Postmortem MR imaging of formalin-fixed human brain
Introduction
MRI has been used to acquire images of the postmortem human brain for the past 15 years. These images offer a valuable complement to the work of the neuropathologist by providing a third dimension for histological specimens, documentation of the whole brain before it is sectioned, and a flexible tool for teaching (Boyko et al., 1994). One use of this technique has been to compare MRI sections acquired postmortem with equivalent physical sections prepared for neuropathological assessment. This approach has been applied to brains of people with diagnoses of AIDS (Grafe et al., 1990) and Alzheimer's disease Bobinski et al., 2000, Bronge et al., 2002, with a focus on identifying false positives, true positives, and false negatives in the MR images relative to histologically identified lesions such as infarction, necrosis, diffuse HIV-associated microglial nodules in AIDS cases, and various white matter changes in AD. Cross-modality comparison has also been made of qualitatively assessed atrophy in AIDS patients (Everall et al., 1997) and quantitatively assessed hippocampus volume in normal and AD patients (Bobinski et al., 2000). While these comparative studies have highlighted many similarities, they have also noted discrepancies attributable to the relative sensitivity of each technique to the characteristics of formalin-fixed postmortem tissue.
MR imaging of a postmortem brain specimen needs to accommodate the significant differences between a living and a dead brain. Ideally, the postmortem brain is intact and has been minimally damaged by removal from the skull; as such, it is drained of cerebrospinal fluid (CSF), blood no longer flows through the capillaries and it is devoid of its encasing skull. Even with formalin fixation, brain tissue requires support to remain stable and not collapse on itself. Furthermore, formalin fixation changes the microstructure of tissue and affects water mobility. Differences in water mobility between different tissue types are key to tissue discriminability by MRI. T1 (longitudinal) and T2 (transverse) relaxation times of water protons rely on mobility of water within the tissue. Formalin fixation reduces difference in water mobility between gray and white matter. Early declines in T1 and T2 after fixation lead to convergence of T1 values in gray and white (Tovi and Ericsson, 1992) and limit gray and white matter discriminability on T1-weighted postmortem MR images. Some imaging protocols yield a reversal of relative signal intensities between gray matter and white matter in fixed brains compared with in vivo images Boyko et al., 1994, Schumann et al., 2001. T2 also declines over time but not so much as T1. Changes in both T1 and T2 occur rapidly over the short term but stabilize after 3–4 weeks (Tovi and Ericsson, 1992), such that length of fixation has little impact on gray–white conspicuity after 3 weeks (Boyko et al., 1994). By contrast, differences in the relative density of protons in gray and white matter increase during the first weeks of fixation (Blamire et al., 1999). Thus, a spin (proton) density-weighted protocol is preferred for postmortem imaging as it provides excellent gray–white contrast (Schumann et al., 2001).
Initial postmortem MRI studies used conventional acquisition protocols, producing images with relatively thick slices in fixed orientations. Comparisons between MRI and pathological sections were made by matching pathological slices to acquired MR slices. More recent imaging developments (e.g., Schumann et al., 2001) allow acquisition of three-dimensional high-resolution MRI images with in-plane voxel dimensions on the order of 100 μm, allowing the image to be rotated in any orientation and thinly resliced with minimal distortion from interpolation. Improvement in resolution of in vivo imaging is limited by the living subject's tolerance for lengthy scan sessions, whereas the postmortem brain is ideally suited for time-dependent improvements in resolution limited only by computer storage space and monetary cost.
Consistent with postmortem histological findings of age-related degradation of white matter microstructure (Meier-Ruge et al., 1992) are recent results based on in vivo diffusion tensor imaging, which is particularly useful for visualizing white matter bundles and microstructure (Basser and Pierpaoli, 1996). The in vivo studies have revealed age-related disruption of brain white matter microstructural integrity in men and women that varies by region (e.g., Pfefferbaum and Sullivan, 2003; for review, Sullivan and Pfefferbaum, 2003). The mechanism for alcoholism-related white matter volume loss, restoration with alcohol abstinence, and disruption of microstructural integrity remains unclear but probably involves changes in both myelination and axonal integrity, which have been identified postmortem.
