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Transcript
What are we measuring in fMRI? Marieke Schölvinck Overview Physics of BOLD signal Magnetic fields and pulses Magnetic properties of oxygen in blood How we use this to generate a signal Physiology of BOLD signal How neurons cause blood flow increases Implications for cognitive studies Correlation with other measures of neural activity physics It all starts with hydrogen… magnetic dipole moment (MDM) MDMs align with / against uniform magnetic field Bo small % more align with Bo net magnetization physics So what goes on in the scanner? Place hydrogen nuclei (brain) in a uniform magnetic field (scanner) Apply radiofrequency (RF) pulse, normally at 90° to magnetic field. This ‘tips’ MDMs of hydrogen nuclei MDMs of hydrogen nuclei are now at 90° to main field (z), i.e. in x,y plane Terminate RF pulse and let nuclei relax: MDMs return to original (z) orientation; energy released during relaxation is measured by receiver coil physics Phase BEFORE the RF pulse: MDMs are not in phase DURING the RF pulse: MDMs in phase strong signal in x,y plane AFTER the RF pulse: MDMs begin to dephase signal decays Two reasons for this dephasing: inhomogeneities in the magnetic field ‘spin-spin’ interactions between neighbouring nuclei physics Time to relax… T1 relaxation T2 relaxation T2* relaxation time course for MDMs to return to original (z) orientation time course of breakdown of magnetization in the x,y plane due to spin-spin interactions Different tissues have different T1 and T2 relaxation rates time course of breakdown of magnetization in the x,y plane due to inhomogeneities in the magnetic field BOLD… physics We measure O2 ratios in blood BOLD (Blood Oxygenation Level Dependent) contrast: measures inhomogeneities in magnetic field due to changes in the level of O2 in the blood deoxyhaemoglobin = magnetic! oxyhaemoglobin = not magnetic! So we can use the change in fMRI signal to infer the relative oxygenation of the blood! High ratio deoxy:oxygenated blood fast decrease in MRI signal Low ratio deoxy:oxygenated blood slow decrease in MRI signal physics Summary of the physics MDMs of hydrogen nuclei align to magnetic field in scanner RF pulse causes them to spin, in phase, in x,y plane Once pulse has stopped they fall back to direction of magnetic field, dephasing as they do so Dephasing takes various amount of time, depending in part on inhomogeneities in magnetic field Inhomogeneities are caused by variable ratio of deoxygenated : oxygenated blood Assumption: activity in brain area lowers this ratio and thereby decreases speed of decay of MRI signal physiology What causes BOLD? The purpose of the increase in blood oxygenation is to feed neurons… …so, what makes a neuron hungry? (neurons can’t store much energy) …and how does this change the blood flow? physiology 3Na 2K Hungry brains Pre-synaptic neuron 3Na 2K ATP ATP GLN 50-75% of energy use is action potential driven; remainder is spent on housekeeping ATP 3Na+ GLU GLUTAMATE H+ K+ Most energy is spent on the reuptake of 3Na 2K glutamate and reversing ion movements! Na + Na + Ca 2+ ATP Glial cell Post-synaptic neuron physiology Vascular density Vascular density is proportional to synaptic density physiology Active control of blood flow physiology Neural activity BOLD: complicated! No easy relationship going from neural activity to BOLD Relationship cannot only differ between brain area and level of activation, but also between subjects physiology … so what does it imply? No summation of BOLD signal! No directly comparing different areas… Different vasculature Different neuromodulatory control Different circuitry BOLD [X] > BOLD [Y] does NOT mean neural activity [X] > activity [Y]! physiology What BOLD does not measure Output of an area Comparisons of activity between areas Inhibitory synapses (GABA) What BOLD does (presumably) measure Excitatory synaptic activity (input to an area) Local processing (reuptake of glutamate) Changes in neuromodulatory substances physiology What does a blob in area X mean? X has changed its local activity Change of inputs arriving at X physiology Other ways to measure neural activity Kim et al 2004 physiology Summary of the physiology Most energy is spent locally on synaptic processes Blood flow is controlled by monoamines BOLD does not measure output of an area, comparisons between areas, or inhibitory connections Instead, BOLD measures changes in local activity and changes in input to an area BOLD signal is most closely correlated with LFP signal References Physics: Lipton ML (2007) MRI Physics: Understanding the Basics. Springer Verlag Weishaupt et al (2003) How Does Mri Work? An Introduction to the Physics And Function of Magnetic Resonance Imaging. Springer Verlag Physiology: Logothetis NK et al (2001) Neurophysiological investigation of the basis of the fMRI signal. Nature 412:150-157 Attwell D & Iadecola C (2002) The neural basis of functional brain imaging signals Trends in Neurosciences 25:621-625 Kim DS et al (2004) Spatial relationship between neuronal activity and BOLD functional MRI NeuroImage 21:876-885