Synopsis

Functional diffusion MRI has been proposed as an alternative to BOLD fMRI, to offer a more direct read-out of neuronal activation. One aspect of the controversial discussion around functional diffusion is whether these signals follow the neuronal activation more closely. We performed BOLD and diffusion measurements with 250 ms temporal resolution upon sensory stimulation and found similar temporal evolutions for BOLD and diffusion signals. Specifically, both signals slowly returned to baseline after the stimulus. However, diffusion measurements with b=500 showed an earlier onset. We performed heat application to rule out tissue heating as a contribution to functional diffusion signals.

Introduction

Methods

We acquired GE-BOLD (n=14 electric stimulation, n=14 heat application) and SE diffusion measurements with b=500 and b=1000 (each n=14 electric stimulation, n=14 heat application) in 5 naïve female Fischer rats using a 9.4 T small animal MRI and a 1 cm surface coil as receiver coil. All measurements were performed under medetomidine sedation in ventilated rats.

First, standard GE-BOLD measurements with TR 1 s and brain-wide coverage were acquired to verify activated brain regions. In those regions measurements with TR of 250 ms were performed with electric forepaw stimulation (5 s stimulation, 25 s rest, 9 Hz, 1.5 mA, 1-ms pulses) or heat application by laser light ^5,6 (200-µm optic fiber implanted in S1Fl, 5 s stimulation, 25 s rest, 180-200 mW/mm², 9 Hz, 100-ms pulses). Scan parameters for the GE-BOLD measurements were: TR= 250 ms, TE=18 ms, FA=30°. Scan parameters for SE diffusion measurements were: TR=250 ms, TE=35.85 ms, δ=2 ms, Δ=15 ms, b-values=500 and 1000, FA=150° ⁷, single SE preparation. All scans were acquired with an 80 x 80 matrix and a FOV of 26 mm x 28 mm, 2 or 3 slices, 1.2 mm slice thickness.

The data were preprocessed (realigned) and then analyzed with a custom-written MATLAB script. First, a ROI covering S1Fl was drawn in slices with activation. A U-test (Bonferroni corrected) was performed to determine activated voxels. Voxels with negative and positive signal changes were averaged separately, each across slices and stimulation trials. Time courses based on less than 3 active voxels were discarded. In addition to these averaged time courses, summed time courses weighted by the number of active voxels in each measurement were calculated.

Results and Discussion

Electric stimulation

We found a positive signal change upon forepaw stimulation in SE diffusion measurements with b=500 (n=12 of 14 measurements) and b=1000 (n=8 of 14 measurements). Diffusion signal amplitudes increased with increasing b-value (b=500 2.6 % and b=1000 3.6%) (Fig. 1), as observed previously by Tsurugizawa et al. ⁸.

The onset (defined by the first value above baseline after start of the stimulation) for SE diffusion measurements with b=500 was observed immediately after stimulation. Onset for SE diffusion measurement with b=1000 was observed 1000 ms after stimulation and for GE-BOLD 1250 ms after stimulation (Fig. 1 and Fig. 2). Diffusion signal amplitudes for both b-values did not return faster to baseline than the BOLD signal. This was in contrast to a study conducted in isoflurane-anesthetized rats where the diffusion signal returned to baseline directly after the stimulus had ended ⁸. The prolonged signal observed in our study suggests a slow vascular contribution to the DfMRI signal and is in line with another recent study conducted in medetomidine-sedated rats, which also found prolonged DfMRI signals upon stimulation ⁹.

Heat application

Heat application led to a pronounced negative signal change in the SE diffusion measurement. Since electric stimulation only led to positive signal change in the SE diffusion measurement, heat could be ruled out as a signal source for DfMRI (Fig. 3). However, it cannot be ruled out that heat production leads to a reduction of the observed signal.

Conclusion

Acknowledgements

References

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