Postmortem imaging provides a technique to explore and quantify discrepancies between in vivo results and postmortem neuropathology as well as to provide a new tool to complement traditional and emerging neuropathological investigations. The goals of this investigation were to use standard clinical 1.5- and 3.0-T MR scanners and product sequences to establish MR imaging parameters for formalin-fixed human brain specimens that would yield high isotropic resolution, whole-brain coverage, and volumetric images. The target imaging modalities were conventional structural MRI and MR diffusion tensor imaging. Among the issues to address were effects of death and fixation on the microenvironment of brain tissue water protons and its effect on temporal imaging parameters, specimen movement, signal-to-noise ratio (SNR), and cost in terms of time and money.
Section snippets
Subjects
Postmortem material was obtained from two sources. A whole-brain specimen, provided by the New South Wales Tissue Resource Center, Australia, was from a woman who had died of pulmonary failure at age 61 years (Fig. 1, left). Review of medical records revealed that she had been a heavy smoker (20–35 cigarettes per day) and heavy alcohol drinker (seven to nine drinks per day) when younger, but reduced alcohol consumption to two drinks per day and then only an occasional drink; a positive DSM-IV
In vivo vs. postmortem relaxation parameters and tissue contrast
The following studies used conventional product MRI protocols and were conducted on a 1.5-T, whole body scanner with specifications appropriate for clinical use.
Discussion
The principal goals of the this study were achieved for macrostructural imaging of postmortem brain specimens with spin-echo, multi-echo acquisition, product sequences on 1.5- or 3.0-T clinical scanners. Specifically, we obtained higher resolution images than can be obtained in vivo because of virtually unlimited scan time restrictions. Resolution was enhanced by using thin slices, isotropic voxels, multiple excitations, and customized field of view. These improvements came at the expense of
Acknowledgements
James Powers, M.D., of the University of Rochester generously provided tissue samples, and Margaret J. Rosenbloom provided helpful assistance with the literature review. The National Institute of Alcohol Abuse and Alcoholism (AA05965, AA12388, AA13999, AA10723, AA12725) provided support for this project. Partial reports of these data were presented at Research Society on Alcoholism, June 2002, San Francisco, CA, and American College of Neuropsychopharmacology, December 2002, San Juan, Puerto
References (27)
- et al.
Neuronal loss in functional zones of the cerebellum of chronic alcoholics with and without Wernicke's encephalopathy
Neuroscience
(1999) - et al.
Topography of brain atrophy during normal aging and Alzheimer's disease
Neurobiol. Aging
(1996) - et al.
Diffusion tensor imaging in normal aging and neuropsychiatric disorders
Eur. J. Radiol.
(2003) - et al.
Postmortem cryosectioning as an anatomic reference for human brain mapping
Comput. Med. Imaging Graph.
(1997) - et al.
Three-dimensional diffusion tensor magnetic resonance microimaging of adult mouse brain and hippocampus
NeuroImage
(2002) - et al.
Microstructural and physiological features of tissues elucidated by quantitative diffusion tensor MRI
J. Magn. Reson., Ser. B
(1996) - et al.
Optimising imaging parameters for post mortem MR imaging of the human brain
Acta Radiol.
(1999) - et al.
The histological validation of post mortem magnetic resonance imaging-determined hippocampal volume in Alzheimer's disease
Neuroscience
(2000) - et al.
Utility of postmortem magnetic resonance imaging in clinical neuropathology
Arch. Pathol. Lab. Med.
(1994) - et al.
Brain MR: pathological correlation with gross histopathology; 2. Hyperintense white matter foci in the elderly
Am. J. Neuroradiol.
(1988)
Postmortem MRI and histopathology of white matter changes in Alzheimer brains. A quantitative, comparative study
Dementia Geriatr. Cognit. Disord.
Correlation of MRI and neuropathology in AIDS
J. Neurol., Neurosurg. Psychiatry
Abnormalities of the brain in AIDS patients: correlation of postmortem MR findings with neuropathology
Am. J. Neuroradiol.
Cited by (162)
Segmented solenoid RF coils for MRI of ex vivo brain samples at ultra-high field preclinical and clinical scanners
2023, Journal of Magnetic Resonance OpenAlcohol-fixed specimens for high-contrast post-mortem MRI
2021, Forensic Imaging