Strategies for Lowering Screening Radiation Doses

Optimizing CT imaging protocols and applying radiation dose reduction techniques is essential to ensure the best imaging results with the lowest radiation dose, according to an article published in the American Journal of Neuroradiology.

The use of CT is growing, along with the resultant increase in related radiation scanning doses. Because of the concern regarding rising cumulative doses of radiation from testing procedures, researchers from the University of Michigan Health System in Ann Arbor, reviewed the issues of radiation dose measurements, the basics of dose reduction, current strategies in dose reduction and future advancements in the field. 

“Recommendations for reducing radiation dose have mainly focused on limiting the radiation exposure,” the authors wrote. Guidelines, issued by the FDA, provide recommendations on optimizing CT protocols and the elimination of inappropriate testing referrals.

There are several factors that affect radiation dose during imaging tests:

• Beam energy, which is determined by the tube voltage or kilovolt(peak). If there are changes in the tube, there can be substantial change in the CT radiation dose. Lower kilovolt(peak) can provide better contrast, but the effect of improved image contrast with lower kilovolt(peak) is reduced by the increased image noise.
• Photon fluence, which is determined by the tube current and X-Ray tube rotation time. Radiation dose can be increased or decreased by changing the tube current, but this affects image noise.
• Collimation, table speed and pitch, which are interlinked parameters that affect the diagnostic quality and radiation dose of an imaging study.

In order to reduce radiation dosages, the following strategies have been implemented:

• Automated tube current modulation, which is the most widely available technical innovation for significant radiation dose reduction. CT imaging can be done with lower radiation doses, regardless of patient size or which body part is being scanned.
• Adaptive dose shielding, which uses an axial protocol rather than a helical protocol, which can expose tissue that need not be scanned. Using a system that blocks parts of the X-Ray beam not needed for reconstruction, the adaptive dose shield limits the “over-ranging” or “z overscanning” performed by extra gantry rotations before the beginning and after the end of the scanned volume.
• Image reconstruction algorithms.

Although these procedures have been developed, more needs to be done to further reduce radiation doses during diagnostic imaging. New approaches include:

• Automated organ-based current modulation, which is a technique that reduces the tube current for certain projections to avoid direct exposure of the thyroid gland and ocular lens, and other radiosensitive organs.
• Automated (optimized) tube-voltage modulation, which modulates the tube current, while the tube voltage, the kilovolt(peak) setting, is left unchanged. “However, there is a large potential for dose reduction by optimizing the tube kilovolt(peak) setting,” the authors wrote.
• Noise-reduction algorithm with image reconstruction and data processing, which improves the overall image quality. The noise-control techniques that operate on raw projection data, the log-transformed sinogram or images after reconstruction, achieve a lower noise in the CT images.

“There are significant variations between sites and scanners in imaging protocols with a wide range of radiation doses for the same scan indication,” the authors wrote. In addition, some children are undergoing scans using the adult protocols, exposing them to higher radiation doses. “Several dose-reduction techniques have been successfully implemented and have been shown to reduce radiation exposure, including tube-current modulation, reducing tube voltage, adaptive dose shielding, and noise reduction filters,” they concluded.