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One of the exhibitions we are working up at the moment is on the broad
subject of Measurement.. And we have been working on some exhibits in
this area of imaging. We have found a great discription of the
tomographic process in A.K. Dewdney's book "the tinktoy computer" in a
chapter called "scanning the cat"  which in essence steps through the
process David has suggested - highlighting the mathmatical process that
is required to work back from the transmission values to reconstruct a
structure. When reflecting on this process it struck me that it really
is very similar to the good old magic squares.. Imagine the values at
the edge of the magic square are the transmission values, and solving
the squares internal values gives you the internal structures/ or
densities.

Going about it this way, your demo could be to show the light
penetrating a visitors hand.. Get across the the idea of transmission
and changing opacities, then hand out a few 4 by 4 magic squares for the
group to "scan their own cats".. You can even bring in the false
colouring techniques used, by assinging a colour to a value and getting
the younger kids to colour in the squares...
 Finish off by showing a 256 x 256 magic square and ask them to get
started :)
Makes the point about the amount of processing power used to do the real
thing.

Hope this helps.

Dr Stuart Kohlhagen
Leader New Concepts
Questacon
The National Science and technology Centre
Canebrra


-----Original Message-----
From: Informal Science Education Network
[mailto:[log in to unmask]] On Behalf Of David Smith
Sent: Friday, 13 January 2006 2:58 AM
To: [log in to unmask]
Subject: Re: CT Scan demonstration

ISEN-ASTC-L is a service of the Association of Science-Technology
Centers Incorporated, a worldwide network of science museums and related
institutions.
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I heard from an ER nurse recently about a device that is used to help
locate veins for IVs in children by shining a bright light through the
flesh of the arm or hand.  It might work for you.

That said, comparing computed tomography with the image produced by a
source in a single location is not really an accurate representation of
the science behind the technique.  Tomography of any kind uses the
delays or attenuation of a signal in many different directions.  If you
shine a light through an object and see a pattern of light and dark, you
have the classic inverse problem.  If you know the internal structure of
the object, you can easily calculate the image that will be produced,
but you cannot do the inverse.  There are literally an infinite number
of possible configurations of geometry and opacity that will give the
same pattern.  You can rule out many, many impossible configurations,
but you can never determine a single unique configuration that you can
be certain produced the pattern.  Single source images, such as plane
film x-rays are useful only if the internal structure is already
relatively well constrained.  Tomography attempts to solve the inverse
problem by combining different views of the same object, taken from
different angles.  Sophisticated mathematical analysis uses the multiple
views to constrain the multiplicity of inverse solutions and provide a
well-constrained 3-D structure of the object.

Tomography is important to geophysicists studying the internal structure
of the earth and I have thought a lot about how you might convey the
technique through an exhibit.  I don't have any great insights to share,
except that it is remarkably difficult.  I had one crazy idea about
using sound - filling a large diameter tube with a few large blocks or
beams some of which conduct sound very well compared to air and are
coupled to the tube, then having people work in pairs, one tapping and
one listening as they work around the tube, then ask them to predict
what is inside. I don't even know if you could actually hear the
difference enough for it to work. 

Maybe a bright light and a ccd camera hooked up opposite each other with
a recognizable semi-opaque object in between and able to rotate around
the object so visitors could see the light/shadow pattern change as the
angle changes could covey some of the idea behind the CT.

Good luck,

Dave


David L. Smith, Ph.D.
Director of Professional Development
Da Vinci Discovery Center, Allentown, PA http://www.davinci-center.org
"Who will pick up where Leonardo left off?"

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