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ORIGINAL ARTICLE
Year : 2021 | Volume
: 10
| Issue : 3 | Page : 222-226
Normative assessment of renal dimensions from computed tomography
Damodar Rokka1, Sharma Paudel2, Prakash Kayastha2, Saroj Chhetry3, Sudil Paudyal4, Sundar Suwal2
1 Department of Radiology and Imaging Technology, National Trauma Center, National Academy of Medical Sciences, Kathmandu, Nepal
2 Department of Radiology and Imaging, Tribhuvan University Teaching Hospital, Kathmandu, Nepal
3 Department of Radiolology, Shahid Gangalaal National Heart Center, Kathmandu, Nepal
4 Department of Radiology and Imaging, Chitwan Medical College, Bharatpur, Nepal
Correspondence Address:
Dr. Damodar Rokka
Lecturer & Radiographic Technologist, Department of Radiology and Imaging Technology, National Trauma Center, National Academy of Medical Sciences (NAMS), Kathamandu
Nepal
Source of Support: None, Conflict of Interest: None
CheckDOI: 10.4103/ijhas.IJHAS_78_20
BACKGROUND: The values measured by ultrasound for normal renal dimensions in adults are well established, but not much is known about the normal renal size of Nepalese subjects from computed tomography (CT). This study aimed to establish normal CT values for kidney dimensions from patients undergoing contrast-enhanced CT (CECT) abdomen examination.
MATERIALS AND METHODS: This was a cross-sectional study carried out in patients undergoing CECT scan with different clinical history excluding abnormalities with the urinary system. The study consisted of the measurement of 212 bilateral kidneys in 106 adults. The kidney length (KL) from pole to pole, kidney width (KW), and cortical widths (CW) at the upper, middle, and lower pole were measured. For measuring length, axes were adjusted for each kidney in double oblique sagittal planes, for the width of kidney and cortex in double oblique coronal planes using a 3D-software available in the scanner (Neosoft). Analyses for normal distribution, t-tests, and correlation were performed using SPSS version 20 (IBM, USA).
RESULTS: KL was 95.52 ± 8.91 mm for the right kidney (RK), and 98.22 ± 8.85 mm for the left kidney (LK). CW at the upper, middle, and lower poles on RK was 5.33 ± 0.97 mm, 5.29 ± 0.97, and 5.03 ± 0.88 mm, whereas on the LK was 5.30 ± 0.94 mm, 5.19 ± 0.97, 5.03 ± 0.96 mm, respectively. The KW was 49.15 ± 5.76 mm and 49.11 ± 5.52 mm on RK and LK, respectively. The most significant independent factors for KL and KW were age, body surface area (BSA), gender, height, and weight and were found to be statistically significant (P< 0.05 each).
CONCLUSIONS: We established the normal references of various renal measurements which provide the radiologists and the referring clinicians an insight about the normal range of different renal parameters. Since there are several influencing factors on kidney size, the assessment should be made individually. The major influencing factors found are BSA, height, gender, age, and weight.
Keywords: Cortex, computed tomography, kidney, multidetector computed tomography, renal dimensions
Renal size and function reflect the healthy status of the kidney.[1] Change in renal dimensions is an important sign of renal disease as kidney sizes are significantly influenced by different clinical conditions.[2] Renal size is used for assessing patients with diabetes, renal artery stenosis, chronic renal failure.[3],[4] Renal size is also used for evaluating kidney transplant candidates and to differentiate between chronic and acute renal failure.[4],[5] It is also important in the management of patients with vesicoureteric reflux. As many therapeutic decisions are frequently based on renal measurements, accurate and reproducible methods for assessing renal dimensions are of utmost importance. Various studies have established normal renal dimensions for the adult population measured by ultrasound (US).[6],[7],[8],[9],[10],[11],[12],[13],[14],[15] US is easily available, cheaper, and radiation free for measurement of various renal dimensions. However, renal dimensions measured with US are prone to interobserver variability and poor repeatability.[5] US measurements are also sometimes inaccurate, especially in obese patients.
A number of reports have been published on the measurement of renal dimensions in the healthy Asian population,[8],[9],[11],[12],[15] but there are very limited data regarding the measurement of renal dimensions in adults from computed tomography (CT), and no such study was done among Nepalese population from CT scan. Thus, this study was carried out to establish normal reference values on the basis of gender and age group using CT in Nepalese population with no clinical history of renal disease and to correlate the measurements with body characteristics including height, weight, age, gender, body mass index (BMI), and body surface area (BSA).
