A mummy's secrets

Stuart Stock is a research professor of cell and developmental biology at Northwestern University (NU).

In 2018, NU's Block Museum of Art held an exhibit titled "Paint the Eyes Softer: Mummy Portraits from Roman Egypt." These portraits are especially important since few other paintings have survived from the Greco-Roman tradition. The centerpiece of the exhibit was the "Hawara Portrait Mummy Four" — a mummified child buried with a luminous painted portrait. University researchers used computed tomography (CT) to investigate subsurface deterioration and reveal details of the encased body and items within the wrappings.

Stock was a plenary speaker at SPIE Optics + Photonics where he covered the provenance of the mummy, what CT tells us about the person and object enclosed, and what secrets x-ray diffraction can reveal.

You're known as one the pioneers of x-ray micro-computed tomography (microCT) and position-resolved microbeam x-ray diffraction. Could you describe these technologies and your role in developing them?

The first thing to understand is that I am not the person who makes new instruments. Instead, I think of myself as someone who looks at emerging imaging and diffraction technologies and imagines how they might be combined or used to study samples in novel ways. The mummy study described below is an example.

I have been using microCT since 1985, and, in fact, my first paper on the subject was in an SPIE proceedings (X-ray Imaging II, SPIE Vol. 691, 1986). In all forms of (x-ray) CT, one shines x-rays through an object from many different directions. The resulting set of radiographs are then mathematically recombined to give a 3D map of the contents of the object. As the prefix indicates, microCT is a microscopic version of clinical CT scanners.

I have been doing position resolved x-ray diffraction even longer than I have been doing microCT. In x-ray diffraction, a beam of x-rays scatters from the specimen, and crystalline materials reinforce the scattering in certain directions related to the periodicities of the atoms within the crystals. The resulting diffraction pattern is a kind of fingerprint of the material(s) encountered by the beam. In position-resolved x-ray diffraction, one makes a very narrow beam of x-rays, smaller than the diameter of a hair. One scans the beam across the sample and observes the resulting diffraction patterns. Typically, one uses synchrotron x-rays from a storage ring such as the Advanced Photon Source, but people also do this kind of mapping in their home laboratories.

What led to your interest in regenerative medicine?

Although my work relates to regenerative medicine, my focus is a bit different. I am interested in mineralized tissues like bone and tooth. These tissues are composites of nanoparticulate calcium phosphate (a mineral similar to hydroxyapatite) and of collagen; the resulting material has remarkable properties that appear to depend on the way mineral interacts with the collagen. We know very little about this interaction, and hence, how bone actually works as a material. I work on a number of different projects tied to this central idea, including studies of human and primate hand bones and how the mineral phase is organized differently in different parts of the bone and of bone healing, for example, in spine fusion operations.

The Hawara Portrait Mummy No. 4 (also known as the Hibbard mummy) is a portrait mummy of a young girl, approximately 5 years old. Credit: Block Museum of Art/Northwestern University

You've been working with "Hawara Portrait Mummy No. 4" and found that the remains were that of a child and not an adult as was previously thought. What were some of the other unusual discoveries about this mummy?

Before we did the CT scan of the mummy, we saw radiographs showing the image of a circular, highly absorbing, 5-10 mm diameter object superimposed on the mummy's abdomen. We wondered what the object was made of and knew that CT could only tell us how absorbing it was and not what material comprised it. For example, minerals such as calcite, jasper, or malachite have nearly identical CT contrast. The CT data provided us a 3D "roadmap" of where to look, and x-ray diffraction identified the object as calcite, calcium carbonate which was quite unexpected, probably from a quarry near where the mummy was found. We think that the calcite object is a scarab, a sacred symbol of rebirth in the ancient Egyptian culture, often used to spiritually safeguard incisions made during mummification. Much higher resolution, whole body CT scanners need to be developed before we will be able to see whether the putative scarab is, in fact, carved.

What current (or future) projects involving microCT and/or X-ray diffraction are you most excited about?

Unlike many other fish, sharks do not have a skeleton of bone, and my current research concentrates on understanding the mineralized cartilage of the shark backbone. We hope that understanding how this tissue functions mechanically will give insight into bone. My colleagues and I have been busy with microCT imaging of shark vertebrae and developing novel approaches to map the 3D organization of crystalline material in the vertebrae, versions of which we have been applying since 2005.