Computed Tomography (CT) Scanning
Computed Tomography (CT) is a very useful technique for non-destructively examining the interior of museum objects in three dimensions. A regular X-radiograph provides a picture of the interior of an object projected onto a flat plane (the film, phosphor plate, or digital detector). These projection X-radiographs can be difficult to interpret, particularly when the object is made of many pieces with similar x-ray attenuation or when features in the object overlap. Computed tomography requires taking a series of X-radiographs from different angles around an object and then mathematically reconstructing the volume which would have caused this series of projections to appear. Medical CT scanners, like the one shown in the image above, provide a convenient way of CT scanning many of the objects from the Anthropology collections.
The results of CT scanning are usually delivered as a series of grayscale images of evenly spaced 'slices' through the object. The lighter grays represent more attenuating material and the darker grays represent less attenuating material. The stack of slices can be rendered in three dimensions and areas with similar X-ray attenuation can be segmented out and/or false-colored for further study.
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The following images illustrate the technical examination of a Late Period Egyptian mummified bovid. It is thought to be from the Ptolemaic Era (330-32 BCE).
After CT scanning, the slices from the CT scan have been rendered as a volume. You can see that the wrappings are rendered reasonably faithfully.
We can use software to remove the wrappings from the volume and look at what is underneath. Here you can see what remains of the animal that was mummified.
By selectively making some areas more transparent, we can see additional features. There is a metal hook (rendered green) which was left in the animal's hind-quarters. There is also a piece of cane (rendered orange) thrust through two of the cervical vertebrae and into the foramen magnum of the skull, presumably to ensure that the head was kept in the required position.
A maximum intensity projection taken from the CT scan allows us to see detail in the skull. The images of the skull allowed the animal to be identified as an early adult of the North-African sub-group of Gazella dorcas.
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This Japanese polychrome wood statue is dated to the mid c.18th CE. The statue was assembled from multiple pieces of wood which were joined with wooden dowels. The wood structure was then covered with a white gesso and the object was painted. Cracks have opened in the statue and there has been at least one previous (undocumented) restoration attempt.
Film X-radiograph of the statue shows that the eyes are very attenuating, that there are metal nails and staples added to the structure, and suggests that the interiors of the torso and the head are hollow. The metal elements are probably part of an attempt to 'repair' the statue after the wood elements began to separate. The highly attenuating eyes are puzzling.
Portable X-ray fluorescence (pXRF) was used to determine why the eyes were so attenuating. The eyes are difficult to reach with the instrument because of the surrounding eyelids.
We decided to compare the spectrum of surrounding gesso (blue line) with the spectrum taken at the proper right eye (red line) which inevitably also included some gesso in the analyzer's field of view. A thin polyester sheet was used to isolate the instrument and prevent contamination. The gesso is high in calcium (presumably a calcium carbonate white). Comparing the two traces shows that the eye has a significant lead content compared to the surrounding gesso. It is likely that the eyes are so attenuating because they are made from a lead-based glass.
Selected slices from the stack of images produced by a medical CT scanner. The highly attenuating eyes create image 'artifacts' in the reconstructed slices, reducing the amount of information that we get from this interesting area. Since there is no X-ray penetration through the eyes and therefore the mathematical reconstruction has very little data to work with. Similar artifacts are produced by metal construction elements that have sufficient thickness to block X-rays (e.g., the metal nails).
A slice from lower down the statue allows us to see the individual pieces of wood and their grain direction. The painted gesso shows as a white layer surrounding the wood. Some losses of gesso are the result of gaps between wooden elements opening, others are from tangential cracking of the wood. You can also see that the dowel which would have held the statue's left hand in place does not actually make contact with the left forearm.
Volumetric reconstruction of the CT slice data allows us to non-destructively cut open the statue and examine the different pieces of wood and their grain directions. The 'artifacts' from the attenuating eyes have been edited out with the loss of some information, but there are still some interesting features in the construction of the head. There are three pins, probably made of bone, holding a trapezoidal wood block at the inside of the face. The wood block presumably plays some role in keeping the eyes in place.
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