39th Annual Meeting – Research and Technical Studies Morning Session, June 2, “Medical Computed X-Ray Tomography and Volumetric Reconstruction for the Technical Examination of Organic/Composite and Ceramic Objects” by JP Brown

This talk presented case studies to demonstrate the application of computed tomography (CT) scanning to archeological objects. According to Dr. Brown, CT is usually available in most hospitals and can often be used for free in the evening. JP recommended first taking a regular x-ray of the object to elucidate its general construction. He did not go into details about the technical aspects of CT and image processing; however, this topic was covered by Hai-Yen Nguyen’s talk earlier in the session (An Open-Source Workflow for the Visualization of CT Data in Art Conservation and Archaeology).

Resolutions of 0.3-1.0 mm can be obtained by CT. The results are presented in Hounsfield units (HU), a scaled measure of the attenuation of radiation due to the material. Water is defined to have 0 HU, while air is defined to have -1000 HU. Typical HU values for metal and bone are 3000 HU and 1000 HU, respectively. Many image-processing techniques, such as the application of false-color, rely on the different HU values of different materials.

Case 1: Animal mummy

The CT images clearly showed that a piece had been inserted into the mummy, probably to hold the head at a desired angle. The detail obtained of the skeleton allowed for the species to be identified as a type of gazelle common in Egypt.

Case 2: Polychrome Japanese sculpture

The CT showed the grain of the wooden object in great detail, enough that non-invasive dendrochronolgy could be possible. This piece demonstrated the problem of having highly attenuating materials in the object – the bright images caused by leaded glass eyes obscured some of the nearby details. However, false-color rendering was able to show areas of gesso and older conservation treatments on other areas of the sculpture.

Case 3: Moche pottery

One of the aims of this study was to check for the presence of organic residues inside the vessels. This was done by comparing the HU values of the pots to those obtained from samples of various modern food residues. Although the CT of one of the pots did not indicate that food residues were present, the image showed a pattern of holes suggesting that the vessel was designed to produce sound from blowing air. A second pot was simply a conch shell, and the CT confirmed the expected internal structure. Interestingly, material inside a third pot did indeed exhibit attenuation values matching those of the test food samples. The material was subsequently collected on a swab, and SEM images indicated that the organic material was likely charred plant stems.

Case 4: Restored archeological stucco (possibly Sasanian)

The sculpture depicted the head of a king. The crown portion of the object, the shape of which could be used to identify the specific king, had been largely restored. Although it was possible to visualize only the original material by manual segmentation of the restored portions, not enough of the original remained for the identity of the king to be determined.

Several interesting points were raised during the question portion of the talk. It is known that x-rays affect the results of thermoluminescence (TL) dating, though it is not known to what extent. As a precaution, JP recommends removing a sample before CT scanning if TL may be performed later. Another interesting question regarded recommendations for approaching a hospital. JP suggested contacting the chief radiologist first, or ideally, finding a teaching hospital with a research radiologist. He mentioned that eventually an administrative/financial person will also need to be contacted and that having an exciting story about your proposed work will increase the chances that the hospital allows use of their CT machines.