Probing mouse brain microstructure using oscillating gradient diffusion MRI

Abstract

High resolution diffusion tensor images of the mouse brain were acquired using the pulsed gradient spin echo sequence and the oscillating gradient spin echo sequence. The oscillating gradient spin echo tensor images demonstrated frequency-dependent changes in diffusion measurements, including apparent diffusion coefficient and fractional anisotropy, in major brain structures. Maps of the rate of change in apparent diffusion coefficient with oscillating gradient frequency revealed novel tissue contrast in the mouse hippocampus, cerebellum, and cerebral cortex. The observed frequency-dependent contrasts resembled neuronal soma-specific Nissl staining and nuclei-specific 4',6-diamidino-2-phenylindole (DAPI) staining in the mouse brain, which suggests that the contrasts might be related to key features of cytoarchitecture in the brain. In the mouse cuprizone model, oscillating gradient spin echo-based diffusion MRI revealed significantly higher frequency-dependence of perpendicular diffusivity (λ(⊥) ) in the demyelinated caudal corpus callosum at 4 weeks after cuprizone treatment when compared with control mice and mice at 6 weeks after cuprizone treatment. The elevated frequency-dependence of λ(⊥) coincided with the infiltration of activated microglia/macrophages and disruption of axons during acute demyelination in the caudal corpus callosum. The results demonstrate the potential of oscillating gradient spin echo-based diffusion MRI for providing tissue contrasts complimentary to conventional pulsed gradient spin echo-based diffusion MRI.

FIG. 1

The gradient waveforms G(t) used in the PGSE sequence (solid curve in A) and OGSE sequences with oscillation frequencies of 50 Hz, 100 Hz, and 150 Hz (solid curves in B-D, respectively), and their corresponding F(f) (E). The magnitudes of the gradient waveforms have been normalized to the range of −1 to 1, and the durations are equal to the echo time (50 ms). The dotted curves in figures A-D have polarity opposite to that of the solid curves in the second half, which represents the effect of the refocusing pulse. Figure E shows the normalized results of Fourier transform of the gradient waveforms (F(f)) for the PGSE (black) and OGSE (blue: 50Hz, green: 100 Hz, red: 150 Hz) sequences (dotted curves in A-D).

FIG. 2

Coronal T₂-weighted and diffusion tensor images of an ex vivo mouse brain acquired using both PGSE and OGSE sequences at 50 Hz, 100 Hz, and 150 Hz. Anatomical structures are labeled in the T₂-weighted image. Maps of ADC, FA and direction-encoded colormap (DEC) images are generated from the diffusion tensor. The unit of the ADC is mm²/s. The three color arrows at the lower right illustrate the color scheme in the DEC images which uses red for the medial-lateral axis, green for the anterior-posterior axis, and blue for the superior-inferior axis. Structural abbreviations are: cc: corpus callosum; cp: cerebral peduncle; CX: cerebral cortex; H: hippocampus.

FIG. 3

Values of ADC, FA, parallel diffusivity (λ_∥), and perpendicular diffusivity (λ_┴) measured in major structures using the PGSE and OGSE sequences. Data are plotted as mean ± standard deviation for four mouse brains. Structural abbreviations are: cp: cerebral peduncle; fi: fimbria; gcc: genu of the corpus callosum; mcx: motor cortex; scc: splenium of the corpus callosum; scx: sensory cortex.

FIG. 4

Enhanced tissue contrasts in the mouse hippocampus and cerebellum in oscillating gradient diffusion tensor images. A: Coronal and sagittal T₂-weighted images and enlarged ADC images of the hippocampus acquired using the PGSE and OGSE sequences and corresponding Nissl stained sections. B: mid-sagittal T₂-weighted images and ADC images of the cerebellum acquired using the PGSE and OGSE sequences and corresponding Nissl stained section. The Nissl stained sections are from the Paxinos’ mouse brain atlas. C: changes in mean ADC values of selected structures acquired using the PGSE and OGSE sequences. Structural abbreviations are: CBGr: cerebellar granule cell layer; CBML: cerebellar molecular layer; GrDG: granule cell layer of the dentate gyrus; Py: pyramidal cell layer of the hippocampus, CA1: CA1 subfield of the hippocampus.

FIG. 5

The spatial organization of neurons and axons in the mouse hippocampus (A & B) and cerebellum (C & D) in immunostained sections. The sections were stained with DAPI (blue, for nuclei) and SMI-31 (green, for phosphorylated neurofilament in axons). B and D are higher magnification (20X) images of the regions outlined by the dashed boxes in A and C (10X), respectively. Scale bars = 100 μm. Structural abbreviations are: CBGr: cerebellar granule cell layer; CBML: cerebellar molecular layer; GrDG: granule cell layer of the dentate gyrus; Py: pyramidal cell layer of the hippocampus.

FIG. 6

Comparison of T2-weighted images, maps representing the linear fits of ADC versus frequency (Δ_f ADC), and Nissl stained sections of the mouse brain. Three coronal sections (A), one sagittal section (B), and one horizontal section (C) are shown. The Nissl stained sections are from the Paxinos’ mouse brain atlas. Arrows in the figure point to regions that show enhancement in the cerebral cortex (1), piriform cortex (2), dentate gyrus (3), olfactory bulb (4), and cerebellum (5).

FIG. 7

A: T2-weighted images, maps of perpendicular diffusivity (λ_┴) measured using PGSE sequence, and maps of the rate of change of perpendicular diffusivity with frequency (Δ_f λ_┴) measured using the OGSE sequence, of the corpus callosum in the control mice (0 week), mice after 4 weeks of cuprizone diet, and mice after 6 of weeks cuprizone diet. B: Plots of FA, λ_┴, and Δ_f λ_┴ of the caudal corpus callosum. *: p < 0.005.

FIG. 8

Organization of axons in the mouse corpus callosum and infiltration of activated microglia at 4 weeks after cuprizone treatment. Coronal immunostained sections of the caudal corpus callosum of a control mouse and mice after 4 and 6 weeks of cuprizone treatment are shown. The sections were stained with iba-1 (red, for activated microglia) and SMI-31 (green, for phosphorylated neurofilament in axons). Disruption of axons at the 4 and 6 week time points can be seen. Swelling of the corpus callosum, indicated by an increase in the thickness, is apparent at the 4 week time point. Structural abbreviations are: cc: corpus callosum.