The rotation method for correcting renal depth in the determination of glomerular filtration rate using Tc-99m diethylenetriamine pentaacetic acid (DTPA)-based renal dynamic imaging in patients with hydronephrosis

Introduction

Hydronephrosis, a common disease in the urinary system, frequently arises from obstructions in the urinary tract, ureter stones, abnormal stenosis, tumors, and other factors (1). Obtaining an accurate measurement of glomerular filtration rate (GFR) can guide clinical treatment for patients with hydronephrosis (2,3). Renal dynamic imaging is widely used in clinical practice to obtain total and single-kidney GFR (4). Renal dynamic imaging is based on calculating the depth of the kidneys via a formula incorporating height and weight (5-7), but the depth of the kidneys in patients with hydronephrosis is altered, especially in cases of severe hydronephrosis. Therefore, for these patients, accurate renal depth must be obtained to calculate an accurate GFR. To this end, Beijing Novel Medical has innovated a new technology to calculate the depth of the kidneys, the rotation method, which has demonstrated application value in healthy kidneys (8). This study aimed to evaluate the applicability of the rotation method in patients with hydronephrosis. Computed tomography (CT) can observe the shape and position of the kidney directly, which is the most effective approach for assessing renal depth (9). We assessed the accuracy of the rotation method in correcting renal depth, depth difference, and its impact on GFR determination in patients with hydronephrosis through a comparison with CT scanning. We present this article in accordance with the STROBE reporting checklist (available at https://qims.amegroups.com/article/view/10.21037/qims-24-1068/rc).

Methods

Clinical data

The study was conducted in accordance with the Declaration of Helsinki (as revised in 2013) and was approved by the Ethics Committee of First Hospital of Shanxi Medical University (No. 2021BAL0146). Informed consent was obtained from all the patients. We analyzed 66 patients with unilateral hydronephrosis in First Hospital of Shanxi Medical University between January 2022 and June 2023 (38 males and 28 females; mean age 50.6±16.3 years; height 165.52±8.49 cm; weight 67.71±13.83 kg). The inclusion criterion for participant selection was a diagnosis of hydronephrosis conducted via abdominal CT scans within 1 week of renal dynamic imaging. Meanwhile, the exclusion criteria were presence of congenital renal malformation, renal mass, and post transplantation status. Among the 66 patients with unilateral hydronephrosis, there were 20 cases of renal calculi with hydronephrosis, 39 cases of ureteral calculi with hydronephrosis, and 7 cases of kidney combined with ureteral calculi with hydronephrosis (Table 1). Abdominal CT was used to categorize the degree of hydronephrosis into three groups: mild (20 cases), moderate (23 cases), and severe hydronephrosis group (23 cases). The criteria for evaluating the severity of hydronephrosis in abdominal CT scans differed depending on the group. The criterion for the mild hydronephrosis group was a largely unaltered renal parenchyma, with an effusion shadow observed in the renal pelvis and calyx. The criteria for the moderate hydronephrosis group were as follows: an expanded kidney volume, with some renal tissue being somewhat thinner, accompanied by renal pelvis and renal calyx expansion; and obvious effusion shadow. Finally the criteria in the severe hydronephrosis group were as follows: a considerably increased kidney volume, a visibly shrunken renal parenchyma, and a renal sinus space evidently exhibiting cystic effusion (10,11).

