Researchers say the newer, 64-slice CT scans, first introduced in the United States in 2005 and initially tested at Johns Hopkins, won't replace the need for inspecting arteries by cardiac catheterization, also known as coronary angiography, but the scans will help cardiologists more quickly rule out those who can skip the more invasive procedures.Studies suggest that as much as 25 percent of the 1.3 million cardiac catheterizations performed each year in the United States may be unnecessary.
The latest study also showed that early detection with 64-CT is a good predictor of who will need angioplasty or coronary bypass surgery to open up new blood supply routes to the heart.
Results showed that on average 91 percent of patients with blockages were detected by 64-CT and that the scans were able to diagnose 83 percent of patients without blockages. This reliability, researchers say, allows them to accurately identify patients who need angioplasty or bypass surgery. More than a quarter million Americans undergo coronarybypass surgery each year.
In the study, investigators selected 291 men and women over the age of 40 who were already scheduled to have cardiac catheterization to check for blocked arteries. Each underwent a 64-CT scan prior to catheterization. Participants were then monitored through regular check-ups to identify who developed or did not develop coronary artery disease and who required subsequent bypass surgery or did not need surgery.
After the first year of monitoring, to continue annually until 2009, researchers found that results from 64-CT scans matched up 90 percent of the time with results from invasive catheterization in detecting patients with blockages.
In other measures, researchers found that 64-CT scans were 83 percent to 90 percent accurate, while tests using older, 16-CT scans were in some instances only 20 percent to 30 percent as precise.
"This study is the first step to realizing the full potential of CT imaging in predicting coronary artery disease, and these scans complement the arsenal of diagnostic tests available to physicians to prevent heart attacks," says cardiologist Julie Miller, M.D., who led the study at Johns Hopkins.
The new study also suggests that the new scanners, four times quicker than the more widely used 16-CT, may be a good alternative to cardiac stress testing, which evaluates heart function by measuring the effects of hard exercising. Exercise stress testing enerally cannot safely be performed on the weak and elderly.
Miller, an assistant professor at Johns Hopkins, says the advanced scanners are so good that physicians can for the first time measure blockages in blood vessels as small as 1.5 millimeters in diameter. Older 16-CT scanners, she says, are best suited for looking inside bigger arteries, those ranging in diameter from 2 millimeters to 4.5 millimeters, and to calculate the amount of calcium buildup in the arteries, also a predictor of the degree of blockage, "but now we have a more-advanced test that actually measures the amount and volume of blockage present."
The advanced CT scanners, she says, produce pictures within five to 10 seconds, while cardiac catheterization, which also checks the function of heart valves and muscle, takes between 30 minutes and 45 minutes to perform, and requires almost an hour for recovery. Potential complications from the invasive procedure include infection, heart attack and stroke, but they are rare.
"And we no longer need to wait until a patient is stabilized before performing this diagnostic test, as no anesthetic is needed for CT scanning," says Miller.
The CT scanner used in the study was an Aquilion 64 CFX multislice CT scanner, manufactured by Toshiba. Similar devices also are manufactured by Siemens and General Electric. Each machine costs between $1.5 million and $2 million. A single test costs approximately $700.