Clearer view: IR bests FBP for low-dose imaging of stones

Iterative reconstruction (IR) has been shown to improve image quality and diagnostic confidence in the identification of urinary stones with abdominal low-dose CT, according to a study published in the November issue of Academic Radiology.

The findings could rally support for the increased use of abdominal low-dose CT as the primary imaging test for evaluating patients with known or suspected urinary stone disease, suggested authors Anna Winklehner, MD, of University Hospital Zurich, Switzerland, and colleagues.

“Due to concerns regarding increased image noise and thus reduced diagnostic confidence, low-dose abdominal CT has not yet been able to replace standard-dose CT as an initial imaging test in patients with known or suspected urinary stone disease completely,” they wrote.

To see how image quality fared with IR compared to standard filtered back-projection (FBP), Winklehner and colleagues evaluated 50 consecutive patients with suspected or known urinary stone disease. All patients underwent nonenhanced abdominal low-dose CT with an effective radiation dose of around 1.6 mSv. Reconstructions were conducted with sonogram-affirmed IR and with FBP.

Attenuation of urinary bladder and subcutaneous fat was similar among IR and FBP datasets, but image noise was reduced in IR datasets by 40.1 percent.

“Use of IR did not result in differences in the number of calculi detected, but the confidence regarding the identification of urinary stones improved significantly compared to FBP,” wrote Winklehner et al.

The authors noted that obesity was a factor impairing image quality in some of the FBP images, but this was not the case for any IR images. “Our study indicates that the use of IR can result in a decreased dependency of image quality on the patients' BMI [body mass index]. Whereas in FBP datasets, image noise increased significantly in obese patients, there was no such effect for IR, where image noise remained relatively constant across different patients' BMI.”

The fact that attenuation values did not change with IR compared with FBP is also an important finding, according to the authors, because it indicates that quantitative measurements are stable across reconstruction techniques. “In the near future, when IR becomes available for dual-energy CT applications when characterizing stone types, this stability in CT attenuation will be of considerable value.”