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Selective and MultiBand FM pulses for z-shim compensation in fMRI

IP.com Disclosure Number: IPCOM000240845D
Publication Date: 2015-Mar-06
Document File: 2 page(s) / 30K

Publishing Venue

The IP.com Prior Art Database

Abstract

2015ID00294

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Title: Selective and MultiBand FM pulses for z-shim compensation in fMRI

The method relates to an magnetic resonance imaging method for fMRI which is sensitive to the BOLD-effect.

According to the method,  the RF excitation is provided with a quadratic phase variation over the imaged slice notably in regions that are susceptible to susceptibility artefacts. For example, slices located near the air-filled sinus-cavity in the patient’s head may have B0 distortions. The extent of quadratic phase variation applied is determined based on a B0 map.

The method enables to reduce the artefact level due to susceptibility variations.

fMRI sequences are sensitive to through-plane susceptibility gradients, causing signal loss at the long echo times required for BOLD signal sensitization. Quadratic RF pulses can mitigate signal loss. Advanced slice-by-slice optimized RF pulse design is proposed to maximize SNR over the sequence. Extensions to multiband are also proposed.    

ID: Selective and MultiBand FM pulses for z-shim compensation in fMRI

Problem description

fMRI sequences are sensitive to through-plane susceptibility gradients, causing signal loss at the long echo times required for BOLD signal sensitization. Reduction of such artifacts is pursued using local external shims, e.g. a piece of metal in the subject’s mouth, which is quite uncomfortable. Another approach is the so-called z-shim method, where multiple images are acquired with different degrees of signal loss, which are combined as maximum intensity projections prior to fMRI statistical processing. The latter method reduces the temporal resolution of the experiment, and thereby reduces statistical power.

An alternative method to recover susceptibility-induced signal loss is to impose a quadratic phase on the slice by applying an FM pulse. The effect of such pulses is that on-resonance the flip angle is reduced by 60-70%, but this flip angle is maintained over...