[BIC-announce] [Ph.D. Oral Defense] Avery Berman: "Advancing Calibrated Functional MRI Through Biophysical Signal Modelling" (Thursday, August 3, 2017; 14h15; Duff Medical Bldg., room 507/509)

[Zografos Caramanos, Mr]

Date: Thursday, August 3, 2017

Time: 14h15

Location: Duff Medical Bldg., room 507/509
3775 University Street

Thesis Title: Advancing Calibrated Functional MRI Through Biophysical Signal Modelling


Abstract:
This thesis presents novel theoretical modelling and experimental studies of the biophysics of blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) with the central aim of advancing calibrated fMRI. Calibrated fMRI is an imaging technique that measures the changing hemodynamic and metabolic factors that contribute to the BOLD signal, and is reliant on a preliminary calibration procedure that uses hypercapnic or hyperoxic gas challenges. However, the need for specialized gas delivery and monitoring equipment and associated biophysical confounds of the gas challenges have hampered the widespread adoption of calibrated fMRI. One such confound is the magnetic susceptibility of dissolved oxygen, which, like deoxyhemoglobin, is paramagnetic. A theoretical model for calculating the susceptibility of dissolved oxygen in blood was derived and experimentally validated in ex vivo plasma samples, showing excellent agreement between theory and measurement. These findings indicate that the susceptibility of dissolved oxygen has a negligible contribution to the overall susceptibility of blood and are consistent with deoxyhemoglobin being the predominant source of contrast during hyperoxic BOLD studies.



Intravascular signal is known to significantly contribute to the BOLD signal, however, it is difficult to incorporate into BOLD signal simulations due to the vast number of red blood cells in vessels. To address this, a model to describe intravascular signal evolution during free induction decay, a spin echo sequence, or a multi-echo spin echo sequence was derived using a validated analytical model of diffusion-induced decay in weak field inhomogeneities. The derived model was in excellent agreement with simulations under a range of conditions including field offset strength, inhomogeneity extent, and pulse sequence. With its ability to accurately predict the full dephasing and refocusing time course of blood, this model could be applied to better understand intravascular BOLD effects, including during gas-free calibration, and more general blood relaxation properties.

Finally, a gas challenge-free alternative to fMRI calibration was investigated. This was based on measuring the reversible component of signal decay resulting from the field inhomogeneities surrounding deoxygenated blood vessels. Simulations showed that diffusion in the extravascular space resulted in an underestimation of the calibration constant of approximately 15–40%, depending on the underlying vessel-size distribution. A method for characterizing and correcting this underestimation was proposed and validated in silico and in vivo. This work could greatly simplify calibrated fMRI by removing the need for a gas challenge.


Kind Regards,

Daniel Caron<mailto:info.bbme at mcgill.ca>
Student Affairs Coordinator | Biological & Biomedical Engineering <http://www.mcgill.ca/bbme/> | McGill University |
514-398-6736<tel:(514)%20398-6736> | www.mcgill.ca/bbme<http://www.mcgill.ca/bbme> |
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