## 4 Discussion
Although there are some advantages to using elastography in the assessment of the kidneys, such as the fact that the image is obtained in real time without the need for special prior preparation of the subjects and without procedure complications, the use of elastography in the assessment of the kidneys is more difficult than in other organs. This is confirmed by the increased heterogeneity that we have found between the studies that have been included in the present meta-analysis [10, 23]. We found in the literature only two other meta-analyses regarding renal elastography. The first one by [PERSON] et al., 2020 proposes looking into whether ARFI could accurately diagnose kidney masses [48], and the second one by [PERSON] et al., 2021 [49] determined the technical performance of ARFI imaging for evaluating renal parenchymal stiffness. The percentage of technical fault and intrasubject correlation coefficient indicated good agreement in both native and transplanted kidneys, but also that the region of interest location represented a significant factor of heterogeneity in transplanted kidneys. The good inter-operator agreement of p-SWE in native kidney studies is also pointed out in the EFSUMB guidelines published by [PERSON] et al., 2018 [9] for non-hepatic applications. Despite the good reproducibility of renal elastography, no evidence-based recommendation for the practical use of renal elastography can be offered so far. The present study is the first meta-analysis that examines the relationship between kidney SWS and renal function, and the presented results, even when comparing studies performed with the same method, are characterized by high heterogeneity. One of the causes of this could be anisotropy. The architecture of the renal tissue, which is characterized by a high degree of anisotropy, has an impact on the measured kidney SWS, making the results difficult to interpret [41]. In addition to anisotropy, another problem related to renal elastography is the depth of the kidneys. The placement of the ROI within native kidneys is a major challenge in renal elastography and the ability of current elastography methods to evaluate deep tissues is constrained. ARFI-based approaches have been reported to have a 7 cm target anatomical depth limitation [50]. In order to improve the performance of renal elastography, different improvements to the technique have been proposed. An experimental elastography-based method is two-dimensional time-harmonic ultrasound elastography. This method uses a vibration bed that produces continuous vibrations and thus the 2D-SWE elastography allows for deeper measurements (up to 13 cm) and provides elastograms that cover the entire kidney rather than just a region of interest [51]. Using this enhanced elastography method CKD could be diagnosed as early as stage 1, the same method underlying the influence of renal blood flow [52]. Another improvement could be obtained through the analysis of raw data of the different elastography systems used. The results of the study published by [PERSON] et al. in 2020 [53] show that the three machines used for renal elastography do not provide accurate shear wave displacement curves and the conclusion is that improved processing algorithms are required to obtain more accurate renal stiffness data from an elastographic device. Although one vendor has released technology that claims to be able to effectively measure shear wave displacements, there are currently just a few studies using 2D-SWE techniques in kidney assessment for this to be accepted. The main limitation of the present study is the limited number of studies that could be introduced in the meta-analysis. The choice of the main method assessed (VTQ) was made because it was the only method for which the number of published studies so far was sufficient to perform the statistical analysis.