In vivo measurement of renal transplant viscoelasticity

Common clinical practice is to follow-up a renal transplant with regular protocol biopsies of the transplanted organ to assess presence of inflammation or signs of rejection. The objective of this study was to investigate the viscoelastic mechanical properties of patients with renal transplants. Recent studies using shear wave-based techniques have shown that the elastic mechanical properties of the kidney increase in presence of inflammation and fibrosis. Under a protocol approved by the Mayo Clinic Institutional Review Board, we studied the transplanted kidneys of 14 patients and the native kidneys of six control subjects. We used ultrasound radiation force to generate shear waves in the renal cortex and measured the shear waves using compounded plane wave imaging. For this study we used a Verasonics system equipped with a linear array or curved array transducer. We measured the group velocity using a Radon transform algorithm on the displacement data for these subjects to calculate the elastic shear modulus. We also analyzed the motion using a two-dimensional fast Fourier transform (2D-FFT) method to examine the phase velocity dispersion and fit the data with a Voigt model, where μ₁ is the elastic shear modulus and μ₂ is the shear viscosity. Clinical parameters such as serum creatinine (SCr) and Banff scoring from histology of transplant biopsies were also recorded. We observed a positive trend with SCr and μ in patients with kidney transplants and found a Pearson correlation of R = 0.54. We measured the viscoelasticity of renal transplant kidneys. Quantitative shear modulus measurements correlated with clinically significant measurements such as serum tests and biopsy histology.