BURIED AMMONOID: As viewed through megavoltage computerized tomography (CT), a 370-million-year-old ammonoid from the Devonian period, an extinct cephalopod relative of squids, octopuses and the chambered nautilus. Ammonoids died out around the same time as dinosaurs, about 65 million years ago, at the Cretaceous–Tertiary (K–T) extinction event. The TomoTherapy machine's four-million-electron-volt x-rays are energetic enough to penetrate rock, as this image reveals.
ANOTHER AMMONOID?: Although this specimen was obtained under the pretense of being another Devonian ammonoid, the CT scan reveals smooth and uniformly concave septa, or dividing walls, that separate the individual chambers, whereas ammonoids typically had slightly ruffled septa. Additionally, the scan shows a siphuncle, a tube that allowed organisms to adjust buoyancy by increasing or decreasing the amount of gas within the chambers of its shell. The siphuncle was located on the shell periphery in ammonoids but pierced the center of the septum in nautiloids, indicating that this fossil is also around 370 million years old, but a nautiloid.
LIMESTONE SAMPLE BEFORE: Is this ordinary round chunk of limestone solid or hollow? Density measurements showed it to be a bit light, suggesting that it might be hollow (a geode). It is well known, however, that limestone comes in three distinct densities. Thus, the density alone cannot prove whether this is solid or hollow.
LIMESTONE SAMPLE AFTER: Megavoltage CT scanning reveals that this rock is indeed a geode.
SANDSTONE MYSTERY: This megavoltage CT image of a slab of sandstone reveals some imperfections within the rock.
SANDSTONE MYSTERY SOLVED: Upon chipping sandstone away, a shark tooth was discovered within the stony matrix.
Using a novel radiotherapy technology called helical tomotherapy—in essence, the marriage of a computerized tomography (CT) scanner and a radiotherapy linear accelerator—James Welsh, associate professor of medical physics and human oncology at the University of Wisconsin School of Medicine and Public Health, and a group of colleagues have created images of fossil specimens of various types and ages.
TomoTherapy, Inc.'s Hi-Art radiation machine, developed at the University of Wisconsin–Madison, is designed to treat cancer patients with intensity-modulated radiation therapy (IMRT). The CT component provides images of tumor position right before each daily treatment session so that doctors can target malignancies while avoiding healthy cells. The Hi-Art device is designed to rotate around a patient, delivering narrow, potent and precise doses of radiation to tumors without harming healthy tissue nearby—a process that takes 15 to 25 minutes. "I was fortunate to be the first MD to ever treat a patient with this technology," Welsh says. "I suspect these are the world's first megavoltage CT images of rocks and fossils from such a unit."
The use of image-guided IMRT technology has shown promise in treating cancer, and TomoTherapy is expected to soon have competitors. Sharp Grossmont Hospital in La Mesa, Calif., offers patients access to a Hi-Art system, but there are only about 30 medical facilities in the country using helical tomotherapy. This is expected to change after Varian Medical Systems, Inc., in Palo Alto, Calif., gets Food and Drug Administration approval to sell its RapidArc IMRT system. Elekta, AB, in Sweden also offers comparable image-guided radiation therapy technology.
Research is currently being conducted on other possible uses for these devices. "I plan to obtain more images and continue a scientific evaluation of the potential of this clinical machine for paleontological and geological purposes," Welsh says. "I expect a peer-reviewed manuscript will be ready by midyear."
