Blood Oxygen Level Determination

Functional MRI (fMRI) refers to MRI of the brain performed while the subject is performing a specific task. For instance, a patient may be asked to repeatedly touch their thumb and forefinger together while being scanned, and this action causes a small change in blood flow to a specific motor center in the brain. The change in blood flow induced by this action causes a change in the ratio of deoxyhemoglobin to oxyhemoglobin. Deoxyhemoglobin is paramagnetic and relaxes relatively faster on T2*-weighted images than does oxyhemoglobin. This small change in signal can be detected and is known as "blood oxygen level determination" scans as it depends on the deoxy/oxy hemoglobin ratio. The differential in T2* relaxation can be exploited to generate difference maps that are thought to reflect activity in the brain. It is important to note that this technique does not necessarily measure tissue oxygenation, i.e., the partial pressure of oxygen within tissue. Rather, it reflects the available oxygen carrying capacity within the hemoglobin of red blood cells within tissue; therefore, it does not necessarily reflect tissue hypoxia.

BOLD imaging has been used to map motor, language, and sensory regions of the brain prior to neurosurgery in an effort to spare vital functions. It can be performed prior to surgery for removal of a tumor to modify the surgical approach, if the tumor is close to an important functional center.

Instead of functionally challenging the brain, an extrinsic contrast agent can be given to augment hemoglobin saturation with oxygen. Making the patient to alternately breathe a mixture of carbon dioxide and enriched oxygen (5% CO2 and 95% O2), a mixture called carbogen, and room air can depict regions of hypoxia within tumors. Presumably, oxygen-deprived regions of a tumor will demonstrate larger changes in signal than do regions that are already satisfactorily served by an oxygenated blood supply. Thus, this technique may reflect tissue hypoxia.

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