Model-based iterative reconstruction slashes CT dose more than 70%

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 - MBIR abdominal CT
55-year-old woman with treated liver metastasis from pancreatic endocrine tumor who underwent contrast-enhanced abdominal CT--low-dose FBP (top left), ASIR (top right), MBIR (bottom left) and standard-dose FBP (bottom right).
Source: Am. J. of Roentgenology (DOI:10.2214/AJR.12.9382)

Model-based iterative reconstruction (MBIR) has shown great promise for reducing radiation dose at routine abdominal CT, according to the preliminary results of an ongoing prospective trial published online ahead of print in the American Journal of Roentgenology.

Specifically, MBIR demonstrated substantial improvement in terms of image noise, subjective image quality and diagnostic performance when compared with the traditional reconstruction method of filtered back projection (FBP) and the newer adaptive statistical iterative reconstruction (ASIR) algorithm, according to Perry J. Pickhardt, MD, of the University of Wisconsin, Madison, and colleagues.

“As the first true model-based reconstruction method in CT, MBIR has the potential to substantially reduce dose while improving resolution,” wrote the authors.

Whereas ASIR offers the possibility of reducing dose between 25 and 40 percent, Pickhardt and colleagues initially targeted a dose reduction goal in the 70 to 90 percent range for the clinical trial of MBIR.

To validate the use of ultralow-dose abdominal CT with MBIR, the researchers imaged 21 patients with standard-dose contrast-enhanced CT, and 24 without enhancement, and immediately followed with ultralow-dose imaging of a matched series of 45 consecutively registered adults. The ultralow-dose images were reconstructed with FBP, ASIR and MBIR, with FBP reconstructions of the standard-dose series serving as the reference standard.

Results showed the mean effective dose for the ultralow-dose scans was 1.90 mSv, a 74 percent reduction relative to the standard series. Mean multi-organ image noise for MBIR was 14.7 HU, compared with 28.9 HU for standard-dose FBP, 59.2 HU for low-dose FBP and 45.6 HU for ASIR.

The mean subjective image quality score for low-dose MBIR was 3.0 on a five point scale. This was significantly higher than the scores of 1.6 for low-dose FBP and 1.8 for ASIR, reported the authors.

MBIR also demonstrated higher pooled lesion detection rates, identifying 79.3 percent of the 213 focal non-calcific lesions detected in the reference standard, compared with 66.2 percent and 62 percent detection rates for low-dose FBP and ASIR, respectively.

Pickhardt and colleagues were surprised to find ASIR delivered slightly worse diagnostic performance than low-dose FBP, despite showing benefits in terms of image noise and image quality score. “Clearly, image noise and subjective quality measurements alone are insufficient for validating novel ultralow-dose iterative reconstruction techniques. Additional objective quality metrics likely need to be developed and validated to better evaluate iterative reconstruction techniques.”

While the initial results of the trial demonstrated the qualitative benefit of MBIR, the authors noted that lesion detection was still somewhat compromised when compared with standard-dose FBP. MBIR is also limited by prolonged reconstruction time.

“Further investigation is needed to determine the optimal dose level for MBIR that maintains adequate diagnostic performance,” wrote Pickhardt and colleagues.