Post-mortem 7 Tesla MRI detection of white matter hyperintensities: A multidisciplinary voxel-wise comparison of imaging and histological correlates

Abstract

White matter hyperintensities (WMH) occur in normal aging and across diagnostic categories of neurodegeneration. Ultra-high field imaging (UHF) MRI machines offer the potential to improve our understanding of WMH. Post-mortem imaging using UHF magnetic resonance imaging (MRI) is a useful way of assessing WMH, however, the responsiveness of UHF-MRI to pathological changes within the white matter has not been characterized. In this study we report post-mortem MRI sequences of white matter hyperintensities in normal aging, Alzheimer's disease, and cerebrovascular disease. Seven Tesla post-mortem MRI reliably detected periventricular WMH using both FLAIR and T2 sequences and reflects underlying pathology of myelin and axon density despite prolonged fixation time. Co-registration of histological images to MRI allowed for direct voxel- wise comparison of imaging findings and pathological changes. Myelin content and cerebrovascular pathology were the most significant predictors of MRI white matter intensity as revealed by linear mixed models. Future work investigating the utility of UHF- MRI in studying cell-specific changes within WMH is required to better understand radio-pathologic correlations.

Keywords: Small Vessel Disease; Ultra High Field MRI; White Matter Hyperintensities.

Fig. 1

Post-mortem imaging and histological co-registration workflow. A) Coronal FLAIR image of a brain section used for identification of WMH. B) Following imaging, tissue blocks were sectioned from the periventricular white matter and sent for histological analysis. Tissue blocks were stained with C) Luxol Fast Blue and D) neurofilament (Dako M762). E) Tissue sections were scanned using an Aperio slide scanner and the field-fractioned images were co-registered to the corresponding MR slices (F). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 2

Post-mortem 7 T imaging detects pvWMH of varying severity and periventricular infarcts. FLAIR sequences with A) mild amount of pvWMH, B) moderate pvWMH and C) severe pvWMH extending into the deep white matter. Lesion areas are indicated by red asterisk. D) Periventricular infarcts were identified on T1map sequences as fluid-filled cavities within pvWMH (E) H&E staining of the lesion from image D to confirm histologically the presence of periventricular infarction. Scale bar indicates 500 μm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 3

Myelin and Axonal Rarefaction within WMH ROIs. A) ROIs drawn using the post-mortem FLAIR sequence to delineate pvWMH and NAWM. B) Reductions in LFB (p = 0.0059) and NF (p = 0.0050) field-fraction in pvWMH vs NAWM ROIs. C) Increase in normalized T1 (p < 0.001), FLAIR (p < 0.001) and T2 (p < 0.001) signal intensity in pvWMH vs NAWM. Qualitative reduction in LFB staining between D) NAWM and E) pvWMH. Qualitative reduction in neurofilament staining between F) NAWM and G) pvWMH. Scale bars indicate 100 μm. Statistics performed using SPSS software, Mann Whitney test, * indicates significance value of p < 0.05.

Fig. 4

Predicted FLAIR voxel intensity is driven by LFB signal and diagnostic group. Predicted voxel intensities generated from the WMH linear mixed model (A) and the NAWM linear mixed model (B). Predicted values for varying field-fractions of LFB signal are grouped by neuropathological diagnosis, with fixed values of age (85 years) and fixation (7 years). Grey shaded area indicates distribution of actual LFB values for each tissue type (only values within 2 SD of mean were included). Dashed line indicates subjective threshold where hyperintense signal becomes visually obvious on a FLAIR image.