Introduction: APTw imaging has the function of non-invasive and targeted detection of tissue-free proteins and peptides [1]. It has initially obtained good clinical value in the study of acute and chronic kidney diseases [2-3]. The reabsorption of amino acids and other protein substances is one of the kidney's primary functions, which lays a theoretical foundation for applying APTw imaging in the kidney. Targeted understanding of the characteristics of normal renal cortical and medullary protein levels not only can help to recognize the change characteristics of free proteins in normal kidneys but also lays a foundation for the early study of disease. Method: 58 healthy volunteers from September 2021 to February 2022 were enrolled for intermittent breath-hold right kidney axial 3D-APT imaging using 3.0T (Philips Ingenia CX, Best, Netherland). The specific imaging parameters are shown in Table 1. The scores of APTw images were more than 2 points on the 5-Likert scale [4]. (Table 2) They were considered successful images and were used to measure APT values by two radiologists. Three circular regions of interest (ROIs) with an area of 0.2-0.4 cm2 were used to measure the APT values of the cortex and medulla, respectively (Fig. 1), avoiding imaging defects on the pseudo-colored maps, artifacts, kidney sinuses, and visible vessels. ROI delineation was then reviewed and approved by a radiologist with more than 10 years of experience in urinary system imaging. The intraclass correlation coefficient (ICC) was used to evaluate the consistency of APT values between two radiologists, and an ICC of > 0.74 was considered excellent. The t-tests were used to analyze the difference in APT values between cortex and medulla in all data and different genders. All statistical tests were 2 sided. A P value less than 0.05 indicated a statistically significant difference. Results： 9 healthy volunteers were excluded from the study due to claustrophobia (2 cases) and breathing training failure before (3 cases) or during the 3D-APTw imaging scan (4 cases). Finally, 49 volunteers (aged 33.10 ± 1.54 years, 13 male) were enrolled. The inter-observer agreement was excellent for the APT values of the cortex (ICC = 0.951) and medulla (ICC = 0.940). The APT values of the medulla (2.407% ± 0.474%) were higher than the cortex (1.714% ± 0.448%). The difference was statistically significant (P < 0.001) (Fig. 2). There was no significant difference in the renal cortex (P = 0.804) and medulla (P = 0.465) between different genders. Discussion： This study's high consistency of APT values in the normal renal cortex and renal medulla reflected the feasibility of accurate renal APT imaging. The differences in APT values between the renal medulla and cortex suggested that the difference in renal tissue microstructure is the main reason, such as the skeleton protein, blood flow, and other factors. This study also reflected the ability of APTw imaging to detect trace protein changes in tissues, which will have significant potential for future research. Conclusion: APT imaging of the normal kidney medulla and cortex was highly consistent, and APT values of the kidney medulla were significantly higher than the cortex. This study provides a theoretical basis for the study of noninvasive kidney proteins.

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References

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