The feasibility of functional MRI of the brain at low field.

The primary aim of the work described in this thesis was to assess the feasibility of performing functional magnetic resonance imaging (fMRI) of the brain at low magnetic field strengths, that is, at 1.5T or below. A thorough review of the current theories, techniques and applications of functional MRI of the brain is presented. fMRI can potentially provide a noninvasive technique which combines imaging of brain function and anatomy in a single modality, with high spatial resolution and intermediate temporal resolution. It is hoped that fMRI will increase understanding of both normal and abnormal brain function, and aid diagnosis and monitoring of brain pathology. The technical requirements for the performance of fMRI were analysed, and the feasibility of performing fMRI at low field strengths was assessed. It is most desirable to observe functional signal changes which are closely localised to regions of activated brain tissue. To observe such changes at low fields requires signal-to-noise ~100-200, system stability during imaging better than ~0.3%, and good magnetic field homogeneity. The inhomogeneous magnetic field of the 0.15T resistive magnet, which formed part of the original University of Surrey whole-body MRI system, was measured and analysed in terms of its spherical harmonic components. The dominant tesseral components of the inhomogeneity in the central imaging plane were reduced by the strategic placement of ferromagnetic rods within the magnet bore. The homogeneity in this plane was improved by a factor of seven. The stability of the 0.5T superconducting whole-body magnet (which superseded the 0.15T magnet) was investigated. It was concluded that fMRI is, in principle, feasible on low field MRI systems. This should enable the implementation of fMRI on most research and clinical MRI systems, leading to a much wider availability at substantially lower cost, and thus assist in furthering understanding of brain function