Monthly Archives: May 2011

Apocalypto – reading the unreadable

Αποκαλύπτω
I click <<save>> on my computer today and I wonder if that file will be readable 10 years from now, or 20, or 50.  Incredibly, and thankfully, a wealth of historical information has been preserved for hundreds of years, written on parchment; a treated animal skin based material. They didn’t worry about hard disk crashes then, but over the years, the wrong environmental conditions can turn a nicely rolled parchment scroll into a solid lump of gelatine; forever entombing its secrets within.

Perhaps it was just a shopping list, but what if this degradation has caused the loss of some valuable fragment of historical information? There is hope!  Much of the ink used contained enough iron to make it detectable on sensitive X-ray equipment.  With computed tomography of sufficient resolution and contrast sensitivity, we can build up a complete 3-dimensional map of the location of this ink and then use advanced software algorithms to work out how the parchment was rolled or folded.

The final step is then to virtually unroll it and reveal (Greek: αποκαλύπτω/apocalypto) the hidden secrets within.  It’s not an easy task, which is why this team has come together, with the best technology money can buy, to crack the problem.
Of course, if this valuable scroll has already suffered from years of neglect in the wrong environment, the last thing we want to do is cause any more damage. So another vital part of this project is to investigate the effects of X-ray exposure on parchment. So far, it looks pretty safe and unless the parchment is wet, we have been unable to detect any measureable damage caused by X-rays.

XMT

X-ray microtomography (XMT) is a miniaturised version of medical CT or CAT (computed axial tomography)scanning. A series of X-ray projections are recorded at a number of angles around the specimen (usually over a range of either 180 or 360 degrees). In XMT, unlike medical CT, the specimen is usually rotated, rather than the X-ray source and detector.

Typical XMT scanner

If the projections are taken through a single plane in the specimen, it is possible to reconstruct a cross sectional image of that plane. In most XMT scanners today, 2D images are recorded, making it possible to reconstruct a complete 3D map of X-ray attenuation. In such cases, because of the divergence of the X-ray beam, it is necessary to use a conebeam reconstruction algorithm. This generally gives only an approximate reconstruction, with errors increasing with distance from the central plane (normal to the rotation axis). By using a spiral locus (translating the specimen along the rotation axis as it rotates) an exact (barring artefacts) reconstruction is possible.

In the ideal case, each voxel of data represents the X-ray linear attenuation coefficient (LAC) of the corresponding volume in the specimen only. This is related to the composition and density of the material within that volume. Thus XMT studies can be used both for pure geometric studies, where the LAC is used only to determine the presence or absence of a phase, and quantitative studies where the LAC is used to determine density or concentration. The latter generally requires a higher signal to noise ratio, requiring high dynamic range detection and long X-ray exposures, a task for which our MuCAT scanner was designed.