Functional Magnetic Resonance Imaging (fmri) Modelling: Observing the brain in action

Transcription

1 Functional Magnetic Resonance Imaging (fmri) Modelling: Observing the brain in action Peter Drysdale Complex Systems, School of Physics, University of Sydney Brain Dynamics Center, Westmead Millennium Institute, Westmead Hospital and Western Clinical School of the University of Sydney

2 How Physics led to Brain Dynamics Honours in theoretical wave physics in plasmas. Ph.D. in theoretical wave physics including statistical phase changes and random diffusion. Prof. Peter Robinson walked into my office and asked me to do some contour integrals for fmri analysis. Leads to research in fmri and then Postdoc in Brain Dynamics. Where have other people come from? Over 20 people researching Brain Dynamics in Physics. They have come from a variety of areas including: Physics, Applied and Pure Mathematics, Psychology, Psychiatry, Computer Science, Medicine, Bioinformatics, Engineering.

3 What is Magnetic Resonance Imaging MRI is the medical name for Nuclear Magnetic Resonance (NMR) imaging (from Hoiting 2005) 1.5T fmri machine of collaborators at FIL, London (N.B. Earth s B field is 30-60µT)

4 Structural MRI and Functional MRI Structural MRI Image:- Jezzard et al. (2001) Functional MRI superimposed on model brain Image:- Williams, Brain Dynamics Centre.

5 What is the Blood Oxygen Level Dependent signal? Indirect measure of neuronal activity: Neuronal activity depletes metabolic reserves. Localised increase in blood flow to active region mediated by astrocytes. Localised change in blood volume and oxygenation of blood. Deoxygenated hemoglobin (dehb) is paramagnetic as opposed to diamagnetic which changes magnetic environment of H nuclei. Change in dehb volume and concentration alters detected MRI signal. Who uses it? Researchers who want to observe brain s functioning! Several thousand neuroscientists around the world. Huge diversity of worldwide research applications including:- visual perception, Alzheimer's, schizophrenia, emotional processing, language processing, motor function, audio and music perception, cognitive origins of decision making.

6 Why do we need to model? BOLD Signal is 2-4% over baseline MRI signal. Noise fluctuations are typically 1-2% of baseline MRI signal. The indirect process of BOLD generation is not well understood.

7 Linearized Frequency Response Linearizing the hemodynamic model yields the transfer function. BOLD response is weakly resonant low-pass filter Peak response at ~ 0.07 Hz

8 Optimized stimulation patterns by exploiting resonance Optimal stimulus pattern which is superior to stimulating every 12-14s can be derived. The optimal stimulus pattern stimulates repeatedly for ~7s and then rests for ~7s. This is analogous to pushing a swing.

9 Examples of nonlinear model response

10 Balloon Model Models blood vessel as extensible balloon which can temporarily store blood. Models each region as containing a single large blood vessel. Models are voxel based where voxels span entire cortical thickness (voxels are instrument dependent volume pixels). Temporal only no spatial terms. Does not conserve blood between adjacent cortical points. Balloon Models are still being refined. Friston et al. (2003), Obata et al. (2004) blood flow brain matrix Balloon model of cerebral vasculature. Vessel is a single inflatable balloon with voxel the full thickness of cortex. Cortical thickness

11 A new approach a porous elastic model Poroelastic models describe a fluid-saturated porous elastic medium and originated in geophysics. Advantages: imposes continuity of fluid flow. allows instrument independent (non-voxel) description of hemodynamics. models vasculature as pores in the cortical matter rather than as a single vessel in each voxel. Image: Ziokovic & Apuzzo (1998) A porous elastic model allows voxels of smaller scale than capillary length, facilitating study of intervoxel dependence and blood flow in more than one direction.

12 Structure of porous elastic model Blood mass conservation Neural activity z Neuronal signal equation BOLD response y Momentum conservation Hemoglobin conservation The structure of the porous elastic model is built from components using basic physical principles and physiological observations.

13 Brain Dynamics is more than BOLD fmri My research also includes: Development of computational models of EEG and ERP in the brain. Understanding the transitions between wake and sleep in a corticothalamic model of the brain. Other people in the Brain Dynamics group research: Development of analytic models of EEG and ERP in brain, Visual rivalry, Transition to epileptic states in the brain, Effect of neurotransmitters on EEG and ERP, Network and stability characteristics of the brain, Visual Hallucinations, Sleep/Wake switch.