Whole-Body PET/CT: Radiation Dose Reduced Doesn’t Noticeably Affect Images

The amount of radiation given during computed tomography (CT) as part of whole-body positron emission tomography (PET)/CT exams can be safely reduced without comprising the quality of the images taken, a new study indicated.

According to the study published in the American Journal of Roentgenology, Elena Tonkopi, of Dalhousie University, Canada, and colleagues were able to reduce the mean effective CT radiation dose by more than 2.5 mSv, from 8.1 mSv to 5.5 mSv, while maintaining image quality.

“Hybrid imaging, such as PET/CT examination, results in increased radiation exposure due to the combined PET and CT components of the scan,” Tonkopi said. “Patients often undergo multiple follow-up studies, which would contribute to lifetime-attributable risk of cancer incidence. Therefore, it is very important to decrease patient radiation dose and to implement the [As Low As Reasonably Achievable] principle.”

First, the researchers established a baseline radiation dose from a whole-body PET/CT examination in 140 consecutive patients. Then, they implemented an optimization protocol. The protocol gradually increased the noise index from 25 to 27.1, reduced the pitch from 1.75 to 1.35, reduced the rotation time from 0.8 to 0.5, and changed the beam collimation from 16 x 0.625 mm to 16 x 1.25 mm.

After the protocol was in place, an additional 100 patients were surveyed to collect information on radiation dose. In addition, the researchers conducted an imaging analysis on 26 patients who underwent imaging both before and after protocol implementation.

The researchers found it was possible to reduce the patient dose by optimizing CT scanning parameters.

“We have developed the low-dose CT protocol, used for attenuation correction and anatomic localization of PET data,” Tonkopi said. “The implementation of the new protocol resulted in 32 percent reduction of the CT dose or 16 percent reduction in total dose from the examination.”

In addition, these reductions occurred without loss in image quality. The images from the 26 patients were blinded and randomly assigned to an experienced PET and CT reader, who then graded the images based on the quality of anatomic structures. No significant degradation of quality was found for images taken during the low-dose imaging studies, with only two of 15 anatomic areas found to have significant differences: the carotid arteries and the posterior triangle region.

“Our study demonstrated that reduction of potentially excess irradiation of patients can be performed without loss of diagnostic power,” Tonkopi said. “An out-of-box configuration may not be optimized for patient dose and needs to be considered in implementation and clinical procedures. The methodology described in the study provides practical solutions.”