Advanced three‐dimensional tailored RF pulse for signal recovery in T₂*‐weighted functional magnetic resonance imaging

Abstract

T * 2 -weighted functional MR images are plagued by signal loss artifacts caused by susceptibility-induced through-plane dephasing. We present major advances to the original threedimensional tailored RF (3DTRF) pulse method that precompensates the dephasing using three-dimensional selective excitation. The proposed 3DTRF pulses are designed iteratively with off-resonance incorporation and with a novel echo-volumar trajectory that frequency-encodes in z and phase-encodes in x , y . We also propose a computational scheme to accelerate the pulse design process. We demonstrate effective signal recovery in a 5-mm slice in both phantom and inferior brain, using 3DTRF pulses that are only 15.4 ms long. Compared to the original method, the new approach leads to significantly reduced pulse length and enhancement in slice selectivity. Blood-oxygenation-level-dependent T * 2 contrast for functional MRI (fMRI) originates from mesoscopic magnetic spin dephasing. Unfortunately, T * 2 contrast is coupled to signal loss artifacts due to dephasing caused by macroscopic inhomogeneity of the main field. A major cause of macroscopic field inhomogeneity in the human head is the bulk magnetic susceptibility (BMS) differences across the interface between tissue and air-filled cavities, such as the sinuses. During the long echo time (T E ) required for T * 2 contrast, an inhomogeneous field causes intravoxel spin signal dephasing and consequently phase cancellation during readout leads to signal loss. This signal loss artifact hampers fMRI studies of brain regions proximal to air cavities, such as the orbital frontal and inferior temporal cortices (1).Various techniques have been proposed to recover the BMS-induced lost signals, but they have different drawbacks. One class of methods focuses on optimization of slice and acquisition parameters (2-6). These methods generally recover signals only partially, and they interfere with brain coverage preferences that are specific to the functional study. Refs. (7-9) proposed compensation of the dephasing using extra gradient lobes. To achieve acceptable recovery, these methods generally require multiple scans for one slice location, leading to loss of temporal resolution of the fMRI experiment. Inspired by the tailored RF pulse method that creates quadratic throughplane phase variation (uniform in-plane) for excitationstage partial "precompensation" of the dephasing (10), Ref. (11) proposed using three-dimensional TRF (3DTRF) pulses for phase precompensation with in-plane selectivity. However, those pulses are undesirably long and thus compromise temporal resolution as well. On the acquisition side, using spiral in-out (12) or spiral in-in (13) trajectories recovers some signals compared to the spiral-out case, but recovery is usually partial. Last, it has also been demonstrated that intraoral and external localized shimming (14-16) can effectively improve field homogeneity and recover signals. However, placement of extra hardware may lead to subject discomfort and may not be appropriate for u...

Model-Based MR Image Reconstruction With Compensation for Through-Plane Field Inhomogeneity

“…To evaluate the proposed methods, we acquired human brain MR field maps as described in [8]. The scans were 64 by 64 by 40 slices, with 24 cm transaxial FOV and 4 cm axial FOV.…”

“…For such levels of field gradients the 2nd-order approximation may be insufficient. On the other hand, tailored RF pulse design methods aim to precompensate for the dephasing (at the echo time) caused by through-plane gradients [8], so a combination of tailored RF pulses and a 2nd-order Taylor expansion might suffice.…”

Molecular Brain Imaging in the Multimodality Era

“…A larger image acquisition matrix (smaller voxels) improves spatial resolution but reduces the voxel signal, and short TR may improve temporal resolution but this depends on the temporal nature of the hemodynamic response function (HRF) (Huettel et al, 2009). Methods have been proposed to recover signal loss in fMRI, including the use of tailored radiofrequency pulses (Yip et al, 2006). Table 2 summarizes parameters for consideration for combined PET and fMRI.…”

Fast slice‐selective radio‐frequency excitation pulses for mitigating B inhomogeneity in the human brain at 7 Tesla

“…To demonstrate the utility of sparsity-enforced spoke placement we first compare it to Fourier-based spoke placement (21). The latter computes the Fourier transform of the ideal in-plane excitation, |B 1 ϩ (r)| -1 , and places T spokes in (k x , k y )-space where the Fourier coefficients are largest in magnitude.…”

“…Unfortunately, placing many spokes leads to impracticably-long pulses. An alternate method is to compute the Fourier transform of the ideal in-plane excitation and place spokes in k-space where Fourier coeffi-cients are largest in magnitude (21). Unfortunately, this tends to concentrate spokes around (kx ϭ 0 ⅐ ky ϭ 0) DC, analogous to a low-pass filter.…”