Magnetic Resonance Elastography to Assess Fibrosis in Kidney Allografts

Abstract

Background and objectives: Fibrosis is a major cause of kidney allograft injury. Currently, the only means of assessing allograft fibrosis is by biopsy, an invasive procedure that samples <1% of the kidney. We examined whether magnetic resonance elastography, an imaging-based measure of organ stiffness, could noninvasively estimate allograft fibrosis and predict progression of allograft dysfunction.

Design, setting, participants, & measurements: Kidney allograft recipients >1 year post-transplant undergoing an allograft biopsy first underwent free-breathing, flow-compensated magnetic resonance elastography on a 3.0-T magnetic resonance imaging scanner. Each patient had serial eGFR measurements after the elastography scan for a follow-up period of up to 1 year. The mean stiffness value of the kidney allograft was compared with both the histopathologic Banff fibrosis score and the rate of eGFR change during the follow-up period.

Results: Sixteen patients who underwent magnetic resonance elastography and biopsy were studied (mean age: 54±9 years old). Whole-kidney mean stiffness ranged between 3.5 and 7.3 kPa. Whole-kidney stiffness correlated with biopsy-derived Banff fibrosis score (Spearman rho =0.67; P<0.01). Stiffness was heterogeneously distributed within each kidney, providing a possible explanation for the lack of a stronger stiffness-fibrosis correlation. We also found negative correlations between whole-kidney stiffness and both baseline eGFR (Spearman rho =-0.65; P<0.01) and eGFR change over time (Spearman rho =-0.70; P<0.01). Irrespective of the baseline eGFR, increased kidney stiffness was associated with a greater eGFR decline (regression r²=0.48; P=0.03).

Conclusions: Given the limitations of allograft biopsy, our pilot study suggests the potential for magnetic resonance elastography as a novel noninvasive measure of whole-allograft fibrosis burden that may predict future changes in kidney function. Future studies exploring the utility and accuracy of magnetic resonance elastography are needed.

Figure 1.

Magnetic resonance elastography can be used to image transplant kidney stiffness. In magnetic resonance elastography, a gentle vibrational wave is applied on the skin overlying the kidney using an acoustic generator. The magnetic resonance imaging (MRI) scanner then images the small displacements produced by the resulting shear waves generated in the kidney. Because the waves are longer and travel more rapidly through stiff tissue, kidney stiffness can be calculated on the basis of the measured displacements and the amount of vibrational force applied. Postimaging quantitative reconstruction enables the generation of stiffness maps (elastograms) of the imaged kidney. Displacement images are shown to the left and right of the scanner. Below each displacement image is the corresponding pseudocolorized elastogram (stiffness map); blue depicts softer tissue, and red depicts stiffer tissue.

Figure 2.

Representative magnetic resonance elastography images demonstrate the heterogeneous distribution of stiffness in the kidney. (A) A representative magnetic resonance elastography magnitude image of a kidney allograft and (B) the corresponding stiffness map for the same kidney, where blue represents softer tissue and red represents stiffer tissue (the color bar is in kilopascals). (C) A histogram of magnetic resonance elastography stiffness value frequency derived from the stiffness map in B, showing stiffness heterogeneity across the entire slice. As can be seen on this histogram, measured stiffness values (per pixel) exhibit a wide range (from 1.9 to 10.7 kPa) for this representative slice.

Figure 3.

Magnetic resonance elastography–derived stiffness scores increase with increasing Banff fibrosis score. Patients were categorized according to their biopsy-derived Banff interstitial fibrosis score (ci), and mean magnetic resonance elastography–derived whole-kidney stiffness scores for each group are plotted. The numbers of patients in each group were as follows: ci0 (n=2), ci1 (n=8), ci2 (n=4), and ci3 (n=2). *Outlier measurement.

Figure 4.

Magnetic resonance elastography–derived stiffness scores may predict future changes in kidney allograft function. Shown are eGFR trajectories over the follow-up period, normalized to baseline eGFR, for (A) all patients (n=16), (B) patients with baseline eGFR ≥45 ml/min (n=4), and (C) patients with baseline eGFR <45 ml/min (n=12). Data points and best fit lines for eGFR trajectories are color coded to the mean stiffness of each allograft (the color bar on the right shows stiffness values in kilopascals). MRI, magnetic resonance imaging.