Quantitative susceptibility mapping of the human carotid artery: Assessing sensitivity to elastin and collagen ex vivo

Abstract

Purpose: The aim is to establish the relationship between carotid susceptibility and microstructural components in diseased carotid arteries.

Methods: Excised cadaveric carotid arteries (n = 5) were scanned using high-resolution QSM at 7 Tesla. After ex vivo imaging, all samples were brought to histology and stained for elastin, collagen, cells, and calcium. An image registration pipeline was used in combination with semi-quantitative, regional histology analysis to evaluate relationships between MRI and microstructural components.

Results: Weak, non-significant (p > 0.05) correlations were found between all components and regional magnitude and R₂* measurements. A significant, moderate negative correlation between the elastin fraction and regional magnetic susceptibility, r_elastin = -0.63 (p < 0.0001) was found, as well as a significant, moderate negative correlation between collagen and regional magnetic susceptibility, r_collagen = -0.59 (p < 0.0001).

Conclusion: Tissue magnetic susceptibility in diseased human carotid arteries was shown to be significantly correlated with the dominant microstructural components of pathological human cadaver samples-elastin and collagen. Knowing that elastin and collagen are disrupted in vascular disease progression, QSM offers clinically translatable potential for novel disease biomarkers.

Keywords: arterial tissue; atherosclerosis; collagen; elastin; quantitative susceptibility mapping.

FIGURE 1

Cadaveric carotid arteries used in this study. (A) Imaging set‐up overview. (B–F) Five carotids were excised from cadavers and whole carotids were imaged. Sections of the common carotid (inset images) were histologically processed for registration to MRI data. Scale bars of whole carotids are 10 mm, scale bars in inset images are 5 mm.

FIGURE 2

Example of image registration and cluster analysis from an example cadaveric carotid sample. Left to right: Verhoeff's elastin‐stained brightfield (BF) and stained area fraction (SAF) histology slice before non‐linear image registration; MRI echo‐combined magnitude, R₂*, and QSM in the same slice (masked to tissue compartment only); regions of interest (ROIs) determined by k‐means clustering; Verhoeff's BF and SAF post image registration.

FIGURE 3

MRI maps of an example excised human carotid artery in three planes. Tissue‐masked magnitude images, R₂* maps, and susceptibility maps. FOV is 30 × 30 × 30 mm. The common carotid is marked by a red asterisk, the internal by a yellow asterisk, and the external by a green asterisk.

FIGURE 4

A central, axial slice of magnitude images, R₂* maps, and susceptibility maps within the common carotid for each sample. Images and maps are masked to the tissue compartment.

FIGURE 5

Pre‐registered histological slices; one slice per stain. Scale bars are 1 mm.

FIGURE 6

Pearson's correlations between MRI data and microstructural tissue components. Susceptibility values are presented in the top row, R₂* in the middle, and magnitude values and their correlation with components are given in the bottom row. The first column is elastin, the second is collagen, the third is cell density, and the last is calcium. Different colored dots correspond to different samples (n = 5) and each individual dot corresponds to a singular region of interest (ROI) as defined by k‐means clustering (n = 9 per sample). Pearson's correlations were performed and presented for the entire dataset. Pearson's coefficients (r) and p values are presented within each plot; yellow boxes represent significant correlations.