Magnetic Resonance Imaging for Evaluation of Interstitial Fibrosis in Kidney Allografts

Abstract

Interstitial fibrosis (IF) is the common pathway of chronic kidney injury in various conditions. Magnetic resonance imaging (MRI) may be a promising tool for the noninvasive assessment of IF in renal allografts.

Methods: This prospective trial was primarily designed to investigate whether the results of T1-weighted MRI associate with the degree of IF. Thirty-two kidney transplant recipients were subjected to 1.5-Tesla MRI scans shortly before or after routine allograft biopsies. MRI parameters [T1 and T2 relaxation times; apparent diffusion coefficient (ADC)] were assessed for cortical and medullary sections.

Results: Advanced IF (Banff ci score >1) was associated with higher cortical T1 (but not T2) values [1451 (median; interquartile range: 1331-1506) versus 1306 (1197-1321) ms in subjects with ci scores ≤1; P = 0.011; receiver operating characteristic area under the curve for prediction of ci > 1: 0.76]. In parallel, T1 values were associated with kidney function and proteinuria. There was also a relationship between IF and corticomedullary differences on ADC maps (receiver operating characteristic area under the curve for prediction of ci ≤ 1: 0.79).

Conclusions: Our results support the use of MRI for noninvasive assessment of allograft scarring. Future studies will have to clarify the role of T1 (and ADC) mapping as a surrogate endpoint reflecting the progression of chronic graft damage.

FIGURE 1.

Representative examples of kidney allograft T1 mapping and corresponding biopsy results. Color scales indicate T1 relaxation times (range: 0–2000 ms). Cortical T1 relaxation time in the first case (A) was 1566 ms. For analysis of interstitial fibrosis (IF), acid fuchsin-orange G staining was used. Histologic assessment revealed chronic allograft injury with severe IF (ci3), tubular atrophy (ct3), glomerular capillary double contours (cg3), and vascular fibrous intimal thickening (cv). The second case (B) showed no features of chronic injury (ci0, ct0, cg0, and cv0) and cortical T1 values were by far lower (1191 ms).

FIGURE 2.

Results of T1 mapping in relation to groups defined according to Banff ci scoring. Cortical T1 relaxation times are shown for the whole transplant (A) and the cranial pole (B), as well as coronal (C) and axial (D) sections of the renal allograft. Box plots indicate the median, interquartile range, and the minimum and maximum of T1 values. Outliers are indicated as circles. The Mann–Whitney U test was used for group comparisons. The prediction of ci scores >1 by T1 mapping results is presented by receiver operating characteristic (ROC) curves and the corresponding area under the curve (AUC) including 95% confidence interval (CI) and P values.

FIGURE 3.

Scatter plots with regression lines illustrating correlations of (A) cortical T1 relaxation time, (B) cortical T2 relaxation time, and (C) ΔADC with semiquantitative Banff ci scores. Spearman tests were used for correlation analysis. ΔADC, corticomedullary difference of apparent diffusion coefficient.

FIGURE 4.

Results of cortical diffusion-weighted imaging (DWI) and T2 mapping in relation to groups defined according to Banff ci scoring. Cortical-medullary differences of the apparent diffusion coefficient (ΔADC) (A) and cortical T2 relaxation times (B) are shown. Box plots indicate the median, interquartile range, and the minimum and maximum of parameters. Outliers are indicated as circles. The Mann–Whitney U test was used for group comparisons. The prediction of ci scores >1 by ΔADC and T2 mapping results is presented by receiver operating characteristic curves and the corresponding area under the curve (AUC) including 95% confidence interval and P values.

FIGURE 5.

Comparison of graft function and proteinuria between patients with high or low T1 (A), cortical-medullary differences of the apparent diffusion coefficient (ΔADC) (B), and T2 values (C). Patients were grouped according to the median of measured magnetic resonance imaging parameters. Box plots indicate the median, interquartile range, and the minimum and maximum of the measures [estimated glomerular filtration rate (eGFR), protein/creatinine ratio]. Outliers are indicated as circles. The Mann Whitney U test was used for group comparisons.