Post mortem brain MRI at 9.4 T
Ultra-high field Magnetic Resonance Imaging of whole post-mortem brains is complementary to histology and pathology studies and can be useful for tissue characterization, sectioning, 3D-reconstruction of sectioned tissue data, and for diagnosis and investigation of neurodegenerative diseases. To preserve the tissue, the cerebrospinal fluid must be replaced by a fixative agent such as formalin. In this paper we show how such agents can be tailored to achieve a) dielectric properties that ensure a homogeneous B1-field, b) a magnetic susceptibility matching the tissue to improve B0 homogeneity, and c) a singulet 1H-NMR spectra to prevent chemical-shift-artefacts.
Dielectric properties of formalin-based agents were assessed (100MHz-4.5GHz), and four candidate fixatives with/without polyvinylpyrrolidone (PVP) and different salt concentrations were formulated. B1-field and MR-properties (T1, R2*, R2, R2’ and magnetic susceptibility (QSM)) were observed in white and grey matter of pig brain samples during immersion fixation. The kinetics was fitted using exponential functions.
The dielectric properties of formalin were non-linearly modified by increasing amounts of additives. With 5%PVP and 0.04%NaCl, the dielectric properties and B1-field reflected in-vivo-conditions. The highest B1 values were found in white matter with PVP and varied significantly with tissue depth and embedding media, but not with immersion-time. MR-properties depended on PVP yielding lower T1, higher R2*, more paramagnetic QSM values, and a lower Medawar-coefficient(0.9mm/hours; without PVP: 1.5). Regardless of fixative, switching to PBS as embedder caused a paramagnetic shift in QSM and decreased R2* progressively during 1month of storage, while no differences were found with Fluorinert.