Galleries / The Cruelty-Free Dissection

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Anne Casselman; published February 5, 2008

Scan, Baby, Scan

The Modern Machine

Beyond Medicine

Consumer Applications: From Sap to Homes

A Biologist's Dream

Scanning for Darwin

A Closer Look at Fossils

Scans from Outer Space

The Majestically Tiny Seahorse

<p>Today, computed tomography (CT) scans are a staple of medicine, giving doctors a glimpse of one's insides without having to cut someone open. Generated by collecting hundreds of x-rays around one axis of rotation, the scans are compiled to give an in-depth snapshot of the body's insides, like this scan of a patient's upper chest, showing the shoulder blades, part of the spine, and both lungs.</p><p>CT scans were originally referred to as EMI scans since they were developed by at EMI's Central Research Laboratories in Hayes, England, by Sir Godfrey Hounsfield. His work on the CT scanner was indirectly bankrolled by the Beatles, whose singles were making EMI a fortune in royalties at the time.</p>

<p>Since it's Beatles-funded inception, CT technology has steadily improved resolution, magnification, and speed.</p><p>In this snapshot, Ghent University's micro CT scanner, one of the most sophisticated CT scanners in the world, occupies a 12-by-10-foot room at the Centre for X-ray Tomography. The set up is similar to that of a medical CT scanner, only instead of the X-ray source rotating around the still patient's body, the sample is rotated in front of a stationary X-ray source. The X-ray source protrudes from the left of the photo and the black cylinder at the bottom of the center of the photo is the rotation motor, while the bottom right corner shows the X-ray detector. </p><p>In the same way that a shadow puppet will grow larger the closer you move your hands to the light source, the resolution of the scan can be improved by moving the sample closer to the X-ray source. This particular CT scanning equipment, which was built two-and-a-half years ago, can scan objects down to a resolution of just under one micron, or one millionth of a meter.</p>

<p>As technology improved, CT scans expanded beyond the medical arena to find countless applications in science and engineering. After all, the human body is not the only thing that modern science wants to peek inside without cutting open. Under a scan, the average cell phone is a thing of complicated beauty. This virtual model illustrates how CT can be used for quality control of consumer goods--like spotting failed batteries or loose wiring--without tearing them apart. This cell phone is just about the largest sample that Ghent's scanner can handle. </p><p>To take an image of, say, a cell phone at Ghent, roughly 1,000 images of the sample are shot around an axis of rotation. This usually takes about one hour. Special image-rendering software then takes these images and compiles them into one three-dimensional image. "At that point you really have a virtual object and you can do anything with it as if it were a real object," explains Manuel Dierick, a researcher at the University of Ghent Centre for Tomography. "The basic advantages are of course, first of all, it's non-destructive."</p>

<p>Here is a micron-size cube from the wood of an Elm tree. This miniscule cube was scanned to study the three-dimensional vessel structure and the micro-scale morphology of the wood. Wood scans can also be used to model sap-flow simulations, to study fungus development, and even to model the penetration of paint layers.</p>

<p>A great variety of scientific samples come through Ghent's facility, ranging form medical stents to blocks of concrete. The most common type is biological specimens, often from cases where there is only one specimen that the researchers are loathe to dissect. </p><p>"The CT data is of course very interesting and very powerful because it can help us to get, in a rather quick fashion, non-distorted and detailed information on the 3D skeletal system," says Dominique Adriaens, director of the Evolutionary Morphology of Vertebrates group at Ghent, who has used the university's CT-scanning facilities on several occasions. </p><p>Pictured is a high-resolution scan of a Candiru catfish (Vandellia cirrhosa). This Amazonian fish is actually a parasite that latches onto the gills of other fish and sucks their blood for sustenance. This index-finger-sized parasite is best known for its horrid habit of confusing unsuspecting urethras for prey, where its specialized feeding apparatus will lodge itself. </p><p>"It's a very special fish," say Adriaens, who is interested in learning how its specialized gill cover works and evolved. "Let's say it's the only fish that has made it into the Journal of Urology." The fish's soft tissue is depicted in red while its skeleton is in grey.</p>

<p>What's helpful for current biology can be just as helpful in tracing evolutionary origins, such as Darwin's famous obsession with finch beaks. Everyone knows about the beaks of the Darwin finches, says Adriaens. "Our interest is to go beyond the beak and look at morphology of the whole musculoskeletal system." </p><p>His colleague Anthony Herrel, an evolutionary biologist at Harvard University, is taking the 3D reconstructions of Darwin finch (Geospiza fortis) skulls and using them for something called "finite element modeling," an approach used in engineering that applies virtual force to objects. In this way Herrel can play with various Darwin finch beaks and skulls to understand the forces they have evolved to withstand while feeding.</p>

<p>Tim Senden, from Australia National University in Canberra, was fortunate enough to discover the world's most intact specimen of the Devonian fossil fish, Gogonasus andrewsi, in the Kimberley, Western Australia, in 2005. He chalks the remarkable 384-million-year-old find up to "beginner's luck." </p><p>"This specimen is unique and so a non-destructive technique is crucial," writes Senden. Handily enough, Senden works at the university's X-ray CT facility. After a scan, Senden found that not only could this fish breathe air, but its bone development suggests that it could walk on the bottom of a shallow sea. The specimen was remarkably well preserved. In this composite image the actual fossil is shown in the lower left corner while the CT image is depicted above it.</p>

<p>The University of Texas High-Resolution X-ray CT Facility in Austin, Texas, has been operating for eleven years, during which time it has been providing CT as an invaluable imaging tool for the U.S. scientific community. </p><p>"I think it's been a sort of kid-in-the-candy-store experience of having so much great and new science come here and originate here," says Richard Ketcham, a senior research scientist who manages the facility. They have received the most press for their work in paleontology, says Ketcham, but are busy helping other disciplines too. </p><p>"We've applied scan data to every sub-discipline of geology that we can think of so far," says Ketcham. These two images depict a 19-centimeter-wide meteorite discovered in the Patuxent Range, Antarctica, during a 1991-1992 expedition.</p><p>The CT scan allows geologists to peer inside the meteorite and get a glimpse the chemistry within: Blobs of metal (yellow) and troilite, a form of iron sulfide (in purple), are caught in the act of separating and giving off gas during a melting event, probably caused by an asteroid-meteorite impact.</p>

<p>For more on CT scanning and the seahorse, visit the <a href="http://discovermagazine.com/2008/mar/using-x-rays-to-do-cruelty-free-dissection" />story</a> in the March 2008 issue.</p>

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