Schäper, Jessica. Multi-Parametric brain tissue characterization using magnetic resonance imaging. 2024, Doctoral Thesis, University of Basel, Faculty of Science.
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Abstract
Magnetic resonance imaging (MRI) is one of the most powerful imaging modalities in clinical diagnostics and intervention to date. It employs strong static magnetic fields in combination with weaker, varying magnetic fields and radio frequency (RF) waves to produce an image of the body's interior. The longitudinal $T_1$ and transverse $T_2$ relaxation times, describing the tissue-type-dependent MR signal decay, are of fundamental importance for this. They are the guiding processes for image contrast between tissue types and the reason why MRI contrast is extremely versatile and often superior in comparison to other imaging modalities. However, this versatility also leads to one of MRI's biggest disadvantages, namely the fact that images do not obey an absolute scale like e.g. in computed tomography. MRI images show relative gray values which depend on imaging parameters, field strength and even the imaging device itself. Therefore, the desire to establish a reliable method to obtain quantitative images with an absolute scale is high and it comes as no surprise that the research field around quantitative imaging is one of the biggest in MRI. The main concern is the development of fast and reliable methods to quantitatively measure a variety of absolute tissue parameters. These provide the individual voxel values with a physical meaning which, ideally, is independent of the MR protocol and hardware, thus offering the possibility to directly compare the results from studies across multiple subjects, time-points, and imaging sites. In this thesis, we focus on various aspects of multi-parametric quantitative brain imaging with immediate connection to relaxometry, the study of $T_1$ and $T_2$.
First, features of the balanced steady-state free precession (bSSFP) sequence are investigated. Phase-cycled bSSFP shows great potential in terms of fast, simultaneous quantification of $T_1$ and $T_2$. However, systematic discrepancies between bSSFP quantification and other methods are observed, especially in brain tissue. One of the major suspected causes, the asymmetry of the bSSFP frequency response, is investigated here.
Furthermore, the magnetization-prepared rapid gradient echo (MP-RAGE) sequence, which serves as a powerful ally in quantitative imaging by offering fast $T_1$-weighted anatomical references, is optimized for the new generation of clinical 0.55 T scanners. For this, potential sequence modifications are tested.
Staying at 0.55 T, myelin water imaging is investigated at this field strength in order to assess its advantages at lower field strengths and its viability. Myelin is the protective sheath around the axons of nerve cells. Its loss causes neurodegenerative diseases and fast, reliable imaging could help to understand the disease progression better.
Lastly, the method of bSSFP phase-cycled quantification is applied to $^{23}$Na. In contrast to $^1$H imaging, the field of sodium MRI is concerned even more with the topic of quantitative imaging due to the way in which images usually need to be acquired and the minor knowledge which can be gained by relative sodium contrast. Here, phase-cycled bSSFP could offer a more efficient alternative to the commonly used methods. Its viability is tested at 3 T.
First, features of the balanced steady-state free precession (bSSFP) sequence are investigated. Phase-cycled bSSFP shows great potential in terms of fast, simultaneous quantification of $T_1$ and $T_2$. However, systematic discrepancies between bSSFP quantification and other methods are observed, especially in brain tissue. One of the major suspected causes, the asymmetry of the bSSFP frequency response, is investigated here.
Furthermore, the magnetization-prepared rapid gradient echo (MP-RAGE) sequence, which serves as a powerful ally in quantitative imaging by offering fast $T_1$-weighted anatomical references, is optimized for the new generation of clinical 0.55 T scanners. For this, potential sequence modifications are tested.
Staying at 0.55 T, myelin water imaging is investigated at this field strength in order to assess its advantages at lower field strengths and its viability. Myelin is the protective sheath around the axons of nerve cells. Its loss causes neurodegenerative diseases and fast, reliable imaging could help to understand the disease progression better.
Lastly, the method of bSSFP phase-cycled quantification is applied to $^{23}$Na. In contrast to $^1$H imaging, the field of sodium MRI is concerned even more with the topic of quantitative imaging due to the way in which images usually need to be acquired and the minor knowledge which can be gained by relative sodium contrast. Here, phase-cycled bSSFP could offer a more efficient alternative to the commonly used methods. Its viability is tested at 3 T.
Advisors: | Bieri, Oliver |
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Committee Members: | Bruder, Christoph and Kozerke, Sebastian |
Faculties and Departments: | 03 Faculty of Medicine > Bereich Querschnittsfächer (Klinik) > Radiologie USB > Radiologische Physik (Bieri) 03 Faculty of Medicine > Departement Klinische Forschung > Bereich Querschnittsfächer (Klinik) > Radiologie USB > Radiologische Physik (Bieri) 05 Faculty of Science > Departement Physik > Physik > Theoretische Physik (Bruder) |
UniBasel Contributors: | Bieri, Oliver and Bruder, Christoph |
Item Type: | Thesis |
Thesis Subtype: | Doctoral Thesis |
Thesis no: | 15508 |
Thesis status: | Complete |
Number of Pages: | ix, 104 |
Language: | English |
Identification Number: |
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edoc DOI: | |
Last Modified: | 23 Oct 2024 04:30 |
Deposited On: | 22 Oct 2024 08:10 |
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