Functional Magnetic Resonance Imaging (fMRI) in Oncology

The measurement of neural activity is done indirectly. It is known that active neurons require more oxygen than passive ones; therefore, the cerebral blood flow in active areas is increased and the demand for oxygen will even be overcompensated. Due to the different magnetic properties of oxygenated and deoxygenated blood the MR scanner's main magnetic field is slightly differently disturbed in areas of neuronal activity than compared to a rest-state with no activation. Proton-spins in the vicinity of such areas exhibit different relaxation properties in both neuronal states. Even though this so-called "blood-oxygenation level dependent" effect, or BOLD effect is very small (a few percent of the normal signal), it can be measured.

Usually two data sets are acquired in an fMRI experiment: one high spatial resolution image data set, and a BOLD-sensitive data set while the test person in the scanner fulfils a predefined task in the MR scanner. For the second data set fast imaging sequences are necessary (e.g. whole brain scan within 2.5s) to achieve a good temporal resolution of the BOLD affected MR signal. As the BOLD effect is very small, the experiment is repeated several times to collect sufficient data points for a profound statistical analysis. The result of this last step is a map of statistical parameters which is overlaid on the reference image as bright areas that e.g. correspond to active brain areas.

fMRI at 7 Tesla is promising due to the the signal gain from the higher magnetic field which is accompagnied by a higher BOLD effect. Thus, less signal averaging is necessary at 7 Tesla, and more subtle changes in neuronal activity might become detectable. In the presence of a tumor, the localization of an active brain area often changes. At higher magnetic fields this shift might be easier to detect, so that a subsequent tumor therapy can be planned more efficiently with an improved outcome for the patient.