Spicer, Abi
(2025)
Magnetic Resonance Imaging and Spectroscopy Methods for Studying Obesity: Applications for Bariatric Surgery.
PhD thesis, University of Nottingham.
Abstract
The prevalence of obesity in the population is increasing globally, leading to a rise in metabolic disorders such as Type-II Diabetes (T2D). Magnetic Resonance Imaging (MRI) and Spectroscopy (MRS) provide safe non-invasive techniques for investigating the physiology and metabolism of the human body. Whilst multinuclear MRS allows metabolic pathways that cannot be measured using 1H methods to be probed, in this thesis 13C and 31P MRS are used to study liver metabolism.
This thesis first provides an outline of the necessary biology and NMR theory to introduce the four experimental chapters. The first experimental chapter, Chapter 3 describes a clinical study using multiparametric 1H MRI and MRS to investigate the changes that occur in T2D and pre-diabetic patients across the bariatric surgery journey. Participants are scanned at 4 timepoints: baseline, following a Very-Low Calorie Diet (VLCD) that precedes surgery, and 6 weeks and 24 weeks post-surgery. MRI findings of the changes in the volume of Subcutaneous Adipose Tissue (SAT) together with volume, fat fraction and MR relaxometry of the liver, spleen, and pancreas measures, are discussed.
The field of multinuclear MRS is ever-growing. However, there are many challenges to applying this research technique to clinical groups with further challenges in obese individuals. Chapter 4 describes the optimisation of RF excitation pulses for abdominal multinuclear MRS using a single-loop surface coil. Block and Half-Passage Hyperbolic Secant pulses for non-localised 13C and 1D ISIS 31P MRS acquisitions are explored including B1 field simulations. Simulations using 13C were compared to experimental data taken in a phantom. 31P MRS measures are then tested in vivo to study the change in Signal-to-Noise Ratio (SNR) with distance from the surface coil. Methods are then used to inform 31P MRS saturation transfer experiments investigating changes to ATP flux in the liver in two intervention studies, the first on the effect of consuming Inulin Propionate Ester (IPE), a dietary fibre, and the second studying changes following bariatric surgery.
A high Number of Signal Averages are needed for natural abundance multinuclear MRS due to low SNR, resulting in spectra typically acquired whilst free breathing. Chapter 5 explores the effects of respiration on liver 1H spectra as an analogue for understanding the effect of respiration on multinuclear MRS. The shifts in the 1H MRS liver water peak central frequency, Full-Width Half Maximum and amplitude are evaluated and compared to shifts seen in imaging-based dynamic and breathheld liver B0 maps. Implications of the results for multinuclear MRS are discussed.
Ensuring the reproducibility of any data is key to its scientific value. In the final experimental chapter, Chapter 6, the reproducibility of 13C liver MRS after a tailored meal and overnight fast is assessed. Measures of inter subject and intra-subject reproducibility are assessed, as well as intra-rater and inter-rater reproducibility, for two raters and an automatic analysis pipeline.
Finally, a summary of key findings and proposals for future work are provided.
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