Iterative reconstruction maintains CT quality while allowing 71% drop in dose

At low tube voltage settings, the iDose4 iterative reconstruction algorithm results in higher signal-to-noise ratio, contrast-to-noise ratio and better overall image quality in contrast-enhanced chest CT images, according to a study published in the May issue of Clinical Radiology.

The findings support previous research that have shown iterative reconstruction techniques could be an alternative to the traditional filtered back projection (FBP) method of reconstruction, according to Shi-Yuan Liu, MD, of Changzheng Hospital, Second Military Medical University, Shanghai, and colleagues.

“With increasing investigation of the use of low doses, it has become apparent that images reconstructed with FBP techniques are often inadequate. Although this technique is very fast and robust, FBP images are prone to high noise, streak artifacts, and poor low contrast detectability in low-dose protocols,” wrote the authors.

In the study, Liu and colleagues tested the iDose4 method of iterative reconstruction against FBP. iDose4 applies an iterative maximum likelihood de-noising algorithm, based on Poisson noise distribution, on raw projection data, according to the authors. It differs from image-based adaptive filtering techniques in that it consists of a dual-modal—noise and anatomical—technique, and makes it possible to remove noise across all frequencies.

A total of 80 patients with normal BMI referred for contrast-enhanced chest CT were randomly assigned into two groups: a commonly used tube voltage of 120 kVp and a reduced voltage of 80 kVp. FBP was used to reconstruct image sets in both groups, while iDose4 was used to reconstruct just the reduced voltage group.

Results showed a 26.74 percent reduction in noise level when the low voltage group was reconstructed with iDose4 compare with FBP. There was no difference in subjective image quality between the low-voltage iDose4 group and the standard voltage FBP group. iDose4 resulted in the highest signal-to-noise and contrast-to-noise ratios.

Radiation dose in the low-voltage protocol was reduced 71.35 percent compared with the standard protocol, reported the authors.

One downside to iterative reconstruction is that it is a more complex and time-consuming process, acknowledged Liu and colleagues, though they noted that reconstruction time is clinically acceptable making it “a viable technique that is worthy of further study.”