The number of overweight and obese individuals in the U.S. is increasing, and studies have shown that CT radiation dose is affected by patient size. New research has quantified this effect and shown that forced change of operation parameters to increase image quality for obese patients can result in an increase of up to 62 percent in organ radiation exposure compared with lower weight patients, according to a study published April 6 in Physics in Medicine & Biology.
Study authors from Rensselaer Polytechnic Institute in Troy, N.Y., created a series of computerized phantoms representing overweight and obese patients to examine the effect of obesity on radiation dose to organs and tissues from CT and to help optimize CT scanning procedures to produce safe, but effective, medical images.
“When a morbidly obese patient undergoes a CT scan … tube potential needs to be increased to make sure there are enough x-ray photons passing through the body to form a good image,” said Aiping Ding, PhD, lead author of the study. “So far, such optimization has been done by trial and error without the use of patient-specific quantitative analysis."
Ten phantoms were created in all, five males and five females, with weight classification ranging from normal (BMI of 23.5 for males and 23.9 for females) to morbidly obese (BMI of 45 for males and 46.4 for females).
The researchers found that the same chest-abdomen-pelvis scanning parameters delivered 59 percent lower radiation doses to deep organs of obese individuals compared to those of normal body weight. This effect was less significant for shallow organs.
However, increasing tube potential from 120 to 140 kVp, which is often done to improve image quality, increased organ doses for the same obese individual by as much as 56 percent for organs in the scan field and 62 percent for those outside of the scan field.
Another finding of the study was the effect that different types of body fat have on organ dose estimates. The phantoms modeled visceral adipose tissue (VAT) that lies deeper in the body around the organs, and the authors found that if VATs were replaced in the models by normal body tissues, dose to organs deep in the abdomen increased by as much as 16 percent and 32 percent for obese males and females, respectively.
“The results from our simple VAT model suggest that VAT plays an important role in the organ dose estimates and should always be considered for obese individuals,” wrote the authors.
The prevalence of overweight and obese individuals has increased over the past 20 years and a recent survey suggests that nearly 60 percent of the adult American population can be diagnosed as being clinically overweight or obese. Because of this trend, the researchers hope their models can be used to better accommodate obese patients.
“The ultimate goal of this on-going work is to include dose data for these obese phantoms in a CT dose reporting software tool called VirtualDose,” concluded the authors. VirtualDose is a software package that will contain a comprehensive database of data on normal-sized, obese and child patients. It will enter clinical testing this summer.