Munich, Germany - Using dose-saving algorithms during multidetector computer tomography (MDCT) assessments of CAD can radically cut radiation exposure without compromising image quality, authors of a new study say [1]. In a paper published online in Circulation March 6, 2006, Dr Jörg Hausleiter (Deutsches Herzzentrum, Munich, Germany) and colleagues report that higher spatial and temporal resolution of 64-slice scanners can improve on the diagnostic accuracy of 16-slice MDCT, but at the price of a near doubling in radiation dose.

"The point is that, if you apply these two algorithms, you can reduce the dose by almost 50% to 60%," Hausleiter told heartwire. "This is a pretty important issue, because a lot of centers don't use these algorithms as intensively as we do."

Hausleiter et al estimated radiation dose in 1035 patients undergoing CT angiography according to different algorithms. For the entire cohort, radiation dose was an estimated 6.4 mSv among patients undergoing 16-slice CT and 11.0 mSv among patients undergoing 64-slice CT (p<0.01). Published estimates from large groups of patients suggest that radiation exposure from standard coronary angiography is somewhere in the range of 4 to 5 mSv, Hausleiter told heartwire.

In Hausleiter et al's study, radiation dose was reduced by 37% in the 16-slice group and by 40% in the 64-slice group when ECG-dependent dose modulation was used—as was done in more than 80% of the cohort. ECG-dependent dose modulation uses high tube current during mid-diastole to obtain images of optimal quality but a reduced amount of current during the rest of the cardiac cycle, when cardiac motion produces blurred images. By contrast, in non-ECG-dependent modulation algorithms, radiation tube current remains constant, even though images taken when the heart is in motion are discarded during reconstruction in a process called retrospective gating.

"The heart moves all the time," Hausleiter explained, "but we need to acquire these images when the heart is almost standing still, in the diastolic phase. During this phase we need a high dose to acquire good images, but in the systolic phase we don't need as high a dose. So by using the ECG we can calculate when the diastolic phase is coming. It's a clever algorithm and it's very effective."

When ECG dose modulation was used in combination with a reduced tube voltage setting (100 kV instead of 120 kV), radiation exposure was further reduced to an estimated 53% and 64%, respectively. Importantly, reduced dose estimates at the 100-kV setting were not associated with compromised image quality, Hausleiter et al note. "Further studies are needed to investigate the balance between dose savings and maintained diagnostic image quality for coronary CT angiographic investigations," the authors say.

While there are no scientific studies specifically linking radiation from different cardiac imaging modalities to cancer, estimates extrapolated from other sources suggest that the risk is in the range of 25 cases of cancer within 100 000 exposures, over a 40-year period, Hausleiter commented. In the setting of CT angiography, as with standard angiography, the benefits of identifying coronary stenoses likely outweigh the risks of radiation exposure if used appropriately.

"CT is certainly not a screening method," he emphasized. "You would not, at this stage, use MDCT on every patient to screen for CAD: this would probably add some significant risk to the patient population in terms of cancer. But in patients where you have some indication that they have CAD . . . then you can see with MDCT whether they have coronary stenosis or not, and this is very helpful, because otherwise they'd need to go for invasive coronary angiography."

As yet, however, no large studies have established whether MDCT might be superior to coronary angiography in certain patients, although small studies have demonstrated that MDCT can produce diagnostic images of the coronary tree. The attraction of MDCT is that it avoids an invasive procedure and associated risks and expense of a hospital stay. More research is also needed to prove that MDCT can lead to improved outcomes in CAD patients.

"So far, there are no good studies out there for CT showing in which patients the methodology is useful or not," Hausleiter commented. "Nobody has proven that a particular patient population has a benefit of CT over coronary angiography, although my belief is that there are some types of patient populations who would benefit from not needing to undergo coronary angiography. It might also be important for healthcare systems, because multislice CT might come out cheaper in the end than coronary angiography."

While more studies are warranted, physicians currently using MDCT should be aware that CT does indeed confer higher radiation exposure than standard angiography, but this dose can be minimized if dose-reducing algorithms are applied, Hausleiter et al conclude.