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The Pittsburgh machine has also made a difference in the diagnosis and treatment of cervical and ovarian cancers. Blood tests of some women who had already been successfully treated for these malignancies began to show markers suggesting that tumors were recurring. CT scans showed nothing amiss, but PET/CT surveys revealed the precise locations of small new tumors in the pelvic area that surgeons were able to remove before they metastasized.
Despite the limitations of a standard CT, it does a superb job of picturing the internal anatomy of the body. "The CT shines a fan beam of X rays though the human body," Nutt explains, "and makes a series of slices. Almost what you'd see if you took a knife and cut a person in two." Literally slices of life.
These slices are made possible by the CT's detectors, which gauge the attenuation of the X-ray beam as it passes through the body. The attenuation, in turn, is a measure of the mass that the X rays encounter in their passage.
And that measurement is translated into precise images of the body's internal structure by an algorithm programmed into the CT computer. The newest machines, called spiral CTs, take many slices in rapid succession and can now image the entire torso in seconds, a procedure that once took as long as an hour.
The PET scan operates on an entirely different principle. In one application, a solution of sugar, its molecules tagged with a radioactive chemical isotope (usually fluorine 18) is injected into a patient's veins. Like any sugar, it migrates to metabolically active vital organs and tumors, if any, which use it for energy.
As the radioactive fluorine isotope, now concentrated in the organs and tumors, decays, it gives off positrons, the antimatter counterpart of electrons. And when an ejected positron collides with an electron, which occurs almost simultaneously, both particles are annihilated, their mass instantly converted into gamma rays, which the pet machine detects and turns into an image.
If there is a metabolically active tumor in the region being scanned, says Meltzer, "you can see an area where there has been sugar uptake. But it looks like a blob and it's difficult, especially in some parts of the body, to tell exactly where it is." Dr. Steven Larson, director of the PET program at Manhattan's Sloan-Kettering, has his own description of the blob: "It's a little like lighting a match in the blackness of a vast cavern. We detect the match, but the location is imprecise."
In designing the PET/CT to remedy this imprecision, says Townsend, one of the problems that he and Nutt faced was the engineering of the scanner tunnel into which the patient is rolled. "You don't want a very long tunnel that's frightening to patients," he explains. To further minimize the claustrophobic effects, they increased the diameter of the PET/CT tunnel to 28 in., making it far more spacious than the familiar typical magnetic resonance imaging (MRI) tunnels. "For the patient, it's very comfortable and convenient," says Townsend. "They arrive, they have a single scan, and then we have all the information."
A far greater problem came in writing the code to run PET/CT's computer. "We needed and finally created software to control two different imaging systems from one computer console," says Townsend, "something that hadn't ever been done before."