The aim of this work was to implement a thorough method for quantifying the errors introduced to frame-based neurosurgical stereotactic procedures by the use of MRI. Chang & Fitzpatrick's reversed gradient distortion correction method was used, in combination with a phantom, to measure these errors. Spatial distortion in MR images of between 1 mm and 2 mm was measured. Further analysis showed that this typically introduced an additional error in the coordinate of the actual treatment point of 0.7 mm. The implications of this are discussed. The main source of distortion in the MR images used for stereotaxis was found to be the head ring. A comparison between imaging sequences and MR scanners revealed that the spatial distortion depends mainly on the bandwidth per pixel of the sequence rather than other differences in the imaging sequences. By comparison with a phase map distortion correction technique, the imaging parameters required to allow successful distortion correction with the reversed gradient method were identified. The most important was the use of full Fourier spin echo acquisitions. The reversed gradient correction method was applied to two contemporary EPI techniques. Considerable improvement was seen in the production of ADC maps after the images had been corrected for distortion. The method also was shown to be valid in application to BOLD fMRI data.

Interest in coherent remote sensing systems has stimulated investigations in the properties laser propagation through extended atmospheric turbulence. This thesis investigates the statistics of phase, and phase related, observables using analytical and computational techniques, together with experimental results. The phase screen technique is used to simulate perturbations to the refractive index of a medium through which the radiation propagates. Several different turbulence models (Gaussian correlated noise, Kolmogorov turbulence, Tatarski and Von Karman spectral models) are investigated, and their relative merits for describing experimental conditions and descriptive statistical measures are compared and contrasted. The phase power spectrum is crucial to an understanding of the practical operation of a coherent imaging system, and later part of the thesis is devoted to the investigation of a LIDAR system in particular. Several turbulence regimes are investigated, from an analytical treatment of a weakly turbulent, extended atmosphere, to large 3D computations designed to simulate experimental arrangements. The 3D simulation technique presented herein has been developed to allow for the investigation of temporal statistics. New power law behaviours are found to appear in temporal frequency spectra which differ from the -8/3 power law form that has been accepted in much of the literature. Strongly turbulent regimes result in a -2 power law while the use of a Gaussian beam profile in an extended medium gives a -11/3 power law under weak turbulence conditions. Please note: Pagination in electronic reproduction differs from print original. The print version is the version of record.

This thesis addresses the problem of radiofrequency probe design for Ultra High Frequency Magnetic Resonance Imaging (7T). The signal-to-noise ratio available in Magnetic Resonance Imaging (MRI) is determined by the static magnetic field strength, causing a continued drive toward higher fields to enable faster image acquisition at finer spatial resolution. The resonant frequency increases linearly with static field strength. At 7T the proton resonant frequency is 300MHz, with a wavelength of approximately 13cm in tissue. As this is smaller than the dimensions of the human head, the phase of the radiofrequency (RF) signal varies considerably across the sample, producing field cancellation due to interference. A full wave electromagnetic simulator, using the Transmission Line Matrix (TLM) method, was developed to investigate RF probes at high frequency. A Birdcage probe operating at 64, 128 and 300MHz (corresponding to 1.5, 3 and 7T) was simulated, loaded with an anatomically detailed human head model. A half-wave microstrip was investigated for use as a high frequency probe element. Magnetic and electric fields produced by a single microstrip were simulated, and the strip dimensions varied to investigate the effect on field penetration into the head and Specific Absorption Rate (SAR). A transmit-receive array probe using four microstrip elements was then developed. Bloch simulations were run, using TLM generated magnetic fields, to investigate imaging at short wavelength. Parallel receive probes are demonstrated to offer considerable advantage over volume probes, as signals from receive elements can be combined without interference. There is no transmit equivalent to parallel reception; simultaneous excitation of independent probe elements causes interference in exactly the same manner as a volume probe. A new imaging sequence was developed using a Burst-like encoding to allow sequential excitation of probe elements, without interference, which can be recalled in a single readout. An improvement in image homogeneity was demonstrated, and SENSE acceleration of the new imaging sequence is shown. The sequence was implemented at 3T using a purpose built four element microstrip probe. An RF multiplexer was also built to enable transmit element switching during the imaging sequence. It was demonstrated that images due to different RF excitations, acquired in a single EPI readout, can be separated.

This thesis describes the development and application of quantitative echo planar magnetic resonance imaging techniques to the study of human placental development in normal and compromised pregnancies. Initially, a method of rapidly and accurately measuring the transverse relaxation time is proposed using a multi-echo measurement sequence. The method is described, validated on CUS04 phantoms and applied in the study of the human placenta and gastric dilution. It is shown that the inversion provided by sinc pulsesis insufficient to generate an accurate measurement but using adiabatic refocusing pulses yields a measurement that is comparable with a single spin echo. Subsequently, a rapid magnetisation transfer method is presented that allows the quantification of the relative size of the bound proton pool. An experimental pulse sequence is proposed, along with a theoretical model, that permits the investigation of the bound proton pool's transition towards the steady state. The sequence and model are validated using agar gel phantoms and shown to agree well with literature values. When applied in the study of the human placenta, it is shown that there is no significant variation in the fitted value of the bound proton pool size with increasing gestational age or in compromised pregnancies. Finally, several methods of measuring the oxygenation level of blood within the human placenta are investigated. The signal intensities of cardiac gated T~• and T~ weighted images acquired at various points in the maternal cardiac cycle are explored but no significant variation is shown through the cycle. A pulsed gradient spin echo sequence that utilises anti-symmetric sensitising gradients is validated and then applied in the human placenta. Oxygenation measurements with this technique are shown to be unfeasible but the potential of the sequence to monitor blood flow in the placenta is demonstrated.

