Functional magnetic resonance imaging

From Psy3242

MRI- The Basics

To understand Functional Magnetic Resonance Imaging (fMRI), a basic understand of Magnetic Resonance Imaging (MRI) is necessary. MRI works (in the most basic sense) by placing an individual into a giant magnetic coil, which aligns hydrogen nuclei (protons) in the body in a certain uniform direction. From there, harmless radio waves are directed towards the individual which knock the protons off axis. The protons gradually realign themselves with the magnetic field, and in doing so send out a radio wave of their own, which a computer detects and analyzes certain properties about those waves. The basis of the recording equipment lies in how the previously aligned hydrogen nuclei respond to the radio waves.

Progression to fMRI

The basis of the first version fMRI, on the other hand, goes all the way back to 1890, when Charles S. Roy and Charles S. Sherrington postulated in On the Regulation of Blood Supply of the Brain that neural activity correlates with blood supply of the brain. It would take another hundred years before Ogawa and Lee’s research on rodents indicated a relationship between blood oxygenation levels and contrast in magnetic resonance images. Basically, since the brain does not have energy reserves of its own, activity in a given neural network will require an additional supply of blood in order to keep functioning at higher levels. Since blood uses iron to carry oxygen to these active areas of the brain, the blood vessels around the active area will dilate and cause oxygenated iron to rush in (and deoxygenated iron to rush out). Since deoxygenated iron creates small magnetic disturbances, it makes it easy for radiologists using an MRI scanner to see a decrease in these disturbances, an indicator of which sites in the brain are being activated during a given task.

Variations of fMRI

There are now four different types of fMRI, each with its own set of advantages, disadvantages, and specific uses. A brief description and example of each follows:

(1) BOLD-fMRI which measures regional differences in oxygenated blood. Best used for processes rapidly turned on and off (i.e. hearing, vision). It is limited by its slow onset and relative units in measurement. It is also very sensitive to movement. Together, these limitations make it difficult to study anything that involves movement, even to the level of speaking. Areas of activation that show up in the scan may also be someone removed from the actual active neural network, since larger veins can dwarf the smaller (and more accurate) capillary bed locations.

• Breiter et al. (1996) scanned OCD patients vs. healthy controls to compare brain activity during rest states and “activity states” (i.e. holding a dirty washcloth).
• The technique is beginning to be used clinically to noninvasively map language, motor and memory functions in candidates for neurosurgery.

(2) Perfusion fMRI which measures regional cerebral blood flow

• Unlike the relative measurements obtained from BOLD, perfusion fMRI obtains absolute measurements, which are more useful in that they can be compared to separate testing sessions.
• This is useful in such circumstances as measuring anxiety, which is much harder to turn on and off, thus impossible for use with BOLD.
• The technique is limited in that it often takes several minutes to acquire the information from a given area of the brain that the researcher is looking for.

(3) Diffusion-weighted fMRI which measures random movement of water molecules

• Due to the higher level of sensitivity in this technique, it can detect myelination and so has great potential in developmental neuropsychology.
• Although not particularly useful in studying psychiatric disorders as of now, future development may lead to better management possibilities for acute ischemic stroke patients.

(4) MRI spectroscopy which can measure certain cerebral metabolites noninvasively.

• Has been used to identify regional biochemical abnormalities. One such discovery was the fluctuating levels of the cell membrane building blocks in the frontal lobes of bipolar patients.
• Can also help diagnose certain psychiatric diseases like Alzheimer’s Dimentia.
• Can measure changes in metabolic rate in individuals with panic disorder or other psychiatric conditions.
• Can measure drug levels of certain psychotropic drugs that contain lithium and/or fluorine, as these are magnetic and do not naturally occur in the human body.
• Perhaps the most useful of the four, MRS is not only a research tool but is also starting to be used for presurgical planning, and identifying characteristics of tumor, stroke, epileptogenic tissue.