Introduction

Renal stiffness quantification via magnetic resonance elastography (MRE) and perfusion assessment through arterial spin labeling (ASL) enable noninvasive, early renal function evaluation in patients with chronic kidney disease (CKD)^[1-2].A few studies^[3-4] indicate that shear wave speed (SWS, m/s) is a marker for renal stiffness assessed by MRE and decreases with increased fibrosis, as opposed to the liver^[5]. This negative correlation is likely due to perfusion variations of renal, which is rich in blood supply. However, the hypothesis that MRE results are impacted by perfusion has not been validated in human experiments. This study investigated the influence of perfusion on renal SWS in patients with CKD to elucidate the distinct behavior of renal stiffness values compared with liver stiffness values in fibrosis.

Methods

Nine healthy volunteers (HVs) and 9 patients with CKD undergoing MRE and a research ASL sequence on a 3T system (MAGNETOM Prisma, Siemens Healthineers AG, Erlangen, Germany) were recruited. The participants fasted for over 2 hours before the examination. During the examination, passive drivers were placed behind both kidneys. The scanning parameters are listed in Table 1.The raw MRE images were uploaded to the official elastography postprocessing website of Humboldt University, Germany (https://bioqic-apps.charite. de),and the kidney shear wave velocity C-map was obtained by postprocessing with wavenumber-based multifrequency dual elasto-visco (k-MDEV) inversion algorithm^[6]. A radiologist (L.Q.M, with 3 years of experience in MRE of the kidney) manually delineated the ROIfor the quantitative analysis of renal stiffness value using C-map images and renal blood flow(RBF, mL/min/100g) usingrenal ASL images (see Fig. 1).
The paired-samplet-test was used to compare the SWS measured in the renal cortex and medulla. The two-sample t-test was used to compare the differences in the MRI parameters measured in the renal of HVs and patients with CKD. The receiver-operating characteristic (ROC) curves were plotted to evaluate the diagnostic value of the two techniques for renal fibrosis. Spearman’s correlation was used to assess the relationship between clinical indicators and MRI parameters in patients with CKD, as well as the relationship between RBF and SWS of the unilateral renal cortex (without distinguishing HVsfrom patients with CKD).

Results

As depicted in Table 2, the paired-sample t-test revealed that SWS of the medulla was considerably lower than that of the cortex. The two-sample t-test revealed that the MRI parameters of the kidney in patients with CKD (SWS: whole kidney 2.43 ± 0.18 m/s, cortex 2.56 ± 0.20 m/s, and medulla 2.15 ± 0.15 m/s; RBF: cortex 151 ± 67 mL/min/100g) was lower than that in HVs (SWS: whole kidney 2.60 ± 0.10 m/s, cortex 2.80 ± 0.15 m/s, and medulla 2.31 ± 0.07 m/s; RBF: cortex 265 ± 61 mL/min/100g) (P =.030/.010/.014/.002). The area under the curve (AUC) of MRI parameters distinguishing different groups are illustrated in Figure 2. The relationship between clinical indicators and MRI parameters in patients with CKD were exhibited in Figure 3. The SWS of the unilateral kidney cortex (without distinguishing HVs from patients with CKD) was positively correlated with the RBF (r = 0.363, P = .030).

Discussion and Conclusion

Both diagnostic techniques effectively distinguished HVs from patients with CKD. ASL slightly outperformed MRE, with higher combined diagnostic efficacy.
Previous studies^[3] have found that renal SWS is negatively correlated with the degree of renal fibrosis, which is consistent with this study, but contrary to the similar study on liver, it may be caused by the influence of perfusion on renal blood supply. The results showed that RBF of renal cortex was negatively correlated with the degree of renal fibrosis, and SWS of unilateral renal cortex was positively correlated with RBF, which further supported this hypothesis. In other words, the SWS in patients with CKD may be more significantly affected by perfusion than by fibrosis.
In summary, ASL and MRE demonstrated favorable diagnostic efficacy in distinguishing between HVs and patients with CKD, even have the potential to predict the stages of CKD. The combined diagnostic approach yielded even superior results. This preliminary study affirmed a positive correlation between renal stiffness values and renal perfusion capacity. Patients with CKD exhibited a reduction in SWS with the increase in the fibrotic degree, which was associated with their declining perfusion capacity. Hence, considering the influence of perfusion factors is essential when examining renal stiffness values in patients with CKD.

Acknowledgements

We thank Bingsheng Huang of Shenzhen University for his excellent help in data statistics. We thank Jing Guo of Humboldt University in Berlin, Germany for their excellent help in MRE post-processing and analysis.

References

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