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How Low Can We Go in Contrast-Enhanced CT Imaging of the Chest?

Posted Feb 28, 2015 Updated Apr 10, 2023
By Fabian G. Mueck MD , Sebastian Roesch , Michael Scherr MD , Florian Fischer MD , Lucas Geyer MD , Oliver Peschel MD , Daniel Maxien MD , Silke Grabherr MD PhD , Stefan Wirth MD PhD MSc
17 min read

Rationale and Objectives

Dose reduction may compromise patients because of a decrease of image quality. Therefore, the amount of dose savings in new dose-reduction techniques needs to be thoroughly assessed. To avoid repeated studies in one patient, chest computed tomography (CT) scans with different dose levels were performed in corpses comparing model-based iterative reconstruction (MBIR) as a tool to enhance image quality with current standard full-dose imaging.

Materials and Methods

Twenty-five human cadavers were scanned (CT HD750) after contrast medium injection at different, decreasing dose levels D0–D5 and respectively reconstructed with MBIR. The data at full-dose level, D0, have been additionally reconstructed with standard adaptive statistical iterative reconstruction (ASIR), which represented the full-dose baseline reference (FDBR). Two radiologists independently compared image quality (IQ) in 3-mm multiplanar reformations for soft-tissue evaluation of D0–D5 to FDBR (−2, diagnostically inferior; −1, inferior; 0, equal; +1, superior; and +2, diagnostically superior). For statistical analysis, the intraclass correlation coefficient (ICC) and the Wilcoxon test were used.

Results

Mean CT dose index values (mGy) were as follows: D0/FDBR = 10.1 ± 1.7, D1 = 6.2 ± 2.8, D2 = 5.7 ± 2.7, D3 = 3.5 ± 1.9, D4 = 1.8 ± 1.0, and D5 = 0.9 ± 0.5. Mean IQ ratings were as follows: D0 = +1.8 ± 0.2, D1 = +1.5 ± 0.3, D2 = +1.1 ± 0.3, D3 = +0.7 ± 0.5, D4 = +0.1 ± 0.5, and D5 = −1.2 ± 0.5. All values demonstrated a significant difference to baseline ( P < .05), except mean IQ for D4 ( P = .61). ICC was 0.91.

Conclusions

Compared to ASIR, MBIR allowed for a significant dose reduction of 82% without impairment of IQ. This resulted in a calculated mean effective dose below 1 mSv.

Computed tomography (CT) is widely available, fast, and offers high-resolution imaging. Consequently, the number of CT examinations is rising , and thus, radiation exposure is increasing as well. As the latter is causing concerns , strategies for effective reduction of radiation dose become necessary .

Various methods are available to reduce dose rate, such as accurate patient positioning, automatic exposure control, and tube-voltage adoption to patient size, as well as iterative image reconstruction methods such as hybrid (adaptive statistical iterative reconstruction [ASIR], sinogram affirmed iterative reconstruction [SAFIRE], and iDose) or fully iterative reconstruction algorithms (model-based iterative reconstruction [MBIR] and iterative image reconstruction [IMR]) . ASIR, a hybrid iterative reconstruction method, allows for dose reductions of >30% in brain CT, 57% in chest CT, and 38% in abdominal CT compared to filtered back projection (FBP) . For the diagnostic evaluation of high-contrast structures such as lung parenchyma, low dose and even ultra-low dose protocols are well established . In contrast, strategies to reduce dose in contrast-enhanced chest CT are limited by challenges concerning the evaluation of soft tissues. Therefore MBIR, a fully iterative reconstruction method with a dedicated data reconstruction optimized for soft tissues, seems promising for further dose rate reduction while keeping image quality (IQ) stable .

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Material and methods

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Cadavers and Study Design

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Recorded Data

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CT Protocol and Image Reconstruction

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Table 1

Computed Tomography Examination and Reconstruction Parameters

Scan Protocol HD scan mode No Scan type Helical full Collimation (mm) 40 Rotation time (s) 0.4 Pitch 0.984 Table feed (mm/rotation) 39.37 Tube voltage (kV) 120 Tube current, range (mA), auto mA, smart mA 10–400 Scan field of view (cm) Large body, 50.0

Dose Level Noise Index ∗ Variable Thickness (mm) Reconstruction Method Image Reconstruction (mm) Image Reformation (mm) FDBR † 39 0.625 ASIR soft tissue 50% 0.625 slice 3.0 all planes D0 39 0.625 MBIR D1 35 2.5 MBIR D2 70 0.625 MBIR D3 35 5.0 MBIR D4 70 2.5 MBIR D5 70 5.0 MBIR

ASIR, adaptive statistical iterative reconstruction; FDBR, full-dose baseline reference; HD, high definition; MBIR, model-based iterative reconstruction.

