Target-specific contrast agents for magnetic resonance microscopy

Abstract

High-resolution ex vivo magnetic resonance microscopy (MRM) can be used to delineate prominent architectonic features in the human brain, but increased contrast is required to visualize more subtle distinctions. The goal of this thesis is to employ target-specific MR contrast agents to regionally alter relaxation rates, resulting in increased contrast in ex vivo MRM of the human brain, with the aim of providing richer information about cyto- and/or myelo-architechtonics than is currently achievable. To accomplish this goal, a traditional optical myelin stain, luxol fast blue (LFB) MBSN with a paramagnetic copper core, has been introduced as a white-matter-selective MR contrast agent in ex vivo brain tissue. The solution relaxivity of LFB was measured at high (4.7 Tesla) and ultra-high (14 Tesla) field strengths. A methodology was developed for staining large tissue samples, enabling MR imaging. Longitudinal (R1) and transverse (R.2) relaxation rates in LFB-stained tissue increased proportionally with myelination at both field strengths. Ri changes produced larger contrast-to-noise ratios (CNR), per unit time, on Ti-weighted images between the deeper, more myelinated cortical layers (IV-VI) and adjacent, superficial layers (I-III) at both field strengths. Specifically, CNR for LFB-treated samples increased by 229± 13 per cent at 4.7T and 269± 25 per cent at 14T when compared to controls. Also, additional cortical layers (IVca, IVd, and Va) became resolvable in 14TMR images after en bloc staining with LFB. After imaging was completed, the LFB-stained sample was prepared for light microscopy.

(cont.) Both macroscopic and microscopic distributions of LFB were found to mimic those of traditional histological preparations. Next, the LFB-MR method was employed to investigate microstructure in X-linked adrenoleukodystrophy (ALD), a confluent demyelinating disorder characterized by accumulation of abnormal lipids. LFB-MR revealed an additional zone, unseen in formualin preparations and best visualized in T2*-weighted images, which produced four-fold increases in contrast-to-noise ratio. Immunohistological analysis identified a corresponding area of perivascular macrophages, and ultrastructural examination suggested LFB particulates bound to lipids within these macrophages. We thus conclude that LFB-MR is able to detect the actively demyelinating edge in cerebral ALD. The results presented in this thesis suggest target-specific contrast agents will 1) enable more detailed MR images, permitting the construction of better MR atlases and advancing the field of MR histopathology, and 2) guide the design of future in vivo contrast agents.