ASRT: Increasing source-to-image distance cuts rad dose, maintains quality

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Increasing the source-to-image distance for direct digital radiography (DR) study of the pelvis reduces the radiation dose while maintaining image quality, according to a study published in the September/October 2011 issue of Radiologic Technology, a journal of the American Society of Radiologic Technologists.

Andrew England, BSc, of the University of Liverpool, and colleagues aimed to investigate distance, dose and image quality for DR exams of the pelvis, which is the second most common bucky procedure.

England and colleagues positioned an anthropomorphic pelvic phantom for a standard anteroposterior exam using a single DR unit with a flat panel detector. The initial source-to-image distance was set at 100 cm and used an antiscatter focused radiation grid. Researchers then varied the distance from between 60 cm and 147 cm, while the x-ray beam was collimated for a standard pelvic exam and was kept constant.

At total of 80 images were sent for quality analysis, as two exposures were taken in 10-cm intervals to determine image quality, and then the experiment was repeated without a grid. Researchers calculated the entrance surface dose and effective dose based on distances of 60 cm, 80 cm, 100 cm, 120 cm, 140 cm and 147 cm.

Results showed that increasing the source-to-image distance decreased radiation dose, both on the entrance surface and the effective dose, when using the antiscatter radiation grid. Dose was further reduced when the grid was removed. At 147 cm, the decrease in dose equated to a 25 percent reduction compared with standard parameters.

“Increasing the [source-to-image distance] is simple, cost-effective and easy to implement in any clinical environment,” wrote the authors.

The results also showed that any difference in image quality from increasing source-to-image distance was statistically insignificant.

“The optimization of radiation dose for both computed radiography and DR examination is an essential part of radiographic practice,” England said in a statement. “Our lab-based phantom experiments have indicated that dose reduction is possible without loss of any image quality.”

England suggested that the radiographic technique should continue to be evaluated in a clinical setting to further examine the dose reduction potential of increasing source-to-image distances.