Implementation of 23Na MRI

Sodium magnetic resonance imaging (23Na MRI) has the potential to provide insight into cellular, tissue and organ health. Proton (1H) MRI is well established, but 23Na MRI faces several challenges, principally due to the low inherent 23Na signal from the body and the need for specialist hardware. The aim of this thesis was to lay the groundwork for carrying out 23Na MRI in humans on a clinical MRI scanner, to determine quantitative 23Na measures in the kidneys, muscle and skin. Firstly, work was carried out to install the 23Na RF coils to allow for the study of the 23Na distribution in the leg, muscle and skin, and abdomen. This required modelling of the specific absorption rate (SAR), phantom construction, temperature testing, coupling checks, power calibration and measurement of the coil's transmit/receive switching time. Secondly, the implementation and assessment of scanning techniques and calibration methods for the measurement of tissue sodium concentration (TSC) in the calf is described. This work includes 3D gradient recalled echo (GRE) imaging and sequence optimisation and the development of improved B1 mapping for correction of sodium images to more accurately compute TSC. Comparisons of measures using 3D GRE and ultrashort echo time (UTE) schemes are made, and measurement of the bi-exponential 23Na transverse relaxation time performed. An optimised 1 hour scan protocol for future studies is outlined. Thirdly, 23Na imaging of the abdomen, specifically of the kidney, is outlined. Results of initial scans performed in vivo using the dual loops coil to assess TSC in the kidney are shown. The limitations of such methods in terms of image homogeneity are described. This is followed by the assessment of a 6-channel body coil for improved homogeneity of sodium measures, and the evaluation of receive sensitivity correction. Finally, work on proton (1H) imaging using a skin coil to assess changes in skin hydration, lays the ground work for future studies combining 1H measurement of skin water content with 23Na imaging of the skin