Multiband RF Excitation for Accelerating Magnetic Resonance Imaging in the Presence of Metal

Target Audience: Researchers interested in MR methods for imaging near metallic implants. Purpose: Recent 3D multispectral imaging (MSI) techniques have dramatically reduced off-resonance artifacts near metal by acquiring multiple 3D-FSE acquisitions at different RF offsets and summing over all RF bins. In-plane distortion, however, remains a problem in areas with extreme local B0 gradients due to frequency-encoding. To avoid these artifacts, our group developed a spectrally-resolved, fully phase-encoded (SR-FPE) 3D fast spin-echo technique. For a single RF bin, feasibility for reducing a four hour SR-FPE scan to 12 min using parallel imaging in all three spatial dimensions was recently demonstrated. However, when imaging near metallic prostheses that cause severe Bo inhomogeneities (eg. cobalt/chromium alloys), multiple RF excitations with different offsets are required to excite all spins over a wide range of frequencies. This exacerbates scan time further. Multiband (MB) RF excitation offers an opportunity to collect multiple off-resonance bins simultaneously to accelerate metal insensitive, fully phase-encoded methods. The purpose of this work was to develop and test multiband RF excitation to accelerate SR-FPE when multiple RF excitations are required. Theory: Using a standard slice-selective gradient, Muller et al. demonstrated simultaneous excitation of different slices (ie. RF bands), which has been exploited to accelerate multi-slice imaging. MB excitation is compatible with multi-slice applications because off-resonance induced by the slice selective gradient is removed prior to frequency encoding. In contrast, metal-induced off-resonance persists throughout frequency encoding so 3D-MSI methods restrict excitation to a single RF band centered at the receiver’s center frequency. Simultaneously exciting other off-resonant RF bands centered at, for example, -4000kHz and +4000kHz compared to the receiver’s center frequency would lead to spatial encoding errors for those two off-resonant bands, analagous to chemical shift artifact, if frequency encoding is used. Fully phase-encoding all three dimensions removes restriction and allows for uncorrupted spatial encoding when multiple RF bands are excited simultaneously. There are two main aspects to consider. First, the receiver BW needs to be large enough to capture the range of excited frequencies. Second, for a given flip angle, conventional MB pulses may require a higher peak B1 strength, although some strategies can be used to mitigate this effect. 7,8