INTRODUCTION: There are studies that have shown a decrease of the ADC value in the fetal kidneys as the gestational age increases. This has been related to the normal renal development. Normal kidneys have a gradual increase in the number of glomeruli between the 10th and 18th weeks of gestation, accelerating until the 32nd week. The renal perfusion is about 20 ml/min at 25 weeks gestation reaching to 60 ml/min at the end of gestation. 1. Renal function has been assessed by indirect signs such as the volume of amniotic fluid, the degree of pulmonary maturation and the degree of bladder repletion; however, more recently, diffusion sequences (DWI) have begun to be used as a tool to predict renal function 5,6. DWI is considered the only method to assess molecular diffusion in vivo. The apparent diffusion coefficient (ADC) values of the renal parenchyma depends on Brownian movement, perfusion in the capillary network and tubular flow (1).. Recently, MRI has begun to demonstrate its value for obtaining additional findings in patients with urinary system disorders that are not detected by ultrasound which result in a change in postnatal management.1,2, 3, 4. Some authors have demonstrated the usefulness of diffusion for the detection ADC values change as gestational age progresses, demonstrating the potential of magnetic resonance to detect developmental variants. 7,8. The evaluation of renal functionality together with lung maturity contributes to establishing the prognosis of the fetus.1, 8. The values of the ADC of the fetal kidneys promises to be a good way to predict the functionality, however, the rate of decrease of the ADC during the third trimester has not yet been established. For these reasons, the present study aims to know the values of the normal diffusion coefficient of fetal kidneys in our population, as well as estimate the decrease rate of ADC during the third trimester. METHODS: The images we used were obtained at the MRI Department of the Ángeles Lomas Hospital, in México from July to November 2019. We used a 1.5T scanner (Siemens Aera) with a phased arrayed coil placed over the mother´s pelvis. A DWI technique was used with b values of 700 and 1000 s/mm2 and 12 directions. We studied 16 patients that were referred for pathology outside the kidneys. We processed the images using 3 different software: dcm2nii, to convert DICOM (digital imaging and communication on medicine) images to NifTI (neuroimaging informatics technology initiative) format and extract the gradient directions of diffusion; FSL (functional magnetic resonance imaging of the brain software library), where we correct eddy currents and movement effects, in addition to solve the diffusion tensor and obtain the ADC for every voxel; and Image J, where subsequently, the ADC mean value was obtained by performing a region of interest (ROI) in axial slides covering the entire fetal renal parenchyma avoiding the renal pelvis. Then we compared the ADC values corresponding to each gestational age. Based on the previous studies, we assumed that the ADC values have a linear decrease behavior with gestational age, so we show in the results section the linear equations of the graphs with the ADC rate of decrease. RESULTS: We found a correlation between fetal kidneys ADC values and gestational age. ADC values decrease with gestational age. This values ranging from 31 to 36 weeks and from 1.15 to 2.67 × 10−3mm2s−1, standard deviation (SD) 0.24. In Figures 1 and 2, we can observe the behavior of ADC with gestational age, the equation that describe the curves was obtained by linear regression. An analysis of Pearson's correlation coefficient was performed between quantitative variables of ADC and gestational age. In Figure 2, we obtained a decrease rate of 0.03 × 10−3mm2s−1 /week.
DISCUSSION: As we described previously, our results coincide with the works previously published. The ADC values show a decrease behavior with gestational age in normal fetal kidneys. There was a decreasing performance of ADC regardless of the b values that were used. and of the number of patients that were studied. (Figures 1 and 2). In order to conclude a standard behavior of ADC with gestation age, we need to increase our sample. Our results suggest that we can assume an abnormal fetal renal function if we find an abnormal ADC. CONCLUSION: Normal fetal kidneys have a decrease rate of 0.03 × 10−3mm2s−1 per week during the third trimester. This may provide important information related to renal function and development. Currently we are including more patients to increase the sample and determine a more precise standard behavior of ADC with gestational age in normal fetal kidneys. In the future we intend to study abnormal kideney´s ADC and compare them to gestational age paired controls.

Acknowledgements

We would like to thank all radiological technical assistants for their practical support.