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Scatter Correction Associated with Dedicated Dual-source CT Hardware Improves Accuracy of Lung Air Measures

Posted Oct 31, 2013 Updated Apr 10, 2023
By Sean D. Mobberley BS , Matthew K. Fuld PhD , Jered P. Sieren BS , Andrew N. Primak PhD , Eric A. Hoffman PhD
14 min read

Rationale and Objectives

Accurate assessment of air density used to quantitatively characterize amount and distribution of emphysema in chronic obstructive pulmonary disease (COPD) subjects has remained challenging. Hounsfield units (HU) within tracheal air can be considerably less negative than –1000 HU. This study has sought to characterize the effects of improved scatter correction used in dual-source pulmonary computed tomography (CT).

Materials and Methods

Dual-source dual-energy (DSDE) and single-source (SS) scans taken at multiple energy levels and scan settings were acquired for quantitative comparison using anesthetized ovine ( n = 6), swine ( n = 13), and a lung phantom. Data were evaluated for the lung, inferior vena cava, and tracheal segments. To minimize the effect of cross-scatter, the phantom scans in the DSDE mode were obtained by reducing the current of one of the tubes to near zero.

Results

A significant shift in mean HU values in the tracheal regions of animals and the phantom is observed, with values consistently closer to −1000 HU in DSDE mode. HU values associated with SS mode demonstrated a positive shift of up to 32 HU. In vivo tracheal air measurements demonstrated considerable variability with SS scanning, whereas these values were more consistent with DSDE imaging. Scatter effects in the lung parenchyma differed from adjacent tracheal measures.

Conclusion

Data suggest that the scatter correction introduced into the dual-energy mode of imaging has served to provide more accurate CT lung density measures sought to quantitatively assess the presence and distribution of emphysema in COPD subjects. Data further suggest that CT images, acquired without adequate scatter correction, cannot be corrected by linear algorithms given the variability in tracheal air HU values and the independent scatter effects on lung parenchyma.

Quantitative computed tomographic (QCT) imaging is increasingly used for the characterization of the lung . Yet, reliable, repeatable, and accurate quantification of volumetric computed tomography (CT) data for assessment of lung density, particularly for longitudinal and multicenter studies, remains a challenge. It has been observed that air in the trachea of a chest CT scan is often significantly different from its true value of −1000 Hounsfield units (HU), and this varies between scanner makes and models . The variability of intrathoracic air HU measurements across sites, scanner makes, and models limits the accuracy when measuring air trappings and the extent of emphysema-like lung parenchyma, which has been defined as the percentage of voxels below −950 HU within the lung field on volumetric CT scans or the HU value below which 15% of lung voxels fall . The variability in intrathoracic air values makes comparisons between CT scanner makes and models challenging.

The motivation for this study was our preliminary observations in animal experiments that air in the trachea was consistently closer to −1000 HU when using Siemens SOMATOM Definition Flash dual-source dual-energy (DSDE) scan mode compared to the same scanner’s single-source (SS) scan mode. The primary difference between the DSDE and SS modes is the implementation of a dedicated hardware-based scatter correction in the DSDE mode, whereas the SS mode simply uses an antiscatter grid that is expected to block all scattered radiation. Therefore, the goal of this study was to test the hypothesis that the scatter correction employed in the DSDE mode results in more accurate (closer to the nominal −1000 HU) CT numbers of air in the trachea compared to the antiscatter grid solution typical for conventional single-source CT.

Materials and methods

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Study Outline

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Animal models

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Figure 1, Swine ( top left panel ) have a more vertically dimensioned thorax compared to the more human-like thorax present in ovine ( top right panel ). A positive Hounsfield unit (HU) shift is present in the histograms of both the trachea (32–35 HU, middle row ) and whole lung regions (10–12 HU, bottom row ) between single-source and dual-source dual-energy modes. The normalized density histograms shown are derived from 140-kVp scans from example animals.

