| Abstract|| |
Ultrasound (US) is a noninvasive, simple and safe imaging investigation that can be done as many times as needed. Therefore, it is the primary imaging modality for evaluating kidneys. We carried out a literature review of information about ultrasonography for clinicians, especially nephrologists. US utilization from prenatal time till adult life with various measurements including, length, width, depth, and volume was searched during 2019. US identifies 90% of fetal kidneys by 20 weeks of gestational age. Kidney weight and volume at birth are approximately only 10% of the mature kidney. Kidney growth is most rapid during the first few weeks of life, with the kidney length increasing by as much as 15–20% in full-term neonates. There is a good correlation between relative function shown by scintigraphy and relative volume estimated from sonography. The most accurate measurement of kidney size is provided by kidney volume, which is correlated with subject’s height, weight, and total body area. Kidney length is the most easily reproduced. Kidney volume is a better approximation of size than length because of the shape of the kidney varies considerably, but it is technically more demanding and needs four measurements in two different planes. It has been shown that in normal adult kidneys, the sonography measurements of kidney length differ by values of between about 1 cm and 1.85 cm in 95% of the cases, irrespective of whether the measurements are performed by the same or by different sonographers. Measuring the renal parenchyma with US is a novel method to assess fetal kidney development and predict future renal function.
|How to cite this article:|
Al Salmi I, Al Hajriy M, Hannawi S. Ultrasound Measurement and Kidney Development: a Mini-Review for Nephrologists. Saudi J Kidney Dis Transpl 2021;32:174-82
|How to cite this URL:|
Al Salmi I, Al Hajriy M, Hannawi S. Ultrasound Measurement and Kidney Development: a Mini-Review for Nephrologists. Saudi J Kidney Dis Transpl [serial online] 2021 [cited 2021 Dec 4];32:174-82. Available from: https://www.sjkdt.org/text.asp?2021/32/1/174/318520
| Introduction|| |
Ultrasound (US) is a non-invasive imaging investigation. It is a well-established medical imaging technique with the pioneering work conducted by Professor Ian Donald et al at the University of Glasgow, from the mid-1950s onwards, in terms of introducing it as a diagnostic tool in the field of obstetrics and gynecology., It is used widely in clinical medicine because of its lack of ionizing radiation and hence its suitability for pregnant women and children. It avoids the possible hazards, such as bleeding, infection, or reactions to chemicals, of other investigative and diagnostic methods.
It is a multi-planer cross-sectional imaging modality in which sonographic information is obtained and simultaneously displayed on a video monitor in a real-time format. Its moving image can be frozen for in-depth analysis and in obtaining various measurements of kidney size.,,
US estimation of kidney size is unaffected by the problem of magnification that occurs with other radiological investigations. Other radiological investigations, such as plain X-ray, intravenous pyelogram (IVP), and kidney angiogram, tend to overestimate the true size due to several factors including magnification.,,,
| Kidney Development and Growth: Role of Ultrasound|| |
Kidneys can be seen on US examination by 16–17 weeks of gestation. However, ultra-sonography identifies 90% of fetal kidneys by 20 weeks of gestational age.
Nephrogenesis increases progressively from the end of the first trimester and rapidly in the mid trimester and is fully complete by 34 weeks gestation. During the period of 20–30-week gestation, there is exponential growth in kidney mass reflecting the speedy development of renal tubules, especially in the third trimester, as shown in [Figure 1]. Kidney weight compared with total body weight, increases linearly during the first two trimesters. The rate of kidney growth is exponential and it is proportional to the overall rate of fetal weight gain. The ratio between fetal kidney volume and body weight remains constant throughout the course of gestation.
Kidney growth follows a non-linear pattern. Kidney weight and volume at birth are only approximately 10% as much as that at maturity., Kidney growth is most rapid during the first few weeks of life, with the kidney length increasing by as much as 15%–20% in full-term neonates.,, The kidney length increases by 17% in the first week of life, and by 37% in the first four months. [Figure 2] shows the age-related growth curve for the left kidney in 291 subjects.
|Figure 2: Age related growth curve for left kidney in 291 subjects by Oswald et al, 2004). The outer represent the 95% CI around the fitted middle line.|
A Nigerian study put the African figures in the upper range of normal (Odita and Ugbodaga, 1982), and Caucasians somewhere in the middle (Gremigni et al, 1984).
