Monday, May 12, 2014

Open Source CT

The Tricorder Project is working on an open source, cheap CT scanner. A CT (or computed tomography) scanner takes a series of pictures using x-rays so you can see the inside of the object.  It also created volumes of scanned objects; that's why its often called "volumetric imaging."

This new open source CT scanner isn't meant to scan humans, instead, it is meant to scan objects. The Tricorder's CT scanner takes several days to scan an object, and it also it scans at a very low resolution. The CT scanner uses very low radiation (barely above background levels) to scan the objects and uses an Arduino to control the motors.

The Advanced Light Source in Berkeley, California also uses a type of CT scanner on its Beamline 8.3.2, except its radiation would kill you in an instant. Our 7th grade class was able to go to the ALS and use their scanner (or beamline).  (You can read about our field trip to the ALS here.)  This open source CT scanner is an interesting idea, since it would allow us (and other classrooms) to do what we did at the ALS, without the synchrotron particle accelerator!  And it would be safer, but much, much slower and give much lower resolution images.

From a high-level technical standpoint, a computed tomography or CT scanner takes a bunch of absorption images of an object (for example, x-ray images) from a variety of different angles, and then backs out 3D volumetric data from this collection of 2D images taken from different angles. In practice, this is usually done one 2D “slice” at a time, first by rotating an x-ray scanner around an object, taking a bunch of 1D images at tens or hundreds of angles, and then using the Radon transform to compute a given 2D slice from this collection of 1D images. One can then inspect the 2D slices directly to see what’s inside something, or stack the slices to view the object in 3D. (source page)

You can read more about the open source CT project on MAKE, Hack a Day, or on the Tricorder Project blog.

- Sam (7th grade)

PS - In the comments section of the article, the maker discusses more specific detail about his plans for a radiation source:

I have a 1uCi Cadmium-109 check source on its way (as well as a pound of lead shielding to help put together a rough collimator). Cd-109 is the lowest-energy radioisotope that I could find, that emits around 22keV if I remember correctly — which is absorbed about 50% by 2cm of tissue, so I think there should be usefully contrastive absorption for things like vegetables. I was also considering a Barium-133 source, which is higher energy (80-120keV, I think), so perhaps more suitable for things that have some small amount of metal, but Ba-133 is not monochromatic. These radioisotope check sources are sealed in epoxy, and are of such low intensity that they’re not licensed, generally considered pretty safe unless you eat them or tape them to your body for long periods, and can apparently be disposed of in the trash. 
In order for the imaging to work, the source has to be completely shielded such that it only emits in about a 1-2mm dia cylinder outward, facing the detector — something like the shape of an uncooked noodle of spaghetti. This dramatically reduces the intensity of an already very low intensity source. If the numbers I’ve read are correct, this should give around one high energy photon per second in that 1mm cylinder to the detector at 30cm away, and zero everywhere else. That should give a signal-to-noise ratio of about 10:1 if you sample each point for a minute, as background is about 5 counts per minute on my desk. Hopefully that will be enough to give an okay image. 
So ... the radiation level should be zero above background outside of the bore, and so slightly above background in an area the size of a spaghetti noodle within the bore that you could measure it in bananas-worth of exposure. The trade off is of course extremely long imaging times.

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