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Readers and Environment

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Objective Assessment of IQ

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Figure 1, Positioning of the regions of interest (ROI) for objective image quality (IQ) evaluation. Objective IQ defined as mean values of Hounsfield Units (HU) and standard deviations. Four circular ROI ( red circles ) of similar size (90–110 mm 2 ) defined in air, aorta, muscle, and fat, respectively in the sagittal (a) , coronal (b) , and axial planes (c) . (Color version of figure is available online.)

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Subjective Assessment of IQ

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Figure 2, Example images for subjective image quality ratings in axial plane, semiquantitative scale (−2 = diagnostically inferior, −1 = inferior, 0 = equal, +1 = superior, and +2 = diagnostically superior). (a) full-dose baseline reference (FDBR); (b) dose level D0 (D0-FDBR comparison: reader 1 = +2, reader 2 = +1); (c) dose level D4 (reader 1 = 0, reader 2 = −1); (d) dose level D5 (reader 1 = −2, reader 2 = −1).

![Figure 3, Examples of subjective image quality (IQ) ratings in different planes. Both readers compared overall subjective IQ for soft-tissue evaluation of dose levels D0–D5 to the full-dose baseline reference. Ratings were done with the help of a semiquantitative score (IQ, −2 = diagnostically inferior; −1 = inferior; 0 = equal; +1 = superior; +2 = diagnostically superior). Comparison of a full-dose scan reconstructed with adaptive statistical iterative reconstruction [ (a) , (c) , (e) ] to the data set acquired with dose level D4 [ (b) , (d) , (f) ] which was about 20% of the full dose. In axial plane (a , b) , ratings were (reader 1 = 0/reader 2 = −1]; in sagittal plane (c , d) (+1/+1); and in coronal plane (e , f) (+1/+2).](https://storage.googleapis.com/dl.dentistrykey.com/clinical/HowLowCanWeGoinContrastEnhancedCTImagingoftheChest/2_1s20S1076633214003985.jpg)

Figure 4, Example for subjective image quality (IQ) ratings of shoulder girdle artifacts in the coronal plane. Both readers were blinded to the dose level and independently compared overall subjective IQ for soft-tissue evaluation of dose levels D0–D5 to the full-dose baseline reference (FDBR). Ratings were done with the help of a semiquantitative score (IQ, −2 = diagnostically inferior; −1 = inferior; 0 = equal; +1 = superior; +2 = diagnostically superior). (a) shows the FDBR. (b) shows a sample picture of dose level D0. Ratings were (reader 1 = +2/reader 2 = +2). (c) shows a sample picture of dose level D4 (0/+1), whereas (d) shows a sample picture of dose level D5 (−1/−2). The evaluation was not limited to the anatomy only as the examined bodies were human cadavers that sometimes showed coagulated blood clots.

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Radiation Dose

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Statistical Analysis

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Table 2

Computed Tomography Dose Index, Dose–Length Product, and Effective Dose for the Different Dose Levels and Corresponding Intended and Observed Mean Dose Levels

CTDI (mGy) DLP (mGy*cm) ED (mSv) Mean Dose Level (%) Mean SD Mean SD Mean SD Intended Observed Dose levels D0 10.1 1.7 340.5 67 4.9 1 100 100 D1 6.2 2.8 213.4 102 3.1 1.4 65 63 D2 5.7 2.7 186.9 96 2.7 1.4 50 55 D3 3.5 1.9 120.1 69.3 1.7 1 35 35 D4 1.8 1 60.6 35.5 0.9 0.5 20 18 D5 0.9 0.5 30.5 17.5 0.4 0.2 10 08

CTDI, computed tomography dose index; DLP, dose–length product; ED, effective dose.

Differences between dose levels were significant for CTDI, DLP, and ED, respectively ( P < .001).