Materials and methodsThis was a prospective cross-sectional study involving the patients undergoing contrast-enhanced CT (CECT) of the abdomen scan at the Department of Radiology and Imaging, Institute of Medicine, Tribhuvan University Teaching Hospital, Kathmandu, for the period of 3 months between August 1, 2014 and November 1, 2014. One hundred six patients were enrolled in this study ranging from age of 17 to 86 years.
Nonprobability purposive sampling technique was used. Since the study was aimed at establishing normal renal dimensions, Nepalese patients attending the department for CECT scan of the abdomen with varied indications other than urinary tract were included. The other criteria of inclusion were contrast in the aorta of >100 HU in the arterial phase and the existence of a venous contrast medium phase with collimation of 5 mm. The HU values <100 were considered to be of inadequate enhancement of the kidney and thus were difficult for the measurement of different parameters. An incomplete image of the urogenital tract, images with movement artifacts, or technical defects, hypertensive and diabetic patients, subjects with a creatinine level >2 mg/dl (abnormal renal function test), patient too ill to allow weighing and measuring, and diseases related to urinary tract found during the examination were excluded from the study.
Imaging
All examinations were conducted on the same 16-slice multidetector CT scanner (Neosoft, China). First, noncontrast CT of the abdomen was obtained. Different phases of contrast study were obtained based on clinical indications. Most of the CT scans were performed in the triple phase, especially for the evaluation of liver and pancreatic pathologies. After a standardized weight-based administration[15] of Iohexol 350 mgI/ml concentration with a flow of 3 ml/s (350 mgI/kg body weight), a bolus tracking procedure was used to obtain a scan of the upper abdomen in an early arterial phase at the delay of 15–20 s with 5 mm slice thickness (ST), tube voltage (TV) 120 kV, scan field of view (SFOV) ~500 mm, and pitch 0.86:1. The second series was scanned in a portal venous phase (5 mm ST; TV 120 kV, SFOV 505, pitch 0.86:1) at the delay of 40–60 s, and the third of the abdomen 200 s later (5 mm ST).
To obtain coronal and sagittal reformatted images with axial images, the arterial phase images were reconstructed at the ST 2 mm and slice interval 1 mm using a standard convolution kernel. The arterial phase images were chosen for the measurement of cortical width (CW) as the corticomedullary differentiation was most pronounced on the arterial phase. The reconstructed images were temporarily saved on the workstation from Neosoft.
Calculation of body mass index and body surface area
The patient height and weight were taken before the examination and the following equations were applied for the calculation of BMI and BSA:
For BSA:
BSA (m2) = (height [cm] × weight [kg]/3600)½
For BMI:
BMI = Height (m)/(weight)2
Execution of the study
Taking all preliminary examinations into consideration, image analysis was carried out after exclusion of urinary tract abnormalities by experienced radiologists. The software used was Neuviz 16 software by Neusoft Medical Systems, China, with which individual reconstructions were also carried out at 2 mm ST and 1 mm slice interval. All image analyses were carried out using a diagnostic LCD monitor 19 inches (model HL 1916).
Parameters measured
Both the left and right kidneys of the patients were measured, which was done using multiplanar reformations on Neosoft Advanced Software named Neuviz 16. The parameters measured were the RK and LK length (RKL and LKL) in oblique sagittal, kidney width (RKW and LKW), as well as CW at the upper (RK upper cortical width and LK upper cortical width), middle (RK middle cortical width and LK middle cortical width), and lower pole (RK lower pole cortical width and LK lower pole cortical width) of both the RK and LKs in oblique coronal reformatted images. Standard coronal reformatted images were individually tilted along the longitudinal axes for both left and right kidneys, using a reference line on sagittal images and standard sagittal planes were tilted along the longitudinal axes of both the kidneys using reference line on coronal images. Influencing factors considered were age, gender, height, weight, BMI, and BSA.
