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. 2018 Nov 7;2(1):30.
doi: 10.1186/s41747-018-0064-3.

Emphysema quantification using chest CT: influence of radiation dose reduction and reconstruction technique

Annemarie M den Harder  1 , Erwin de Boer  2 , Suzanne J Lagerweij  3 , Martijn F Boomsma  2 , Arnold M R Schilham  3 , Martin J Willemink  3 , Julien Milles  4 , Tim Leiner  3 , Ricardo P J Budde  5 , Pim A de Jong  3
Affiliations

Emphysema quantification using chest CT: influence of radiation dose reduction and reconstruction technique

Annemarie M den Harder et al. Eur Radiol Exp. .

Abstract

Background: Computed tomography (CT) emphysema quantification is affected by both radiation dose (i.e. image noise) and reconstruction technique. At reduced dose, filtered back projection (FBP) results in an overestimation of the amount of emphysema due to higher noise levels, while the use of iterative reconstruction (IR) can result in an underestimation due to reduced noise. The objective of this study was to determine the influence of dose reduction and hybrid IR (HIR) or model-based IR (MIR) on CT emphysema quantification.

Methods: Twenty-two patients underwent inspiratory chest CT scan at routine radiation dose and at 45%, 60% and 75% reduced radiation dose. Acquisitions were reconstructed with FBP, HIR and MIR. Emphysema was quantified using the 15th percentile of the attenuation curve and the percentage of voxels below -950 HU. To determine whether the use of a different percentile or HU threshold is more accurate at reduced dose levels and with IR, additional measurements were performed using different percentiles and HU thresholds to determine the optimal combination.

Results: Dose reduction resulted in a significant overestimation of emphysema, while HIR and MIR resulted in an underestimation. Lower HU thresholds with FBP at reduced dose and higher HU thresholds with HIR and MIR resulted in emphysema percentages comparable to the reference. The 15th percentile quantification method showed similar results as the HU threshold method.

Conclusions: This within-patients study showed that CT emphysema quantification is significantly affected by dose reduction and IR. This can potentially be solved by adapting commonly used thresholds.

Keywords: Densitometry; Emphysema; Radiation dosage; Thorax; Tomography (x-ray computed).

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Conflict of interest statement

Ethics approval and consent to participate

Institutional Review Board approval was obtained. Written informed consent was obtained from all subjects (patients) in this study.

Consent for publication

All authors provided consent for publication.

Competing interests

Julien Milles is an employee of Philips Healthcare. All other authors declare that they have no competing interests.

Publisher’s Note

Springer Nature remains neutral 12with regard to jurisdictional claims in published maps and institutional affiliations.

Figures

Fig. 1
Fig. 1
Example of the semi-automatic software which was used for emphysema quantification. First, the airways, lungs and lung lobes are segmented (a). Subsequently, a histogram is made which displays the number of voxels with a certain density (b). In this example the percentage of voxels below -950 HU is displayed
Fig. 2
Fig. 2
Scatterplots of the effect of radiation dose and reconstruction on the percentage emphysema. The y-axis displays the percentage emphysema with FBP at routine dose (reference), while the x-axis displays the percentage emphysema at reduced dose with FPB (a) and with HIR (b) and MIR (c). Values below the diagonal represent an overestimation of the percentage of emphysema as compared to FBP and routine dose, while values above the diagonal represent an underestimation. FBP filtered back projection, HIR hybrid iterative reconstruction, MIR model-based iterative reconstruction
Fig. 3
Fig. 3
Scatterplots of the effect of radiation dose and reconstruction on the perc15. The y-axis displays the perc15 with FBP at routine dose (reference), while the x-axis displays the perc15 at reduced dose with FBP (a) and with HIR (b) and MIR (c). Values below the diagonal represent a higher HU value compared to the reference, while values above the diagonal represent a lower HU value compared to the reference. FBP filtered back projection, HIR hybrid iterative reconstruction, MIR model-based iterative reconstruction
Fig. 4
Fig. 4
Effect of different thresholds (a) and percentiles (b) on emphysema quantification. For FBP at reduced dose a lower threshold is more appropriate, while with HIR and MIR a higher HU threshold should be used. With the percentile quantification method, FBP at reduced dose requires a higher percentile while with HIR and MIR a lower percentile should be used to achieve the same results as with FBP at routine dose. FBP filtered back projection, HIR hybrid iterative reconstruction, MIR model-based iterative reconstruction
Fig. 5
Fig. 5
Noise (a and b) and CNR (c) measured at different dose levels with FBP, HIR and MIR. Noise was measured in the aorta (a) and subcutaneous fat (b). The dotted line represents the reference (FBP at routine dose). CNR contrast-to-noise ratio, FBP filtered back projection, HIR hybrid iterative reconstruction, MIR model-based iterative reconstruction
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