Imaging white matter microstructure with gradient‐echo phase imaging: Is ex vivo imaging with formalin‐fixed tissue a good approximation of the in vivo brain?

Abstract

Purpose Ex vivo imaging is a commonly used approach to investigate the biophysical mechanism of orientation‐dependent signal phase evolution in white matter. Yet, how phase measurements are influenced by the structural alteration in the tissue after formalin fixation is not fully understood. Here, we study the effects on magnetic susceptibility, microstructural compartmentalization, and chemical exchange measurement with a postmortem formalin‐fixed whole‐brain human tissue. Methods A formalin‐fixed, postmortem human brain specimen was scanned with multiple orientations to the main magnetic field direction for robust bulk magnetic susceptibility measurement with conventional quantitative susceptibility imaging models. White matter samples were subsequently excised from the whole‐brain specimen and scanned in multiple rotations on an MRI scanner to measure the anisotropic magnetic susceptibility and microstructure‐related contributions in the signal phase and to validate the findings of the whole‐brain data. Results The bulk isotropic magnetic susceptibility of ex vivo whole‐brain imaging is comparable to in vivo imaging, with noticeable enhanced nonsusceptibility contributions. The excised specimen experiment reveals that anisotropic magnetic susceptibility and compartmentalization phase effect were considerably reduced in the formalin‐fixed white matter specimens. Conclusions Formalin‐fixed postmortem white matter exhibits comparable isotropic magnetic susceptibility to previous in vivo imaging findings. However, the measured phase and magnitude data of the fixed white matter tissue shows a significantly weaker orientation dependency and compartmentalization effect. Alternatives to formalin fixation are needed to better reproduce the in vivo microstructural effects in postmortem samples.

An improved asymmetric susceptibility tensor imaging model with frequency offset correction

“…Furthermore, although the microstructure effects are mainly because of the heterogeneity of the magnetic susceptibility, they cannot be fully explained by the effects of bulk anisotropic magnetic susceptibility 18,19 . In the QSM field, a few attempts have been made to separate the bulk magnetic susceptibility and non‐bulk‐magnetic‐susceptibility (NBMS) contributions 20–25 . Recently, Jeong et al proposed to simultaneously estimate susceptibility tensor and chemical exchange maps 26 .…”

“…18,19 In the QSM field, a few attempts have been made to separate the bulk magnetic susceptibility and non-bulk-magnetic-susceptibility (NBMS) contributions. [20][21][22][23][24][25] Recently, Jeong et al pro-posed to simultaneously estimate susceptibility tensor and chemical exchange maps. 26 Therefore, these NBMS effects should be considered when solving the susceptibility tensor from measured MRI frequency signals.…”

Incorporating the effect of white matter microstructure in the estimation of magnetic susceptibility in ex vivo mouse brain

“…As imaging was performed on ex vivo mouse brains, effects related to fixation may also affect the estimated parameters due to structural alterations, increased chemical shifts and changes in chemical composition 14,19,47,70 . Susceptibility values have earlier been found to be numerically smaller in vivo compared to ex vivo 14,71 .…”

“…As imaging was performed on ex vivo mouse brains, effects related to fixation may also affect the estimated parameters due to structural alterations, increased chemical shifts and changes in chemical composition. 14,19,47,70 Susceptibility values have earlier been found to be numerically smaller in vivo compared to ex vivo. 14,71 For example, PBS and PFA solution can lead to increased macro-molecular exchange, earlier found to lead to shifts on the order of −0.013 ppm and 0.05 ppm, respectively.…”

Segmentation of supragranular and infragranular layers in ultra-high resolution 7Tex vivoMRI of the human cerebral cortex

“…This makes it well-suited for characterizing neuroanatomy at high resolution and providing finer macroscopic morphometric measures, such as cortical thickness, of the underlying cytoarchitecture. Although imaging the intact human brain ex vivo at high magnetic fields is challenging due to the need for specialized hardware [13], recent progress in high-field scanner and coil technology, and imaging protocols [16], has enabled full-brain scanning with voxel sizes as small as 100 µ m [13, 17], helping bridge the gap between histology and MRI.…”