Table 3

Objective Image Quality Measurements in the Different Regions of Interest Air, Muscle, Fat, and Aorta Using Dose Levels (D0–D5) and the Full-Dose Baseline Reference

Axial Sagittal Coronal All Planes Mean HU SD CNR Mean HU SD CNR Mean HU SD CNR Mean HU SD CNR_P_ (CNR) Air Dose levels FDBR −988.7 13.3 −95.5 −987.1 13.0 −113.7 −975.2 16.1 −76.8 −983.7 14.1 −95.3 — D0 −989.9 11.3 −185.9 −987.3 10.5 −207.9 −972.7 11.9 −129.6 −983.3 11.2 −174.5 .00 D1 −989.4 10.5 −178.5 −986.4 11.0 −198.2 −973.4 11.6 −126.6 −983.1 11.0 −167.8 .00 D2 −989.3 11.6 −169.5 −986.9 10.7 −195.7 −971.8 12.1 −122.0 −982.7 11.5 −162.4 .00 D3 −989.1 11.7 −165.5 −987.0 10.4 −190.3 −971.4 11.8 −121.2 −982.5 11.3 −159.0 .00 D4 −985.2 12.8 −136.4 −985.8 11.4 −157.8 −976.3 12.1 −117.5 −982.4 12.1 −137.2 .01 D5 −987.8 12.5 −135.2 −983.1 12.0 −141.4 −977.0 12.4 −117.3 −982.6 12.3 −131.3 .02 Muscle Dose levels FDBR 56.3 15.4 3.9 55.2 15.3 4.2 66.4 13.8 5.2 59.3 14.8 4.4 — D0 54.6 11.6 5.2 53.8 9.4 6.3 67.9 10.3 7.0 58.8 10.4 6.2 .00 D1 55.6 12.7 4.6 53.1 9.9 5.8 67.5 10.3 7.1 58.7 11.0 5.8 .01 D2 53.9 13.3 4.3 52.6 10.3 5.5 64.6 11.3 6.1 57.0 11.6 5.3 .04 D3 54.0 13.9 4.2 54.6 11.8 5.2 63.9 11.5 5.8 57.5 12.4 5.1 .19 D4 51.5 16.2 3.4 55.6 13.0 4.8 62.7 13.3 5.0 56.6 14.2 4.4 .26 D5 49.6 18.2 2.9 59.6 16.0 4.3 55.2 16.1 3.7 54.8 16.8 3.6 .20 Fat Dose levels FDBR −88.1 16.0 −6.3 −68.6 17.7 −4.4 −70.8 13.7 −5.5 −75.8 15.8 −5.4 — D0 −89.5 13.3 −8.1 −73.2 16.0 −5.8 −66.9 10.4 −7.0 −76.5 13.2 −7.0 .00 D1 −88.3 14.0 −7.6 −72.9 16.2 −5.6 −69.0 11.6 −6.4 −76.7 13.9 −6.5 .01 D2 −89.4 14.2 −7.2 −73.7 16.7 −5.4 −69.5 12.4 −6.1 −77.5 14.4 −6.2 .05 D3 −88.5 14.4 −7.1 −74.5 17.1 −5.4 −69.7 12.5 −6.1 −77.6 14.7 −6.2 .13 D4 −90.4 16.5 −6.2 −73.5 18.1 −4.9 −71.3 14.7 −5.3 −78.4 16.4 −5.5 .44 D5 −89.1 18.2 −5.4 −71.8 18.8 −4.5 −81.0 16.6 −5.4 −80.6 17.9 −5.1 .55 Aorta Dose levels FDBR 482.5 34.2 18.3 499.8 39.4 17.4 498.6 20.3 28.5 493.6 31.3 21.4 — D0 484.4 31.8 21.3 503.9 33.7 24.5 503.1 15.1 41.7 497.1 26.9 29.2 .00 D1 477.6 36.3 18.7 496.1 34.8 22.4 487.2 17.7 31.3 487.0 29.6 24.1 .27 D2 472.3 45.5 18.8 495.6 35.9 20.4 481.6 23.4 30.3 483.2 34.9 23.2 .37 D3 469.5 43.4 17.5 485.2 38.3 18.2 468.9 28.7 26.4 474.5 36.8 20.7 .44 D4 456.9 40.8 15.3 475.0 34.2 17.8 462.8 23.9 23.3 464.9 33.0 18.8 .23 D5 435.9 42.1 13.4 454.5 38.8 14.3 441.8 26.1 20.0 444.1 35.7 15.9 .02

CNR, contrast-to-noise ratio; FDBR, full-dose baseline reference; HU, Hounsfield units; SD, standard deviation.

