New adaptive statistical iterative reconstruction ASiR-V: Assessment of noise performance in comparison to ASiR

Abstract

Purpose: To assess the noise characteristics of the new adaptive statistical iterative reconstruction (ASiR-V) in comparison to ASiR.

Methods: A water phantom was acquired with common clinical scanning parameters, at five different levels of CTDI_vol . Images were reconstructed with different kernels (STD, SOFT, and BONE), different IR levels (40%, 60%, and 100%) and different slice thickness (ST) (0.625 and 2.5 mm), both for ASiR-V and ASiR. Noise properties were investigated and noise power spectrum (NPS) was evaluated.

Results: ASiR-V significantly reduced noise relative to FBP: noise reduction was in the range 23%-60% for a 0.625 mm ST and 12%-64% for the 2.5 mm ST. Above 2 mGy, noise reduction for ASiR-V had no dependence on dose. Noise reduction for ASIR-V has dependence on ST, being greater for STD and SOFT kernels at 2.5 mm. For the STD kernel ASiR-V has greater noise reduction for both ST, if compared to ASiR. For the SOFT kernel, results varies according to dose and ST, while for BONE kernel ASIR-V shows less noise reduction. NPS for CT Revolution has dose dependent behavior at lower doses. NPS for ASIR-V and ASiR is similar, showing a shift toward lower frequencies as the IR level increases for STD and SOFT kernels. The NPS is different between ASiR-V and ASIR with BONE kernel. NPS for ASiR-V appears to be ST dependent, having a shift toward lower frequencies for 2.5 mm ST.

Conclusions: ASiR-V showed greater noise reduction than ASiR for STD and SOFT kernels, while keeping the same NPS. For the BONE kernel, ASiR-V presents a completely different behavior, with less noise reduction and modified NPS. Noise properties of the ASiR-V are dependent on reconstruction slice thickness. The noise properties of ASiR-V suggest the need for further measurements and efforts to establish new CT protocols to optimize clinical imaging.

Keywords: computed tomography; iterative reconstruction; noise power spectrum.

Figure 1

Scheme of the four ROIs used for the NPS evaluation.

Figure 2

Standard Deviation (SD) as a function of CTDI_vol.

Figure 3

Noise reduction for the ASiR‐V as a function of slice thickness (0.625 mm solid lines −2.5 mm dashed lines) for kernel STD (a) and BONE (b).

Figure 4

Noise reduction for the ASiR with the STD kernel with 0.625 mm (a) and 2.5 mm (b) slice thickness.

Figure 5

Comparison of noise reduction between ASiR and ASiR‐V with different kernels and different slice thickness: STD 0.625 mm (a) and 2.5 mm (b); SOFT 0.625 mm (c) and 2.5 mm (d); BONE 0.625 mm (e) and 2.5 mm (f).

Figure 6

Dose‐dependent behavior below 4 mGy for the CT Revolution Normalized NPS (nNPS) are shown for kernel STD (a) and BONE (b).

Figure 7

Noise Power Spectra of the ASiR‐V with different kernels at increasing IR level: kernel STD (a), SOFT (b), BONE (c), BONE normalized to the maximum (d).

Figure 8

FBP spectra with STD kernel for CT Discovery and CT Revolution at different dose levels: 1–4 mGy (a), 4–15 mGy (b).

Figure 9

FBP spectra with SOFT kernel for CT Discovery and CT Revolution at different dose levels:1–4 mGy (a), 4–15 mGy (b).

Figure 10

FBP spectra with BONE kernel for CT Discovery and CT Revolution at different dose levels: 1–4 mGy (a), 4–15 mGy (b).

Figure 11

NPS with STD kernel for ASiR‐V and ASiR at different IR levels (CTDI_vol = 7 mGy).

Figure 12

NPS with SOFT (a) and BONE (b) kernel for ASiR‐V and ASiR at different IR levels (CTDI_vol = 7 mGy).