Technical Note: Impact on central frequency and noise magnitude ratios by advanced CT image reconstruction techniques

We use central frequency ratio and noise magnitude ratio from noise power spectrum (NPS) to evaluate the noise reduction techniques of ASiR and ASiR-V of GE, SAFIRE and ADMIRE of Siemens, and PixelShine of AlgoMedica. ASiR, ASiR-V, SAFIRE and ADMIRE use a combination of image and projection data whereas PixelShine uses artificial intelligence neural network for noise reduction.

The homogeneous module of the ACR computed tomography (CT) phantom was scanned on a GE Revolution HD 64-slice CT for ASiR and ASiR-V, a Siemens Somatom Force for ADMIRE, and a Siemens Definition AS 64-slice for SAFIRE for NPS calculation. The baseline filtered back-projection (FBP) reconstructions were derived from the standard kernel on Revolution HD, Hr44f on Force and D40s on Definition AS. The central frequency ratio (CFR) indicates the degree of shift in the central frequency of NPS after noise reduction. A smaller CFR means a larger shift of the NPS curve, or a larger degree of image blurring. The noise magnitude ratio (NMR) indicates the amount of noise removed. A smaller NMR means a larger degree of noise reduction. An ideal noise reduction shall maintain a CFR close to 1 and a NMR close to 0.

The ideal noise reduction by increasing radiation exposure did not shift the central frequency when the image noise was reduced. PixelShine was the closest to the ideal noise reduction in CFR, and was followed by SAFIRE, ASiR-V, ADMIRE and ASiR, in sequence. Similarly, PixelShine had the smallest NMR, and was followed by SAFIRE, ASiR-V, ADMIRE and ASiR in sequence. Overall, PixelShine had the least central frequency shift for the same amount of noise reduction or the most noise reduction for the same amount of central frequency shift. For the same CFR, ASiR-V reduced more noise than ASiR; and SAFIRE reduced more noise than ADMIRE.

We introduced two new parameters of CFR and NMR from NPS to compare the reconstructions from different manufacturers. PixelShine had the least central frequency shift for the same amount of noise reduction or the most noise reduction for the same amount of central frequency shift. For the same central frequency shift, ASiR-V reduced more noise than ASiR, and SAFIRE reduced more noise than ADMIRE.