This thesis describes an experimental study of the electronic properties of semiconductor heterostructure tunnel devices. InAs self-assembled quantum dots (QDs) are incorporated into the barrier layer of a GaAs/AlAs/GaAs tunnel diode. When a voltage, V, is applied across the device, we observe resonant features in the tunnel current, I, whenever an electron state in one of the qds comes into resonance with an occupied electron state in the emitter. We employ an electron state of a single qd as a spectroscopic probe of a two-dimensional electron system (2DES), from the Fermi energy to the subband edge [1]. For magnetic field B applied parallel to the current, we observe peaks in the I(V) characteristics corresponding to the formation of Landau levels in the 2DES. We obtain quantitative information about the energy dependence of the quasiparticle lifetime, Tqp, of the 2DES. We find that Tqp ~ 2.5 hbar=(Ef - E), in contrast with the expectation for a normal Fermi liquid, but in agreement with predictions for a Fermi liquid state of a disordered 2DES. Close to filling factor nu = 1 we observe directly the exchange enhancement of the g factor. This thesis also describes the design, realisation and measurement of a tunnel diode incorporating InAs QDs and a series of 4 planar electrostatic gates. By applying a bias to the gates, it is possible to selectively inject current into a particular QD. We use magneto-tunnelling spectroscopy to determine the energy levels of the ground and excited state of a single QD, and to map the spatial form of the wave functions of these states [2]. The effect of pressure on the resonant tunnelling of the QDs is also described. [1] P. C. Main et al., Phys. Rev. Lett. 84, 729 (2000) [2] R. J. A. Hill et al., Appl. Phys. Lett. 79, 3275 (2001)

This thesis describes an experimental investigation and analysis of two topical problems in condensed matter physics: 1.) the effect of a magnetic field on quantum states of an electron bound to a shallow donor impurity in a quantum well heterostructure and 2.) the breakdown of the quasi-dissipationless state of the integer quantum Hall effect. Two introductory chapters describe important material parameters and the experimental equipment and techniques used. Magneto-tunneling spectroscopy (MTS) is used to probe the spatial form of the eigenfunction of electrons bound in the ground state of a shallow Si-donor impurities in a GaAs/(A1Ga)As quantum well. An in-plane magnetic field, B[subscript] |, acts to tune the k-vector of the tunnelling electron through the effect of the Lorentz force. The variation with B [subscript] | of the tunnel current through the donor ground state provides a map of the Fourier transform, |ψ(k)| [superscript]2, of the probability density of the ground state donor wavefunction in real space. By applying a strong magnetic field component, B [subscript] ||, parallel to the direction of tunnel current, it is possible to magneto-compress the donor function in real space. The magneto-compression is investigated using MTS and the data are analysed in terms of a simple model, which is critically discussed. The breakdown of the integer quantum Hall effect is investigated by measuring the variation of the voltage drop Vxx along the direction of current flow for a range of currents and magnetic fields and for a number of sample geometries including Hall bars with narrow channels. The data are discussed in terms of two complementary models of breakdown: the bootstrap electron heating model and magneto-exciton formation at a charged impurity. Evidence is found for both types of breakdown depending on the type of sample used and on experimental parameters. For samples with constrictions, it is found that in the breakdown region the value of Vxx measured across a pair of contacts on one side of the Hall bar can differ substantially from that measured on the other side. A model based on magneto-exciton formation at impurities is proposed to explain this unexpected effect. The thesis concludes with a brief summary and suggestions for future work.

BURST is a fast single-shot imaging technique used in magnetic resonance imaging. Most previous implementations of BURST on whole body systems have been carried out at fields of 1.5T and lower. In this work BURST has been implemented on a 3T whole body system. The signal and attenuation characteristics are discussed, leading to an approximate expression for the optimum echo time. A novel method for controlling the pulse amplitude envelope is described. It is shown that this can lead to a modest gain in signal with little loss of resolution, or to a much greater gain in signal with more severe blurring of the image. Frequency modulated RF pulses have been introduced in order to reduce the peak RF power required, which was found to be beyond the range of a 2kW amplifier. The resulting images show a quadratic phase roll in the readout direction, with no reduction in magnitude or image artefacts. A reduction in peak power of more than ten-fold is demonstrated. The maximum reduction factor possible is shown to be approximately equal to the number of pulses applied in the RF train. The total RF power has been reduced by superimposing a sinusoidal oscillation onto the constant excitation gradient. Each pulse is applied at a gradient minimum, and hence need be of lower bandwidth, while the overall excitation is unchanged. The RF pulses are transformed using VERSE. A reduction in SAR of 32% is demonstrated. Greater reductions can be achieved by increasing the amplitude of the oscillation. However, this technique introduces a moderate amount of acoustic noise into the sequence.

