Evaluation of renal microstructure is pivotal for diagnosing and monitoring chronic renal disease, being currently performed by an invasive procedure like renal biopsy with its risk of complication and sampling errors [15]. DWI has been proved to be practicable and reliable examination for the evaluation of renal function and parenchymal damage in some renal diseases [16,17,18,19,20].
In this study, we aim to evaluate the role of DWI in the assessment CKD and investigate the correlation between different stage of the CKD and renal parenchymal ADC value changes.
The optimal b value for abdominal DWI has not been determined exactly. In this study, we performed DWI sequence using b values of 0 and 600 s/mm2, which has been used in several previous studies [4, 5, 8, 20] to yield a sufficient SNR, reduce “T2 shine through” and avoid perfusion effect.
In all cases and control, DW sequence was performed during free breathing. Kocyigita et al. in their study on 46 children with VUR reported reproducible ADC values on DWI without pulse triggering during free breaching, which is a considerable advantage in children [8].
In some of the previously published studies [19, 21], the authors had evaluated the ADC of renal cortex and renal medulla in each kidney and had reported a significant difference between ADC value of renal cortex and medulla. Because of lower resolution of images with higher b values, it was difficult to reliably discriminate between cortex and medulla; hence, ROIs were placed on the renal parenchyma. This was supported by the findings of Thoeny et al. [22] and Xu et al. [23], and they have reported that there is no statistical difference between the ADC values of the cortex and medulla.
The mean ADC value of renal parenchyma in our patients with CKD was (1.85 ± 0.24 × 10−3 mm2/s) which is significantly lower than that of the control cases (2.21 ± 0.12 × 10−3 mm2/s) with p value < 0.001. This is similar to the findings of several previous studies [21, 23, 24]. The low renal parenchyma ADC values in CKD are due to reduced perfusion as well as reduced water diffusion. Different pathological factors including glomerulosclerosis, interstitial fibrosis, and tubular atrophy restrict the free water molecules movement in both the extracellular and intracellular space [4].
The results of this study were in agreement with several authors who have reported that renal ADC value has a potential correlation with renal function, showing lower ADC values with a decrease in GFR [23, 24]. Thus, DWI can be utilized as non-invasive technique to monitor renal function changes [25].
A study by Xu et al. on a 1.5-T MR unit with b values of 0 and 500 seconds/mm2 found that the ADC was significantly lower in impaired kidneys than in normal kidneys, and there was a positive correlation between ADCs and split GFR (r = 0.709) [10].
In another study performed on 72 healthy volunteers and 43 patients with CKD (chronic glomerulonephritis) using b value 0 and 500 s/mm2, they have found that patients with CKD had significantly lower renal ADC (r = − 4.383, p = 0.000) than age and sex-matched volunteers, and a negative correlation was found between ADC and stages of CKD (r = − 0.492, p = 0.000) [23].
In our study, the mean ADC values of kidneys in patients with CKD were significantly lower than normal at most stages of CKD, except in stage 1. We found an overlap of ADC value in stage 1 CKD patients with that of the normal control. This was in concordance with the findings of other previous studies [23, 24].
We have found that the mean RI of group A (0.66 ± .11) was significantly higher than that of group B (0.57 ± .05) with p value < 0.001. This was matching with the findings of Hanamura and his colleagues who have reported increased RI with the progression of CKD stage. Additionally, RI was correlated with age, eGFR, and renal histological changes, including glomerulosclerosis, arteriosclerosi,s and tubulointerstitial damage. They concluded that RI in CKD patients was considered a marker for assessment of renal function, histological damage and disease prognosis as well as a possible determinant of indication for steroid [26].
Sugiura and colleagues also stated that high RI (> 0.70) was an independent risk factor for renal function deterioration in CKD, and elevated RI was linked to lower renal survival [27].
In our study, there was strong positive correlation between RI and stage of CKD in group A. This was consistent with the findings of other studies [26, 28] including Parolini et al. who showed that the patients with an initial RI of 0.70 or higher showed a rapid decline of renal function independent of initial eGFR. Initial RI in their study not only correlated significantly with eGFR of the initial groups representing its correlation with CKD stage but also with final eGFR on follow-up [29].
There are few limitations in our study: First, relatively small sample size with limited cases of each cause (e.g. HUS, LN). Second, wide range of pediatric population ranging from age 2 to 18 years, with the ADC not compared with the control group of the respective age group. Lastly, no clear cutoff of ADC values can be used in the differentiation between CKD and normal kidney. Further studies are warranted to evaluate the diagnostic accuracy of DW MRI in a larger pediatric patient cohort compared to healthy volunteers of a wide age range.