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Reducing Radiation Dose at Chest CT

Posted Sep 30, 2016 Updated Apr 10, 2023
By Constance de Margerie-Mellon MD , Cédric de Bazelaire MD PhD , Claire Montlahuc MD , Jérôme Lambert MD PhD , Antoine Martineau PhD , Philippe Coulon PhD , Eric de Kerviler MD , Catherine Beigelman MD
12 min read

Rationale and Objectives

The study aimed to evaluate the performances of two iterative reconstruction (IR) algorithms and of filtered back projection (FBP) when using reduced-dose chest computed tomography (RDCT) compared to standard-of-care CT.

Materials and Methods

An institutional review board approval was obtained. Thirty-six patients with hematologic malignancies referred for a control chest CT of a known lung disease were prospectively enrolled. Patients underwent standard-of-care scan reconstructed with hybrid IR, followed by an RDCT reconstructed with FBP, hybrid IR, and iterative model reconstruction. Objective and subjective quality measurements, lesion detectability, and evolution assessment on RDCT were recorded.

Results

For RDCT, the CTDIvol (volumetric computed tomography dose index) was 0.43 mGy⋅cm for all patients, and the median [interquartile range] effective dose was 0.22 mSv [0.22–0.24]; corresponding measurements for standard-of-care scan were 3.4 mGy [3.1–3.9] and 1.8 mSv [1.6–2.0]. Noise significantly decreased from FBP to hybrid IR and from hybrid IR to iterative model reconstruction on RDCT, whereas lesion conspicuity and diagnostic confidence increased. Accurate evolution assessment was obtained in all cases with IR. Emphysema identification was higher with iterative model reconstruction.

Conclusion

Although iterative model reconstruction offered better diagnostic confidence and emphysema detection, both IR algorithms allowed an accurate evolution assessment with an effective dose of 0.22 mSv.

Introduction

Over the past decades, the average annual radiation dose delivered has raised significantly worldwide . Reducing computed tomography (CT) radiation dose has become a major concern because of the potential risk of radiation-induced cancer .

Various strategies have been developed to reduce radiation exposure while maintaining image quality, including tube potential selection and tube current modulation. Unfortunately, radiation dose decrease is limited when using filtered back projection (FBP) reconstruction. FBP assumes that each pixel value perfectly represents the attenuation of the object at this location, setting aside system hardware details and photon noise statistic information . Because of those idealized assumptions, a lowered radiation dose is accompanied by an increased noise and often increased artifacts.

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Materials and Methods

Population

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TABLE 1

Characteristics of Included Patients

Characteristics Patients ( n = 36) Age (y) \* 59[28.5–66] Sex ratio (M/F) 21/15 Body mass index (kg/m 2 ) \* 21.9[19.8–24.2] Effective diameter † (cm) \* 27[25.6–29.0] Delay since the initial chest computed tomography (d) \* 28[14–50] Initial pulmonary diagnosis Invasive aspergillosis (proven or probable) 7(19%) Bacterial pneumonia (proven or probable) 7(19%) Nonspecific lesions, probabilistic antifungal or antibiotic therapy 11(31%) Nonspecific lesions, no therapy 7(19%) All trans-retinoic acid syndrome 1(3%) Organizing pneumonia 1(3%) Lymphoma tumoral extension 1(3%) Alveolar proteinosis 1(3%)

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Imaging Protocol

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Reconstruction Process

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Radiation Dose Assessment

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Objective Measurements

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Subjective Measurements

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TABLE 2

Grading for Subjective Image Quality

1 2 3 4 5 Subjective noise Minimal image noise Less than average noise Average image noise Above average noise Unacceptable image noise Normal anatomic structure detectability \* Excellent visualization Above average visibility Acceptable visibility Unacceptable visualization of small structures Not applicable Artifacts † No artifacts Minor artifacts not interfering with diagnosis decision making Major artifacts but possible diagnosis Artifacts affecting diagnostic information Not applicable Lesion conspicuity Well-seen lesion with sharp margins Well-seen lesion with poorly demarcated margins Subtle lesion Probable artifact mimicking a lesion Definite artifact mimicking a lesion Diagnostic confidence Completely confident Probably confident Confident only for limited clinical situations Poor confidence Not applicable

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Statistical Analysis

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Results

Technical Parameters

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Objective Measurements

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Figure 1, Boxplots representing the trachea and aorta noise of the standard-of-care protocol and of the three reconstructions of reduced-dose chest computed tomography. Boxes indicate interquartile range; whiskers extend the data extremes, and horizontal lines the median value. All pairwise comparisons were statistically significant.

