Pediatric Imaging Original Research

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1 Pediatric Imaging Original Research Kadioglu Normal Renal Measurements in Children Pediatric Imaging Original Research Alev Kadioglu 1 Kadioglu A Keywords: children, kidney, medullary width, thickness, ultrasound DOI: /AJR Received April 29, 2009; accepted after revision July 16, ALKA Radiologic Diagnosis Center, Buyukdere Cad. Levent Apt. 36/16, Mecidiyekoy-Sisli, Istanbul, Turkey. Address correspondence to A. Kadioglu (alevkadioglu@yahoo.com). AJR 2010; 194: X/10/ American Roentgen Ray Society Renal Measurements, Including Length, Parenchymal Thickness, and Medullary Pyramid Thickness, in Healthy Children: What Are the Normative Ultrasound Values? OBJECTIVE. The objective of our study was to develop, by use of ultrasound, nomograms of renal thickness, medullary pyramid thickness (height), renal length, and the ratio of medullary pyramid thickness to thickness in healthy children. SUBJECTS AND METHODS. This prospective study included 292 consecutive children (136 boys and 156 girls) who were referred between October 2008 and March 2009 for problems other than urinary tract symptoms or underlying kidney disorders. The children were between 1 month and 18 years old (mean age, 6.1 ± 5.9 years). Real-time gray-scale sonography was performed with a linear or curved array transducer. All examinations were performed by the same experienced radiologist (16 years of experience in pediatric sonography at the time the study began). All the children were well hydrated and had full bladders at the time of examination. Renal length measurements were performed in the sagittal view, and the maximum length of each kidney was measured. Measurements of thickness and medullary pyramid thickness were performed on the same image on which length measurements were made. Parenchymal thickness and medullary pyramid thickness were measured at the middle third portion of the kidney. The Wilcoxon s signed rank test was used for statistical analysis. RESULTS. Nomograms of renal thickness, medullary pyramid thickness, renal length, and the ratio of medullary pyramid thickness to thickness were developed. When all age groups were pooled together, statistically significant differences were observed between right and left kidneys in terms of thickness (p < 0.001), medullary pyramid thickness (p < 0.001), and renal length (left kidneys were longer, with thicker medullary pyramids and parenchyma; p < 0.001). A slight but significant difference in the ratio of medullary pyramid thickness to thickness was observed (p = 0.045). CONCLUSION. By use of renal sonography, nomograms of renal thickness, medullary pyramid thickness, renal length, and the ratio of medullary pyramid thickness to thickness were established in healthy children. T he length of normal kidneys in children has been extensively studied with ultrasound and excretory urography [1, 2] and with nomograms [3]. Ultrasound has completely replaced excretory urography as the primary technique for evaluating kidneys in children. Many renal disorders are associated with changes in kidney size; therefore, in patients with chronic problems, such as recurrent urinary tract infection, vesicoureteric reflux, or a neurogenic bladder, renal growth is monitored. Renal length is the most commonly used quantitative measure of renal size for comparison with established standards [1, 3]. Renal volume measurement is used less frequently because it requires calculation based on multiple measurements, and observer error may approach 25% [4]. Relatively recently, 3D ultrasound has been studied for use in the measurement of renal volume [5]. Deviation in renal size from established normal values indicates alteration in normal renal growth and is an important criterion in the diagnosis of renal disease. Research on prenatally detected hydronephrosis and monitoring of its clinical course have increased our knowledge and insight, shedding new light on the accepted traditional imaging algorithms [6]. To facilitate general acceptance of the algorithms, the European Society of Uroradiology and the European Society of Pediatric Radiology published a consensus statement AJR:194, February

