Quantitative susceptibility mapping of kidney inflammation and fibrosis in type 1 angiotensin receptor-deficient mice

Abstract

Disruption of the regulatory role of the kidneys leads to diverse renal pathologies; one major hallmark is inflammation and fibrosis. Conventional magnitude MRI has been used to study renal pathologies; however, the quantification or even detection of focal lesions caused by inflammation and fibrosis is challenging. We propose that quantitative susceptibility mapping (QSM) may be particularly sensitive for the identification of inflammation and fibrosis. In this study, we applied QSM in a mouse model deficient for angiotensin receptor type 1 (AT1). This model is known for graded pathologies, including focal interstitial fibrosis, cortical inflammation, glomerulocysts and inner medullary hypoplasia. We acquired high-resolution MRI on kidneys from AT1-deficient mice that were perfusion fixed with contrast agent. Two MR sequences were used (three-dimensional spin echo and gradient echo) to produce three image contrasts: T1, T2* (magnitude) and QSM. T1 and T2* (magnitude) images were acquired to segment major renal structures and to provide landmarks for the focal lesions of inflammation and fibrosis in the three-dimensional space. The volumes of major renal structures were measured to determine the relationship of the volumes to the degree of renal abnormalities and magnetic susceptibility values. Focal lesions were segmented from QSM images and were found to be closely associated with the major vessels. Susceptibilities were relatively more paramagnetic in wild-type mice: 1.46 ± 0.36 in the cortex, 2.14 ± 0.94 in the outer medulla and 2.10 ± 2.80 in the inner medulla (10(-2) ppm). Susceptibilities were more diamagnetic in knockout mice: -7.68 ± 4.22 in the cortex, -11.46 ± 2.13 in the outer medulla and -7.57 ± 5.58 in the inner medulla (10(-2) ppm). This result was consistent with the increase in diamagnetic content, e.g. proteins and lipids, associated with inflammation and fibrosis. Focal lesions were validated with conventional histology. QSM was very sensitive in detecting pathology caused by small focal inflammation and fibrosis. QSM offers a new MR contrast mechanism to study this common disease marker in the kidney.

Keywords: AT1; fibrosis; inflammation; magnetic susceptibility; pathology; quantitative susceptibility mapping; renal structures; small animal preclinical imaging.

Figure 1

Quantitative susceptibility mapping (QSM) reconstruction process for a kidney from a Agtr1a−/− mouse. (A) Phase from complex data. (B) Local phase extracted using Laplacian-based three-dimensional phase unwrapping. (C) Background phase removed using spherical-mean-value filtering. (D) QSM image computed by inverting the background-removed phase (least-squares algorithm using orthogonal and right triangular decomposition). Color bar: ppm of B₀.

Figure 2

MRI (T₁-weighted) coronal slices for kidneys from wild-type (A), Agtr1a−/− (B) and Agtr1a−/− Agtr1b−/− (C) mice. Oblique slices were chosen from the datasets to show all three regions of the kidney. Scale bars, 1 mm.

Figure 3

Identification of cortical pathologies confirmed by Masson’s Trichrome histology. Kidney images from wild-type (top row) and Agtr1a−/− Agtr1b−/− (bottom row) mice. (A, D) Histology images. (B, E) T₂*-weighted (magnitude) images. (C, F) Quantitative susceptibility mapping (QSM) images. Clusters of glomeruli with wall wrinkling can be seen (white arrows in D–F). The magnitude image shows susceptibility artifacts on the borders of the cluster. Focal regions of cortical interstitial inflammation and fibrosis were identified (yellow arrows in A, C, D and F). Scale bars, 300 µm. Color bar: ppm of B₀.

Figure 4

Kidney images from wild-type (top row), Agtr1a−/− (middle row) and Agtr1a−/− Agtr1b−/− (bottom row) mice. MIPs from (A, C, E) T2* magnitude images. (B, D, F) MIPs from quantitative susceptibility mapping (QSM) images. White arrows point to structures in the most outer cortical region. Scale bars, 300 µm.

Figure 5

Maximum intensity projections (MIPs) from quantitative susceptibility mapping (QSM) images of kidneys from wild-type (A), Agtr1a−/− (B) and Agtr1a−/− Agtr1b−/− (C) mice. White arrows point to structures in the most outer cortical region. These are kidneys without contrast agent. Scale bars, 300 µm.

Figure 6

Segmentation of renal structures confirmed by hematoxylin and eosin (H&E) histology. Images from wild-type mouse kidney. Top row: histology image. Second row: MR image. Third row: segmentation results overlaid on MR image. Bottom row: volume renders of the segmented structures. (A) Segmentation for the veins and arteries using T₁-weighted images. (B) Segmentation for the renal regions using T₂*-weighted images. Cortex shown in red, outer medulla shown in light blue and inner medulla shown in dark blue. Scale bars, 1 mm.

Figure 7

Plots comparing kidneys from the three cohorts: (A) kidney mass; (B) total kidney volume; (C) mean vessel segment lengths (arterial); (D) cortex volume; (E) outer medulla volume; (F) inner medulla volume. *Significance between wild-type and Agtr1a−/− (p < 0.05). *,*Significance between wild-type and Agtr1a−/− Agtr1b−/−, and between Agtr1a−/− and Agtr1a−/− Agtr1b−/− (p < 0.01). *,⁻Significance between wild-type and Agtr1a−/− Agtr1b−/− (p < 0.01) and no significance between Agtr1a−/− and Agtr1a−/− Agtr1b−/− (p > 0.05).

Figure 8

Volume rendering of focal lesions of inflammation and fibrosis. Cortical lesions shown in relation to the vasculature and medullary regions. Kidneys from Agtr1a−/− (A) and Agtr1a−/− Agtr1b−/− (B) mice. Insets show a zoomed-in view of the cortex from an axial plane. The proximity of lesions to the vasculature and the extent of lesions in the cortex can be appreciated in the insets. Yellow arrows point to lesions. Inset scale bars, 300 µm. (A) and (B) scale bars, 1 mm.