Renal dynamic imaging

The NET632 SPECT (Beijing Novel Medical Company, Beijing, China) was used for imaging and was equipped with a low-energy, high-resolution collimator (LEHR). The image acquisition parameters were as follows: energy peak, 140 KeV; window width ±20%; and matrix 64×64. Diuretics were discontinued 3 days before the examination, and no drugs were taken on the examination day. Patients were instructed to drink 7–10 mL/kg of water 30 minutes before imaging (4). The height and weight of the patients were recorded. The bladder was emptied before the examination. For full needle measurement, a syringe of Tc-99m diethylenetriamine pentaacetic acid (DTPA) was placed 30 cm from the posterior probe for 6-second tracer counting. The patient was placed in the supine position with both renal areas in the collimator field. The probe field included both kidneys and the bladder. Furosemide (0.5 mg/kg) was injected into one side of the elbow vein, and then a Tc-99m (molybdenum-technetium generator: radiochemical purity >95%, dose 185 MBq, volume <1.0 mL; Atomic High-Tech, Beijing, China) DTPA (Beijing Shihong Pharmaceutical Research Center, Beijing, China) imaging agent was injected via bolus injection method. The posterior probe was used for 20-minute posterior image acquisition. The blood perfusion phase was first acquired at 2 s/frame for 1 minute, and then the renal parenchymal function phase was acquired at 1 min/frame for 19 minutes. At the third minute of the renal dynamic imaging, the anterior probe was rotated 15°, 30°, 45°, and 60° to collect renal projection images for a total of 8 minutes (Figure 1). After the imaging, the injection point and syringe were counted for the 6-second tracer. The operation of empty-needle measurement was the same as that of full needle measurement.

Figure 1 Schematic diagram of the renal depth acquisition process with the rotation method. The top left corner of (A-D) contains a simulation diagram. (A-D) Illustration of the anterior probe sequentially rotating at 15°, 30°, 45°, and 60° during renal depth measurement under the rotation method.

Kidney depth measurement

Kidney depth was measured using four methods: (I) CT; (II) rotation method; (III) the Tonnesen formula; (IV) and the Li-Qian formula.

• For CT measurement, CT images of the renal hilum were selected, the vertical distance from the anterior and posterior edges of the kidney to the dorsal skin was measured, and the arithmetic mean of these two distances was used as the renal depth (Figure 2).
• The rotation method is based on the principle of tomosynthesis, with the kidney’s position being determined by the acquisition of kidney projections from different angles. In conventional renal dynamic imaging, the posterior probe is typically used. In this study, an anterior probe was introduced and rotated 3 minutes after the start of the renal dynamic imaging to collect projection data from four angles (15°, 30°, 45°, and 60°), with each angle recorded for 2 minutes. These data were then used to estimate the renal depth. The projection images obtained by the anterior probe at each angle were segmented and combined with the positional information of the anterior probe to calculate the true coordinates of the kidney’s center. The depth of the kidney was determined based on the position of its center relative to the examination bed (Figure 1).
• The Tonnesen formula (12) was derived from ultrasound measurements of the depth of the kidneys of European and American adults by Tonnesen et al. The formula is as follows:

  $$\text{Depth}\text{of}\text{right}\text{kidney}\text{(DR)}=\text{13}\text{.3}\times \text{W/H}+0.7$$[1]

  $$\text{Depth}\text{of}\text{left}\text{kidney}\text{(DL)}=\text{13}\text{.2}\times \text{W/H}+0.7$$[2]

  
  where W is the weight (kg), and H is the height (cm).
• The Li-Qian formula (13) is based on the depth of the kidney measured by CT in Chinese adults. The formula is as follows:

  $$\text{DR}=\text{15}\text{.449}\times \text{W/H}+0.009637\times \text{age}+0.782$$[3]

  $$\text{DL}=\text{16}\text{.772}\times \text{W/H}+0.0\text{1025}\times \text{age}+0.224$$[4]

  
  where W is the weight (kg), and H is the height (cm).

Figure 2 Renal depth measured with CT. (A-C) The respective renal depth measurements for the mild, moderate, and severe hydronephrosis with the CT method. Renal depth = (a + b)/2. a, the distance from the anterior edge of the kidney to the dorsal surface of the skin; b, the distance from the posterior edge of the kidney to the dorsal surface of the skin. CT, computed tomography.

Image processing and GFR calculation

A novel medical image processing workstation was used to process renal dynamic images. The patient’s gender, age, height, and weight were input into the processing system. The kidney depth measured by the Tonnesen formula, the Li-Qian formula, and the rotation method were automatically processed by the workstation, and the depth values of the kidneys measured by CT were manually input into the workstation. In the functional phase, the region of interest (ROI) of the kidney was delineated according to the image, the background ROI of the meniscus was delineated along the lower lateral side of the kidney, and the ROI of the abdominal aorta was delineated above the bifurcation of the renal artery. When there was effusion at the ureteropelvic junction, ROI delineation did not include this area (Figure 3). The ROI was delineated three times by the same nuclear medicine physician, and the mean values of the total and separate renal GFR obtained three times were recorded (Figure 4).

