Abstract
Chemical Exchange Saturation Transfer (CEST) MRI is a non-invasive imaging method that can assess many molecules without the need of additional metallic and radioactive labelling, such as neurofluid dynamics, lipids, proteins, and various exchangeable protons of endogenous molecules, e.g. hydroxyl protons of glucose, guanidine protons of creatine. Neurofluids, including cerebrospinal fluid (CSF) and interstitial fluid (ISF), play a vital role in maintaining brain homeostasis, while neuropathologies often indicate disruptions in brain function. Understanding them is essential for studying brain health and disease progression. Other imaging tools, such as positron emission tomography (PET) and diffusion MRI, provide valuable may require ionizing radiotracers, and lack the ability to directly detect subtle molecular-level changes. CEST MRI can bridge this gap by offering non-invasive, radiation-free, and molecular-sensitive imaging. Over the past two decades, CEST MRI has been increasingly applied in neuroscience and neurodegeneration research to map molecular changes in vivo, enabling early detection and monitoring of diseases such as Alzheimer’s disease (AD), multiple sclerosis (MS), and brain tumors. By probing exchangeable protons in endogenous molecules, CEST MRI has been used to study brain metabolism, myelin integrity, and protein aggregation, offering complementary information to conventional MRI. Dynamic Glucose-Enhanced (DGE) MRI and Magnetization transfer Indirect Spin Labelling (MISL) MRI are advanced imaging tools that assess neurofluid dynamics, providing insights into CSF influx, clearance, and water exchange between CSF and brain tissue. Additionally, CEST MRI includes specific contrasts such as the rNOE signal, which evaluates lipid contents. Amide Proton Transfer (APT) CEST targets amide protons to detect protein changes. The CEST@2ppm contrast is sensitive to creatine levels. These features enable CEST MRI to assess cerebral abnormalities associated with aging and neurodegenerative diseases, offering insights into their underlying mechanisms. In this thesis, we include the studies on (1) dynamic neurofluid assessment using DGE and MISL MRI at 3T, (2) CEST MRI to detect myelin changes in cuprizone-induced demyelination model, and (3) molecular changes of amide, creatine and lipids assessed by CEST in Alzheimer’s disease. These findings contribute to the promotion of non-invasive imaging approaches for assessing glymphatic function and the application of CEST in neurodegenerative diseases.Firstly, we examine CSF dynamics and glucose uptake and utilization in the rodent brain using DGE MRI, providing insights into glymphatic function related to gender, ageing and neurodegeneration. The cerebrospinal fluid (CSF) glucose clearance rate, which partially reflects CSF drainage and glymphatic function. We observed the clearance rate is faster in young female than male (female: 19.66±8.640 vs male: 11.08±5.997, p = 0.0417), slower in aged mice than young cohorts (2 months: 0.22±0.03 vs 13 months: 0.02±0.01, **p = 0.0065), and showed a declining trend in AD mice than WT. To further confirm the feasibility of DGE in assessing CSF dynamics, we include interventions, including IL-33 that could activate microglia phagocytic function and dobutamine that could increase the heart rate, which could potentially affect the CSF dynamics. We observed that upon IL-33 treatment in AD mice, CSF clearance rate significantly increase (pre: 0.84±0.45 vs day 1: 3.33±0.16, p<0.05), while the CSF clearance rate do not change much upon dobutamine treatment. These findings highlight the potential of DGE MRI in assessing CSF dynamics in mouse brain, offering valuable insights that could aid in the early AD diagnosis. Additionally, MISL MRI provides an approach by assessing neurofluid dynamics through water exchange between CSF and brain tissue. To enhance its applicability in rodents at 3T, we optimized parameters in both phantoms and in vivo experiments, and we investigated the impact of respiration on MISL and MT signals. By leveraging these advanced imaging tools, we aim to deepen our understanding of neurofluid dynamics and their implication to brain health.
Second, we detect the myelin changes in a cuprizone-induced demyelination mouse model using CEST MRI. Amide and rNOE CEST have been reported in detecting proteins and lipids, which are major components of myelin. These features make CEST MRI a versatile imaging tool for detecting myelin-related pathologies and assisting in neurodegenerative diseases diagnosis such as MS. We observed significantly decreased rNOE (control: 4.85% ± 0.09%/s vs. cuprizone: 3.88% ± 0.18%/s, p = 0.007) and amide pool (control: 3.20% ± 0.10%/s vs. cuprizone: 2.46% ± 0.16%/s, p = 0.02) in the myelin-enrich region, corpus callosum, after 8 weeks on cuprizone diet (p < 0.05). Additionally, the rNOE in the cuprizone group recovered to a level comparable to the control group at week 14 (p = 0.39), while amide remained at a level as low as that for the control group (p = 0.051). These significant changes in rNOE signal were validated by immunohistochemistry results for demyelination and remyelination, highlighting the potential of rNOE CEST for uncovering myelin pathology and its implications for the identification of MS at a clinical field strength of 3T.
Third, we investigated molecular changes in young AD mice using CEST MRI to explore the potential early alterations associated with AD. As a progressive disorder, AD requires early intervention and prevention, yet the early biomarkers for AD remain unclear. CEST MRI offers a non-invasive approach to detect subtle molecular changes, including variations in proteins, creatine and lipids, enabling the observation of early molecular dysfunction associated with AD. Interestingly, we consistently observed a significantly lower CEST@2ppm signal in AD mice (WT: 14.64±0.23 vs AD: 13.81±0.24, p = 0.026) at age of 4 months. This could be related to either a change in pH or a reduction in molecules with guanidinium protons, such as creatine. Meanwhile, no significant differences were observed in APT and rNOE signal referring to proteins and lipids. These results demonstrate that CEST MRI is a promising tool for investigating molecular changes at early stages of neurodegenerative diseases such as AD.
In summary, the utilization of DGE and MISL techniques enables the assessment of fluid dynamic and water exchange in brain, which can provide insights into the CSF drainage system and glymphatic function. Moreover, by employing advanced CEST methods, we can non-invasively detect the molecular alterations, such as lipid changes that may reflect the demyelination pathology associated with MS, and creatine alterations that could contribute in AD. These CEST methods showed potential for the assessment and monitoring of neurodegenerative diseases, allowing for improved understanding of their underlying mechanisms and potential therapeutic targets.
| Date of Award | 5 Sept 2025 |
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| Original language | English |
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| Supervisor | Wai Yan Kannie CHAN (Supervisor) |