Essentially, fMRI is the “ultrafast” MRI. Similar
to PET methodology, fMRI is based on the tight relationship among
neuronal synaptic activity, metabolism, and blood circulation. Conventional MRI
was first performed in the 1940s, and today its principle is used
routinely in the clinical setting in the form of the MRI scanner.
When a patient is placed in the MRI scanner, hydrogen nuclei from
water molecules are oriented along the magnetic fields of the scanner.
By administering radiofrequency pulses to these nuclei, their alignment
can be disturbed. After the radiofrequency pulse ceases, the nuclei
realign themselves by transmitting a radiofrequency signal that can
be detected in a receiver coil placed around the patient’s
body. Different structures in the human body have different water
content, and, therefore, reflect different intensities of radiofrequency
signal. The map of these transmitted signals represents a magnetic
resonance image. However, obtaining each slice of the magnetic resonance
image requires a new radiofrequency pulse to be transmitted and
new excitation of water molecules to occur. For that reason, it
may take a long time to obtain a magnetic resonance image of the
human brain, containing multiple slices.