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C. Satan Rigl a' ~ patient lies quietly CAT scan's x-ray tube As from hundreds of .,..~.~.~.~.~.~.~.~,~.~.~'~'~'~'~'~'~'~'~'~'~'~'~'~'~'~'. "''' '''''"';"'''''"''"'";'"'";": .......................................................... .................................................................... .................. ......................... I : ..~;~nce 1895) when .-~nan physicist Konrad ...... passing through the patient. With this data, the CAT scan computer generates an image showing the structure in that cross section of her head. A young man's vertebrae, broken in a motorcycle accident, appear almost lifelike in ~ three-dimensional image created from dozens of I CAT scans. CAT scans are particularly good for showing the fine detail of bone structure. I 22 1 roenigenogram, x-ray pictures have been clouded by shad- ows of bones, teeth, and tissue piled together on one piece of film. But the marriage of modern computers to this old technology has given birth to a machine that produces clear, ~ detailed images of things i inside the body. The machine is called a CAT scan, for computerized axial tomography scanner. The wedding of computers to other diagnostic techniques has subsequently led to ultra- sound imaging, magnetic resonance imaging (MRI), and other new procedures that peer inside the body without resorting to surgery. Indeed, many doctors believe that more progress has been made in medical diagnos- tics since the CAT scan was introduced than in all previous medical history. CAT has so far had the widest impact of the new imaging technologies. It has saved untold numbers of lives by quickly finding tumors, infections, bleeding, and blood clots that would have been found too late or missed altogether by regular x-rays or other diagnostic techniques. A normal series of 30 scans is performed in about 20 minutes and usually without injections. CAT scans have reduced the need for some diagnostic I techniques that require uncomfortable injections and a stay in the hospital. The procedure has also revealed to researchers brain abnormalities in some persons suffering from schizophrenia, alcoholism, manic-depressive illness, and Alzheimer's disease. CAT scans locate brain tumors precisely for radiation treatment and are used to build three-dimensional models for reconstructive surgery. CAT is especially good at finding internal injuries because it easily distinguishes between blood and E N G ~ N E E R ~ N G A N D T H E A D VA N C E M E N T O F H U M A N W E ~ FA R E
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l tissue. Some hospitals now have a CAT scanner in the emergency room, and the U.S. Army is developing a rugged, lightweight one that can be transported by truck or helicopter to medical stations in combat zones. CAT-often shortened to CT, for computed tomography-doesn't make a flat picture on photographic film, like a normal x-ray. Instead, it constructs a television image C AT 5 C A ~ I A cross on this three- dimensional image generated from a series of CAT scans marks the location of a tumor near the center of the patient's head. Color coding shows sensitive areas, such as the eyes, spinal cord, and lungs, that should be protected from unnecessary radiation when the tumor is treated. 23
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An enhanced MRI scan shows a tumor growing in the spinal cord of a 4-year-old girl, who lost her ability to walk. A doctor quickly removed the tumor, and the girl eventually recovered the use of her legs. 24 based on density measurements of a "slice" of the patient. The process is something like seeing a slice of salami without cutting the meat. To measure density, the CAT scan's x-ray tube shoots a thin beam edge-to-edge through a slice while circling the patient. Detectors circling opposite the tube measure the strength of the emerging beam-and therefore the density of the slice from hundreds of points around the patient. With this information, the CAT scan computer calculates the density for each of more than 250,000 tiny bits of tissue in the slice. It then displays these values in a television picture showing density patterns that reveal the fine structure of the slice. The practical CAT scanner emerged from the confluence of several new technolo- gies. The most important developments were low-cost minicomputers and, later, parallel- array computers that could perform many calculations at the same time. These high- speed computers and the mathematically complex programs to operate them made it possible to run millions of calculations for each slice. In 1979 the Nobel Prize in physiol- ogy or medicine was awarded to British research engineer Godfrey Hounsfield for pioneering the CAT scan and to U.S. physi- cist Allan Cormack for work on the mathe- matics behind it. The first CAT scanners were E N G ~ N E E R e N G A N D T H E A D VA N C E M E N T O F ~ U M A N W E L FA R E
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installed in Great Britain in 1971 and in the United States two years later. Today they are used in more than half of U.S. hospitals and in hundreds of clinics. Very small x-ray detectors and ultrasen- sitive signal amplifiers have greatly increased the speed of CAT scanning. An early scan took about 4~/: minutes. Today it takes less than 2 seconds as a scanner's 1,000 detectors each record 2,000 separate measurements. The machine's sensitivity has improved, too. Now, for example, CAT can detect a liver tumor of almost the same density as normal tissue. The difference would not show up on a regular x-ray. Tomorrow's scanner will produce even more detailed images as scan slices become progressively thinner and scan times decrease even more. The patient will move continuously through the scanner, like a car on an assembly line, completing a 40- slice series in about a minute. And it may soon become routine for a treating doctor to beam the scan image via satellite to a consulting specialist thousands of miles away. Small, powerful computers have been applied to other diagnostic imaging tech- niques since the CAT scan. Each uses a different means of probing the patient, but the image is constructed by computer. Low- level radioactive isotopes are injected into the blood during positron emission tomography (PET) and single-photon emission computed tomography (SPECT). With digital subtrac- tion angiography (DSA), a computer sub- tracts an earlier x-ray image from another one taken after a contrast substance has been injected into the blood, leaving the clear contrast outline of blood vessels on the second image. Ultrasound devices bounce high-frequency sound waves off internal surfaces to indicate their position and motion. The most promising computer-assisted technique, though, uses magnetic fields and radio waves to coax information from the nuclei of atoms in body chemicals. This nuclear magnetic resonance imaging, or MRI, is particularly good for examining the brain and spinal cord, whose surrounding bone is invisible to it. CAT culminates the development of Roentgen's technology. And although traditional x-ray pictures still account for 80 percent of medical diagnostic imaging, they may one day be completely replaced by computer-generated video images. The . ~ a_ continued development of CAT, ultrasound imaging, and especially MRI should further reduce the need for exploratory surgery or invasive imaging techniques such as those involving injection of contrast agents that may occasionally cause allergic reactions. In fact, in 10 or 15 years, MRI may be able to perform an entire diagnostic chemical analysis of a patient, without breaking the skin. CAT SCAN I Doctors discuss features revealed by MRI images. MRI employs radio waves and magnetic fields to produce images in which bones are invisible. It is therefore very useful for looking at soft tissue inside the skull and spinal column. 25