Spiral‐in/out BOLD fMRI for increased SNR and reduced susceptibility artifacts

Abstract

BOLD fMRI is hampered by dropout of signal in the orbitofrontal and parietal brain regions due to magnetic field gradients near air-tissue interfaces. This work reports the use of spiral-in trajectories that begin at the edge of k-space and end at the origin, and spiral in/out trajectories in which a spiral-in readout is followed by a conventional spiral-out trajectory. The spiral-in trajectory reduces the dropout and increases the BOLD contrast. The spiral-in and spiral-out images can be combined in several ways to simultaneously achieve increased signal-tonoise ratio (SNR) and reduced dropout artifacts. Activation experiments employing an olfaction task demonstrate significantly increased activation volumes due to reduced dropout, and overall increased SNR in all regions. The most widely used form of fMRI exploits BOLD contrast (1,2) to produce maps of neuronal activation. When the transverse magnetization decay is exponential, changes in BOLD contrast are maximized if the echo time (TE) is made equal to the susceptibility-mediated transverse relaxation time constant, T* 2 . In uniform brain the T* 2 for gray matter is about 50 ms at 3T (3,4); thus, in sensitizing the acquisition to BOLD changes from the microscopic gradients surrounding capillaries, the acquisition is also made exquisitely sensitive to intravoxel dephasing resulting from macroscopic field gradients established near air-tissue interfaces. These susceptibility-induced field gradients (SFGs) cause severe dropout of signal in the frontal orbital and lateral parietal brain regions due to the difference in magnetic susceptibility of tissue and air (ϳ -8 ppm). These dropouts can limit the applicability of fMRI for many cognitive experiments.Several methods have been proposed to reduce the effect of SFGs. One class of techniques corrects for dropouts caused when SFGs shift the center of excitation k-space (k z direction), by applying compensation gradients in the slice-selection direction to refocus the dephased spins (5,6). 3D compensation schemes were introduced by Yang et al. (7,8) in which multiple echoes and Fourier inversion are used to create compensated images, and by Glover (9), who used extended coverage of k z -space with windowed reconstruction to provide efficiency improvements in gathering the compensated images. A related method simply decreases the slice thickness and averages adjacent slices (10,11). However, each of these methods suffers from prolonged scan time and loss of SNR efficiency. Another class of methods uses tailored RF pulses to compensate the dephasing during excitation (12)(13)(14). The design of these pulses is complex and ideally must be tailored for each subject, and their effectiveness is reduced by gradient system limitations that cause the pulses to be lengthy. In addition, all of these compensation methods are effective only for SFGs in the slice-select direction, and they provide no mitigation for intravoxel dephasing caused by in-plane gradients.Spiral methods have several advantages over other techniques for ...

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“…Suppose the histogram of the field map, ∆ω(r i ), has K bins, and ω k and h k are the center and occurrence frequency of the kth bin. 3 Define matrices E = {e kj }, e kj = e ıω k t j , [11] P = {p kl },p kl = e ıω k τ l , [12] Q = {q lj }, [13] H = diag{h k }, [14] where i = 0, . .…”

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