2 Kadioglu for the most common and important conditions in pediatric uroradiology [6]. Both of these algorithms, for ultrasound and grading postnatal hydronephrosis, which were adapted from the Society for Fetal Urology fetal hydronephrosis grading criteria, emphasize that renal thickness should be measured and followed up [6, 7]. In addition to renal length and volume, thickness can also be compared with normative values. This necessitates the development of normative standards of thickness, as has been done for renal length. Excretory urography and CT have been used for this purpose [8, 9]; however, both techniques use ionizing radiation, in contrast to ultrasound. Ultrasound can be performed bedside, is readily available, and can be used repeatedly in children, because no ionizing radiation is used, and is accurate in determining kidney measurements [4]. Nonetheless, a search of the literature revealed no reports on the normative values of renal thickness in healthy children based on ultrasound examination. Furthermore, a search of the literature showed that normal values of medullary pyramid thickness (height) that is, the distance between the base and the apex in children have yet to be determined with ultrasound. The medullary pyramid may simply be compressed in earlier phases of hydronephrosis or may totally disappear as a result of atrophy, in severe degrees of hydronephrosis. Thus, knowing the normal range of medullary pyramid thickness could be an important parameter in the grading of hydronephrosis. MR urography is similar to ultrasound in its ability to categorize the degree of hydronephrosis [10]; however, MR urography has several limitations, including sensitivity to patient motion and frequent need for sedation. Additionally, MR urography is relatively expensive. The purpose of this study was to prospectively determine normal values of renal thickness, medullary pyramid thickness, and renal length in children using ultrasound, as well as to determine the ratio of medullary pyramid thickness to thickness. Subjects and Methods This prospective study was performed between October 2008 and March 2009 and included 292 consecutive children (136 boys and 156 girls) without urinary tract symptoms or underlying kidney problems. Patients were excluded from the study if they had a history of premature birth, malignancy, use of steroids, upper urinary tract abnormality, vesicoureteral reflux, or urologic surgery, or if ultrasound of their kidneys showed any abnormality, such as hydronephrosis, dysplastic kidney, or solitary kidney. Kidneys with a complete bridge in the sinus, which is diagnostic for a duplex collecting system, were also excluded from the study. The children were between 1 month and 18 years old (mean age, 6.1 ± 5.9 years). Real-time gray-scale ultrasound was performed with a linear array transducer equipped with a 4 9-MHz emission frequency or a curved array (convex) transducer with a 1 4-MHz emission frequency connected to an ultrasound unit (S2000, Siemens Healthcare) that was released to the market in September The gray-scale amplification gain and the time-gain compensation curve were adjusted to acquire the best images of the kidneys. Focus number was automatically adjusted with this ultrasound unit, but one focus mode was preferred. Focus was adjusted at the level of the kidney. Tissue harmonic imaging was routinely used. All the examinations were performed by the same experienced radiologist (16 years of experience in pediatric sonography at the time the study began). All the children were well hydrated and had full bladders at the time of examination. Verbal informed consent was provided by all the patients parents, and the study was performed in accordance with the Helsinki Declaration. There was no formal institutional review board at the institution where the study was performed. Renal length measurements were performed in the sagittal view, with patients in the supine position or in the contralateral decubitus position. The maximum length of each kidney was measured between the uppermost edge of the upper pole and the lowest edge of the lower pole. Sagittal plane images were obtained either from the long-axis view, using a subcostal approach with the patient in the supine position, or from the contralateral decubitus position view, using a posterior approach with the patient in the contralateral decubitus position. A linear transducer was used in the trapezoid mode. When both poles of a kidney extended beyond the field of view of the linear transducer, a convex transducer was used. All measurements in children 5 years and younger were obtained with a linear transducer. In children older than 5 years, the primary determinant for which transducer should be used was body habitus. Approximately 60% of the measurements were obtained with a linear transducer. Both the thickness line and medullary pyramid thickness line were parallel to each other but were perpendicular to the length measurement line on the same image (Fig. 1). Parenchymal thickness was defined as the distance between the cortex Fig. 1 Sagittal renal sonographic image of 1-month-old boy. Technique used to measure renal length, medullary pyramid thickness, and thickness is shown (between calipers: kidney length, 42.5 mm; medullary pyramid thickness, 6.5 mm; and thickness, 8.7 mm). Fig. 2 Sagittal renal sonographic image of 11-year-old girl. Kidney length, medullary pyramid thickness, and thickness are easily observed (between calipers: kidney length, 91.8 mm; medullary pyramid thickness, 5.6 mm; and thickness, 12.6 mm). 510 AJR:194, February 2010

