Comprehensive ultrasonographic evaluation of normal and fibrotic kidneys in a mouse model with an ultra-high-frequency transducer

Purpose: This study aimed to establish baseline morphological and functional data for normal mouse kidneys via a clinical 33 MHz ultra-high-frequency (UHF) transducer, compare the data with the findings from fibrotic mice, and assess correlations between ultrasonography (US) parameters and fibrosis-related markers. Methods: This retrospective study aggregated data from three separate experiments (obstructive nephropathy, diabetic nephropathy, and acute-to-chronic kidney injury models). Morphological parameters (kidney size, parenchymal thickness [PT]) and functional (shear-wave speed [SWS], stiffness, resistive index [RI], and microvascular imaging-derived vascular index [VI]) were assessed and compared between normal and fibrotic mouse kidneys. Semi-quantitative histopathologic scores were calculated and molecular markers (epithelial cadherin), Collagen 1A1 [Col1A1], transforming growth factor-beta, and alpha-smooth muscle actin [alpha-SMA]) were evaluated using western blots. Correlations with US parameters were explored. Results: Clinical UHF US successfully imaged the kidneys of the experimental mice. A three-layer configuration was prevalent in the normal mouse kidney parenchyma (34/35) but was blurred in most fibrotic mouse kidneys (33/40). US parameters, including size (11.14 vs. 10.70 mm), PT (2.07 vs. 1.24 mm), RI (0.64 vs. 0.77), VI (22.55% vs. 11.47%, only for non-obstructive kidneys), SWS (1.67 vs. 2.06 m/s), and stiffness (8.23 vs. 12.92 kPa), showed significant differences between normal and fibrotic kidneys (P<0.001). These parameters also demonstrated strong discriminative ability in receiver operating characteristic curve analysis (area under the curve, 0.76 to 0.95; P<0.001). PT, VI, and RI were significantly correlated with histological fibrosis markers (rho=-0.64 to -0.68 for PT and VI, rho=0.71-0.76 for RI, P<0.001). VI exhibited strong negative correlations with Col1A1 (rho=-0.76, P=0.006) and alpha-SMA (rho=-0.75, P=0.009). Conclusion: Clinical UHF US effectively distinguished normal and fibrotic mouse kidneys, indicating the potential of US parameters, notably VI, as noninvasive markers for tracking fibrosis initiation and progression in mouse kidney fibrosis models.