Results represented as mean Hounsfield units and standard deviation, and respective contrast-to-noise ratio for all planes (axial, sagittal, and coronal).

Table 4

Subjective Image Quality Ratings for Soft-Tissue Evaluation of Dose Levels D0–D5 Compared to Full-Dose Baseline Reference for Mean Image Quality Ratings, Influence of Skin–Air–Transition Artifacts, and Shoulder Girdle Artifacts on Image Quality

Axial Sagittal Coronal All Planes Mean SD Mean SD Mean SD Mean SD_P_ (Mean) IQ Dose levels D0 1.56 0.51 1.80 0.35 1.98 0.10 1.78 0.23 .00 D1 1.28 0.46 1.50 0.41 1.74 0.33 1.51 0.29 .00 D2 0.86 0.42 1.00 0.41 1.50 0.48 1.12 0.33 .00 D3 0.38 0.68 0.54 0.59 1.02 0.64 0.65 0.55 .00 D4 −0.36 0.57 0.06 0.55 0.44 0.77 0.05 0.51 .61 D5 −1.42 0.57 −1.22 0.52 −0.92 0.66 −1.19 0.47 .00 Skin–air–transition artifacts Dose levels D0 −0.18 0.64 0.74 0.65 0.98 0.60 0.51 0.45 .00 D1 −0.26 0.50 0.52 0.51 0.84 0.61 0.37 0.34 .00 D2 −0.22 0.50 0.36 0.42 0.80 0.56 0.31 0.31 .00 D3 −0.30 0.56 0.10 0.32 0.42 0.45 0.07 0.25 .13 D4 −0.48 0.62 0.00 0.25 0.38 0.55 −0.03 0.32 1.00 D5 −0.80 0.54 0.00 0.54 −0.28 0.58 −0.61 0.32 .00 Shoulder girdle artifacts Dose levels D0 1.96 0.14 1.80 0.32 1.96 0.14 1.91 0.14 .00 D1 1.46 0.54 1.38 0.51 1.80 0.35 1.55 0.35 .00 D2 1.16 0.51 1.10 0.38 1.72 0.43 1.33 0.33 .00 D3 0.38 0.87 0.58 0.51 0.96 0.68 0.64 0.55 .00 D4 −0.42 0.77 0.04 0.59 0.54 0.68 0.05 0.57 .50 D5 −1.56 0.46 −1.00 0.63 −0.82 0.58 −1.13 0.47 .00

IQ, image quality; SD, standard deviation.

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Institutional Review Board Approval

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Results

Cadaver Examinations

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Radiation Dose

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Figure 5, Mean dose values for the different dose levels D0–D5. (a) Mean computed tomography dose index for different dose levels (D0–D5). (b) Mean dose–length product. (c) Effective dose. CTDI, computed tomography dose index; DLP, dose–length product; FDBR, full-dose baseline reference.

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Objective IQ

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Figure 6, Contrast-to-noise ratios (CNR) in the regions of interest as a function of dose level. CNR as a measure of objective image quality. For better visualization, scales for air and fat are shown inverted and a red line representing the CNR of the full-dose baseline reference. (Color version of figure is available online.)

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Subjective IQ

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Figure 7, Subjective mean image quality (IQ) results. Ratings for soft-tissue evaluation as a function of dose levels (D0–D5) for mean IQ ratings, skin–air–transition artifacts and shoulder girdle artifacts. Zero on the y-axis (subjective rating score) corresponds to the full-dose baseline reference ( red line ). Subjective rating score range, −2 = diagnostically inferior; −1 = inferior; 0 = equal; +1 = superior; and +2 = diagnostically superior. FDBR, full-dose baseline reference. (Color version of figure is available online.)

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Inter-reader Correlation

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Discussion

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Appendix A

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Academic Radiology, Volume 22, Issue 3
Journals General Radiology
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