Figure 3 ROI delineation for hydronephrosis of varying degrees. (A-C) ROI delineation in mild, moderate, and severe hydronephrosis. Green line: ROI delineation of the left kidney. Red line: ROI delineation of the right kidney. Blue and yellow represent background delineation. ROI, region of interest.

Figure 4 Renogram and split renal function values were ultimately obtained through image processing. L, left; R, right; KP, kidney perfusion; KF, kidney function; GFR, glomerular filtration rate.

Statistical methods

Statistical analysis was conducted using SPSS 25.0 (IBM Corp., Armonk, NY, USA). Quantitative data following a normal distribution are presented as the mean ± standard deviation (x¯±s). We analyzed the kidney depth, depth differences, and renal GFR values of participants using four different methods. As the data met the assumptions of normality and homogeneity of variance, a one-way analysis of variance (ANOVA) was employed. In cases where statistically significant differences were observed, a post hoc least squares difference test was conducted to further explore specific differences between the four methods. P<0.05 was considered statistically significant.

Results

Comparison of renal depth

For normal kidneys (NKs), the Tonnesen formula led to a notable underestimation of kidney depth compared to CT, and the difference was statistically significant (F: 29.839; P<0.05). There were no statistical differences in the renal depth measured by the rotation method, the Li-Qian formula, and CT (P>0.05). For hydronephrotic kidneys (HKs), compared to that in CT, the depth of hydronephrosis measured by the Tonnesen formula and the Li-Qian formula was significantly underestimated (F: 54.110; P<0.05) (Table 2). There was no statistical difference in the depth of hydronephrosis measured by the rotation method and CT (P>0.05). The Tonnesen formula undervalued the depth of NKs and HKs. The Li-Qian formula undervalued the depth of HKs. The renal depth measured by rotation method was similar to that of CT in NKs and HKs.

Comparison of GFR

For NKs, compared to CT, the Tonnesen formula significantly underestimated the GFR of NKs (F: 19.520; P<0.05). GFR measured by the rotation method, the Li-Qian formula, and CT showed no statistical differences (P>0.05). For HKs, the GFR of hydronephrosis measured by the Tonnesen formula and the Li-Qian formula was underestimated (F: 16.551; P<0.05). There was no statistical difference between the rotation method and CT (P>0.05) (Table 2). In summary, the Tonnesen formula underestimated the kidney depth and GFR of both the NKs and HKs; meanwhile, the Li-Qian formula underestimated the kidney depth and GFR of HKs.

Comparison of renal depth difference

The Tonnesen formula and the Li-Qian formula significantly underestimated the renal depth difference, and the difference was statistically significant (F: 109.611; P<0.05). The renal depth difference measured by the rotation method was not significant different compared with that measured by CT (P>0.05) (Table 3).

Comparison of renal depth in patients with different degrees of hydronephrosis

For patients in the mild hydronephrosis group, the depth of hydronephrosis measured by the Tonnesen formula was significantly underestimated (F: 10.383; P<0.05), and there was no significant difference in the depth of hydronephrosis measured by the rotation method and CT (P>0.05). In patients with moderate or severe hydronephrosis, the depth of hydronephrosis measured by the Tonnesen formula and the Li-Qian formula was significantly underestimated (F: 19.592, 28.395; P<0.05). No variance was observed between the rotation method and CT for the three groups (P>0.05) (Table 4).

Comparison of GFR in patients with different degrees of hydronephrosis

The GFR of the hydronephrosis kidney measured by the Tonnesen formula was significantly underestimated in the mild hydronephrosis group (F: 3.763; P<0.05). The Tonnesen formula and the Li-Qian formula underestimated the GFR of hydronephrosis kidneys in the moderate and severe hydronephrosis groups, and the differences were statistically significant (F: 7.70 and 15.538; P<0.05). There was no statistical difference between the rotation method and CT for the three groups (P>0.05) (Table 4).

