Sep. 26, 2001 COLLEGE STATION, - Tumors, seizure disorders and cardiac diseases are best detected at early stages with a technique called Positron Emission Tomography, but each PET scan costs about $2500 - until recently at the patient's personal expenses.
Now PET scans have been approved for medical insurance reimbursement, and physicists at Texas A&M University and the University of Texas at Austin are working to improve PET scanners to make them even more accurate in the detection of abnormalities in human organs.
PET acts like a camera that produces images of an ailing organ by imaging radiation produced from energy related to the metabolic rate of the organ; the harder the organ works, the brighter the image.
"About 60 percent of these devices are used for brain research," says John A. McIntyre, a professor emeritus at Texas A&M who has been working on PET scans for the last 25 years. "Physicians inject glucose labeled with a radioactive chemical in the patient's body. The brain burns glucose, so the glucose goes to where the brain is working, and since the glucose produces radiation, the PET scan can image it. So it is a powerful way to look inside the brain."
A radiation-producing chemical emits particles called positrons. When a positron meets an electron from a nearby atom, the positron and the electron annihilate, creating two gamma rays moving in opposite directions.
"When scientists realized that the two gamma rays come out opposite to each other," McIntyre says, "they realized that if you could detect them, then somewhere on the line between them was the source."
To detect the pairs of gamma rays emitted by the radioactive source - and subsequently the damaged part of the brain - scientists surround the source with gamma ray detectors called scintillators.
"The instrument is a big ring," McIntyre says, "with slices inside the ring, just like inside a CAT (Computer-Assisted Tomography) scanner, each containing a scintillator.
"The challenge is to find out precisely where the source is," he adds. "When gamma rays have been detected in two opposite detectors in the ring, you draw a line between the detectors. Then you take another pair of detectors that have detected two other gamma rays, and you draw a second line. The source is at the intersection of these two lines."
Sharp pictures of the source depend on the size of the detectors. The larger the detector, the broader the reconstructed line and the fuzzier the picture.
"All you know is that the gamma ray hit the detector somewhere, but you do not know precisely where," McIntyre says, "so if you have a detector that is one centimeter across, the source of the gamma ray will always look one centimeter across. The width of the reconstructed point cannot be any smaller than the size of the detectors."
So McIntyre and his collaborators decided to make an instrument with more than one ring of detectors. They built a prototype with eight concentric rings of detectors so that, in addition to knowing the angular position of the gamma rays, they could also determine the gamma rays' radial positions.
The detectors in the radial direction are each tilted by a small angle so that each detector gives unambiguously the positions of eight points instead of one, enhancing the quality and sharpness of the picture.
To stop the gamma rays, high-density detectors are needed. Current PET scanners use crystal scintillators usually made of bismuth germanate oxide (BGO). Since McIntyre uses plastic scintillators which are one-eighth as dense as crystal scintillators, his device is radially eight times as thick as current PET scanners.
Plastic scintillators have many advantages over crystal scintillators: they are cheaper, they collect more light, and they collect it faster, McIntyre says. Indeed the ratio of the amount of collected light over the collection time is 480 times larger for a plastic scintillator than for a crystal scintillator, he adds.
"It is important to stop as many gamma rays as possible," says John Paulson, research associate of physics at the University of Texas at Austin, who recently finished his doctoral thesis on McIntyre's prototype and is now developing computer simulations to improve it. "These gamma rays provide the information for the image produced by the PET. And, the number of gamma rays available is limited by the amount of radioactivity that can safely be injected into the patient."
"We are now working on publishing all the results that we have," Paulson says. "New concepts are not accepted by the scientific community until they have passed the inspection of peer review and have been published in a scientific journal. After publications, the capabilities of the Texas A&M tomograph will be recognized."
Because of its cost (about $2 million), a PET scanner is usually affordable only for use in large medical centers. But PET scanners are also being shared by smaller hospitals. Trailer-mounted, they travel from one hospital to the next throughout one state and/or across neighboring states.
For example, a PET scanner is available at St. Joseph's Medical Center in Bryan, Texas, on Thursday of every other week. The scanner is mounted on a trailer and is traveling throughout Texas and Colorado.
"Cancer is the primary target of the scanner, but we also use it for brain disorders such as seizures" says Michael L. Wallin, nuclear medicine technologist at Radiation Corporation of America, the company operating the PET scanner.
The patient lies on a table that slides into the middle of the scanner. The scanner's electronics record the gamma rays produced by the scintillators and map an image of the area where the radioactive source is located.
"Once we get the patient positioned, we tell him or her just to relax, we turn down the lights, and we go in the control room where data are automatically processed in the computer," Wallin says. "The patient usually needs to lie still for 45 minutes to an hour."
It could take a few years before a PET scanner becomes available to more patients, time for McIntyre and Paulson to demonstrate the advantages of the novel design of the Texas A&M PET.
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