One of my research areas involves the study of clinical applications of
fMRI. Probably the best way to discriminate the methods is just to look
at the
different scans. The only scans that really look similar are PET and
SPECT scans. They use a similar process and the spatial resolution of
each is
similar. CT Scans are very sharp structural images that are constructed
from X-Ray images. CT scans have greatly improved in recent years and
show structural lesions, such as skull fractures and hemorrhages, very
well. They are also still less expensive than MRI scans. They have been
improved by greater post-processing of the CT data. CT is still the
first level of scanning for traumatic brain injury. The spatial
resolution of
MRI and CT scanning is much higher than PET, SPECT or EEG scanning. The
temporal resolution of EEG is much higher than the others.
MRI scans are built from our water molecules suspended in a very strong
magnetic field. Most of them line up with the field. Send in a radio
transmission and they precess away from the field because they absorbed
some of the energy from the radio transmission. They would really like
to line back up with the field. When they do this, they send out a tiny
radio transmission that is picked up by an antenna coil wrapped around
your head and body. This small radio transmission is analyzed and
mapped in gray scale. This is obviously simplified but this is the basic
process. MRI scans also render a structural image that depicts the soft
tissues extremely well. It is the scanning workhorse of the hospital and
medical diagnosis made a great leap forward when it was invented. The
story of its invention is very entertaining and important for students to
learn. It represents the best of American pragmatism and
inventiveness. The guys involved were great characters. PBS did a nice
story on its
invention that you can use in class:
http://www.pbs.org/wnet/brain/scanning/mri.html
The best part was when they "scanned" the first person, one of the
research team members. They got no meaningful data. They figured out that
the reason was because he was too fat. They all then turned to the thin
guy on the team. He stated that he would get in the magnet if nothing
happened to the first guy for 6 weeks. Since no one had been scanned
before, he felt there might be some harm caused by the magnet. After a
time, they scanned him and graphed the data by hand coloring darker and
lighter cells in graph paper. When they did this, his internal organs
became visible. Then they knew they had it. The rest was history. MRI
scans have been greatly improved by faster data collection, larger, open
magnet cores, faster computer processing and stronger magnets. The
images have become higher and higher resolution constructed faster and
faster. Millions of MRI scans are conducted each year in the US.
fMRI uses the same magnetic resonance data as the MRI scan. Sherrington
discovered 100 years ago that neural activity causes a local increase
in the flow of oxygenated blood. Ogawa showed that this increase in
blood flow causes an increase in the MRI signal (BOLD). By subtracting the
signal level at the a time when the brain is active to the signal level
when it is not, I can map the locations of activity. By controlling the
cognitive activity, I can image the locations of the activity. fMRI
methods are improving with the improvements in MRI data collection in
general.
fMRI is also improving with better data analysis methods. The current
emphasis is on methods to study changes in BOLD responses over time
that reveals connectivity of brain areas that function over time.
DTI imaging is also something I have studied and this involves the
detection of movement of the water molecule. You can render the orientation
of white matter pathways in a very detailed map. DTI imaging still uses
the same MRI data collected using a special protocol. We used it to
study lesions in seizure disorder.
I think many people who are disparaging of fMRI are reacting to the
sensational stories using it. I have used fMRI to study individual
patients and
I have been very impressed by its utility. We discovered patients who
had receptive language (Wernicke's area) in the left hemisphere and
expressive language (Broca's area) in the RIGHT frontal lobe, language
areas that defined the margins of a tumor and clear evidence of a left
upper
visual defect caused by a tumor in the right temporal lobe.
We are still in the early stages of developing fMRI. It is already the
dominant method of studying brain function and this will only get better
as the
method improves. When I started in this area, it took about 3 hours to
analyze the data for a single subject, and this involved a lot of staring at
the computer while it crunched numbers. Now, the new scanners analyze
the data on the fly and render an image after each active period. You
can adjust the protocol while the scanner is collecting data and see the
results in the next iteration of active/rest periods. All the scanners now
use the DICOM data format, enabling me to view the scans on my
iPhone/iPad or PC. The best DICOM viewer app I found is a free one called
Osirix.
Depending how you define a thought, we are imaging them now.
Mike Williams
Subject: Re: Types of brain scans From: "David Wheeler, Ph.D. "
<[email protected]> Date: Mon, 24 Oct 2011 12:08:56 -0400 X-Message-Number:
12 Differences between the way PET and fMRI look: fMRI looks like it is
superimposed on a MRI image of the brain PET is just the PET data and a
wire frame sketch of the brain may be superimposed on it. fMRI:
http://pnrc.cchmc.org/images/hearing/hearing1.jpg PET:
http://www.biochem.arizona.edu/classes/bioc462/462bh2008/462bhonorsprojects/462bhonors2004/navratilovaz/Overview.htm
It sounds like it would be too difficult (and not necessary) for our students
to be able to tell the difference between an image of an fMRI scan and a PET
scan.
Michael A. Britt, Ph.D.
[email protected]<mailto:[email protected]>
http://www.ThePsychFiles.com<http://www.ThePsychFiles.com/>
Twitter: mbritt
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