Variations in CT exam technique can alter dose rates dramatically, by up to a factor of 10, according to a presentation at the 2009 meeting of the European Congress of Radiology (ECR) in Vienna, Austria, this week.

Unjustified CT exams and non-optimized exam protocols are significant contributors of unnecessary dose. Appropriate selection of exposure parameters, reduction of scanning length and minimization of the number of scans included in a single imaging study would contribute to dose optimization. Also, automatic exposure control, if used appropriately, is a technique for patient dose reduction, according to John Damilakis, PhD, associate professor at the University of Crete, Iraklion, who presented the research.

Typically, the over-scanning doses are not included in calculations of effective dose, which are usually based on the planned scan volume and not the actual volume exposed, Damilakis said. He recommended appropriately selecting three parameters--beam collimation, pitch and reconstruction slice width--to limit the extent of over-scanning.

"The relative contribution of extra x-ray exposure due to over-scanning may be considerable, especially when the planned image volume is limited, as in pediatric studies for example," Damilakis said.

PET/CT has now replaced standalone PET for many oncology applications. The level of radiation exposure from CT can be several times higher than that from PET, but patient exposure can vary depending on the clinical indication and combined imaging protocol, according to Thomas Beyer,PhD, a teaching professor at the University Hospital Essen in Essen, Germany, and CEO of cmi-experts, a Swiss-based company focused on cross-modality imaging.

Beyer suggested using PET/CT instead of CT or PET when clinically indicated. Perhaps most importantly, radiologists and nuclear medicine physicians must cooperate to define the most appropriate imaging parameters for different clinical indications for PET/CT, he noted.

It is crucial to consider risks together with the benefits brought by advances made in both modalities, according to Reinhard Loose, MD, head of the institute for diagnostic and interventional radiology at the Nuremberg North Hospital in Nuremberg, Germany. Technological progress has helped reduce the likelihood of patient morbidity and/or mortality in some areas, for instance in patients with severe polytrauma. Twenty years ago, the diagnostic work up used to take 35 to 45 minutes for separate imaging exams. Today, polytrauma victims can be imaged within 20 seconds thanks to multidetector CT, allowing more patients in a critical condition to undergo this exam, Loose said.

Finally, patients who receive CT scans regularly are very ill, and the likely reduction in life expectancy from their medical condition is much greater than that associated with the CT examination they undergo. Loose said that the justification of CT use is more critical when it comes to outpatients and participants in screening programs.

But the development of referral criteria for imaging studies should help to reduce the number of inappropriate CT and x-ray examinations, as is the case in pediatric radiology, Loose stressed.