Exogenous and endogenous contrast agents

The fact that neural activity is linked to local blood flow provides the opportunity for functional MRI. Broadly, there are two ways in which altered blood flow can produce measurable change in magnetic resonance signals.

The first involves the administration of an exogenous contrast agent, such as gadolinium diethylenetriaminepentaacetic acid ( Gd-QTPA). Gd-DTPA has marked paramagnetic properties causing a local change in the effective magnetic field. When given intravenously as a bolus while the subject is exposed to experimental stimulation, Gd-DTPA causes locally increased field strength in those regions (and only those regions) that are neurally activated by the stimulus. This effect on the field alters the magnetic resonance properties of protons, namely the spin-spin relaxation time, creating tissue contrast between brain regions activated by the stimulus and regions that are not activated.

The second approach involves exploiting the existence of an endogenous contrast agent. Iron in deoxygenated haemoglobin has paramagnetic properties similar to Gd-DTPA. It also does not diffuse out of the vascular compartment. Neural activity causes a local reduction in the ratio of deoxygenated to oxygenated haemoglobin, so that the paramagnetic effects of deoxyhaemoglobin are 'diluted'. Since apparent spin-spin relaxation or dephasing is accelerated by microscopic inhomogeneities in the magnetic field due to the presence of paramagnetic contrast agents, the net effect of diluting deoxyhaemoglobin will be to prolong T2* times in areas of the brain that receive an increased blood flow as a consequence of neural activity. The haemodynamic effect on spin-spin relaxation can be measured by a T2*-weighted signal which is blood oxygen level dependent (BOLQ).

Note that gadolinium infusion causes a somewhat larger signal decrease (about 5 per cent) than the signal increase (about 3 per cent) associated with the activity-dependent dilution of deoxyhaemoglobin. However, endogenous contrast is generally preferable on the grounds that it does not require an intravenous infusion.

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