Researchers from Japan have found they can significantly reduce radiation exposure to the eye lens during sinus MDCT scans using an iterative reconstruction method. The technique could reduce damage such as cataracts to this radiosensitive organ.
CT produces the high-quality images of the sinus region often needed for diagnosis, but radiation exposure to the eye lens is a concern inasmuch as "several recent studies have shown a lower threshold for damage to the eye tissues for radiation-induced injury," said Haruhiko Machida, MD, PhD, from Tokyo Women's Medical University Medical Center East. Cataracts are the principal concern.
Routine MDCT generates doses to the eye of approximately 20 mGy, compared to conventional radiography at 0.5 mGy, but the latter modality doesn't provide sufficient image quality. Instead, low-dose MDCT images could be acquired by optimizing kVp and mAs, increasing the pitch, and using multiple combinations of adaptive statistical iterative reconstruction (ASIR) and kVp -- with equivalent image quality, Machida said in a presentation at the 2010 RSNA meeting.
Machida and colleagues wanted to determine the optimal tube current and ratio of ASIR to a traditional filtered back projection (FBP) algorithm for modulating the dose to the eye lens while maintaining the signal to noise, he said. Image noise is inversely proportional to the tube current, and radiation dose is proportional to the tube current, he noted, indicating that radiation dose can be reduced by reducing tube current.
The researchers used a Rando (Phantom Laboratory, Salem, NY) quality assurance phantom to determine the optimal tube current and ratio of ASIR to filtered back projection. Using tube currents of 25, 50, 100, and 200 mA, they scanned the phantom with 120-kV tube voltage, 0.6-sec rotation speed, and 1.375 helical pitch.
Glass dosimeters were placed on both of the phantom's eyelids at 120-kV tube voltage, 0.5-sec rotation speed, and 0.984 helical pitch on a 64-detector-row CT scanner (GE Healthcare, Chalfont St. Giles, U.K.).
Axial images were reconstructed with varying ASIR-to-FBP ratios ranging from 0% to 100% at 10% intervals. The standard deviation of the CT value was measured within five regions of interest in each of three images, and the average standard deviation was calculated under each condition. The average dose was measured at each tube current.
"We optimized the tube current to minimize the radiation dose and achieve similar image noise to that of our routine protocol [of 200 mA]," Machida said. "Eye dose was proportional to tube current."
They found that image noise at 30 mAs and ASIR of 80% was similar to that of the hospital's routine protocol. At the 50-mAs level, dose to the eye was reduced to 10.6 mGy.
As for limitations, the optimal image quality was not thoroughly assessed, and the current routine protocol wasn't fully optimized, he said. Additional studies should be performed to answer these questions.
Fifty mAs represents a major improvement, an audience member commented. In response to a question, Machida said his radiologists don't have a problem with the odd textural representations in ASIR at 80%, though 100% ASIR was a problem.
By Eric Barnes
AuntMinnie.com staff writer
January 28, 2011
Related Reading
High cataract rates found in interventional cardiologists, August 5, 2010
Young adults face higher CT radiation risk than seniors, May 4, 2010
Cardiac cath delivers high radiation doses to operators, April 5, 2010
New paper updates guidelines on fluoro radiation injuries, February 18, 2010
Exposure to low doses of ionizing radiation linked to increased risk of cataracts, October 3, 2008
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