CT: X-rays - sees bone best. Used mostly to show subdural hematomas or
ventricular enlargement.
MRI: Extremely strong magnetic fields align hydrogen molecules. When
magnet is turned off, the hydrogen relaxes back to it's normal state and
give off radiation. Good for seeing water and fat. Water has different
relaxation time than fat so detectors can be tuned to different
relaxation times. Best for showing myelin in brain.
PET: Positron emission tomography. Uses a short-lived (must be made on
site the same day as test)radioactive form of GLUCOSE. It is
deoxyglucose so it does not get metabolized but accumulates in places of
highest metabolic activity. Positrons are emitted during the decay of
the radioactive Flourine-18. The positron hits an electron in the brain
and annihilation occurs (transformation of the mass of positron and
electron into energy (E=mc^2)). The annihilation generates two photons
traveling at exactly 180 degrees apart. These two photons arrive at
detectors at slightly different times depending on the distance from the
annihilation to the detector. The origin of the photons can be
calculated from the differential in arrival time. In practice, the
coincidence of photons arriving at the detector is used without figuring
out the distance. Annihilations occurring at different angles can be
used to calculate the intersection of the coincidence lines to figure
out the 3 dimensional location of the source.
Measures glucose utilization: active parts of brain use more glucose.
Disadvantages: Usually long exposures are required (45 min). This means
you must be doing a task for that whole time. Cannot measure
instantaneous thinking. Also, annihilation does not occur at exactly the
spot where glucose is being utilized; so, this could never be used to
localize small areas of brain.
fMRI: Measures blood flow which is an even more remote marker of brain
function than glucose utilization. Also requires prolonged exposure to
task in order to get the blood flowing to that area.
Evoked Potentials: Best way to get at functional activity in one area of
the brain. This measures the electrical activity of one area of the
brain in response to one stimulus. Usually have to do a number of
stimuli to get the signal out of the noise. Can be measured with
implanted electrode or EEG.
Conclusion. We do not have the technology to see an individual thought
occurring in the brain yet.
=David Wheeler, Ph.D.
Robert Morris University
On 10/24/11 8:37 AM, Michael Britt wrote:
Since my experience with brain scans is limited, can someone tell me if
I have the following correct:
* CT scan: uses xrays, the scan is good for revealing the structure of
the ear perhaps, but very little detail of the brain
* MRI: uses magnets, reveals more detail of the brain's structure
* fMRI: also using magnets, but reveals the activity of the neurons in
the brain by detecting changes in oxygen in the blood (oxygen used
by active neurons)
* PET: requires the injection of a radioactive substance that contains
glutamate, detects activity in the brain via neurons using up the
glutamate
Question: fMRI and PET scans look very similar. How can students tell
them apart? When is one of these scans preferred over the other?
Michael
Michael A. Britt, Ph.D.
[email protected] <mailto:[email protected]>
http://www.ThePsychFiles.com
Twitter: mbritt
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