FMRI Davind Norris

11 important questions on FMRI Davind Norris

What is paramagnetic, oxygenetad hemoglobin or deoxygenated hemoglobin?

Deoxygenated hemoglobin is paramagnetic.

What is the effect of deoxygenated hemoglobin on T2*?

A shorter T2*.

Deoxy Hb distort surrounding magnetic field --> protons experience different magnetic field strangths --> protons precess at different frequencies --> more rapid decay in transverse magnetization --> shorter T2*

When is there more T2* signal?

Where blood is highly oxygenated.
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What contrast is shortened by the presence of deoxy-Hb, T2 or T2*

BOTH are effected, however the effect on T2* is the greatest. This generally utillised for fMRI.

Which three parameters change upon activation and affect the amount of deoxy-Hb present?

  1. Oxygen consumtion.
  2. blood flow
  3. blood volume

What has the strongest effect on the levels of deoxy-Hb?

Blood flow is the dominant effect.
There is a net reduction in the amount of deoxy-Hb during activation.

What happens during activation to T2* and the MRI signal?

T2* increases, and the MRI signal increases.

What determines the spatial resolution of fMRI?

The spatial spread of the haemodynamic response. It's physiological and not techological.

What are three explanations for the post-stimulus undershoot?

  1. slow return of venous blood volume to baseline
  2. sustained elevated oxygen metabolsim.
  3. flow undershoot


flow undershoot is considered least likely. Sustained oxygen metabolism is favoured by many.

It's still not clear why there's an undershoot.

How is the temporal resolution defined?

The ability to distinguish successive activations in the same region.

How is the spatial resolution defined?

The ability to distinguish activations in neighbouring regions.

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