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Analysis

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Phantom protocol

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Figure 2, A variant of the COPDGene phantom ( top panel ), with regions representative of the trachea (a) , lung parenchyma (b) , air (c) , acrylic (d) , and water (e) , was scanned in single-source (SS), dual-source dual-energy (DSDE), and DSDE-SS modes at 80, 100, and 140 kVp. A Hounsfield unit (HU) shift between SS and DSDE modes is demonstrated in the histograms of both the “tracheal” (26 HU, middle panel) and the “lung” regions (10 HU, lower panel ). The normalized density histograms shown are derived from SS and DSDE 140-kVp scans. Similar shifts are also seen in the 80- and 100-kVp scans.

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Analysis

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Results

Animal Study

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Table 1

Six Ovine and 13 Swine Scanned in SS and DSDE Modes with Lungs Inflated at 5, 15, 20, or 25 cm H 2 O Airway Pressure

Swine Ovine SS DSDE SS DSDE Trachea Mean −949 −981 −954 −986 Difference ± SD 32 ± 6 32 ± 7t -test_P_ < .001 ( n = 18)P < .001 ( n = 12) Lung Mean −709 −733 −773 −794 Difference ± SD 24 ± 15 19 ± 5t -test_P_ < .001 ( n = 18)P < .001 ( n = 12) IVC Mean 24 39 23 37 Difference ± SD −15 ± 1.6 −14 ± 1.5t -test_P_ < .001 ( n = 18)P < .001 ( n = 12)

DSDE, dual-source dual-energy; HU, Hounsfield unit; IVC, inferior vena cava; SD, standard deviation; SS, single source.

The tracheal lumen, inferior vena cava, and lung parenchyma were segmented for determination of the respective mean HU and standard deviations. Two-tailed paired-difference t -tests were used to determine the P values for the SS versus DSDE comparisons across subjects. Shown are the HU values from the animal models from 80-kVp scans, with all scanning parameters matched except for scanning mode. A significant shift in HU is seen when comparing the trachea, IVC, and lung tissue in the paired SS versus DSDE scans for both swine and ovine.

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Figure 3, (a,b) Linear regression plots of average slice densities comparing single-source (SS) and dual-source dual-energy (DSDE) at 80 kVp are shown for the in vivo trachea ( a , left panel) and lung parenchyma ( b , left panel) from an example swine. Similarly, Bland-Altman plots show the relationship of the mean of the average slice densities from the SS and DSDE scans versus the difference between values obtained from the two scanning modes operated at 80 kVp. Data obtained from the trachea ( a , right panel) and lung parenchyma ( b , right panel) are of the same swine as depicted in the left panels. HU, Hounsfield unit.

Figure 4, (a,b) Mean of tracheal and lung Hounsfield unit (HU) values along the z-axis from single-source (SS) and dual-source dual-energy (DSDE) (80 kVp) scans. SS data are in gray and DSDE are in yellow for an example swine (a) and ovine (b) . Anatomical reference volume rendered images ( right panel ) in the dorsal-ventral and lateral projections of the swine (a) and ovine (b) are provided to demonstrate the anatomic basis for the SS-DSDE differences. Color-coded background bars are provided to help link the anatomic locations to the positions on the density graphs. Note that SS versus DSDE differences in the trachea are reflected in the lung and the greatest SS versus DSDE differences in both the lung and tracheal regions occur in an anatomic location associated with the sternum and scapula regions.

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Phantom Study

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Table 2

Variant of the COPDGene Phantom with “Trachea”- and “Lung”-Like Regions and Air, Water, and Acrylic Regions