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Subjective Measurements

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TABLE 3

Subjective Quality Scores (Consensus Reading)

Standard-of-care CT Ultra Reduced-dose CT FBP iDose 4 IMR Subjective noise (1–5) 2 7(19) 0(0) 0(0) 32(89) 3 29(81) 0(0) 10(28) 4(11) 4 0(0) 18(50) 26(72) 0(0) 5 0(0) 18(50) 0(0) 0(0) Normal anatomic structure detectability(1–4) 2 10(28) 0(0) 0(0) 0(0) 3 26(72) 0(0) 0(0) 0(0) 4 0(0) 36(100) 36(100) 36(100) Artifacts(1–4) 1 21(58) 0(0) 0(0) 0(0) 2 15(42) 2(6) 25(69) 36(100) 3 0(0) 32(88) 11(31) 0(0) 4 0(0) 2(6) 0(0) 0(0) Lesion conspicuity(1–5) 1 35(97) 0(0) 0(0) 25(69) 2 1(3) 14(39) 31(86) 11(31) 3 0(0) 22(61) 5(14) 0(0) Diagnostic confidence(1–4) 1 36(100) 0(0) 0(0) 15(42) 2 0(0) 20(56) 35(97) 21(58) 3 0(0) 16(44) 1(3) 0(0)

CT, computed tomography; FBP, filtered back projection; IMR, iterative model reconstruction.

Data are expressed as count (percentages).

Figure 2, Example of normal anatomic lung structure detectability scoring. Corresponding images for (a) standard-of-care CT, (b) RDCT, FBP reconstruction, (c) RDCT, iDose 4 reconstruction, and (d) RDCT, IMR reconstruction. Arrows show pulmonary fissures, which are well seen as a complete thin line in (a) , but which are ill-defined in (b) , (c) , and (d) . Detectability of normal anatomic lung structures was ranked 3 in (a) , and ranked 4 in (b) , (c) , and (d) . CT, computed tomography; FBP, filtered back projection; IMR, iterative model reconstruction; RDCT, reduced-dose chest computed tomography.

Figure 3, Example 1 of lesion conspicuity scoring. Corresponding images for (a) standard-of-care CT, (b) RDCT, FBP reconstruction, (c) RDCT, iDose 4 reconstruction, and (d) RDCT, IMR reconstruction. Arrows show a ground-glass opacity in the left lung. Definition of this opacity grows from (b) to (d) . Lesion conspicuity was ranked 1 in (a) , 3 in (b) , 2 in (c) , and 1 in (d) . CT, computed tomography; FBP, filtered back projection; IMR, iterative model reconstruction; RDCT, reduced-dose chest computed tomography.

Figure 4, Example 2 of lesion conspicuity scoring. Corresponding images for (a) standard-of-care CT, (b) RDCT, FBP reconstruction, (c) RDCT, iDose 4 reconstruction, and (d) RDCT, IMR reconstruction. Arrows show emphysema of the right apex. Definition of emphysema grows from (b) to (d) . Lesion conspicuity was ranked 1 in (a) , 3 in (b) , 2 in (c) , and 1 in (d) . CT, computed tomography; FBP, filtered back projection; IMR, iterative model reconstruction; RDCT, reduced-dose chest computed tomography.

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TABLE 4

Lesion Detection

FBP iDose 4 IMR Lesion Number of Lesion with Standard-of-care CT Number of Lesion with RDCT Sensitivity (95% CI) Specificity (95% CI) Number of Lesion with RDCT Sensitivity (95% CI) Specificity (95% CI) Number of Lesion with RDCT Sensitivity (95% CI) Specificity (95% CI) Ground-glass opacity 75 53 0.67(0.55–0.77) 0.98(0.94–0.97) 75 0.96(0.89–0.99) 0.98(0.94–1) 77 0.95(0.87–0.99) 0.96(0.91–0.98) Emphysema 48 33 0.65(0.50–0.78) 0.99(0.96–1) 33 0.67(0.52–0.8) 0.99(0.97–1) 45 0.9(0.77–0.97) 0.99(0.96–1) Consolidation 32 30 0.94(0.79–0.99) 1(0.98–1) 30 0.94(0.79–0.99) 1(0.98–1) 32 1(0.89–1) 1(0.98–1) Linear atelectasis 21 22 0.95(0.76–1) 0.99(0.96–1) 21 1(0.84–1) 1(0.98–1) 18 0.86(0.64–0.97) 1(0.98–1) Bronchiectasis 15 8 0.53(0.27–0.79) 1(0.98–1) 11 0.73(0.45–0.92) 1(0.98–1) 9 0.6(0.32–0.84) 1(0.98–1) Septal thickening 7 3 0.43(0.01–0.82) 1(0.98–1) 4 0.57(0.18–0.9) 1(0.98–1) 4 0.57(0.18–0.9) 1(0.98–1)

CI, confidence interval; CT, computed tomography; FBP, filtered back projection; IMR, iterative model reconstruction; RDCT, reduced-dose chest computed tomography.

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Discussion

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Acknowledgments

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Academic Radiology, Volume 23, Issue 10
Journals General Radiology
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