Discussion

Renal dynamic imaging enables rapid, noninvasive calculation of the total and single renal GFR in patients with hydronephrosis, which can provide an accurate basis for the diagnosis and treatment of these patients, and is the only widely used method for monitoring split renal function at present (14). Tc-99m DTPA renal dynamic imaging involves use of the Gates formula to calculate the total and single-kidney GFR. However, the depth of the kidneys, ROI delineation, gender, age, height, and weight influence the precision of GFR as assessed by the Gates formula, with kidney depth exerting a significant effect (15). Research indicates that with each 1-cm increment in kidney depth, there is a 14% variation in overall kidney radioactive levels and a 16% difference in GFR measurements. Consequently, the precise assessment of renal depth is crucial to estimating the GFR (16).

The results of our study are consistent with the conclusions in the literature (7,8,17), in that we found the Tonnesen formula significantly underestimated the renal depth of all healthy kidneys in the mild, moderate, and severe hydronephrosis groups. The Li-Qian formula underestimated the renal depth of the moderate and severe hydronephrosis groups. Neither the Tonnesen formula nor the Li-Qian formula can reflect the renal depth difference. The Li-Qian formula can accurately estimate the renal depth and GFR of healthy kidneys (13). This study also confirmed that the Li-Qian formula can accurately measure the depth and GFR of non-HKs, but there were significant differences in the measurement of renal depth and GFR in kidneys with hydronephrosis between the Li-Qian formula and CT. The renal depth and GFR measured by the rotation method were similar to those measured by CT.

The current formula-based method is often used to estimate kidney depth in clinical practice. However, it tends to underestimate the depth of kidneys affected by hydronephrosis and struggles to account for the depth differences between the two kidneys, particularly in cases of moderate-to-severe hydronephrosis. This underestimation of kidney depth can lead to inaccurate estimations of GFR. The formula-based method’s inability to accurately estimate differences in renal depth leads to further miscalculation split renal function. The rotation method overcomes these drawbacks by improving the accuracy of the Gates formula in estimating both total and individual kidney GFR in patients with hydronephrosis. This is achieved through the precise correction of kidney depth and depth differences, providing a more reliable basis for evaluating renal function in patients with hydronephrosis.

Effect of hydronephrosis on renal depth measurement

The varying degrees of severity in hydronephrosis lead to renal morphological alterations, thereby altering the depth of the kidney (18). Therefore, in our study, 66 patients with hydronephrosis were divided into mild, moderate, and severe hydronephrosis groups according to abdominal CT. The differences in renal depth and renal depth difference determined by the four methods in each group were compared. Our study found that the Li-Qian formula correctly measured the renal depth and GFR in the mild hydronephrosis group. For the groups with moderate and severe hydronephrosis, the Li-Qian formula underestimated the renal depth and GFR. This can be explained by the fact that the methodology employing the Li-Qian formula relies on the computational equation for non-HKs. Mild hydronephrosis does not alter kidney morphology greatly, but the kidney morphology of moderate and severe hydronephrosis is significantly altered (11). Consequently, the Li-Qian formula cannot accurately estimate the depth of hydronephrosis, causing a notable discrepancy between the Li-Qian formula and CT.

The significance of renal depth difference in split renal function

The degree of accumulation of the imaging agent in each kidney reflects the proportional contribution of each kidney to the total GFR. Therefore, split-kidney GFR is obtained by multiplying the percentage of radionuclide uptake in each kidney by the total renal GFR. The GFR of the single kidney can be obtained by the ratio of the single kidney uptake rate to uptake rate of both kidneys: [(C_XK − C_XB)/e^−0.153YX]/[(C_LK − C_LB)/e^−0.153YL+(C_RK − C_RB)/e^−0.153YR] (C, counts; X, the kidney to be measured (left or right kidney); K, kidney; B, background; Y, renal depth; L, left; R, right) (15). Therefore, the depth difference between two kidneys will also affect the split renal function. Weinberger et al. used CT to measure the depth of the kidney and found that about one-third of the participants had a renal depth difference greater than 1 cm, which resulted in an over 5% variance in split renal function before adjustment (19). Similarly, our previous study and Gruenewald’s study also found that renal depth difference could significantly affect renal GFR (8,16). Hydronephrosis leads to a further increase in renal depth difference. Among the 66 patients with hydronephrosis in this study, 41% (27/66) had a renal depth difference exceeding 1 cm as measured on CT. Therefore, the accurate determination of renal depth difference is critical for patients with hydronephrosis. Both the Tonnesen formula and the Li-Qian formula greatly underestimated the renal depth difference in our study; that is, the Tonnesen formula and the Li-Qian formula could not accurately reflect the split renal function of patients with hydronephrosis. The results of the rotation method showed no statistical significance in renal depth difference compared with that of CT.

