Search for a Pet

Find Animal
Welfare Groups

Check us out on:

Science Watch

How X-Rays Work

iStockphoto

Stephen L.Zawistowski, Ph.D., ASPCA

Image is Everything

When I was a kid, the comic books I bought for 12 cents always had an intriguing set of advertisements on the inside back cover. I don't know anyone who actually ordered the stuff, but it was fun to think about what you could do with things like garlic-flavored gum, plastic vomit and most wonderful of all, X-ray vision glasses. In the years since then, I've learned that garlic flavored gum is probably available at a trendy gourmet store, that plastic dog vomit is a much more popular office item in animal shelters than any of us would want to admit, and those X-ray glasses-well, you might be surprised.

In December 1896, experimental physicist Wilhelm Conrad Roentgen announced the discovery of X-rays. He demonstrated that they could pass through solid objects and their presence could be captured on photographic plates. One of the very first "Roentgenograms," an early name for one of these radiographs, or X-ray photographs, was taken of his wife's hand. It clearly showed the outline of her hand and the bones of her wrist and fingers. As X-rays pass through the body, they are absorbed by various structures according to the density of the structure. Since bones are denser then muscle tissue, they absorb more of the X-rays, and the photographic film captures this difference. Within months of discovery, physicians were using X-rays to assist in setting broken bones and detecting the presence of foreign bodies in patients. The emerging field of veterinary medicine also began using X-rays, which helped prove such ideas as large breed dogs' predisposition to hip dysplasia. Roentgen's contribution to science was recognized with the first Nobel Prize in physics in 1901.

Computers Enter the Picture
For the next three-quarters of a century, X-ray radiography progressed as equipment improved and expertise was developed to interpret radiographs. However, X-ray images were still limited to two-dimensional views. That changed in 1972 when British engineer Godfrey Hounsfield and Allan Cormack, a South African physicist, independently developed Computed Axial Tomography, or the CAT scan. CAT scans use computers to assemble a sequential series of X-rays into a three-dimensional (3-D) image. CAT scans take a series of images of a human or animal by rotating around a specific point on the body. Think of the body as a gelatin mold and the fruit suspended in it as the bones and organs. Each of these images shows a slightly different perspective of each organ's placement in that "slice" of the body. As the scanner moves along the body and generates another series of images, it creates another slice. The computer then reassembles these slices into a detailed image of the area scanned. This image can be projected on a monitor or printed on paper, enabling a physician or veterinarian to examine a bone or organ for damage or structural abnormalities, the growth of tumors or the presence of a foreign body. Improvements in CAT scan technology now make it possible for projectors and sensors to spiral around the entire subject, allowing doctors to essentially "walk" through the internal structures of a person or animal. Medical interventions following CAT scans are much more effective, since the location of a problem can be identified with greater precision. Surgical access is also more exact, and radiation therapy for tumors can be more accurately targeted.

The Cost of Advancement
Another important step in medical imaging has been the development of Magnetic Resonance Imaging, or MRI. Whereas CAT scans depend on X-rays passing through the body to generate an image, MRIs utilize electromagnetic radiation, or radio waves, to align the spin angle of protons in the body. Radio waves are directed at the area of interest to excite the protons and alter their spin alignment, and then are quickly turned off. Experience has shown that the protons in different tissues return to alignment at different rates in a characteristic fashion. Special detectors collect this information and computers provide a sophisticated mathematical analysis of these data. The end product is a high resolution image of the area scanned. Unlike a CAT scan, an MRI can take a series of images in any direction without moving the body.

These technologies are being rapidly incorporated into veterinary medicine. They provide valuable data for the treatment of a variety of animal injuries and illnesses. These medical advances come with a price, however, and pet owners may be shocked by the cost of cutting-edge diagnostic tests and treatments. The earnest promise that "I would spend anything to take care of my pet" may be put to the test when "anything" becomes a very large number.

Dr. "Z" is senior vice president of ASPCA Animal Sciences and Animal Watch science advisor.

Courtesy of
ASPCA
424 East 92nd St.
New York, NY 10128-6804
(212) 876-7700
www.aspca.org

Next in How You Can Help Pets: Science Watch:
Foreign Animal Species