Ultra-low dose chest computed tomography: Effect of iterative reconstruction levels on image quality

Mercy Afadzi; Elisabeth Kirkeby Lysvik; Hilde Kjernlie Andersen; Anne Catrine T Martinsen

doi:10.1016/j.ejrad.2019.02.021

Ultra-low dose chest computed tomography: Effect of iterative reconstruction levels on image quality

Eur J Radiol. 2019 May:114:62-68. doi: 10.1016/j.ejrad.2019.02.021. Epub 2019 Feb 18.

Authors

Mercy Afadzi¹, Elisabeth Kirkeby Lysvik², Hilde Kjernlie Andersen², Anne Catrine T Martinsen³

Affiliations

¹ Department of Diagnostic Physics, Oslo University Hospital, Oslo, Norway. Electronic address: merafa@ous-hf.no.
² Department of Diagnostic Physics, Oslo University Hospital, Oslo, Norway.
³ Department of Diagnostic Physics, Oslo University Hospital, Oslo, Norway; The Department of Physics, University of Oslo, Oslo, Norway.

PMID: 31005179
DOI: 10.1016/j.ejrad.2019.02.021

Abstract

Purpose: To optimize image quality and radiation dose of chest CT with respect to various iterative reconstruction levels, detector collimations and body sizes.

Method: A Kyoto Kagaku Lungman with and without extensions was scanned using fixed ultra-low doses of 0.25, 0.49 and 0.74 mGy CTDI_vol, and collimations of 40 and 80 mm. Images were reconstructed with the lung kernel, filtered back projection (FBP) and different ASIR-V levels (10-100%). Contrast-to-noise ratios (CNR) were calculated for 12 mm simulated lesions of different densities in the lung. Image noise, signal-to-noise ratios (SNR), variations in Hounsfield units (HU), noise power spectrum (NPS) and noise texture deviations (NTD) were evaluated for all reconstructions. NTD was calculated as percentage of pixels outside 3 standard deviations to evaluate IR-specific artefacts.

Results: Compared to the FBP, image noise reduced (5-55%) with ASIR-V levels irrespective of dose or collimation. SNR correlated positively (r ≥ 0.925, p ≤ 0.001) with ASIR-V levels at all doses, collimations, and phantom sizes. ASIR-V enhanced the CNR of the lesion with the lowest contrast from 12.7-42.1 (0-100% ASIR-V) at 0.74 mGy with 40 mm collimation. As expected, higher SNR and CNR were measured in the smaller phantom than the bigger phantom. Uniform HU were observed between FBP and ASIR-V levels at all doses, collimations, and phantom sizes. NPS curves left-shifted towards lower frequencies at increasing levels of ASIR-V irrespective of collimation. A positive correlation (r ≥ 0.946, p ≥ 0.001) was observed between NTD and ASIR-V levels. NTD of the FBP was not significantly (p ≤ 0.087) different from NTD of ASIR-V ≤ 20%. The data from the NPS and NTD indicates a blotchier and coarser noise texture at higher levels of ASIR-V, especially at 100% ASIR-V.

Conclusion: In comparison with the FBP technique, ASIR-V enhanced quantitative image quality parameters at all ultra-low doses tested. Moreover, the use of ASIR-V showed consistency with body size and collimation. Hence, ASIR-V may be useful for improving image quality of chest CT at ultra-low doses.

Keywords: ASIR-V; Chest CT; Image quality; Iterative reconstruction; Ultra-low dose.

MeSH terms

Algorithms
Artifacts
Humans
Image Processing, Computer-Assisted / methods
Phantoms, Imaging
Radiation Dosage
Radiographic Image Interpretation, Computer-Assisted / methods
Radiography, Thoracic / methods
Radionuclide Imaging
Signal-To-Noise Ratio
Tomography, X-Ray Computed / methods*