fMRI
Übersicht
Die funktionelle Magnetresonanztomographie, abgekürzt fMRT oder fMRI (englisch: functional magnetic resonance imaging), ist ein bildgebendes Verfahren, um physiologische Funktionen im Inneren des Körpers mit den Methoden der Magnetresonanztomographie (MRT) darzustellen.
Dabei werden Blutströme im Gehirn gemessen. Dort, wo viel Blutstromaktivität herrscht, ist das Gehirnareal aktiv.
Bei dem Verfahren wird das Gehirn durch viel Voxels dargestellt und jeder Voxel representiert eine kleine 3D-Region des Gehirns. (wie ein Array beim Programmieren)
Different origins of the T1 and T2 signals in fMRI.
T1 (T1-weighted (short Time to Echo, TE and Repetition Time, TR))
Longitudinal relaxation time or T1 is the time constant that measures the time taken by excited hydrogen atoms to realign with the external magnetic field.
T2 (T2-weighted (long TE and TR))
transverse relaxation time or T2 is the time constant that measures the time taken by excited hydrogen atoms to either reach equilibrium or go out of the phase (dephasing).
Different interpretations of the T1 and the T2 signal in fMRI
T1: proton density (MRI)
T2: blood-oxygen levels are measured (fMRI)(deoxyhemoglobin)
BOLD (blood-oxygen level-dependent) contrast is resulted from changes in magnetic field depending on the oxygen state of hemoglobin.
Which three components do you need in your machine to measure fMRI and why?
- Gradient coil: transmission coil, apply magnetic field along B1
- RF Coil: receiving coil, measure net magnetisation
- 4T Magnet
Why does the BOLD signal occur at a delay to the neuronal activity? How large is this delay typically?
5-6 seconds after activation is the peak of BOLD signal.
The BOLD signal measures the increase of oxyhemoglobin and decrease of deoxyhemoglobin. This is due to the higher demand of neurons firing.
The BOLD signal occurs at a delay because it reflects the post-synaptic activity and it takes time for the blood to run through the arteries. Especially higher cortices areas are supplied with oxygen through smaller arteries. Therefore it takes longer.
wie genau wird oxyhemoblobin vom gerät erfasst
ses also
Quelle
https://de.wikipedia.org/wiki/Funktionelle_Magnetresonanztomographie