Statistical analysis
All values were calculated as the mean value ± standard deviation (SD) in the normal value table. Descriptive statistics were compiled using the SPSS version 20 developed by IBM Corporation, USA. The mean ± SD was presented for age, KL, KW, CWs at the upper, middle, and lower poles. Frequencies and percentages were computed for gender and age groups. Pearson's correlation coefficient (r) was computed to assess the correlation of the measured dimension KL and KW with BMI, age, weight, height, and BSA. Scatter graphs were also made to assess the linear relationship of BMI, age, height, weight, and BSA with KL and KW for both sides. Correlation analyses were performed using the method of Karl-Pearson to find the correlation of different renal parameters with independent factors such as age, sex, BMI, and BSA. Since the sample size was small, group comparisons were made using parametric and nonparametric two-tailed t-tests where appropriate and indicated. The data followed the normal distribution and P < 0.05 was considered statistically significant.
ResultsThe total number of subjects included in this study was 106. There were 57 males (53.8%) and 49 females (46.2%). The patients included in the study were in the range of 17–86 years old. The mean age was 49.35 ± 17.65 years, with the majority lying in the fourth, fifth, and sixth decade of life.
The range of RKL was 74.55–116.76 mm with the mean length of 95.22 ± 8.91 mm, whereas the range of LKL was 78.89–125.42 mm with the mean length of 98.22 ± 8.85. The range of RKW was 38.34–62.50 mm with the mean width of 49.15 ± 5.76 mm. The mean LKW was 49.11 ± 5.52 mm with a range of 34.27–62.09 [Table 1]. A statistically significant difference was found between dimensions of RK and LK, with the left being slightly larger than the right in length and width (P < 0.05).
Mean KL measurements in males were 96.76 ± 7.82 and 98.77 ± 8.10 mm for RK and LK, respectively. The mean KL in females was 94.06 ± 9.92 and 97.59 ± 9.70 mm for RK and LK, respectively. The mean KW measurements in males were 50.40 ± 6.18 mm on the right and 50.28 ± 5.40 mm on the left. The mean KW measurements in females were 47.69 ± 4.90 on the right and 47.74 ± 5.40 on the left [Table 2]. All these parameters were statistically higher in males as compared to females (P < 0.05).
Table 2: Mean right kidney length, left kidney length, right kidney width, and left kidney width measurement in males and femalesThe mean KL showed a gradual decrease from the fifth decade onward and on the correlation of KL and KW with age, there was a significant negative (P < 0.05) but a relatively weak linear relationship between the two, with the r values of −0.277 and −0.295 when compared with RKL and LKL, respectively. The r values when the age is correlated with RKW and LKW are −0.199 and −0.423, respectively.
The CW did not show any significant correlation with age (P > 0.05). As the height of subjects increased a significant increase in KL and KW was seen, showing a positive but a weak linear relationship. This was found to be comparatively more between “height and KL on both sides,” “height and LKW” than between “height and RKW.” The value of “r” was 0.430 on the right and 0.369 on the left for KL and 0.305 on the right and 0.409 on the left for KW when height was correlated with these variables.
As the weight of subjects increased a significant increase in KL and KW was seen, showing a positive but a weak linear relationship. This was found to be comparatively more on “weight with RKL and RKW,” than on “weight with RKL and RKW” value of “r” was 0.405 on RT and 0.318 on LT for KL and 0.481 on RT and 0.303 on LT for KW when weight was correlated with these variables.
As seen between height and renal size and weight and renal size, a similar significant positive relationship was found on the correlation of subject's BSA with KL and KW (P < 0.01). The value of r was 0.464 and 0.479 when BSA was correlated with RKL and RKW, respectively, and it was 0.374 and 0.379 for LKL and LKW, respectively. It was more stronger on correlating BSA with RKL and RKW than on the left side. No statistically significant correlation was seen with BMI on either side (P > 0.05). The mean BMI was 23.96 ± 3.47.
DiscussionUS is the primary method of choice for examining the kidneys in clinical scenario, but we preferred CT for this study because CT measurements are more accurate and have less interobserver variability as multiplanar reformation and resolutions up to the submillimeter range are possible from the volume data sets in CT. The underestimation of length is the main problem when using US[16] since the longitudinal axis of a kidney is not always perfectly adjustable in US. Moreover, the US technique is also dependent on the sonographer.[6] Our study is carried out in CECT abdomen because it provides better delineation of the renal boundary and the corticomedullary differentiation for the precise measurement of the CW the preferred method for evaluating renal sizes. Coronal and sagittal reformatted CT scans along with axial source images were used for measurement purposes as this approach was previously proven to be the most accurate way of measuring kidney sizes on imaging studies.[17],[18]
The values for KL correspond very closely with those indicated for US[7],[8],[9],[10],[11],[12],[13],[14],[15] and CT.[4],[5] For the most reliable method of determining the KL, KW, and CW, sagittal and individual reformatting were also used depending on the position of the kidneys. Thus, we can assume the greatest possible reliability of the results. The thin slice, multiple detector technique has been chosen in order to obtain very accurate data.
