● The physiology
- Neuronal activity causes an increase in regional Cerebral Blood Flow (rCBF).
- There is an excess of oxyhemoglobin in active brain tissue

● The physics
- Deoxyhemoglobin is paramagnetic
- Oxyhemoglobin is diamagnetic.
- Paramagnetic objects become magnetized in the presence of a magnetic field.
- Inhomogeneities in the magnetic field and signal dephasing.

● Physics meets Physiology
- More oxygenated blood than we did before the activation started
- A net decrease in paramagnetic material.
- A net increase in signal in activated areas due to less dephasing of the signal