Copper filter reduces dose, increases prestige

When Dr. Salwa Haidar picked up her $1000 award from the RSNA last year, she promptly bought gifts for her four children and her husband. He and the three oldest children had come with her from Kuwait to the University of Toronto while Haidar completed her fellowship. The youngest child was born in Canada.

"They deserved the gifts. They were a great support," she said.

The RSNA honored Haidar for her investigation into the use of a copper filter to reduce skin dose in computed radiography for pediatric imaging. The concept is simple. The copper wire absorbs the low energy, which mostly is scatter radiation, and allows the high energy to pass through. The effect on quality is negligible, she said.

Dr. Salwa Haidar

Haidar and colleagues at the Hospital for Sick Children in Toronto used two identical portable x-ray machines. They installed a 0.1-mm copper filter in the collimator port at the x-ray source of one machine. Physicists tested each unit before filter installation to ensure identical radiation output. After filter installation, each unit was checked for radiation output using an ionization chamber.

The researchers performed a series of exposures with and without copper filtration for 20 cadavers undergoing routine skeletal survey prior to autopsy. They placed thermoluminescent dosimeters in the beam for four exposures: head, chest, abdomen, and lower limb. Skin dose was measured for each exposure.

The copper filter reduced radiation dose by 15% to 20%, Haidar said. Three blinded pediatric radiologists agreed that 68% of the images performed using the copper filter were equal or better to those without the filter. The interobserver variability using intraclass correlation method was 89%. Overall, none of the images was evaluated as poor or nondiagnostic.

"The most surprising finding was that a reduction in dose did not translate into a decrease in image quality," Haidar said. "We had intended to adjust the image parameters for quality but were surprised that the image was clinically sufficient to evaluate."

She is especially concerned with repeated exposures for children, particularly those with chronic illnesses. A reduction of 15% to 20% might not seem like that much, she said, but it adds up in terms of a cumulative effect.

"Chronic cases are admitted more often to the hospital. If we can spare them some radiation, their chances of developing cancer in the long run are greatly diminished," she said.

The 39-year-old Haidar and her family returned to Kuwait following her fellowship. She is now a consultant in pediatric radiology at Mubark al-Kabeer Hospital. She said the research project has given her a better chance to join the Kuwait University faculty of medicine, for which she has recently applied.

The next step is to apply the filter for imaging patients, rather than cadavers. Pediatric radiologists at "Sick Kids" in Toronto have already begun that effort, and she is initiating plans for implementation in Kuwait. She would then like to find out how to apply the same technique for adults.

One challenge of the research was finding a sufficient number of cadavers that were in the right age group: up to six years old. The experiment took longer than expected because working with cadavers, "we had to wait for our chances," she said.

Chief technologist Ellen Charcot had a difficult time - technically and psychologically - working with the cadavers, Haidar said. Each time, she had to do a full study twice, and she had to get the positioning just right. Haidar also credits her mentor Dr. Paul S. Babyn, the radiologist-in-chief in the department of diagnostic imaging at the University of Toronto.