Computer Tomography, Volumetric Imaging Reveal Details of Ancient Past
LINKÖPING, Sweden, Jan. 27, 2017 — Computer tomography (CT) scanning and volumetric visualization have been used to create a 3D display of the Gebelein Man, a 5,500 year-old natural mummy buried during the late pre-dynastic period at the site of Gebelein in Upper Egypt. Visitors to the Early Egypt Gallery at the British Museum can now interact with this display, even performing a virtual biopsy on the Gebelein Man in real time with the touch of a hand.
The Gebelein Man. Courtesy of Visualization Center C.
The technology allowing visitors to explore the Gebelein Man is an image-generation technique called "volume rendering," that is, the rendering of images from a volumetric dataset on a computer screen. The technology is also known as direct volume rendering (DVR), as the image is generated directly from CT data without any intermediate representation.
The most common approach to DVR is raycasting, where the stack of images is traversed by simulated rays that collect the color contribution from each sampled point as they traverse the stack. Each ray estimates how simulated light is reflected and attenuated as the ray propagates from the user's eye through the volume; and the final footprint of color and opacity is then deposited onto a pixel on the screen.
Computing such a ray for every pixel on the screen showing the image, and doing so at least 30 times per second, results in an interactive visualization of the interior of the scanned body or artifact. For a realistic high-quality final result, the ray-casting process must consider how light sources affect every sample and how samples globally affect each other in a participating medium. This multiple scattering effect is referred to as volumetric illumination. There are numerous approaches to volumetric illumination, ranging from methods based on approximations of light transport to more physically based methods with higher computational demand.
For the Gebelein Man display, 10,000 virtual slices of the mummy were imaged using CT, some as thin as 0.3 mm. Rapid graphics processors were used to create volumetric images of the CT scans, which were then stored in the display table.
"The table displays 60 images per second, which our brain interprets as continuous motion. Sixty times each second, virtual beams, one for each pixel on the screen, are projected through the dataset and a color contribution for each is determined. We use the latest type of graphics processor, the type that is used in gaming computers," said Patric Ljung, senior lecturer at Linköping University.
The degree of reflection and absorption of the x-rays by the mummy is recorded by the CT scanner and converted with the aid of a specially developed transfer function to different colors and degrees of transparency. Bone, for example, gives a signal that is converted to a light gray color, while soft tissue and metal objects give different signals that are represented by other colors or structures.
Using touchscreen technology, visitors can, for example, peel away the desiccated skin of the mummy to display only the parts of the image that consist of bone. The history of the Gebelein Man can unfold for the visitor in the same way researchers have used visualization technology to establish the Gebelein Man's cause of death (murder, with a metal blade the most likely weapon).
"It was challenging to obtain sufficiently high performance of the visualization such that visitors can interact with the table in real time, without experiencing delays. Further, the interaction must be both intuitive and informative," said professor Anders Ynnerman.
The presence of metallic objects, such as amulets, can pose a challenge to CT scanning, as these objects potentially block x-ray radiation from reaching the sensor array, and the signal reconstruction suffers. Artifacts were reduced through modifications to scanning protocols and through use of multi-energy CT, as well as through reconstruction algorithms.
"Allowing a broader public to visualize scientific phenomena and results makes it possible for them to act as researchers themselves," said Ynnerman. "We allow visitors to investigate the same data that the researchers have used. This creates incredible possibilities for new ways to communicate knowledge, to stimulate interest and to engage others. It's an awesome experience — watching the next generation of young researchers be inspired by our technology,"
The research was published in Communications of the ACM (doi: 10.1145/2950040).
A full-body virtual autopsy of an ancient Egyptian mummy showed visitors he was likely murdered. Courtesy of CACM.
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volumetric imaging
A technique to capture a 3D volume of activity in a single 2D image. This contrasts with conventional imaging techniques that produce two-dimensional replications from one viewing point.
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