Vertical Bell Lab Layered Space-Time (VBLAST) is an emerging spatial modulation technique, that provides high spectral efficiency due to its use of multiple transmit and receiver antennas. We present novel schemes; consisting of combinations of Transmit Antenna Selection (TAS) with Power Allocation (PA), to improve the error rate performance of VBLAST systems. Our schemes provide significant performance gain over VLAST because of enhanced transmit diversity introduced by transmit antenna selection. Our scheme may be used for uplink wireless system, where the total power is limited by the mobile terminal battery. The effect of limited rate feedback link is investigated by sending quantized power weights from receiver to transmitter. The simulation results show that even one-bit, codebook for power weights can achieve performance close to that of unlimited rate feedback link in low and medium SNR regions. We also present a novel PA technique alloting power in the joint space-frequency domain to reduce the error rate, and apply our scheme to frequency selective MIMO-OFDM channels. In addition, an analytical analysis of transmit correlated Successive interference Cancelling Zero Forcing (SIC-ZF) VBLAST in flat fading channels is presented. Over frequency selective channels, the performance of SIC-ZF VBLAST is assessed by computer simulation.

In this thesis, the potential of MRI phase to reveal the magnetic properties of tissue is thoroughly investigated. Extending beyond the well established field of quantitative susceptibility mapping (QSM), consideration is given to the influence of susceptibility anisotropy, chemical exchange, non-spherical susceptibility inclusions and multi-compartmental signal evolution from both a theoretical and experimental viewpoint. The first experimental chapter discusses a collaborative project with Cardiff University Brain Research Imaging Centre (CUBRIC) and the Department of Neurobiology in Tel-Aviv University to investigate neuroplasticity changes in rats. In this chapter the contribution from the SPMIC is described, detailing an analysis pipeline to generate and optimise isotropic susceptibility and R2∗ maps from multi-echo gradient echo (GE) in-vivo rat datasets obtained on a 7T Bruker Biospec 70/30 MR system. The second experimental chapter investigates the potential of frequency difference mapping (FDM), a recently developed phase processing technique which characterises multi-compartmental signal evolution to probe microstructure content. In this chapter, a novel FDM processing algorithm is introduced which does not require the use of sophisticated phase unwrapping and image filtering. To test the effectiveness of the FDM algorithm on experimental data, ten healthy volunteers underwent a single-slice, sagittal, multi-echo GE scan on a Philips Achieva 7T MR system. Results revealed consistent contrast over the corpus callosum relative to the surrounding tissue and a high sensitivity to small changes in microstructure content. Though fitting experimental magnitude and frequency difference evolution to a three-pool white matter model, the signal evolution over the corpus callosum is described in terms of physical properties of the nerve fibres. The final experimental chapter investigates the magnetic properties of skeletal muscle. A skeletal muscle sample (in the form of a section of pork tenderloin) was placed within an agar phantom and scanned via a multi-orientation, dual-echo GE protocol on a Philips Achieva 7T MR system. Measured phase data were post-processed and novel least-squares minimisation algorithms were developed to investigate the isotropic susceptibility, anisotropic susceptibility, chemical exchange and cylindrical microstructure inclusion properties of the sample. Analysis revealed that the skeletal muscle sample was significantly more diamagnetic than agar, χI = (−121 ± 22)ppb, and a large chemical exchange contribution was also observed within the tissue, E = (31 ± 11)ppb, relative to the surrounding agar. The experimental protocol was not sensitive enough to reveal quantitative information about the anisotropic susceptibility properties of the sample, yielding an an upper bound of 10 ppb. No evidence of cylindrical susceptibility inclusions was observed within the experimental field perturbation maps.

Ultra-High field (7T) MRI provides high sensitivity which allows for new qualitative and quantitative methodologies to be developed, that provide clinically useful information. The work presented in this thesis is focussed on developing a quick and reliable quantitative MT and CEST methodology, taking account of the difficulties encountered at high field. The method developed here has been tested on various studies, in both healthy and diseased brain, in an effort to aid the understanding of myelination in the human brain. The work in this thesis uses the quantitative measure of MT as a marker for myelination, and it shows strong correlations between MT-based myelination and functional connectivity, as well as very strong correlation between MT and NOE. These findings showcase the potential of NOE as a myelin marker as well, as long as the MT vs. NOE relationship remains the same in pathology. Myelination is investigated (via MT and NOE) in Multiple Sclerosis (MS) and Glioma, showing a strong coupling between the two exists even in pathology. Amide Proton Transfer (APT) is also investigated in Glioma, showing similar trends to MT and NOE. High resolution anatomical images can provide valuable information on the extend of the pathology, but quantitative information of the NMR properties of tissue (like MT, NOE and APT) has the potential to detect earlier abnormalities, and give a quantitative measure of healing or degeneration caused by pathology.