AJR: CT showed dose variability in National Lung Screening Trial

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CT scans measuring blood flow in the lungs can detect early emphysema-related changes that occur in smokers who are susceptible to the disease.
Image source: Eric Hoffman, PhD, department of radiology, University of Iowa Carver College of Medicine

For quality assurance reasons, CT radiation dose measurement data were collected from all multidetector CT (MDCT) scanners used in the National Lung Screening Trial. According to research published in the June issue of the American Journal of Roentgenology, average normalized CT dose index values varied by a factor of almost two for all scanners used in the National Lung Screening Trial, and more complex scanners showed improved dose efficiency when compared to those with fewer data channels.

According to Dianna D. Cody, PhD, from the department of imaging physics at the University of Texas MD Anderson Cancer Center in Houston, and colleagues, the study focused on machine-specific normalized CT dose index and collected a total of 247 measurements on the 96 CT scanners used in the trial from 2002-2007.

The researchers collected measurements using a standard CT dose index measurement protocol. These parameters, which included tube voltage, milliampere-seconds [mAs] and detector–channel configuration, were set according to trial protocol for average size subjects, they explained.

A statistically significant difference in normalized CT dose index among CT scanner manufacturers was noted by the authors, who attributed the findings to design differences, including filtration, bow-tie design and geometry. “Our findings also indicated a statistically significant difference in normalized CT dose index among CT scanner models from the same manufacturer, and we also found a statistically significant difference in normalized CT dose index among all models and all manufacturers,” said Cody and colleagues.

The authors also noted differences in normalized CT dose index when they compared four or eight data channel scanners to 16, 32 and 64 channel scanners, which may suggest that more complex scanners have improved dose efficiency.  They explained, “It appears that the more advanced technology was associated with a consistently lower normalized dose measurement.” 

Noting that patient dose and image quality were not addressed in their research, it is “unknown whether the scanners that delivered relatively higher dose using the technique charts developed for this trial obtained relatively better image quality,” the authors said.

“Examination of these figures reveals varying levels of consistency for normalized dose reported by sites with the same model of CT scanner. This value would be expected to be very consistent,” wrote the authors. “Unfortunately, this study indicates that simplifying image quality or noise is not likely to ultimately be useful because of the output variability among CT scanners,” the study concluded.