George, an assistant professor at Hopkins where he also serves as director of its CT Perfusion Laboratory, says a CTP takes three seconds or less of actual scanning and, if done correctly, involves an average radiation exposure of about 8 millisieverts. A SPECT test, he says, averages between 10 millisieverts and 26 millisieverts, and cardiac catheterization ranges between 2 millisieverts and 10 millisieverts. The 320-CT scanning device has at least five times the speed and power of the 64-CT scanners in widespread use elsewhere.
The scanner's software compares ratios of brightly dyed blood flows between the innermost and outermost layers of heart muscle, where the effects of arterial narrowing first appear.
As part of CTP imaging, each patient is injected with a chemical dye containing iodine, known to light up on screen when struck by the scanner's X-rays. Lower concentrations of iodine will show up as darker regions, indicating constrained and reduced blood flow, the underlying cause of chest pain, than brighter regions where blood flow is more uniform and free flowing.
To enhance the image, blood flow to the heart is sped up through chemical injections of adenosine, which causes the blood-pumping organ to beat faster.
Previous research by the team among 60 patients with suspected coronary artery disease showed that using dual testing with CTA and CTP had almost the same statistical predictive values as SPECT, prompting the team's latest investigation to see if the dual tests were as clinically useful as SPECT.
George cautions that CT scans are not a substitute for catheterization, but are "an alternative diagnostic tool" physicians can use to "get a real picture" of the extent of coronary blockages and their effects on blood flow, especially when physicians need both sets of information to make treatment decisions.<
|Contact: David March|
Johns Hopkins Medical Institutions