Quantitative Susceptibility Mapping in Ischemic Stroke: Cerebral Veins, Tissue, and Microbleeds

• Author / Creator

  Parisa Badihi Najafabadi

• Quantitative susceptibility mapping (QSM) is an emerging magnetic resonance imaging (MRI) post-processing technique that uses phase images from a gradient echo sequence to create magnetic susceptibility maps. QSM is sensitive to paramagnetic iron found in various states of blood, from deoxyhemoglobin to hemosiderin. QSM can also differentiate between calcium and iron deposits. Unlike phase images, QSM is not strongly dependent on object shape nor orientation with respect to the main magnetic field. A 3D flow-compensated gradient echo was used in this work for reconstruction of QSM from phase images. An introduction to MRI, phase images, susceptibility and susceptibility-weighted imaging (SWI) is given in Chapter 1. This chapter will also explain the details of QSM reconstruction and an overview to various applications.Tissue and draining vein oxygenation level in the brain is a prominent parameter in determining the severity of ischemia. Due to its sensitivity to deoxyhemoglobin, QSM provides a quantitative measure of brain tissue and venous oxygenation. QSM can also distinguish between cerebral microbleeds (CMBs) and calcium deposits and provides valuable information of the size and quantity of CMBs. Chapter 2 introduces stroke, with a focus on ischemic stroke and cerebral microbleeds, as the major clinical applications of this research.Recent studies in ischemic stroke using QSM have only examined severe ischemic strokes. Furthermore, no studies to date have examined all three potential applications of QSM to ischemic stroke: cerebral microbleed quantification, and vein and tissue oxygenation. In Chapter 3, the value of QSM in ischemic stroke patients was investigated in two ways. First, by evaluating susceptibility in tissue and draining veins from infarcted and contralateral regions in acute stroke with a wide range of stroke severity; and second, assessing microbleed burden on QSM in comparison to standard SWI in chronic stroke. QSM analysis in acute ischemic stroke revealed significantly greater susceptibility (p< 0.001) for vein measurements in the infarcted hemisphere compared to the contralateral homologous hemisphere. This can be an indicator of higher blood deoxygenation in veins draining from the infarcted hemisphere, suggesting a compromised arterial blood flow and collateral vascular distribution in the ischemic region. Tissue analysis showed a significantly higher susceptibility (p< 0.036) of white matter in infarcted regions compared to healthy regions which can be an indicator of higher deoxygenation in the infarct, while no significant higher susceptibility was observed in grey matter between the two hemispheres. This may be due to higher sensitivity of white matter to ischemia.Microbleed assessment using QSM revealed key advantages of QSM over SWI, such as smaller size of microbleed (p< 0.0001) as well as potentially improved detectability of microbleeds. This provides quantitative measurement of susceptibility rather than the qualitative nature of SWI and removal of blooming signal artifacts originating from strong dipole field effects seen on SWI.In summary, this thesis has demonstrated the value of QSM in ischemic stroke for measuring both oxygenation and microbleeds. Since QSM can be obtained from raw phase images of standard SWI sequences, no extra scan time is needed to enable QSM studies when SWI is already performed.

• Subjects / Keywords

  • MRI
  • Ischemic Stroke
  • Cerbral Microbleeds
  • Quantitative Susceptibility Mapping
  • Susceptibility Weighted Imaging

• Graduation date

  Spring 2019

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/r3-ajkt-rg19

• License

  Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. Where the thesis is converted to, or otherwise made available in digital form, the University of Alberta will advise potential users of the thesis of these terms. The author reserves all other publication and other rights in association with the copyright in the thesis and, except as herein before provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission.