Development of upright, multimodal, respiratory MRI

Harkin, James (2020) Development of upright, multimodal, respiratory MRI. PhD thesis, University of Nottingham.

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Respiratory diseases can have a severe impact on people's quality of life and life expectancy. To investigate these diseases, CT and spirometry are the current "gold standards" used by clinicians. In tandem, CT and spirometry, offer high resolution images and lung function information. However, CT exposes the patient to ionising radiation, making multiple investigations undesirable. The results of pulmonary function tests, such as spirometry, are also variable and depend on the effort levels of the patient. In addition, the information provided from most pulmonary function tests is a global measure; abnormal values may only occur when a large region of the lung is affected. These issues mean that, using current measures, it can be difficult to diagnose respiratory diseases and track their progression to see if a particular treatment is being effective.

In tissues other than the lungs, proton MRI can offer highly detailed structural, as well as functional information. However, proton MRI is difficult to perform in the lungs due to their low proton density. In order to maximise the information obtainable from respiratory MRI, other techniques need to be employed . Implementing hyperpolarised xenon-129 MRI, fluorine MRI and proton diaphragm imaging are the major focuses of this thesis.

The other problem with most conventional MRI for respiratory imaging is that it is performed supine. Lung function is reduced when supine, as opposed to seated or standing. This causes two problems. Firstly, patients with severe lung disease may struggle to lie down for extended periods of time; arguably these are the people who it is most useful to scan. Secondly, most people spend the majority of their lives upright, therefore images acquired upright should be of greater relevance for clinicians. At Nottingham, the 0.5T Paramed MROpen Upright scanner is the first of its kind to offer proton and multinuclear imaging. This scanner allows for imaging in a variety of orientations. The open design of the scanner also makes it particularly suited to paediatric imaging. Children as young as three have sat comfortably inside the scanner, on a parent or guardian's lap, without need for sedation.

In this thesis, the optimum conditions for polarising xenon-129 in a batch mode system are investigated. Higher rates of polarisation build-up were observed at higher temperatures. Vastly higher temperatures were also observed inside the cell (particularly in the first few cm after the laser strikes the cell), than the oven during runaway. Higher polarisations of xenon-129 were found in gas mixes with lower concentrations of xenon-129 (leaner mixes). However, the largest bulk magnetisation was found with a balance of xenon-129 and nitrogen. For a given Rb vapour density replacing nitrogen with helium-4 appeared to have little effect on the polarisation build-up rate of xenon-129. The polarisation build-up rate was also seen to be highly variable in a cell with a bead of Rb, using the same external conditions. After "spreading" the Rb bead, by vaporising it and forcing it to condense on the walls of the cell, the build-up rate became more consistent.

Before the Paramed MRI scanner was built, hyperpolarised xenon-129 imaging at Nottingham was performed using a 1.5T GE scanner. The original Rapid birdcage xenon-129 coil, was not fit for purpose; larger volunteers couldn't be imaged. The testing of a new coil from Clinical MR Solutions L.L.C. is detailed; with the aim of investigating if it was fit for use. In QTAR mode, an inconsistent signal was seen throughout the lungs, making it unsuitable. In QUAD T/R mode, initial tests were encouraging.

On the Paramed, a sequence has been developed to image a single slice in <0.5s. This allows for the movement of the diaphragm to be characterised throughout the breathing cycle and to track repeating lung density changes. In addition, a stock Paramed sequence was altered so the duration of the scan was reduced from an average of 35s to 21s; the acquired resolution was maintained at 160x128x7. This meant the sequence could be acquired within a breath hold; allowing the diaphragm morphology at full expiration and inspiration to be characterised. Compressed sensing has also been implemented on the upright. A sequence has been developed, at a resolution of 256x200x22 and undersampling by a factor of 3, to acquire an image in 22.6s. By undersampling a resolution of 256x160x10 by a factor of 4.82 an image can be acquired in 5.1s. With both, the diaphragm can be accurately characterised. The shorter duration sequence is tailored for patients with more severe lung disease, who can't hold their breath for as long. The intention is to apply compressed sensing to hyperpolarised imaging in the future.

Using a small surface xenon-129 test coil on the Paramed scanner, a 3D GRE sequence has been developed, which can resolve detail of <3mm in <20s. Also using xenon-129 a flip angle calibration and dissolved phase spectroscopy have been performed. A calibration for an SAR monitoring system has been produced, to allow for in vivo imaging using the test coil once the system has been built. Proton on a hydrofluorocarbon gas has also been imaged, illustrating it should be possible to image fluorine using the Paramed.

MRI has excellent potential to be used clinically to diagnose and track the progression of respiratory disease. By imaging in an upright orientation, fewer patients will be excluded from being scanned using the techniques developed and the images should be of greater diagnostic use for clinicians. By using techniques such as hyperpolarisation, the lower field strength of the upright scanner than a conventional scanner can be counteracted.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Barlow, Michael
Hall, Ian
Keywords: Xenon-129, Xe-129, Hyperpolarised, MRI, Upright, Paramed, Respiratory, Diaphragm, Lung, Fluorine, Compressed Sensing, Fourier Decomposition, SEOP, Raman
Subjects: W Medicine and related subjects (NLM Classification) > WN Radiology. Diagnostic imaging
Faculties/Schools: UK Campuses > Faculty of Medicine and Health Sciences > School of Medicine
Item ID: 60451
Depositing User: Harkin, James
Date Deposited: 13 Oct 2023 12:10
Last Modified: 13 Oct 2023 12:10

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