High field MRI has many advantages including increased signal to noise ratio (1
) and increased Blood Oxygen Level Dependent (BOLD) functional MRI (fMRI) contrast (2
). Unfortunately, several confounds associated with the high field need to be addressed before many applications can reach their full potential. A major problem is image artifacts produced by inhomogeneity in the transmitted RF field (B1+) (3
). The two factors that produce B1+ inhomogeneity at high field are the decreased RF wavelength, further shortened by the dielectric properties of tissue, and the attenuation of RF amplitude due to tissue conductivity (4
). These artifacts appear as regions of increased and decreased brightness at 3T and even regions with no image magnitude at ultra high fields such as 7T. A second confound at high field are susceptibility artifacts that appear as distortions or as large voids in the images. They are particularly problematic in T2*-weighted imaging applications with long TE such as fMRI (5
). The through-plane signal-loss artifact in axial slices is of primary importance due to the close proximity of air/tissue boundaries to the inferior brain areas. The fMRI data in many crucial regions such as the orbitofrontal cortex remain sub-optimal as a result.
Numerous methods have been proposed to mitigate B1+ inhomogeneity artifacts. These include specially designed coils (6
), adiabatic pulses (7
), image post-processing (8
), and small-tip-angle tailored RF pulses (9
). The tailored RF pulse technique has the advantage of being able to compensate for the B1+ inhomogeneity using a predetermined spatial excitation. More recently, it has been shown that “B1+ shimming” can be performed using multiple transmitters (10
). Combining three-dimensional (3D) RF pulses with parallel transmission and sensitivity encoding (transmit SENSE) is currently showing great promise (11
). The use of a parallel transmission has many additional benefits at high fields including managing the Specific Absorption Rate (SAR). Parallel transmission pulse designs have also been optimized for broad band excitations (14
Numerous methods have been proposed to mitigate the signal loss artifact including z
-shim methods (16
), thin slice averaging (17
), passive and active shim coils (18
), and tailored RF pulses (20
). All techniques have advantages and disadvantages; however, a desirable requirement is that the correction be performed in one shot. For example, multi-echo sequences or parallel transmission can be used to perform a single shot z
). Recently, the use of 2D spectral-spatial pulses has been shown to be very promising for compensating for the through-plane signal loss artifact using a single RF pulse (25
). The basic idea is that regions needing the through-plane pre-compensatory phase will also tend to be off-resonance. The spectral-spatial pulse compensates the through-plane phase of the off-resonance spins and leaves the on-resonance spins unaffected.
We propose the use of four-dimensional (4D) spectral-spatial pulses for the simultaneous mitigation of B1+ inhomogeneity and through-plane susceptibility artifacts for high field T2*-weighted imaging. The method is presented using a dual-band design in a numerical framework that includes parallel transmission. The technique is demonstrated with simulations and human brain imaging experiments using a custom four-channel parallel transmission system. Reduction of excitation k-space using transmit SENSE was explored to trade in-plane resolution for an improved slice profile. Two pulse designs consisting of four 15 ms long 4D spectral-spatial pulses for a single parallel excitation are presented. Both sets of pulses were found to reduce B1+ inhomogeneity and signal loss artifact in several slice locations and several subjects.