Renal depth measurement in multiple methods of GFR calculation

Currently, Tc-99m DTPA renal dynamic imaging incorporates the Tonnesen formula, the Li-Qian formula, the nuclear medicine lateral method, and CT for assessing kidney depth (17,20). Although CT as the optimal method for assessing kidney depth (9), its increases the risk of radiation and imposes the financial strain on patients. The Tonnesen formula is derived from the depth of the kidney measured by the ultrasound when the patient is in the sitting position, but renal dynamic imaging is performed with the patient in the supine position. The different positions lead to discrepancies in kidney depth. The Taylor method, a new method for calculating the depth of the kidney, is based on the renal depth measured by CT and is integrated with the age of the patient. However, this approach is based on data from Western populations, and it is unclear if it is applicable in the Chinese context. The application value of the Li-Qian formula has been confirmed in healthy kidneys (13), but there are few studies regarding its value in patients with hydronephrosis. The lateral method of nuclear medicine aims to determine the renal depth via a lateral scan after the completion of renography (21). The disadvantage of this method is that the measured depth of the kidney is not the attenuation distance of the renal cortex, as in the Gates method, and may include the radioactive count of the renal pelvis. In patients with severe hydronephrosis, the kidney contour is blurred, and the kidney depth cannot be accurately measured with the lateral scan. Finally, the rotation method collects projection images of both kidneys in four directions at the third minute of renal dynamic imaging. At this time, the renal cortex is clearly visible, and accurate renal depth can be obtained. This method can measure renal depth while renal dynamic imaging is performed, reducing examination time and radiation exposure.

Strengths and limitations

The rotation method has certain limitations in its scope of application. In patients with severe renal failure, the uptake rate of imaging agents is reduced, making it difficult to clearly identify the kidneys and accurately calculate their depth. Additionally, this method is not suitable for patients with ectopic kidneys or those who have undergone renal transplantation.

Despite these limitations, the rotation method allows for the precise determination of renal depth and depth differences without the need for CT. This cost-effective technique requires no additional time or radiation, making it both practical and easy to implement, with promising potential for future applications. It should be noted, however, that the number of cases in this study was limited, and a single-center design was used. To enhance the generalizability of the findings, multicenter studies with larger sample sizes should be completed.

Conclusions

In patients with hydronephrosis, the renal depth and depth difference measured with the rotation method were similar to those measured by CT. The rotation method can thus accurately correct the renal depth and depth difference without increasing the dose to radiation to the patient and improve the accuracy of total and single-kidney GFR.

Acknowledgments

Funding: This study was funded by the National Natural Science Foundation (No. 82027804), the Basic Research Program Fund of Shanxi Province (No. 20210302123240), and the Shanxi National College Students’ Innovation and Entrepreneurship Training Project (No. 20230243).

Footnote

Reporting Checklist: The authors have completed the STROBE reporting checklist. Available at https://qims.amegroups.com/article/view/10.21037/qims-24-1068/rc

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://qims.amegroups.com/article/view/10.21037/qims-24-1068/coif). All authors report that this study was funded by the National Natural Science Foundation (No. 82027804), the Basic Research Program Fund of Shanxi Province (No. 20210302123240), and the Shanxi National College Students’ Innovation and Entrepreneurship Training Project (No. 20230243). The authors have no other conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. This study was conducted in accordance with the Declaration of Helsinki (as revised in 2013) and was approved by the Ethics Committee of First Hospital of Shanxi Medical University (No. 2021BAL0146). Informed consent was obtained from all the patients.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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