VC radiation dose holds steady overall despite dose modulation

The radiation dose from virtual colonoscopy (VC or CT colonography [CTC]) hasn't dropped much since the introduction of dose modulation software on CT scanners, largely because VC providers use it only half the time, according to a new study from the Netherlands, which also demonstrated significantly lower doses for VC screening protocols compared to daily practice protocols.

The study, by authors Dr. Marjolein Liedenbaum, Dr. Henk Venema, and Dr. Jaap Stoker from Amsterdam's Academic Medical Center at the University of Amsterdam, used a survey of medical practices to determine the effective doses currently used in CTC practice and to search for dose trends since the group's first survey in 2004.

A number of changes have taken place since that time, particularly due to the increasing number of detector rows in MDCT scanners, the group wrote in the online edition of European Radiology (May 20, 2008).

"These technical improvements will have an effect on image quality, but also on radiation exposure," they wrote. "Dose efficiency is improved with an increasing number of detector rows due to the decrease of the effect of overbeaming, which is the additional radiation due to the penumbra effect. On the other hand, dose efficiency is lost with machines with a larger number of detector rows, because of increased amount of overranging, which is the difference between the exposed length and the planned length of the CT examination."

In addition, automated current selector (ACS) functions adjust the tube current to patient size to reduce differences in noise level between thinner and thicker patients, maintaining image quality in different-sized patients. And dose modulation adjusts tube current to changing patient anatomy, reducing dose while maintaining image quality, the authors wrote.

In a search for institutions that performed CTC, the researchers reviewed the PubMed database and abstracts from the largest meetings where CTC studies are presented, including RSNA in Chicago, the European Congress of Radiology (ECR) in Vienna, and the International Symposium on Virtual Colonoscopy in Boston, between 2004 and 2007.

Based on the responses, the effective doses to an adult hermaphrodite for practice and screening protocols were estimated for every responding institution using the ImPACT CT dosimetry spreadsheet. The study assumed a nominal scan trajectory of 43 cm from the diaphragm to the groin, and the effects of overranging as well as effective volume were used to calculate dose. Calculations were based on the average effective mAs value used for a CTC of an average-sized patient of 170 cm and 70 kg.

Liedenbaum and colleagues evaluated protocols for daily practice and screening separately, and for modulated and nonmodulated protocols separately, classified according to the number of detector rows on the scanner. Institutions that had responded to the 2004 questionnaire were asked to detail protocol and scanner changes over time.

In all, 50 of 83 institutions responded, yielding 37 (45%) completed questionnaires, including 21 (62%) that used 64-detector-row CT and 17 (50%) that used dose modulation.

In an analysis of 37 different scanners used, the authors found no significant differences in effective dose between scanners with different numbers of detector rows or between protocols with and without dose modulation.

In an analysis of 39 daily practice protocols, the mean effective dose was 9.1 mSv (range 2.8-22), 5.2 mSv (range 1.0-14.1) for supine and 3.0 mSv (range, 0.6-9.8) for prone CT acquisition, Liedenbaum and colleagues wrote. The median effective dose per institution was 9.1 mSv (range, 2.8-22).

For 25 screening CTC protocols, the mean effective dose was 5.6 mSv (range, 2.6-14.7), 2.8 mSv (range, 1.0-6.1) for supine and 2.5 mSv (range, 0.6-9.8) for prone CT acquisition. The median effective dose per institution was 5.7 mSv (range, 2.6-12.2).

In 17 institutions queried in 2004 who responded a second time, the mean effective dose in practice protocols changed from 11 mSv (range, 4.2-21.0) to 9.7 mSv in the present study (screening doses were not included in the 2004 questionnaire).

Although 17 institutions responding to the questionnaire used dose modulation in the present study compared with none in 2004, the dose reduction from 11 mSv to 9.7 mSv was not statistically significant.

In 2004, 82% of the institutions used a CT system with fewer than 16 detector rows and 18% used a 16-detector-row CT system, the authors noted. In the present survey, only 18% used a CT system with fewer than 16 detector rows and 62% used a 64-detector-row CT scanner.

The lack of significant dose reduction over time may be due to a number of reasons, the authors explained.

"A number of institutions may value a higher image quality more than a lower dose," they wrote. "This hypothesis is supported by the large range of effective doses found in the present study: from less than 3 mSv to more than 20 mSv. Some of these differences may be explained by differences in the CT examination, for example, the use of intravenous contrast medium (which is mostly used for high-risk patients that require higher image quality) necessitates a higher dose."

The authors cited the limited number of institutions and an inherent measure of unreliability associated with questionnaires as the study's main limitations. The difficulty of matching weight to mAs values was cited as another potential cause of error.

"The median effective dose for CTC colonography at present is significantly lower for screening protocols (5.6 mSv) than for daily practice protocols (9.1 mSv), which is important because of differences in benefit-risk ratios for patients in screening and in daily practice," the team wrote. "We found that the use of CT systems with a different number of detector rows does not influence the effective dose. Furthermore, the current effective dose has not significantly changed compared to the dose in 2004, but the number of CTC protocols with dose modulation increased substantially."

By Eric Barnes staff writer
June 2, 2008

Related Reading

CT experts grapple with rising concerns about radiation dose, May 14, 2008

Researchers recommend alternatives to CT, April 15, 2008

Z-axis modulation cuts VC dose, keeps image quality high, April 12, 2005

Even low-dose VC yields extracolonic findings, September 21, 2004

Ultra-low-dose VC appears feasible, August 3, 2004

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