What impacts breast dose during 64-slice CT studies?

Radiation dose is a huge topic in modern imaging, with providers always looking out for new ways to reduce the amount of radiation patients are exposed to during necessary exams. A significant part of this ongoing research is focusing on what does and does not impact dose, including dose to parts of the body that are not necessarily the focus of the exam.  

For example, according to a new study published by Academic Radiology, breast dose from 64-slice CT studies are mainly dependent on the protocol when breasts are entirely included in the primary x-ray field; when breasts are just partially included in that primary x-ray field, however, dose is dependent on both the type of scanner being used and the protocol.

A research team led by Zhihua Qi, PhD, department of radiology at the University of Cincinnati in Cincinnati, Ohio, used four scanner models and an anthropomorphic phantom with breast modules to track breast dose during lung screening CT, chest/abdomen/pelvis CT and virtual colonoscopy CT.

Why track breast dose? Qi et al. explained that, in a recently published International Commission on Radiological Protection Publication, the weighting factor for breast tissue to calculate patient risks was bumped up from 0.005 to 0.12, “which puts an emphasis on breast dose in estimating radiation-induced patient risk.”

Overall, the team found that the mean glandular dose of lung cancer screening CT is equal to less than a single two-view digital mammogram. The mean glandular dose of a chest/abdomen/pelvis CT, meanwhile, is equal to five to seven two-view digital mammograms, while a virtual colonoscopy CT is equal to one to three two-view digital mammograms.

The authors also noted that virtual colonoscopy CT differs from the other two studied protocols, because breast are only partially included in the primary x-ray field. This, they explained, leads to variations in dose depending on which scanner model is being used.

“Out of the four scanner models, two show a linear decrease in dose with respect to the distance, and the other two scanners show an exponential decay,” the authors wrote. “Overranging is believed to be the major contributing factor to the observed difference between the two groups. Overranging is the extension of scan length in helical CT beyond both ends of the imaged volume, and the two most important technical parameters that affect overranging are pitch and detector collimation.”

The two models that displayed exponential delay, the team added, “use either a smaller pitch or a smaller total detector collimation compared to the other two that show the linear decreasing trend.”

While not including breasts in the primary beam field at all is one way to avoid exposure to radiation, the researchers noted that it wasn’t always necessarily that simple: “Ideally, breasts should be excluded from the primary beam field for protocols like colonoscopy; however, because of their close proximity to the anatomy of study and the overranging factor of 64-slice CT, it is very likely that some breast tissue will be included in the primary beam field even when the breasts are pulled away from the field during patient preparation.”