Magnetic Resonance Imaging (MRI) is a non-invasive technique for obtaining images of brain structure. By placing an individual in a very strong magnetic field and passing a radio wave through the field, all of the hydrogen atoms in the individual's head can be induced to spin in the same direction. When the radio frequency is removed the hydrogen atoms all orient in the direction of the magnetic field, and in doing so produce magnetic fields that can be detected and used to construct an image based on the density of hydrogen atoms. Because of the ubiquity of hydrogen in the body, this image gives a very good picture of the structure of brain (or other) tissue. Functional MRI
Functional MRI (fMRI) is based on the same principles as MRI, but relies on the differing magnetic properties of oxygenated and deoxygenated hemoglobin (in red blood cells). When there is greater activity in a brain region there is a corresponding increase in blood flow to that region, which is known as the hemodynamic response. Because the removal of oxygen from the blood does not match the increase in oxygen delivery to the active brain region, the hemodynamic response results in a greater ratio of oxygenated to deoxygenated hemoglobin in that region. This change can be measured with MRI and is called the Blood-Oxygen Level Dependant (BOLD) signal (also known as the T2*-weighted MRI signal).
fMRI allows for the measurement of brain activity with very high spatial resolution (on the order of millimetres). Because fMRI uses the same equipment as MRI, structural images can be obtained from a participant during the same session as fMRI signals are recorded, and so changes in the BOLD signal can be localized on a structural image of the brain of the actual participant. However, because the hemodynamic response is quite slow (occuring over several seconds), the temporal resolution of fMRI is relatively poor. Although in recent years techniques for Event-Related fMRI (ER-fMRI) have greatly improved the temporal resolution of fMRI, it is still not possible to adequately characterize neural activity at the subsecond level with fMRI.
We use the 3 Tesla Phillips MRI system at the Vancouver Hospital and Health Sciences Centre located at the University of British Columbia campus.