Modeling magnitude and phase neuronal current MRI signal dependence on echo time

Abstract

To enhance sensitivity in measuring neuronal current MRI (ncMRI) signal using T(2)*-weighted sequences, appropriate selection of echo time (TE) is vital for optimizing data acquisition strategy. The purpose of this study is to establish the contrast-to-noise ratio of neuronal current MRI signal dependence on TE and determine the optimum TE (TE(opt)) in achieving its highest detection power. The TE(opt) in human brain and tissue preparation at 1.5, 3, and 7 T are estimated with different voxel sizes. Our results show that TE(opt) values are different between magnitude and phase images, and TE(opt) is larger in magnitude than phase imaging. This suggests that a dual-echo data acquisition strategy would provide the best efficiency in detecting magnitude and phase neuronal current MRI signals simultaneously. Our results also indicated that the detection sensitivity will be stronger at lower magnetic fields for human brain, whereas the sensitivity will be enhanced/reduced as field strength increases for phase/magnitude imaging on tissue preparation.

Figure 1

Fitting results for the relationships of relative magnitude change (δ) (a) and phase shift (Δϕ) (b) of ncMRI signal with TE. The data points are selected from the reference .

Figure 2

TE dependences of CNR of magnitude and phase ncMRI signals for voxel size of 3×3×3 mm³ in human brain (a) and tissue preparation (b) at 1.5, 3, and 7 T. In the figure, the CNR data of magnitude/phase signal are normalized to the peak CNR of magnitude/phase signal at 1.5 T. (c) and (d) show the optimum TE values for magnitude and phase signals respectively for different voxel volumes at 1.5, 3, and 7T. The data points indicate the results for voxel size=1×1×3, 2×2×3, 3×3×3, 4×4×3, and 5×5×3 mm³.

Figure 3

(a) and (b) show the dependences of maximum CNR (i.e., CNR at optimal TE) for magnitude (CNR_M) and phase (CNR_ϕ) signals on voxel volume in human brain. The maximum CNR values are normalized to that of voxel size of 3×3×3 mm³ at 1.5T.

Figure 4

(a) and (b) illustrate the T₂*-dependences of optimal TEs for magnitude and phase signals in human brain. Crosses indicate the data points corresponding to the typical T₂* values. (c) shows the field dependences of ncMRI magnitude and phase signal changes at the optimal TEs, and (d) shows the tSNR_M at the TE_opt,M and TE_opt,ϕ for different B₀. All the data in (c) and (d) are normalized to the corresponding results at 1.5 T.