Date: Tue, 6 Jan 1998 17:33:13 -0800
Message-Id: <v01520d03b0d80e338a2b@[205.179.255.149]>
To: pinhole@exploratorium.edu
From: mafifi@redshift.com (Marc Afifi)
Subject: Re: some bio-med questions, kinda
My daughter has had three MR and two CT scans in the last two months. I
tried to find out as much as I could about these two tests but I think
worry got the best of me. Here's my rudimentary understanding, for what
it's worth. Please feel free to correct any inaccuracies.
CT uses the same intensity X-ray that you get for a broken bone or a dental
exam. The inventors were either a mathematician or a computer scientist
(call him A) working with a radiologist (call him B). A was trying to find
a way to handle large amounts of data in such a way as to glean only
important data from the morass of extraneous data. He came up with an
algorithm which allowed precisely that and was so excited about it that he
happened to share his discovery with his friend, B. B got to thinking about
the data that was lost as X-rays pass through soft tissue, realizing that
there must be some useful data in all that energy that passed through. So A
and B worked together to modify the algorithm to retrieve the lost data and
the CT scan was born. For this they won the Nobel Prize. Current technology
uses very fast computers and relatively low intensity X-radiation (I held
my daughter's head still during both of her scans) and is a very quick
process. The whole scan takes less than a minute.
The main idea for CT scans is that even though most of the X-rays pass
through the soft tissues, some of it is absorbed. Using computers and
advanced detectors, the small amount of X-radiation which is absorbed can
be reconstructed and a computer used to render an image. It is truly an
amazing piece of technology.
I don't know the history of MR scans, but my understanding is that when
placed in a strong magnetic field, the magnetic moment of the H nuclei in
water line up in the field. As they randomize once the magnetic field is
shut off they emit radio waves which detectors pick up and then a computer
is used to reconstruct an image based on the emission of these radio waves.
Since soft tissue contains lots of water molecules which are relatively
fixed in place, the MR scan is particularly useful for studying soft
tissues. Blood and other fluids which are in motion give no detail because
the water molecules are not able to be aligned with the external B field
due to their motion. Bone doesn't give much detail due to the relative
paucity of water molecules in bone tissue. During the MR scan, the external
B field is pulsed repeatedly and this requires quite a bit of time. My
daughter's MRIs typically took 45min compared to the one minute required
for the CT scan.
A very simplistic way of visualizing how a MRI machine works might be to
picture a bar magnet suspended by a string in space aligned with a magnetic
field. If you turn off the magnetic field, the bar magnet will probably
rotate. This rotation is caused by releasing potential energy which can be
detected with the proper instrumentation. If your equipment is sensitive
enough, you can specify where in space that magnet is. This is basically
what MRI does. The computer determines where the water molecules are which
are giving off the radio waves and, depending on the strength of the
signal, the type of tissue can be determined. This, too, is a truly amazing
piece of technology. Ted Kaczinsky be damned (if guilty)!
I hope I don't have too many glaring errors here. Please correct any you
see so that I might improve my communication to my students in the future.
They find it fascinating when I put my daughter's films on the overhead to
discuss her pathology and the processes used in diagnosis and correction.
It's nice that something useful has come from her condition. By the way,
she's fine now. Great doctors at UCSF and California Pacific Medical
Center!
-mafifi
______________________________________________________________________________
Favorite Oakland Raiders motto: Marc Afifi
Pacific Grove HS
Just When Baby? 615 Sunset Dr.
Pacific Grove, CA 93950
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