3 Normal Renal Measurements in Children perirenal fat interface (capsule) and the sinus pyramidal apex interface. Medullary pyramid thickness was defined as the distance between the apex and the base where the line intersects the base as pyramid height. All kidneys were completely visible and measurable in all the children. Measurement of medullary pyramids, even in older adolescents, was successful. In larger children, a convex transducer was used (Fig. 2). Parenchymal thickness and medullary pyramid thickness were measured at the middle third portion of the kidney. All measurements were obtained prospectively on static original ultrasound images using electronic calipers at the time of scanning. Renal length, thickness, and medullary pyramid thickness measurements were performed for each kidney. Mean measurements were calculated for each age group. Because more-rapid changes in kidney size and hydronephrosis grade are expected during the first year of life than during other periods, children younger than 12 months were divided into monthly groups. Children between 1 year and 18 years old were divided into yearly groups. Measurements were analyzed according to age group for the right and left kidneys separately. The Wilcoxon s signed rank test was used for statistical analysis of each age group. Statistical significance was set at p < Results Parenchymal thickness, medullary pyramid thickness, renal length, and the ratio of medullary pyramid thickness to thickness in all age groups are shown in Table 1. Renal parenchyma thicknesses in all age groups are shown in Figure 3. In newborns, thickness of 8 mm was the lower limit, but a nonlinear appearance (zigzag) was present in the graph. kidney thickness was slightly greater than that of the at 8 months (p = 0.012) and at ages 4 years (p = 0.007) and 13 years (p = 0.013). Medullary pyramid thicknesses in all age groups are shown in Figure 4. Medullary pyramid thickness was greater in younger children than in older children. The left kidney medullary pyramid was slightly thicker than the medullary pyramid at 9 months (p = 0.015) and at 4 years (p = 0.013) and 9 years (p = 0.047). The ratios of medullary pyramid thickness to thickness in all age groups are shown in Figure 5. This ratio decreased as age increased. Statistically significant differences in the ratio of medullary pyramid thickness to thickness were observed at ages 6 months (p = 0.012), 9 months (p = 0.037), and 11 months (p = 0.047) and at ages 14 years (p = 0.047) and 17 years (p = 0.007). Renal length measurements in all age groups are shown in Figure 6. Renal length gradually increased with age. Between 10 and 16 years of age, renal length versus patient age had some zigzag. Statistically significant differences were observed between right and left renal length at 2 months (p = 0.009) and 5 months (p = 0.028) and at 8 years (p = 0.021), 10 years (p = 0.022), and 18 years (p = 0.022). When data for all age groups were pooled together, statistically significant differences were observed in thickness (p < 0.001), medullary pyramid thickness (p < 0.001), and renal length: kidneys were longer, with thicker medullary pyramids and parenchyma (p < 0.001). In addition, a slight but significant difference in the ratio of medullary pyramid thickness to thickness (p = 0.045) was observed between the right and left kidneys. Discussion The number of infants and children referred for assessment of the urinary tract has grown substantially since fetal ultrasound screening has become routine. This has resulted in an increase in the number of fetuses evaluated for prenatally detected hydronephrosis. Research on these children and monitoring of their clinical course have considerably changed the accepted traditional imaging algorithms [6]. These updated algorithms for ultrasound and postnatal hydronephrosis grading highlight the fact that renal thickness should be measured and monitored [6, 7]. Prassopoulos and Cavouras [8] used CT to measure renal thickness in children. They determined that the lower limit of thickness in the earlier period of life was 11 mm, whereas that in the current study was 8 mm. This difference may exist because Prassopoulos and Cavouras measured thickness in 11 different portions of the kidney, including both poles, which are the thickest portions of the kidney, whereas in the