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Kidney growth then gradually slows throughout the remainder of the 1st year of life, after which the rate of increase in kidney length stabilizes at approximately 2–3 mm/year. After age 10 years, kidney growth again declines until it ceases at maturity. This increase in size or functional reserve is accomplished through enlargement and maturation of existing nephrons, rather than through formation of new nephrons.
| Kidney Length|| |
Chiara et al found that kidney length is strictly related to birth weight and gestational age. The length of the kidneys is significantly correlated with birth weight, as shown in [Figure 3]. A significant correlation was found between gestational age and the length of a kidney during fetal life. The weight of the kidneys increased by 6 g for each increase in total body weight of 1000 g in babies under one year at postmortem examination. The kidney length gradually increases with advancing gestational age in infants from 23 to 42 weeks.,
|Figure 3: The length and width of left kidney plotted against weight of newborns (Soyupak et al, 2002).|
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In a study of bilateral kidney lengths in 20 adult subjects, with no history of kidney disease, suggested that sonographic bipolar kidney length measurements in normal adult kidneys are reasonably reliable. The length of the normal adult kidney is usually given as 10–12 cm but there is a wide range of 7–14 cm in patients with normal kidney function., Because of the extensive use of kidney sonography, normal kidney size standards have been developed which are widely accepted.,, Most standards rely on a graph of kidney measurements, such as volume or length, plotted against age.,,
A number of studies showed that sonographic measurements of kidney volume correlated with urography standards, and also with the volume of nephrectomy specimens determined by water displacement., Sargent and Gupta showed good correlation between relative function shown by scintigraphy and relative volume estimated from sonography. However, many variables other than age can potentially affect kidney size. Correlation studies have been done to evaluate the association of kidney dimensions with patient height, weight, and body mass index.
| The Relative Kidney Length|| |
Because of the well-known individual variation in kidney size according to the height, sex, and age of subjects, the relative kidney length is calculated using the kidney length: Body height ratio (KBR) by dividing the absolute kidney length (in millimeters) by the subject’s body height (in centimeters) for each kidney. In adults younger than 60 years without kidney disease, KBRs should range from 0.60 to 0.74 for the left kidney and from 0.57 to 0.72 for the right kidney. Kidney length decreased with age, and the rate of decrease seemed to accelerate at 60 years and older. Kidney weight decreases 20%–30%; from between 200 and 270 g, to between 180 and 200 g between the age of 30 and 90 years. Kidney length diminishes by 2 cm between the age of 50 and 80 years, which represents a loss of volume of about 40%.
| Kidney Width and Thickness|| |
A study by Buchholz et al utilizes ultrasonographic renal size in a group of individuals with no known renal disease and assessed the effect of age, gender. Side and body mass index (BMI). Of the total 194 patients, 98 were males and 96 females. The mean age was 44.7 [standard deviation (SD) 14] years. The mean kidney width was 4.5 (0.6). Compared with measurement of the kidney length, measurements of the width and thickness of the kidney showed higher relative observer variations. This can be attributed to the difficulty in obtaining an optimal and reproducible transverse image of the kidney. This could also be because the width and thickness of the kidney is shorter than the kidney length so that a given measurement error will result in relatively higher observer variations for these measurements than for kidney length.
| Kidney Volume|| |
The most accurate measurement of kidney size is provided by kidney volume, which is correlated with subject’s height, weight, and total body area., However, kidney length has the lowest inter-observer variation and therefore has better reproducibility than volumetric estimation. Kidney volume is a more sensitive means of detecting kidney abnormalities than any single linear measurement. Kidney volume correlates well, although indirectly, with the number of functioning nephrons.
Kidney volume determinations in adults by US scanning have been used in normal subjects and in kidney transplant patients., Sonographic studies of kidney size in children have dealt with parameters such as kidney length and thickness. Kidney bipolar length has been reported to correlate well with the weight of kidney substance, which correlates closely with kidney volume.
Relative observer variation of 16% in the calculation of kidney volume means that a repeated calculation should show a difference of more than 32% before it can be considered a real and reliable change. The corresponding figures for the kidney length measurements are much smaller, i.e. 5% and 10%, respectively.
Currently, there is no acceptable in vivo method of counting nephrons. However, kidney weight has been shown to correlate with nephron number., Although human fetal kidney weight cannot be measured in utero, estimates of kidney volume appear to be a reasonable substitute.
| Racial Differences|| |
Despite expectations that smaller body sizes in children of developing countries might equate to smaller kidneys than in western children, a sonographic study of kidney length found no significant difference between Chinese, American, and Australian children. Others found that the two kidneys were longer in white than in black children with a mean difference of 0.33 cm. on the left and 0.28 cm. on the right sides (P <0.001 and 0.004, respectively). Chin et al showed that an accurate description of the variability of kidney length should include gender, race, and other relevant covariates (such as height and weight) in addition to age. Indio-Asian populations have sizes at the lower end of normal., This is probably a reflection of the relatively small body size of most Indio-Asians. Organ size is related to body size.
| Ultrasound Underestimation of Kidney Measurements|| |
Kidney size measurements with US, computerized tomography (CT), and magnetic resonance imaging (MRI) results in a 24% underestimation of the kidney volume. A single study comparing MRI imaging with US, found MRI to be a better alternative for measuring kidney volume. It concluded that the US underestimates kidney volume and has a poor repeatability compared to the MRI. Studies on kidney length and size have also been made by means of CT and MRI. But although these methods show a better repeatability, the US technique is the tool of the first choice due to its ubiquitous availability, its easy handling and its cost-effectiveness.
