2.3 From X-Rays to Computed Tomography

2.3.1 X-Rays and Computed Tomography: An Introduction

Wilhelm Conrad Röntgen and the First X-Ray; the hand of his wife, Anna Bertha (Image from the Röntgen Museum). On the right is an X-Ray taken in c. 1896. Click image to enlarge.
Did you ever have a röntgenogram? The answer is probably yes. A röntgenogram is nothing more than an X-ray, which many of you are familiar with. In 1985, Wilhelm Conrad Röntgen, a German physicist, discovered the so-called X-ray, when in an experiment studying the phenomena accompanying the flow of electric currents through a partially evacuated glass tube (cathode-ray tube), he realised that a piece of barium platinocyanide became fluorescent. When he started further investigating this phenomenon, he discovered that many materials of varying thickness, including aluminum wood, and paper become (variably) transparent to this new form of radiation when recorded on a photographic plate. As he couldn't fully understand the nature of this radiation produced by cathode rays, he named them X-Rays. The first X-Ray he took was that of his wife's, Anna Bertha, hand wearing a ring. In that X-Ray the flesh was more permeable than the bones and the ring, and therefore, the latter cast a harder shadow than the former (Nobel Lectures 1967). In 1901, Röntgen was awarded the first Nobel Prize in Physics 'in recognition of the extraordinary services he has rendered by the discovery of the remarkable rays subsequently named after him.' (The Nobel Prize in Physics 1901).
Mummies

X-Rays of a cat sarcophagus (left) from the Manchester Museum, University of Manchester; of a mummified cat (bottom right) and of a dog and a bovine (top right) from The Brooklyn Museum Sources: Kahn, E. M. (2015). Animal Mummies Unwrapped. The New York Times. Oct. 2, 2015, Section C, Page 27 ttps://www.nytimes.com/2015/10/02/arts/design/animal-mummies-unwrapped.html / Eric A. Powell (2014). Messengers to the Gods. Archaeology Online, March/April 2014. https://www.archaeology.org/issues/124-1403/features/1724-egypt-animal-mummies-brooklyn-museum
In 1898, three years after the invention of X-Rays, the method started being used in archaeological investigations, firstly for mummified bodies. This was especially critical for the field of archaeology, since in order to investigate the inside of wrapped mummified body, researchers had to conduct a direct examination, the so-called autopsy. However, the side-effect of that is that in the case of mummies, they had to be unwrapped and essentially destroyed, since they cannot be reassembled. There is also one other drawback with regular X-Rays, since they only produce flat, two-dimensional representations. For example, if a mummy included a piece of jewelry, the X-Ray wouldn't be able to show where it lies, e.g. on the surface, below the wrappings or inside the body (Hughes 2011, p. 59).

Computed Tomography was invented by Godfrey Hounsfield, who used to tinker as a child by taking apart electronics and devices. As he said in at the time of his Nobel Prize award:

At a very early age I became intrigued by all the mechanical and electrical gadgets which even then could be found on a farm; the threshing machines, the binders, the generators. But the period between my eleventh and eighteenth years remains the most vivid in my memory because this was the time of my first attempts at experimentation, which might never have been made had I lived in a city...I constructed electrical recording machines; I made hazardous investigations of the principles of flight, launching myself from the tops of haystacks with a home-made glider; I almost blew myself up during exciting experiments using water-filled tar barrels and acetylene to see how high they could be waterjet propelled (Nobel Lectures 1979).


As an electrical engineer, Hounsfield joined the EMI Laboratories after WWII. His invention of CT Scanning was based on the idea that the contents of a box can be determined by taking X-Rays around an object. Although his idea was originally rejected by many radiologists, Dr. James Ambrose, collaborated with him, also providing him with samples to test his prototype on. In the early 1970s, only head scanning prototypes were developed, while in 1976 body scanners were also produced and distributed to medical facilities in England and the USA. In his 1973 publication at the British Journal of Radiology, Hounsfield describes CT scanning:

a technique in which X-ray transmission readings are taken through the head at a multitude of angles: from these data, absorption values of the material contained within the head are calculated on a computer and presented as a series of pictures of slices of the cranium. The system is approximately 100 times more sensitive than conventional X-ray systems to such an extent that variations in soft tissues of nearly similar density can be displayed. (p. 1016)

 
Contrary to X-Rays, which can only produce a two-dimensional representation of the scanned object/body, Computed Tomography (from the Greek tomos meaning 'slice' and grapho meaning 'describe') produces volumetric data, which can be used to generate 3D models. CT works similarly to X-Ray devices; there is an X-Ray source and X-Ray detectors opposite the X-Ray source. The device takes hundreds or even thousands of scans (2D digital radiography projections) in a 360-degree rotation around the object. In other words, the object is scanned in consecutive slices which together form a complete image of the object in cross-section. In a post-scanning phase, the 2D images are processed and reconstructed in a three-dimensional volume, therefore allowing the observation of the object at any 'slice' and angle (Applbaum & Applbaum 2005, p. 233). According to Hughes (2011, p. 60), CT scanning is ideally suited for heritage artefacts, since techniques such as Magnetic Resonance Imaging (MRI) require enough moisture to work, which in most cases is not preserved in such objects.  

CT Scanning

Computed Tomography: From 2D projections to 3D volumetric models.
Click image to enlarge

CT Scanning, although mostly used in medicine, as a diagnostic tool, has become very popular in cultural heritage (and beyond, e.g. in engineering) due to its non-destructive nature. It is a relatively and extremely accurate technique, which however, is not always easy to be applied to heritage artefacts. Such equipment, can typically only be found in hospitals and medical laboratories, as its very high cost of obtaining and maintaining it, make it unaffordable for non-medical related applications. There have been many cases, where heritage projects develop collaborations with medical facilities, but of course, the bureaucracy involved in transporting the artefacts (e.g. from a museum) to such a facility, often poses insurmountable difficulties. In the following page, you will explore some characteristic case studies for a range of materials and object types that demonstrate the potential of the method in the study of material culture.   


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References

Further Reading