80 kVp 140 kVp SS DSDE SS DSDE “Trachea” Mean −987 −1003 −987 −1004 |Difference| ± SD 16 ± 2 17 ± 2t -test_P_ < .001 ( n = 10)P < .001 ( n = 10) Air Mean −999 −1002 −998 −1004 |Difference| ± SD 3 ± 4 6 ± 2t -test_P_ < .05 ( n = 10)P < .001 ( n = 10) “Lung” Mean −858 −860 −856 −863 |Difference| ± SD 2 ± 5 7 ± 3t -test_P_ = .1701 ( n = 10)P < .001 ( n = 10) Water Mean −2 2 −2 2 |Difference| ± SD 4 ± 8 4 ± 6t -test_P_ = .1816 ( n = 10)P = .0972 ( n = 10) Acrylic Mean 94 111 125 154 |Difference| ± SD 17 ± 15 29 ± 8t -test_P_ < .01 ( n = 10)P < .001 ( n = 10)

DSDE, dual-source dual-energy; SD, standard deviation; SS, single source.

The resultant mean values of the 80 and 140 kVp scans are listed for each region, in addition to the difference in values between the two modes and the level of significance using a two-tailed paired difference test ( Fig 2 , top panel). The results from the 100 kVp showed similar HU shifts between SS and DSDE scans. Reference computed tomography number for internal “trachea” air is −1000 HU, internal air is −1000 HU, “lung” is −860 HU, water is 0 HU, and acrylic is 120 HU.

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Table 3

The “Trachea”-Like Region of a Variant of the COPDGene Phantom Scanned in SS Mode (80, 100, 120, and 140 kVp), DSDE Mode (80/140Sn kVp, 140/80 kVp, and 100/140Sn kVp), and DSDE-SS Mode (i.e., Same Combinations as in the DSDE Mode but with Tube A or B Set at Minimum mAs)

Phantom “Trachea” Values Scan Mode Mean Channels Pitch Rotation Time Kernel Tube Sn filtered 80 kVp SS −987 128 0.55 0.5 B35f A — DSDE-SS −1002 64 0.55 0.5 B35f A — DSDE-SS −1002 128 0.55 0.5 B35f A — DSDE −1002 128 0.55 0.5 B35f A — DSDE −1002 128 1.0 0.5 B35f A — DSDE −1001 128 0.55 0.33 B35f A — DSDE −1003 128 0.55 0.5 D30f A — DSDE-SS −1003 64 0.55 0.5 B35f B — DSDE-SS −1004 128 0.55 0.5 B35f B — DSDE −1003 128 0.55 0.5 B35f B — DSDE −1002 128 1.0 0.5 B35f B — DSDE −1003 128 0.55 0.33 B35f B — DSDE −1002 128 0.55 0.5 D30f B — 100 kVp SS −988 128 0.55 0.5 B35f A — DSDE-SS −1004 64 0.55 0.5 B35f A — DSDE-SS −1007 128 0.55 0.5 B35f A — DSDE −1002 128 0.55 0.5 B35f A — 120 kVp SS −985 128 0.55 0.5 B35f A — 140 kVp SS −987 128 0.55 0.5 B35f A — DSDE-SS −1002 64 0.55 0.5 B35f A — DSDE-SS −1007 128 0.55 0.5 B35f A — DSDE −1002 128 0.55 0.5 B35f A — DSDE-SS −1000 64 0.55 0.5 B35f B Yes DSDE-SS −1001 128 0.55 0.5 B35f B Yes DSDE −1003 128 0.55 0.5 B35f B Yes

DSDE, dual-source dual-energy; SD, standard deviation; Sn, tin; SS, single source.

A consistent CTDIvol of 12 (±0.1) mGy was used for each. The results here show the mean Hounsfield units (HU) for a sampling of the analyzed parameter changes. Additional controlled scans comparing slice thickness (0.6 or 0.75 mm), slice spacing (0.5 or 0.6 mm), and scan direction (craniocaudal or caudocranial) were also analyzed but not shown any meaningful difference in mean values compared to the control. Reference computed tomography number for internal “trachea” air is −1000 HU.

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Figure 5, (a,b) These plots are similar to those shown in Figure 3 but are derived from a variant of the COPDGene Phantom. SS, single-source; DSDE, dual-source dual-energy.

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Discussion

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Acknowledgments

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Academic Radiology, Volume 20, Issue 11
Journals General Radiology
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