The reference values obtained showed the mean renal length of 95.52 ± 8.91 mm and 98.22 ± 8.85 mm for the RK and LK, respectively, in the Nepalese population, which lies in almost the same range as that of Indian[8] and Filipino population[10] but is relatively shorter as compared to population residing in Korea,[15] African,[4],[16] Pakistan, [7],[11],[15] and European countries.[5] Similarly, the mean renal width obtained was 49.15 ± 5.76 mm and 49.11 ± 5.52 mm, respectively, for the RK and LK, which are similar to the previous studies done in the Asian population.[7],[8],[10],[11],[15].
The main attraction in this study was the reference values for CW at the upper, middle, and lower poles of both kidneys unlike other studies where the CW was measured in only one place in the kidney.[5],[7] CW/thickness is better correlated with renal function than renal length. Hence, it is recommended to measuring the cortical thickness in patients with chronic renal disease.[19] No significant difference was found statistically in the CWs measurement in the RK and LK, respectively. The range of CWs measurement was found to be from about 3 mm to as high as 8–9 mm on both the RK and LK giving us the general idea of the range of normal CWs measurement in the adult population in CT.
The differentiated observation of kidney sizes is of great significance clinically.[4] The normal range is large,[5] and what is “normal” depends on many factors. The factors having an impact on renal size must be viewed individually before arriving at any relevant conclusions and information.
The age-related decrease of the kidney dimensions of length and width is well known.[5],[7],[8],[11],[12] This study showed decrease in renal length after the fifth decade in both the genders. It should also be noted that only age was the greatest negative influencing factor on both KL and KW in our study and it also showed no significant correlation with the CWs on the both sides.
The renal dimensions measured were higher in males in comparison to females which supported the previous established norms and studies[4],[5],[8],[11],[12],[13],[15] which may be probably because of difference in height or body size which is greater in males than in females. Interestingly, the study showed no significant correlation between BMI and measured kidney dimensions (P > 0.05), unlike other studies where it showed a significant positive correlation.[4],[5],[7],[10]
When the KL and KW were correlated with height and weight individually, they showed a significant positive correlation as in previous researches done in other population.[5],[7],[8],[10],[12] Similarly, the measured dimensions like KL and KW showed a significant positive correlation when BSA was taken into account which also supported the previous theories. KL and KW showed a stronger correlation with BSA than with height and weight. Interestingly, these body parameters showed no statistically significant correlation with CWs at the upper, middle, and lower poles bilaterally. This could facilitate arriving at conclusions as to the extent of arteriosclerotic renal disease based on the CW.[3] From a physiological perspective, this finding would make sense, as patients with a bigger body habitus will have a larger blood volume requiring larger kidneys for filtration.
Limitations of the study
As US is an established method to measure renal dimensions, especially renal length, correlation of CT measurements with US would have given better results. Correlation of CT findings with US was not done in this study. The patient group was not a randomly selected sample. Selecting a real random sample was not possible due to the danger of radiation exposure from the CT. In order to estimate the normal value of the kidney dimensions, diseases of the kidney and lower urinary tract in the imaging were excluded if they were not known or symptomatic in the patient history but have affected the renal parenchyma. Due to the exclusion of clinically conspicuous “maximum variants” of these diseases, we can assume only the normal reference values of these dimensions. We did not calculate the kidney volume which is a well-known factor getting affected by body parameters such as BSA, BMI, height, and weight.[5],[7],[8],[10] This was the major limitation of our study. The patients with the normal creatinine levels were only included. Therefore, the possible correlation of kidney function and size measurements is not possible in the present study. Results from ultrasonographic volume measurements of kidneys are promising, so further research is necessary in order to improve these opportunities in CT.
ConclusionsWe established the normal references of various renal measurements which provide the radiologists and the referring clinicians an insight about the normal range of different renal parameters. Since there are several influencing factors on kidney size, the assessment should be made individually. The major influencing factors found are BSA, height, gender, age, and weight.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
References
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