Improved precision of noise estimation in CT with a volume-based approach

Hendrik Joost Wisselink, Gert Jan Pelgrim, Mieneke Rook, Ivan Dudurych, Maarten van den Berge, Geertruida H de Bock, Rozemarijn Vliegenthart, Hendrik Joost Wisselink, Gert Jan Pelgrim, Mieneke Rook, Ivan Dudurych, Maarten van den Berge, Geertruida H de Bock, Rozemarijn Vliegenthart

Abstract

Assessment of image noise is a relevant issue in computed tomography (CT). Noise is routinely measured by the standard deviation of density values (Hounsfield units, HU) within a circular region of interest (ROI). We explored the effect of a spherical volume of interest (VOI) on noise measurements. Forty-nine chronic obstructive pulmonary disease patients underwent CT with clinical protocol (regular dose [RD], volumetric CT dose index [CTDIvol] 3.04 mGy, 64-slice unit), and ultra-low dose (ULD) protocol (median CTDIvol 0.38 mGy, dual-source unit). Noise was measured in 27 1-cm2 ROIs and 27 0.75-cm3 VOIs inside the trachea. Median true noise was 21 HU (range 17-29) for RD-CT and 33 HU (26-39) for ULD-CT. The VOI approach resulted in a lower mean distance between limits of agreement compared to ROI: 5.9 versus 10.0 HU for RD-CT (-40%); 4.7 versus 9.9 HU for ULD-CT (-53%). Mean systematic bias barely changed: -1.6 versus -0.9HU for RD-CT; 0.0 to 0.4HU for ULD-CT. The average measurement time was 6.8 s (ROI) versus 9.7 (VOI), independent of dose level. For chest CT, measuring noise with a VOI-based instead of a ROI-based approach reduces variability by 40-53%, without a relevant effect on systematic bias and measurement time.

Trial registration: ClinicalTrials.gov NCT02477397.

Keywords: Data accuracy; Noise; Pulmonary disease (chronic obstructive); Thorax; Tomography (x-ray computed).

Conflict of interest statement

RV is supported by an institutional grant from Siemens Healthineers. The other authors have no competing interests to be declared.

© 2021. The Author(s).

Figures

Fig. 1
Fig. 1
Flow chart of the steps to determine the ground truth noise and the isocenter for the measurements. ROI, region of interest; VOI, volume of interest
Fig. 2
Fig. 2
Subsection of the CT images around the carina (window width 1600 HU, window level −700 HU). The red part is the position of the region of interest, the blue is the volume of interest, the yellow is used to measure the ground truth, and the green area was removed from the segmentation to prevent edge artifacts like the partial volume effect. This image shows the measurement with the isocenter 1.0 cm above the carina ridge. a Axial images. b Coronal images, interpolated to account for the anisotropic dimensions of the voxels. c Volume render of the yellow segmentation
Fig. 3
Fig. 3
Results of the Bland-Altman analyses. Each plot shows the difference between the noise measured with either ROI or VOI and ground truth noise on the y-axis, versus ground truth on the x-axis. Regular radiation dose computed tomography protocol measured with an ROI (a) or a VOI (b), same data for ultra-low dose protocol (c and d, respectively). ROI, region of interest; VOI, volume of interest; LoA, limits of agreement; HU, Hounsfield units

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Source: PubMed

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