Computerized Axial Tomography
Since the invention of the X-ray machine, it has been apparent to physicians that an efficient and precise method for viewing patients' internal structures was essential. Fluoroscopes could outline bone structure and help locate foreign objects, but were not sensitive enough to show in detail organs such as the brain. During the late 1960s, Alan Cormack (1924-), an American, and Godfrey Hounsfield (1919-), an Englishman, independently developed a method called computerized axial tomography (CAT) scanning by which the internal structure of the body could be seen by assembling X-ray cross-sections, taken along a body axis, into a three-dimensional picture.
Cormack was the first to construct a tomographic device. His initial model used a thin beam of X-rays aimed at one section of the body but repeated from many different angles. This method allowed him to combine the many X-ray pictures into one complete view. Though Cormack published his findings and theories, he received little recognition, chiefly because he lacked a system to process the volumes of information that went into a single CAT scan.
The solution to this problem was the computer. Hounsfield began working on his own CAT scanner in 1967, using a system remarkably similar to Cormack's with one important difference: Hounsfield used computers to collate the X-ray data and create a tomographic picture. The prototype CAT scanner used gamma rays to view inside the body; they required very long exposures, and the first CAT scans (of a preserved human brain) took nine days to complete. This was far too long for patients to endure, so Hounsfield began to improve his design. Later models required nine hours, then nine minutes. His final apparatus could complete its X-ray scan in 4.5 minutes, with an additional 20 minutes for the computer to assemble the information.
Hounsfield predicted that his scanner would not only provide the most accurate view of the body's internal structure, but that it could also identify areas of diseased tissue. The first CAT machine was installed at Atkinson Morley's Hospital in Nimbledon, England, in 1971, and Hounsfield's claims were tested on a human patient a year later. A woman, whose symptoms indicated the presence of a brain tumor, was scanned, and the results indeed showed a dark spot of diseased tissue on her brain. After several other equally successful experiments, Hounsfield patented the CAT scanner in 1972.
The design of most CAT scanners is relatively simple. A series of X-ray scanners are placed around the periphery of a tube-like cavity (large enough for a patient to insert his head, for example). Each X-ray scanner has a detector placed exactly opposite it, and both scanner and detector are built to move together so that, wherever the scanner moves, it is always aimed precisely at its detector. During the CAT scan, the X-ray scanners emit short pulses, usually lasting no more than a few milliseconds. Once this X-ray snapshot has been taken, the scanners and detectors rotate slightly (providing the same view but from a different angle) and another pulse is emitted. After all of the snapshots have been taken, a computer collates the information and constructs from it a complete picture. This can be a monumental task, since many CAT machines use up to 300 X-ray scanners taking 300 snapshots each, resulting in almost 90,000 X-ray "slices." Newer machines have also been designed to scan a patient's entire body.
In the 1970s, a more advanced tomography technique was developed by Michael Phelps and Edward Hoffman, a pair of biophysicists from the UCLA School of Medicine. Their technique, called positron emission tomography (PET) uses radioactive tracers injected into a patient that emit positrons and gamma rays. The scanner "reads" the gamma rays in much the same way that X-rays are scanned in a CAT machine, but a much finer image can be obtained by tailoring the radioactive tracers used to the organs being viewed. For instance, certain tracers bond to neurotransmitters in the brain, so a detailed image of brain functions can be obtained.
CAT and PET scanners have significantly reduced the need for dangerous exploratory surgery, particularly on the brain. They also view soft tissue in unprecedented detail. However, tomographic technology is prohibitively expensive: the machines cost hospitals up to one million dollars each, and for this reason they have been criticized as expensive toys for wealthy establishments. However, CAT scanners are nearly one hundred times more efficient than X-ray machines, since they require less energy per view and use all of the information gathered, rather than the estimated one percent recorded by conventional X-ray machines.
Cormack and Hounsfield shared a 1979 Nobel Prize for physiology or medicine for their work on computerized tomography. It is interesting to note that neither scientist held a doctoral degree at the time of their ground-breaking work. Research continues into various uses for CAT scans. One of the most recently developed uses for CAT scans is to help create a virtual three-dimensional reality via the computer that allow doctors to "fly" through areas of the body, in effect, doing a reconnaissance of diseased areas. The technique will also allow surgeons to "practice" procedures in a virtual reality setting.
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