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Title: Exploration, Development and Application of Z-Shim and Allied Methods for Signal Recovery in Single-Shot Echo Planar Imaging
Author: Marshall, Helen
ISNI:       0000 0001 3619 6293
Awarding Body: Imperial College London (University of London)
Current Institution: Imperial College London
Date of Award: 2007
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Echo planar imaging (EPI) is widely used for applications where speed and/or sensitivity are essential, such as functional MRI (fMRI) and perfusion MRI. Gradient echo EPI suffers from signal drop-out due to through-slice magnetic field (Bo) inhomogeneity, which prevents certain brain areas from being imaged, e.g. around the frontal sinus and auditory canals. Signal loss can be corrected using the z-shim method, which approximates local 80 variations as linear and recovers signal by applying an opposite refocusing gradient. However, the required gradient is spatially varying and unknown, and. each refocused image requires a separate acquisition. Multiple z-shims are required for full correction making the method inefficient. Signal loss increases with increasing magnetic field strength. The need for an effective correction strategy is driven by the current trend towards high field strengths of 3T and above. This work explores signal loss and recovery in the brain at 3T, and investigates signal loss correction methods culminating in a method which achieves successful signal recovery from two optimally chosen z-shims. The linear approximation made by z-shim methods was found to be valid for the majority of pixels in the brain ,at 3T. Parallel imaging in the through-slice direction to reconstruct points of the k-space slice profile not originally measured was investigated but found to be limited by current hardware. An efficient signal loss correction method was developed which requires only two optimally spaced zshims, positioned by a rapid calibration method for maximum signal recovery. Full signal restoration (to within 2% of the correct value) was achieved in 96% of all brain pixels for 3mm slices, and partial correction in pixels outside this range. This method was applied to a language fMRI study which suffers from signal loss, and recovered activation in regions of 80 inhomogeneity revealing language activation not detectable by conventional fMRI.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available