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Huetel et al. - Chp 7 Summary Notes

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McGill University
NSCI 201
Evan Balaban

CHP7BOLD fMRI ORIGINS AND PROPERTIES SUMMARYalmost all fMRI studies rely on an endogenous measure BOLD contrastearly research found that deoxygenated Hb is paramagnetic and altered the T2 properties of nearby tissueBOLD response to brief neuronal activity ie HDR consists of a short onset delay rise to a peak after a few sec return to baseline and a prolonged undershootsome researchers report the presence of an initial dip in the BOLD signal due to O2 extraction before the large inc in blood ow but this effect is not always seenfMRI spatial resolutiondetermines our ability to separate adjacent brain regions with different functional propertiescollecting data using smaller voxels improves the spatial resolutioncost of inc spatial resolution is a dec BOLD signalthe vascular systems organization and response properties introduce spatial constraintsfMRI temporal resolutionability to estimate the timing of neuronal activity from the measured hemodynamic changesusing a short repetition time improves temporal resolutionHDR slowness limits researchers ability to make precise temporal measurements nonlinearities in the fMRI HDR reect the temporal dynamics of activation in a single spatial locationeg if the same brain region becomes active twice in rapid succession within a 6s interval the HDR of the second event is dec in amplituderefractory effectsoften present as an analysis challenge in studieshave been used to study functional adaptation within a brain regionstrengthslimitationsUses a simple physiological mechanism Poor temporal resolution many minutes Provides absolute quantitative data PETPoor spatial resolution several centimeters Allows imaging of anything that can be tagged Requires injection of radioactive materialNoninvasive replicable Mediocre temporal resolution seconds fMRIPotentially good spatial localization Complex highly variable data analyses Common wellvalidated technique Expensive and timeconsumingfMRIenergy is supplied to the brain via the vascular system more hemoglobin is supplied than needed causing a decrease in deoxygenated hemoglobinDeoxygenated hemoglobin reduces some forms T of MR signal CHP7BOLD fMRI ORIGINS AND PROPERTIES energy glucoseO is required to maintain and restore neuronal membrane potentials2this energy must be continuously supplied by the vascular systemHistory of BOLD fMRIbloodoxygenatedlevel dependent BOLD contrastdifference in signal on T2 weighted images as a function of the amount of deoxygenated Hb oxygenated Hb is diamagnetic property of a weak repulsion from a magnetic field no unpaired electrons and zero magnetic moment deoxygenated Hb is paramagnetic property of being attached to a magnetic field both unpaired electrons and a significant magnetic moment completely deoxygenated blood has a magnetic susceptibility 20 greater than fully oxygenated bloodmagnetic susceptibilityintensity of magnetization of a substance when placed within a magnetic fieldnearby protons experience different field strengths because paramagnetic substances distort the surrounding magnetic fieldtherefor protons will precess at different frequencies resulting in a quicker decay of transverse magnetization shorter T2in a test tube of blooddecay of transvers magnetization depends on the proportion of oxygenated HbMR pulse sequences sensitive to T show more MR signal where blood is highly oxygenated2magnitude of the effect increases with the square root of the strength of the static magnetic fieldDiscovery of BOLD ContrastBOLD contrast depends on the total amount of deoxygenated Hb present in a brain region total deoxygenated Hb depends on the balance between O2 consumption and supplyOgawa hypothesized that when neuronal activity increases oxygen consumption increases resulting in more deoxygenated Hb and a darker MR image false this does not happen what really happensinc neuronal activity increases the signal of T images2
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