The majority of chest, abdominal and pelvic CTs are scanned off-center—a phenomenon that could impact both dose and image quality, a group of Atlanta researchers reported this month in Current Problems in Diagnostic Radiology.
Courtney C. Moreno, MD, and a team at Emory University School of Medicine in Atlanta said that, since per capita radiation dose has nearly doubled in the U.S. over the past two decades, clinicians are grappling with ways to lower national exposure. Technologies like automated tube current modulation, tube voltage selection software and bow-tie filters have all shown promise in decreasing radiation dose, but Moreno et al. wrote something as simple as off-centering in the CT gantry could minimize any of those benefits.
Off-centering can impact radiation dose through magnification effects in the topogram with the use of topogram-based tube current modulation and tube voltage selection software or malalignment with the x-ray beam shape with the use of a bow-tie filter.
“A linear relationship exists between the distance separating the topogram x-ray source and the patient and the resultant magnification in the topogram,” the authors wrote. “If the patient is off-centered closer to the x-ray source, the patient’s magnified size in the topogram will result in selection of higher maximum tube currents and tube voltages than would be selected based on the patient’s true size.”
When patients are off-centered and the technician is using a bow-tie filter, they explained, more x-rays than are necessary are delivered to that patient’s periphery, increasing surface radiation dose by up to 49 percent.
Moreno and co-authors evaluated a database of more than 56,000 CT acquisitions in an effort to quantify the issue and its commonality. They also imaged a water phantom and an anthropomorphic phantom in the centered position in the CT gantry and at several off-centered positions.
The researchers found the majority of patient acquisitions—83.7 percent—were performed while a patient’s centroid was positioned below isocenter by an average of 1.7 centimeters, though some patients’ readings were off by as much as 12 centimeters. Off-centering on the x-axis was less severe, the authors said, and distance between the centroid and isocenter on the y-axis did not differ as a function of sex but differed based on scan region, patient position and gantry aperture.
“In the largest series published to date, we have documented the tendency of CT technologists to center patients’ centroids below the isocenter in the CT gantry when centering is performed based on visual estimate,” Moreno and colleagues wrote. “Further investigation of the prevalence and severity of geometric off-centering is needed as (it) has implications for patients dose based on topogram magnification effects.”
After graduating from Indiana University-Bloomington with a bachelor’s in journalism, Anicka joined TriMed’s Chicago team in 2017 covering cardiology. Close to her heart is long-form journalism, Pilot G-2 pens, dark chocolate and her dog Harper Lee.