Pet Columns, Office of Public Engagement, College of Veterinary Medicine at the University of Illinois at Urbana-Champaign

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Demystifying Diagnostic Imaging


Pet Column for the week of March 25, 2002


Office of Public Engagement
2001 S. Lincoln Ave.
Urbana, Illinois 61802
Phone: 217/333-2907
Jennifer Stone
Information Specialist

When we go to the doctor to have a X-Ray taken, do we really know what is happening to us? Radiographs (x-rays), CAT scans, Magnetic Resonance Imaging (MRI) and Ultrasound help both human doctors and veterinarians diagnose disease every day, but for most people these imaging techniques are shrouded in mystery.

The most well known imaging technique is the radiograph or x-ray. The x-ray machine uses a beam of x-rays, which are projected through the patient onto x-ray sensitive paper film. Dr. Michael Thomas, a veterinary specialist in diagnostic imaging at the University of Illinois Veterinary Teaching Hospital says, "X-rays are able to pass through some tissues such as skin and fluid more readily than other tissues (like bone). The X-rays that pass all the way through and reach the film, make the film turn dark when it is developed while the areas that are not exposed to the x-rays remain clear. Because the x-rays are blocked almost completely by bone, they look clear while a gas cavity looks dark and a fluid/tissue area appears gray.

While the patient is not in any significant danger from a one time exposure to x-rays, repeated exposure can cause problems such as the induction of cancer. For this reason, the veterinarian, doctor, or technician who operates the machine regularly must always wear protective gear.

The CAT scan differs from the radiograph in that it consists of a very thin beam of x-rays, which pass through a cross-section of the body in a rotational manner. The beam can be varied in speed and slice thickness and allows a detailed look at a very specific section of the body. This method is particularly useful for viewing complicated structures. "These image 'slices' can be utilized to view the skull, one section at a time," says Dr Thomas, "or by using a computer, the images can be stacked to create a three-dimensional image of the entire head." This technique can show subtle variations in materials found there such as different types of brain tissue, abnormal tissue, blood and fluid.

The MRI or "magnetic resonance imaging" uses radio waves and strong magnetic fields to create images. The MRI machine produces images by creating resonance (vibrations) in the water found in the body. This causes a radio signal to be emitted from the water in the body, which can be interpreted by a receptor called a coil. The coil produces a cross-sectional image of body water content found in the tissues. This methodology can pick up tiny differences in water density that would be missed by using other methods. Bones are not shown because of their relative low water content, but this is often preferred when the problem lies within tissues other than bone (such as the brain). While it is an effective diagnostic tool, it can be cumbersome to own because it requires a special room equipped with copper shielding in order to block out other waves that could cause interference, such as signals from local radio stations.

Ultrasound uses high frequency sound waves to visualize the body. In studies of the heart, it is used primarily for motion analysis and can be also used to detect turbulence in the blood flow or a change in "texture" in the spleen or liver that can indicate disease. In human medicine, it is probably best known for its use in imaging of unborn fetuses in pregnant women. While all of these techniques are used in veterinary medicine, there are times when some techniques are preferable to others. Ultrasound is sometimes preferable to MRI because ultrasound allows movement of the subject. While it is possible to politely ask your human patients to remain motionless for an extended period of time, this may not be possible with conscious veterinary patients. Dr. Thomas says, "While it is a useful tool, ultrasound equipment is expensive and requires upkeep."

While many veterinarians have x-ray machines and some have ultrasound, few have MRI or CT capabilities. These machines are very expensive to maintain so they are usually found at larger referral institutions such as the University of Illinois Teaching Hospital.

If you have any questions about diagnostic imaging, please contact your local doctor or veterinarian.