Caution needed in applying CT noise reduction in obese patients

While noise reduction significantly improves the quantitative image quality in simulated large patients undergoing abdominal CT in comparison with filtered back projection, no improvement was seen for low-contrast detectability, according to a study published in the February issue of the American Journal of Roentgenology.

CT examinations of obese patients are often challenging due to poor image quality. Modified CT protocols for these patients tend to result in a substantial increase in radiation dose, catalyzing the advent of several noise reduction methods. Lead author Sebastian T. Schindera, MD, of the University of Toronto, and colleagues designed a phantom that enabled multiple repeatable CT studies with different technical parameters to assess the impact of a new noise reduction method on image quality, radiation dose, and low-contrast detectability in obese patients undergoing abdominal CT.

The liver phantom designed by the researchers included 12 different tumors, as well as fat rings to mimic the size of intermediate and large patients. The phantoms were scanned with a standard abdominal CT protocol and the large phantom was then scanned using five different noise levels. Once the datasets from the large phantom were reconstructed with filtered back projection and the noise reduction technique, the image noise and contrast-to-noise ratio were examined.

Results from the study revealed that the measured image noise decreased 14.5 percent to 37 percent after application of the noise reduction method. The contrast-to-noise ratio increased in comparison with the filtered back projection at the same noise level.

Noise reduction was unable to improve the sensitivity for tumor detection in the large phantom compared with filtered back projection at the same noise level. With a noise level of 15 HU, the overall sensitivity for tumor detection in the intermediate and large phantoms with filtered back projection was 75.5 percent and 87.7 percent. Radiation doses respectively measured at 42 and 32.6 mGy.

“Because CT noise reduction primarily improves the image quality and not lesion detection, we recommend applying CT noise reduction with caution for radiation dose reduction in obese patients to avoid a negative influence on the low-contrast detectability,” wrote Schindera and colleagues.

“Furthermore, it is important to point out that the CT Accreditation Program of the American College of Radiology requires low-contrast assessment in a phantom, which is primarily based on contrast-to-noise ratio values," they continued. “Unfortunately, our study shows that contrast-to-noise ratio is not a meaningful parameter for evaluating the potential for radiation dose reduction with CT noise reduction methods in comparison with a filtered back projection.”

The authors suggested the need for future investigations to validate their study’s findings in patients.