New developments in high speed MR imaging

Abstract

The work presented in this thesis covers two areas of research; firstly, the measurement of myocardial perfusion using spin-labelling techniques and secondly design of gradient coils to limit neural stimulation. Chapter 1 gives an introduction to MRI in general and chapter 2 outlines the hardware used in MRI generally and refers more specifically to hardware used in this thesis. Chapter 3 describes techniques and methodology used in order to measure myocardial perfusion. Techniques aimed at tackling the problems faced with MR perfusion measurement were investigated, including ECG gating to prevent image misregistration and the use of spin-echo EPI, half-Fourier EPI and alteration of switch gradient frequency in order to improve image signal to noise ratio. A modified FAIR (Flow Sensitive Inversion Recovery) sequence was used to obtain an image series of a cross section of the myocardium. Average signal intensities from a selected ROI in the myocardium were obtained and the signal behaviour compared to a theoretical model in order to estimate perfusion. An average perfusion value (n=7) of 84.3#+-#18.4 ml/100g/min was obtained for healthy volunteers. In Chapter 4 methods of producing axial gradient coils which are designed to limit neural stimulation are investigated. This was done by producing coils with B (magnitude of the magnetic field) suppressed in a given region. Techniques examined for their effect on B minimisation were removal of coil symmetry and specification of gradient rather than magnetic fields in the region of desired homogenous gradient. It was found that the latter of these two techniques was the most effective. For coil designs possessing identical regions of gradient homogeneity a reduction in B of 70% in a specified region, was achievable with the gradient specification technique. In chapter 5 transverse gradient coils were designed that limited neural stimulation in a similar fashion to that described in chapter 4. In this case removal of coil symmetry was the most effective approach. In this case for coils of identical regions of gradient homogeneity a reduction of B of 83% is achievable for a specified region. (author)Available from British Library Document Supply Centre- DSC:DXN054724 / BLDSC - British Library Document Supply CentreSIGLEGBUnited Kingdo