Dose information proves vital to calm fears, bolster CT's image

The increasing use of multislice CT has raised questions about patients' rising radiation burden. But CT can-and should-be a low-dose modality, according to Prof. Dr. Willi Kalender, director of the Institute of Medical Physics at the University of Erlangen in Germany. Often the problem rests not with the modality but with misconceptions about the doses delivered during imaging. Address this lack of knowledge, optimize scanning parameters correctly, and CT will no longer be feared, is Kalender's point.

Dose is no secret, and accurate information on examination doses is now readily available, he said. Dose data just need to be properly understood and communicated more effectively.

"We have to get rid of the situation that often happens in the clinic where patients ask: 'How high is the dose? What is the risk?' and we don't know. We think of CT as a high-dose modality, even when it is working in the low-dose range," Kalender said.

Today's MSCT consoles provide a readout known as the volume CT dose index (CTDIvol). This describes the average dose over the total volume scanned. Reducing the pitch will cause the CTDIvol to increase unless the tube current (mAs) is also reduced. This relationship means that examinations with very low pitch values, for example in cardiac CT, will have a higher dose than exams with higher pitch levels.

Patient-specific dose information is also available, thanks to mathematical modeling techiques. Organ dose and effective dose values calculated with these tools are typically accurate to within 10% of measured values. This level of accuracy is perfectly acceptable, given that standard dosimeters are not much better, he said.

At ECR 2007's special session, "CT and radiation dose: How to make this devil an angel?," Kalender illustrated the power of computer modeling with a simulation of dose distribution across an anthropomorphic phantom. This revealed the highest doses to be at the periphery of the phantom. These outer dose values were typically 50% higher than the lowest dose values. He showed another simulation based on a 30-sec CT scan of the trunk. The dose to the thyroid gland was calculated as 22 mSv, while the effective dose came out as 17 mSv.

Kalender recommends that dose values be expressed in terms of years of background radiation when talking to patients. Organ doses for the International Commission on Radiological Protection's standard man typically range from 1 to 30 mSv, and effective doses from below 1 mSv to 20 mSv. The United Nations reference value for natural background radiation is 2.4 mSv per year.

"This provides a reference, so patients know the order of magnitude for their examination. They have no idea what you mean if you talk to them in terms of milliSieverts or if you tell them about GigaBecquerels," he said.

Understanding dose is only half the battle, though. Armed with this knowledge, radiologists should optimize protocols. Many modern scanners use a technology known as tube current modulation to control noise levels and reduce dose. Methods of fixing the noise level are also being investigated as a means of reducing dose. For example, there is now consensus in cardiothoracic CT that coronary calcium measurements should be performed at a noise level of around 30 HU. With this figure as the fixed reference point, the tube current can then be adjusted according to the patient's size.

Dose-lowering strategies are being developed for interventional CT, too. Three-D navigation tools can reduce the need for repeated scanning. Radiologists at the University of Erlangen now take only two CT scans when performing interventions. Data from an initial scan are input into a software program to define the target and the best entry point. Optical tracking then advises practitioners how to orient the needle and how deep to insert it. The second scan is used as a control.

"According to official sources in Germany, the average effective dose per scan is 10 mSv. That means we are in the low-dose range already, but we can reduce it further," Kalender said. "In interventional CT we can limit the dose because we are using only two scans. In pediatric CT, I think we can come down to around 1 mSv and in coronary CT the same. That is the goal for the future."