Ex vivo MRI facilitates localization of cerebral microbleeds of different ages during neuropathology assessment

Cerebral microbleeds (CMBs) identified by in vivo magnetic resonance imaging (MRI) of brains of older persons may have clinical relevance due to their association with cognitive impairment and other adverse neurologic outcomes, but are often not detected in routine neuropathology evaluations. In this study, the utility of ex vivo MRI in the neuropathological identification, localization, and frequency of CMBs was investigated. The study included 3 community dwelling elders with Alzheimer’s dementia, and mild to severe small vessel disease (SVD). Ex vivo MRI was performed on the fixed hemisphere to identify CMBs, blinded to the neuropathology diagnoses. The hemibrains were then sliced at 1 cm intervals and 2, 1 or 0 microhemorrhages (MH) were detected on the cut surfaces of brain slabs using the routine neuropathology protocol. Ex vivo imaging detected 15, 14 and 9 possible CMBs in cases 1, 2 and 3, respectively. To obtain histological confirmation of the CMBs detected by ex vivo MRI, the 1 cm brain slabs were dissected further and MHs or areas corresponding to the CMBs detected by ex vivo MRI were blocked and serially sectioned at 6 µm intervals. Macroscopic examination followed by microscopy post ex vivo MRI resulted in detection of 35 MHs and therefore, about 12 times as many MHs were detected compared to routine neuropathology assessment without ex vivo MRI. While microscopy identified previously unrecognized chronic MHs, it also showed that MHs were acute or subacute and therefore may represent perimortem events. Ex vivo MRI detected CMBs not otherwise identified on routine neuropathological examination of brains of older persons and histologic evaluation of the CMBs is necessary to determine the age and clinical relevance of each hemorrhage

Ex-vivo quantitative susceptibility mapping of human brain hemispheres.

Ex-vivo brain quantitative susceptibility mapping (QSM) allows investigation of brain characteristics at essentially the same point in time as histopathologic examination, and therefore has the potential to become an important tool for determining the role of QSM as a diagnostic and monitoring tool of age-related neuropathologies. In order to be able to translate the ex-vivo QSM findings to in-vivo, it is crucial to understand the effects of death and chemical fixation on brain magnetic susceptibility measurements collected ex-vivo. Thus, the objective of this work was twofold: a) to assess the behavior of magnetic susceptibility in both gray and white matter of human brain hemispheres as a function of time postmortem, and b) to establish the relationship between in-vivo and ex-vivo gray matter susceptibility measurements on the same hemispheres. Five brain hemispheres from community-dwelling older adults were imaged ex-vivo with QSM on a weekly basis for six weeks postmortem, and the longitudinal behavior of ex-vivo magnetic susceptibility in both gray and white matter was assessed. The relationship between in-vivo and ex-vivo gray matter susceptibility measurements was investigated using QSM data from eleven older adults imaged both antemortem and postmortem. No systematic change in ex-vivo magnetic susceptibility of gray or white matter was observed over time postmortem. Additionally, it was demonstrated that, gray matter magnetic susceptibility measured ex-vivo may be well modeled as a linear function of susceptibility measured in-vivo. In conclusion, magnetic susceptibility in gray and white matter measured ex-vivo with QSM does not systematically change in the first six weeks after death. This information is important for future cross-sectional ex-vivo QSM studies of hemispheres imaged at different postmortem intervals. Furthermore, the linear relationship between in-vivo and ex-vivo gray matter magnetic susceptibility suggests that ex-vivo QSM captures information linked to antemortem gray matter magnetic susceptibility, which is important for translation of ex-vivo QSM findings to in-vivo

Correspondence of in-vivo and ex-vivo data within the same participants.

<p>In-vivo and ex-vivo magnetic susceptibility and gradient-echo magnitude maps for a section of the basal ganglia of three hemispheres from Dataset 2 imaged both in-vivo and ex-vivo.</p

Dataset 1 characteristics.

<p>Dataset 1 characteristics.</p

Magnetic susceptibility over time postmortem for gray and white matter regions.

<p>Plots of ex-vivo magnetic susceptibility in selected gray and white matter regions as a function of time postmortem, for all hemispheres of Dataset 1. Error bars around individual data points represent the 95% confidence interval of the susceptibility values within the region at that specific time point.</p

Ex-vivo magnetic susceptibility as a function of in-vivo magnetic susceptibility.

<p>Plot of regional gray matter magnetic susceptibility values measured ex-vivo as a function of the corresponding susceptibility values measured in-vivo in the same hemispheres, for all hemispheres of Dataset 2. Each point in the scatter plot represents a single gray matter brain region of a single hemisphere. Ex-vivo magnetic susceptibility values shown in the plot have been corrected for the effects of lower temperature during ex-vivo imaging [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0188395#pone.0188395.ref004" target="_blank">4</a>].</p

Postmortem brain hemisphere in imaging container.

<p>Example of a participant’s brain hemisphere submerged in formaldehyde solution.</p

Ex-vivo magnetic susceptibility maps.

<p>Examples of axial ex-vivo magnetic susceptibility maps of human brain hemispheres from five participants.</p

Voxel-wise differences in magnetic susceptibility maps of consecutive time-points.

<p>Ex-vivo magnetic susceptibility difference maps between consecutive time-points, for all hemispheres of Dataset 1. The corresponding susceptibility map and spin-echo image of the last time-point are displayed on the rightmost two columns. Note: For hemisphere D, the difference map marked with an asterisk represents the difference between the third and fifth time-points.</p

Measurements from 3T as a function of measurements from 1.5T.

<p>Plot of magnetic susceptibility in selected gray and white matter regions of the same hemisphere imaged postmortem using both in-vivo (1.5T) and ex-vivo (3T) QSM methods (Dataset 3).</p