current study, the middle third portion of the kidney was used for thickness measurements with ultrasound. A search of the literature revealed that there is not an established normal range for renal thickness or standardized measurement techniques using ultrasound. The results of the current study can be used as a reference for this purpose. Postnatal hydronephrosis grading, adapted from Society for Fetal Urology grading, is primarily based on the shape of the pelvis, calyces, fornices, and apices of the medullary pyramids, also known as the papilla [6, 11]. The medullary pyramid has a hypoechoic appearance on ultrasound. It is also prominent and larger in volume, especially in the earlier periods of life [12]. Han and Babcock [3] studied 122 children (from newborns to age 17 years) and reported that all neonates and 62% of infants up to 6 months old had prominent and anechoic medullary pyramids with accentuation of corticomedullary definition on ultrasound. Macroscopic morphometry of the kidneys showed that in neonates the medullary pyramid occupies a proportionally larger corticomedullary volume than it does later in life [13, 14]. This fact may explain why, in the current study, the ratio of medullary pyramid thickness to thickness decreased as age increased. Personal observation shows that calyceal and forniceal shapes mirror the severity of damage in the papilla and medullary pyramid. For example, when fornices are rounded and calyces are clubbed, it is expected that the medullary pyramid will be shortened. Similarly, when a severely dilated pelvicalyceal system with thinning of the parenchyma is detected, the medullary pyramid is barely seen, sometimes not at all. Thus, the normal range of medullary pyramid thickness could be an important parameter in the grading of hydronephrosis. As with thickness, a search of the literature revealed that there is not an established normal range for medullary pyramid thickness or standardized measurement techniques using ultrasound. These results can be used as a reference for this purpose. The results of this study show that renal length gradually increased with age. Between the ages of 10 and 16 years, the graph of renal length versus the patient age had a zigzag. A similar zigzag in the graph of the renal length versus patient age, between ages 10 and 16 years, was reported by Rosenbaum et al. [1] as well. The limitation of sonographically measured renal length and its interpretation in the context of published normative standards have been reviewed in detail elsewhere [15]. One of the major potential limitations discussed in the literature is the possible influence of patient position and imaging projection on measured renal length [16 18]. AJR:194, February

4 Kadioglu TABLE 1: Kidney length, thickness, medullary pyramid thickness, and the ratio of medullary pyramid thickness to thickness No. of patients Kidney Length Kidney Length p a thickness kidney thickness p b medullary thickness kidney medullary thickness p c medullary thickness/ thickness left kidney medullary thickness/left kidney thickness p d 1 mo ± ± ± ± ± ± ± ± mo ± ± ± ± ± ± ± ± mo ± ± ± ± ± ± ± ± mo ± ± ± ± ± ± ± ± mo ± ± ± ± ± ± ± ± mo ± ± ± ± ± ± ± ± mo ± ± ± ± ± ± ± ± mo ± ± ± ± ± ± ± ± mo ± ± ± ± ± ± ± ± mo ± ± ± ± ± ± ± ± mo ± ± ± ± ± ± ± ± y ± ± ± ± ± ± ± ± y ± ± ± ± ± ± ± ± y ± ± ± ± ± ± ± ± y ± ± ± ± ± ± ± ± y ± ± ± ± ± ± ± ± y ± ± ± ± ± ± ± ± y ± ± ± ± ± ± ± ± y ± ± ± ± ± ± ± ± y ± ± ± ± ± ± ± ± y ± ± ± ± ± ± ± ± y ± ± ± ± ± ± ± ± y ± ± ± ± ± ± ± ± y ± ± ± ± ± ± ± ± y ± ± ± ± ± ± ± ± y ± ± ± ± ± ± ± ± y ± ± ± ± ± ± ± ± y ± ± ± ± ± ± ± ± y ± ± ± ± ± ± ± ± Note. Except for p values, all data are mean millimeters ± SD. a p indicates statistical comparison of right and left kidney length. b p indicates statistical comparison of right and left kidney thickness. c p indicates statistical comparison of right and left kidney medullary pyramid thickness. d p indicates statistical comparison of right and left kidney medullary pyramid to thickness ratio. 512 AJR:194, February 2010