| Kidney Length versus Kidney Volume|| |
In 196 children aged between four weeks and 16 2/12 years a sonographic investigation of kidneys was performed and put into relation to somatometric data including body weight, body length, length of the trunk and the upper and the lower limb, breadth of the thorax and pelvic, head circumference, maximum cranial length, and maximum cranial breadth. None of the children had signs of renal disorder and five kidney parameters (length, depth, width, and areas of longitudinal and transverse section) and calculated kidney volume using a prolate ellipsoid volume (PEV). Although the closet correlations were found between kidney parameters and pelvic breadth, arm length and leg length, similar correlations existed between kidney length and body length and between volume and body weight.
The kidney length may not be an accurate predictor of kidney volume, as kidney volume best correlates with body surface area (BSA), whereas kidney length correlates with height., In addition, the kidney becomes wider and thicker with age, where the kidney length decreases with little change in kidney volume. A study of aboriginal Australians found that both kidney length and kidney volume correlated significantly and positively with weight, height, BMI, and BSA.
A study of 18 healthy volunteers aged 34.4 ± 2.61 years found that kidney length does not reliably predict kidney volume. However, length (L) multiplied by width (W); L × W is a better predictor of kidney volume., This discrepancy between length and volume of the kidney, occurs in older populations. It may be that the absolute reproducibility of the measurement of children is smaller than those of adults. Because of changing body proportions during growth, the correlation coefficients may vary and should be calculated separately for each age group. It was found that the longitudinal and transverse section areas of the kidney, subtracting the pelvis and the sinus fat of the kidney, showed close correlation to the PEV.
| Norm Measurements|| |
In 1962 Hodson et al established the first set of norms for kidney size based on normal pyelograms. With the acceptance of the US for pediatric applications others established similar norms based on US parameters.,,,, Kidney US is now the modality of choice for measuring kidney size. US was found to measure the kidney more accurately (mean difference between estimated size and actual = -3.4 mm ± SD; 6.96, than plain X-ray (mean difference from actual 13 mm ± SD 5.24), IVP (mean difference from actual 16.9 mm ± SD 5.74), and kidney angiogram (mean difference from actual 15.2 mm ± SD 5.77).
| The Accuracy and Repeatability of the Kidney Measurements by Ultrasound|| |
The poor accuracy and repeatability of kidney measurements may be due to an inadequate depiction of the kidney borders, for example, by overlying bowel gas, ribs, interposed scars, or patient motion. Errors in the accuracy of ultrasonographic caliper measurements of distance may explain the discrepancy in the measured kidney lengths.
Furthermore, several reports have shown that, depending on the patient’s position, the measured maximal longitudinal length varies. It was shown that supine, coronal views yield longer kidney measurements than prone, sagittal views.,, In addition, rotational and positional changes in the kidney because of changes in the patient position and/or changes in the degree of inspiration/expiration may also influence the accuracy of distance measurements.
A study by Emamian et al showed that a relative observer variation of 5% for a kidney measured to be 100 mm long means that an SD of 5 mm could be expected in a repeated measurement. From this it follows that (1) the probability is >95% that repeated measurements will result in a value within ±2 SD, i.e., the repeated measurement of kidney length will be within ± 10 mm of the first measurement, and (2) the difference of <2 SD (± 10 mm) in two successive measurements of kidney length may be due to observer variation in the measurements and may not represent a real change in kidney size.
The observer error in the sonographic measurement of kidney length in children is equivalent to about two years of normal growth.,, Nonetheless, kidney length is the most easily reproduced. Kidney volume is a better approximation of size than length because the shape of the kidney varies considerably, but it is technically more demanding and needs four measurements in two different planes. It has been shown that in normal adult kidneys; replicate sonography kidney length measurements differ by values of between about 1 and 1.85 cm in 95% of the cases, irrespective of whether the measurements are performed by the same or by different sonographers.
| Conclusion|| |
US is an imaging modality with no radiation hazards with wide spectrum of usage worldwide. It is easily utilized in all fields of medical practices. Importantly, it is vastly used in obstetrics health care with its great safety and accessibility. Assessment of kidney sizes may vary upon operator experience. The most accurate measurement of kidney size is provided by kidney volume, which is correlated with subject’s height, weight, and total body area. Currently, there is no acceptable in vivo method of counting nephrons. However, Kidney weight has been shown to correlate with nephron number. Kidney volume correlates well, although indirectly, with the number of functioning nephrons.
Conflict of interest: None declared.
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Issa Al Salmi
Department of Renal Medicine, The Royal Hospital, P. O. Box: 1331, Code 111, Muscat
[Figure 1], [Figure 2], [Figure 3]