Six iterative reconstruction algorithms in brain CT: a phantom study on image quality at different radiation dose levels

Objective:

To evaluate the image quality produced by six different iterative reconstruction (IR) algorithms in four CT systems in the setting of brain CT, using different radiation dose levels and iterative image optimisation levels.

Methods:

An image quality phantom, supplied with a bone mimicking annulus, was examined using four CT systems from different vendors and four radiation dose levels. Acquisitions were reconstructed using conventional filtered back-projection (FBP), three levels of statistical IR and, when available, a model-based IR algorithm. The evaluated image quality parameters were CT numbers, uniformity, noise, noise-power spectra, low-contrast resolution and spatial resolution.

Results:

Compared with FBP, noise reduction was achieved by all six IR algorithms at all radiation dose levels, with further improvement seen at higher IR levels. Noise-power spectra revealed changes in noise distribution relative to the FBP for most statistical IR algorithms, especially the two model-based IR algorithms. Compared with FBP, variable degrees of improvements were seen in both objective and subjective low-contrast resolutions for all IR algorithms. Spatial resolution was improved with both model-based IR algorithms and one of the statistical IR algorithms.

Conclusion:

The four statistical IR algorithms evaluated in the study all improved the general image quality compared with FBP, with improvement seen for most or all evaluated quality criteria. Further improvement was achieved with one of the model-based IR algorithms.

Advances in knowledge:

The six evaluated IR algorithms all improve the image quality in brain CT but show different strengths and weaknesses.Iterative reconstruction (IR) algorithms are one of the most recent advances in CT. Since the introduction of the first IR algorithm in 2008 1], multiple clinical studies have shown the potential of such algorithms to improve the image quality and allow for the reduction of radiation dose while maintaining diagnostic acceptability 2–7].Although all IR algorithms perform iterative image optimisation at some point in the CT image generation process, there are considerable technical differences between the available IR solutions. Furthermore, some vendors even offer more than one type of IR algorithm in their product range. Although detailed mechanisms of the current algorithms remain undisclosed, they can be classified into two basic categories 8,9] (