5 Normal Renal Measurements in Children Renal Parenchyma Thickeness (mm) Medullary Pyramid Thickness (mm) Ratio of Medullary Pyramid Thickness to Parenchymal Thickness Carrico and Zerin [16] compared supine (sagittal), contralateral decubitus (coronal), and prone positions and reported that renal length measured in children in the supine or contralateral decubitus position was longer than that measured in children in the prone position. Michel et al. [17] compared the supine with the prone position and reported that renal length measured in children in Fig. 3 Graph shows renal thicknesses in all age groups. Fig. 4 Graph shows medullary pyramid thickness in all age groups. Fig. 5 Graph shows ratio of medullary pyramid thickness to thickness in all age groups. the supine position is greater than that measured in the prone position. They suggested that this difference was due to position-dependent changes in the degree of filling of the renal calyces and pelvis, as well as errors in caliper distance measurements for the different scan depths (supine vs prone). Similarly, De Sanctis et al. [18] compared sagittal, coronal, and prone positions and reported that coronal and sagittal views yielded the longest measurements, whereas prone views yielded the shortest measurements. It should be noted that these investigators did not exclude patients with renal disorders. In the current study, renal length was measured in the sagittal view, with healthy children either in the supine position or in the contralateral decubitus position. The main AJR:194, February

6 Kadioglu Fig. 6 Graph shows renal length in all age groups. Renal Length (mm) aim was to determine the maximum kidney length in both planes. The sagittal plane was the long-axis view obtained from a subcostal approach with the patient in the supine position. The contralateral decubitus position view was obtained from a posterior approach with the patient in the contralateral decubitus position. Both patient positions were imaged in same plane (sagittal view). Other measurements thickness and medullary pyramid thickness were performed on the same image plane as well. When all the age groups were pooled together, statistically significant differences were observed in thickness, medullary pyramid thickness, and renal length; a slight but significant difference in the ratio of medullary pyramid thickness to thickness was observed between the right and left kidneys. Some investigators [4, 19] have observed a statistically significant difference between right and left kidneys in terms of length; however, one previous report [3] reported that there was not a significant difference in renal length between the right and left kidneys. A search of the literature revealed that there are no reports on the effect of differences in the right and left kidneys in terms of renal thickness and medullary pyramid thickness. This study has some limitations. Sex was not taken into consideration. The effect of sex on renal length has been discussed in the literature [3, 19]. Sex does not affect kidney dimensions in the pediatric age group [3, 19]; yet, in practice, renal length charts according to age are most commonly used. The primary aim of this study was to develop nomograms of thickness, medullary pyramid thickness, and renal length according to age in healthy children. All kidneys were completely visible and measurable in all children. Measurement of the medullary pyramid, even in older adolescents, was successful as well. This result could be due to the ultrasound unit used in this study, which was recently released to the market. This may be a potential limitation of the study. Interobserver and intraobserver variations were not evaluated in the current study. All measurements were made by the same radiologist. Radiologists may prefer rounding up to rounding down during measurements. One previous report [20] on renal length measurement indicated that significant interobserver and intraobserver errors occur in sonographic measurement of renal length. The observed variability in sonographic measurement of renal length is comparable to the expected annual increase in length of the kidneys during childhood [20]. Some zigzags were observed in the graphs of renal length, thickness, and medullary pyramid thickness. This fact might be the result of the number of patients included in the study. Larger series might result in the disappearance of zigzags from the graphs. Future studies with larger samples and longitudinal follow-up could lead to more valuable insights and overcome these limitations. In conclusion, establishing normal parameters is necessary for defining the pathologic changes in any organ using ultrasound. For this purpose, nomograms of renal thickness, medullary pyramid thickness, renal length, and the ratio of medullary pyramid thickness to thickness were established in healthy children by use of renal sonography. Acknowledgment I thank Omer Uysal for his invaluable aid with the statistical analysis during this study. References 1. Rosenbaum DM, Korngold E, Teele RL. Sonographic assessment of renal length in normal children. AJR 1984; 142: Klare B, Geiselhardt B, Wesch H, Schärer K, Immich H, Willich E. Radiological kidney size in childhood. Pediatr Radiol 1980; 9: Han BK, Babcock DS. Sonographic measurements and appearance of normal kidneys in children. 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