports which hypothesize this to be a cause-and-effect relationship, as e v e n m a r k e d l y h y p o e c h o i c p y r a m i d s are present in n o r m a l patients whose R C E is less than the adjacent hepatic p a r e n c h y m a . T h e reasons w h y we are n o w able to visualize h y p o e c h o i c p y r a m i d s in older age groups t h a n previously reported are not entirely clear. O u r study was not designed to c o m p a r e the a p p e a r a n c e o f the kidney on older generation and state-of-the-art e q u i p m e n t in individual patients n o r to c o m p a r e the visualization o f p y r a m i d s with different transducer frequencies or designs. Still, it seems reasonable to conclude that recent i m p r o v e m e n t s in ultrasound technology a n d transducer design resulting in i m p r o v e d contrast a n d spatial resolution n o w perm i t i m p r o v e d sonographic visualization o f n o r m a l renal a n a t o m y . W e conclude that p r o m i n e n t h y p o e c h o i c m e d ullary p y r a m i d s , e v e n to the degree seen in neonates, are a n o r m a l sonographic finding o f older children a n d young adults a n d should not raise suspicion o f early medical renal disease. While the incidence o f
165 their visualization is greatest in n o r m a l kidneys with R C E equal to liver, they are also often seen w h e n the R C E is less t h a n t h a t o f the liver. These results are p r o b a b l y due to recent i m p r o v e m e n t s in ultrasound e q u i p m e n t design.
References 1. Hailer JO, Berdon WE, Friedman AP: Increased RCE: A normal finding in neonates and infants. Radiology 142:172174, 1982 2. Hayden CK Jr, Santa-Cruz FR, Amparo EG, et al.: Ultrasonographic evaluation of the renal parenchyma in infancy and childhood. Radiology 152:413-417, 1984 3. Hricak H, Slovis TL, Callen CW, CaUenPW, Romanski RN: Neonatal kidneys: Sonographic anatomic correlation. Radiology 147:699-702, 1983 4. Han BK, Babcock DS: Sonographic measurements and apl~arance in normal kidneys in children. A JR 145:611-616, 1985 5. Vade A, Lau P, Smick J, Harris V, Ryva J: Sonographic renal parameters as related to age. Pediatr Radiol 17:212215, 1987 6. Cook JH III, Rosenfield AT, Taylor KJW: Ultrasonic demonstration of intrarenal anatomy. AJR 129:831-835, 1977 7. Rosenfield AT, Taylor KJW, Crade M, DeGraaf CS: Anatomy and pathology of the kidney by gray scale ultrasound. Radiology 128:737-744, 1978 8. Sarti DA: Diagnostic Ultrasound." Text and Cases, 2nd ed. Chicago: Year Book Medical Publishers, 1987, p 352 9. Coleman BG: Genitourinary Ultrasound--A Text~Atlas, 1st ed. New York: Igaku-Shoin, 1988, pp 19-21 10. Mittelstaedt C: Abdominal Ultrasound, 1st ed. New York: Churchill-Livingstone, 1987, pp 221-227 11. Hricak H: Renal medical disorders: The role of sonography. In Sanders RC (ed): Ultrasound Annual 1982. New York: Raven Press, 1982, pp 43-80 12. Rosenfield AT, Siegel NJ: Renal parenchymal disease: Histopathologic-sonogr.aphic correlation. A JR 137:793-798, 1981 13. Hricak H, Cruz C, Romanski R, et aL: Renal parenchymal disease: Sonographic--histologiccorrelation. Radiology 144: 141-147, 1982 14. Platt JF, Rubin JM, Bowerman RA, Marn CS: The inability to detect kidney disease on the basis of echogenicity. A JR 151:317-319, 1988