Az fMRI alapjaiBOLD fiziológia

Dr. Kincses TamásSzegedi Tudományegyetem

Neurológiai Klinika

T2*

Az obszervált transzverzális relaxáció (T2*) több különböző komponens összege

Many physical effects result in dephasing. The process that dephases the spins most quickly dominates the contrast in the final images, thus the net observed T2 (T2*, “T2-star”) is shorter than any of the individual T2’s. Most MR imaging is concerned chiefly with three dephasing effects: molecular interactions (T2), local variations in field from, for example, tissue boundaries (T2’) and dephasing from molecular motion from diffusion (T2D).

T2*

A lokális mező-inhomogenitások a spinek fáziseltolódását okozzákez gyorsítja a transzverzális relaxációt (T2*)

A deoxy-haemoglobin tartalmú erek környékén mágneses grádiens keletkezikMRI signal depends on the coherence of individual proton spins. In the presence of magnetic field distortions, the MR signal is reduced. Deoxygenated blood is paramagnetic and results in such field distortions in the immediate vicinity of the capillaries.

Intrinsic kontraszt: Deoxy-haemoglobin

A paramágneses deoxy-haemoglobin intrinsic kontrasztanyagA BOLD válasz a venulákban váltakozó deoxy-haemoglobint jellemzi

Nagyobb térerő a kisseb erek felé tolja el a méréstThe BOLD signal is composed of CBF contributions from larger arteries and veins, smaller arterioles and venules, and capillaries. Experimental results indicate that the BOLD signal can be weighted to the smaller vessels, and hence closer to the active neurons, by using larger magnetic fields. For example, whereas about 70% of the BOLD signal arises from larger vessels in a 1.5 tesla scanner, about 70% arises from smaller vessels in a 4 tesla scanner.

Neuro-vaszkuláris kapcsolat

A neuronális aktivitást a vasculáris válasz némi késéssel követi

The fMRI response takes some time to occur. Much of this delay is thought to be related to the signalling from neurons to the vascular system. At this time it is not entirely clear how this signalling takes place, though there are many candidate signalling molecules and mechanisms that are probably all involved to varying degrees.

Nyugalmi dinamika

Nyugalomban a grádiensnek megfelelően az O2 az agyszövetve diffundál

This series of slides outlines the dominant hypothesis as to why the BOLD signal is increased when brain metabolic demand is increased. Some believe, however, that this demand is driven not by oxygen but by glucose. The effects would be the same, however.

Kezdeti aktiváció

Neuronális aktiváció hatására az O2 extrakció fokozódik

This series of slides outlines the dominant hypothesis as to why the BOLD signal is increased when brain metabolic demand is increased. Some believe, however, that this demand is driven not by oxygen but by glucose. The effects would be the same, however.

Aktiváció

Az regionális agyi vérátáramlás fokozódása nagyobb mint az O2 extrakció üteme

This series of slides outlines the dominant hypothesis as to why the BOLD signal is increased when brain metabolic demand is increased. Some believe, however, that this demand is driven not by oxygen but by glucose. The effects would be the same, however.

Haemodinamikus válasz

A haemodinamikus válasz a stimulushoz képest időben késik

CMRO2: Cerebral Metabolic Rate for OxygenCBF: Cerebral Blood FlowCBV: Cerebral Blood VolumeBOLD: Blood Oxygen Level Dependent Response

Példa

fMRI Signal increases relatively slowly (several seconds), and returns to baseline slowly. No exogenous contrast needed. One of the more interesting and important features of fMRI is its ability to collect multiple experiments on the same subject to tease apart the responsiveness of various brain regions to complex stimuli. In this study, Roger Tootell looked at the fMRI signal change when a subject observed a visual stimulus containing moving vertical bars. Multiple brain areas showed signal increases, but his hypothesis was that the detection of the bars and of their motion were represented separately in the brain.

So long, and thanks for all the fish...

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