Method May Pinpoint Blocked Heart Arteries
Scientists have developed a technique that shows promise of being able to pinpoint blocked coronary arteries, identifying patients at risk for heart attacks before symptoms appear.
The method, based on studies by researchers at Lawrence Berkeley Laboratory, uses a nuclear imaging technique called positron emission tomography, or PET.
“Over the past 10 years, our team has developed a technique for evaluating the flow of blood to the heart muscle--a measurement that reflects the health of the coronary arteries,” said Dr. Thomas Budinger, head of the Research Medicine and Radiation Biophysics Division.
Complicated Procedure
Currently, only a coronary angiogram can show arterial blockage. This is a complicated, expensive and risky procedure that requires hospitalization and involves running a tube called a catheter up an artery from the groin to the heart and pumping in dye, Budinger said.
In contrast, he said, PET scans could be performed in the doctor’s office as part of an annual checkup and could point out potential trouble long before any outward symptoms appeared.
In this procedure, the patient inhales, ingests or is injected with a positron-emitting radioactive tracer that concentrates in a particular organ or tissue. The tracer is detected through a ring of crystals placed around the patient. The resulting three-dimensional image of the tracer enables doctors to follow the course of organic processes in progress.
In the coronary artery studies, researchers used the tracer rubidium-82 because of its metabolic resemblance to potassium, an important biological element absorbed so quickly into the heart muscle that within minutes of an injection the heart can be seen clearly on a PET scan.
In addition, rubidium-82 has only a 75-second half-life and disappears from the heart almost instantly. This allows researchers to perform a scan, test a particular treatment such as a drug or exercise and then follow up with another scan to measure the effect of the therapy.
2 Years of Tests
Budinger’s team began human tests of the procedure two years ago on patients with arteriosclerosis and heart disease.
The patient first exercises briefly or, at times, is given a drug that mimics the effect of exercise, dilating the coronary arteries and allowing increased blood flow to heart muscles. A PET scan is then taken.
The images showed that in the arteriosclerotic patients, certain areas of the heart received decreased--rather than increased--blood flow.
“These are the areas that are served by diseased arteries that cannot dilate,” Budinger explained.
After a short rest, a second PET scan is taken.
“In the diseased patients, the difference between the two images is remarkable,” Budinger said.
“The patients not only have a decrease in blood flow during exercise but also show a greater increase than normal patients immediately afterwards during the recovery period. This reversal was a striking and unexpected observation.”
Such ability to measure blood flow has become particularly important with the introduction of drugs designed to be injected into coronary arteries to dissolve clots during a heart attack.
“With PET you can see within seconds--while a heart attack is still going on--whether the drug has cleared the blockage,” Budinger said. “PET can be just as important for evaluating drugs that slow the rate of cholesterol buildup in the arteries. No other technique offers as much.”
