Effects of CT Section Thickness and Reconstruction Kernel on Emphysema Quantification

Rationale and Objectives

Computed tomography (CT) section thickness and reconstruction kernel each influence CT measurements of emphysema. This study was performed to assess whether their effects are related to the magnitude of the measurement.

Materials and Methods

Low-radiation-dose multidetector CT was performed in 21 subjects representing a wide range of emphysema severity. Images were reconstructed using 20 different combinations of section thickness and reconstruction kernel. Emphysema index values were determined as the percentage of lung pixels having attenuation lower than multiple thresholds ranging from −960 HU to −890 HU. The index values obtained from the different thickness-kernel combinations were compared by repeated measures analysis of variance and Bland-Altman plots of mean versus difference in all subjects, and correlated with quantitative histology (mean linear intercept, Lm) in a subset of resected lung specimens.

Results

The effects of section thickness and reconstruction kernel on the emphysema index were significant ( P < .001) and diminished as the index attenuation threshold was raised. The changes in index values from changing the thickness-kernel combination were largest for subjects with intermediate index values (10%–30%), and became progressively smaller for those with lower and higher index values. This pattern was consistent regardless of the thickness-kernel combinations compared and the HU threshold used. Correlations between the emphysema index values obtained with each thickness-kernel combination and Lm ranged from r = 0.55–0.68 ( P = .007–.03).

Conclusion

The effects of CT section thickness and kernel on emphysema index values varied systematically with the magnitude of the emphysema index. All reconstruction techniques provided significant correlations with quantitative histology.

Quantitative computed tomography (CT) analysis of lung attenuation increasingly has been used in clinical research on chronic obstructive pulmonary disease for the objective measurement of emphysema. Emphysema has been quantified from CT data obtained in clinical trials to identify associations between emphysema and demographic factors ( ), genetic factors , and body habitus ; to assess drug treatment effects ; to distinguish chronic obstructive pulmonary disease phenotypes with and without emphysema ; and to evaluate the contribution of emphysema to airflow obstruction . Studies supporting the validity of measurements obtained using low-radiation-dose CT technique may further expand applications in clinical research.

As the use of CT for emphysema quantification has grown, the potential for measurement variability related to certain CT technical parameters has been recognized . It has been reported that CT estimates of emphysema severity increase as section thickness decreases , and that higher resolution (edge-enhancing, sharper) image reconstruction kernels (also referred to as algorithms or filters) result in higher CT measurements of emphysema than lower resolution (smoothing) kernels . These effects may impact the ability to combine or compare CT measurements of emphysema in cross-sectional or longitudinal studies in which section thickness and reconstruction kernel are not held constant. This is particularly relevant to retrospective studies in which these parameters can no longer be changed, to prospective multicenter studies using multiple CT scanner models with different section thickness and kernel options, and to longitudinal studies during which scanners are replaced with new models.

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Materials and methods

Subjects

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Table 1

Subject Characteristics

Transplant ( n = 12) Lobectomy ( n = 9) All ( n = 21) Female: male 5:7 5:4 10:11 Age, y (range) 58 ± 5 (46–64) 65 ± 5 (58–74) 61 ± 6 (46–74) Smoking history, pack years (range) 58 ± 19 (35–90) 60 ± 39 (13–135) 59 ± 29 (13–135) FEV 1 , % of predicted (range) 20 ± 7 (13–37) 78 ± 29 (45–128) 43 ± 34 (13–128) FVC, % of predicted (range) 52 ± 16 (36–90) 99 ± 25 (61–140) 71 ± 30 (36–140) FEV 1 /FVC (range) 0.32 ± 0.12 (0.19–0.67) 0.63 ± 0.14 (0.37–0.81) 0.44 ± 0.20 (0.19–0.81) TLC, % of predicted (range) 139 ± 19 (109–171) 130 ± 24 (111–180) 135 ± 20 (109–180) RV, % of predicted (range) 295 ± 64 (146–395) 177 ± 72 (111–320) 249 ± 88 (111–395) DLCO, % of predicted (range) 31 ± 11 (15–47) 81 ± 17 (53–102) 52 ± 29 (15–102)

FEV 1 , Forced expiratory volume in 1 second; FVC, Forced vital capacity; TLC, Total lung capacity; RV, Residual volume; DLCO, Diffusing capacity for carbon monoxide.

Values are mean ± SD.

The following data were not available: FEV 1 in one lobectomy patient, FVC in one lobectomy patient, TLC and RV in one transplant and two lobectomy patients, and DLCO in two transplant and two lobectomy patients.

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Imaging and Image Analysis

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Lung Fixation and Quantitative Histology

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Statistical Analysis

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Emphysema index mean=[(Index from thickness-kernel combinationa)+(Index from thickness-kernel combinationb)]÷2 Emphysema index mean

[

(

Index from thickness-kernel combination

)

(

Index from thickness-kernel combination

)

]

The emphysema index difference was calculated according to the formula:

Emphysema index difference=(Index from thickness-kernel combinationa)−(Index from thickness-kernel combinationb) Emphysema index difference

(

Index from thickness-kernel combination

)

−

(

Index from thickness-kernel combination

)

The emphysema index mean vs. difference was examined at all HU thresholds. Curves were fit to these plots using linear and polynomial models with JMP 6.0 (SAS Institute, Cary, NC). Models having both the highest R 2 and F ratio were considered the best empirical fit.

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Results

Effects of Section Thickness and Reconstruction Kernel

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Table 2

Mean Computed Tomography Emphysema Index Values in all 21 subjects shown at 4 Representative Attenuation Thresholds (−950, −930, −910, and −890 HU) for each Combination of Section Thickness (1mm, 2mm, 5mm, and 10mm) and Reconstruction Kernel (B20f, B30f, B40f, B50f, and B60f)

B20f B30f B40f B50f B60f −950 HU 1 mm 32 ± 16 33 ± 15 34 ± 15 41 ± 9 42 ± 7 2 mm 29 ± 17 30 ± 16 31 ± 16 37 ± 11 37 ± 9 5 mm 26 ± 16 26 ± 16 27 ± 16 32 ± 13 34 ± 11 10 mm 22 ± 15 22 ± 15 22 ± 15 27 ± 14 28 ± 13 −930 HU 1 mm 45 ± 17 45 ± 16 46 ± 15 49 ± 10 49 ± 8 2 mm 43 ± 18 44 ± 17 44 ± 17 47 ± 12 46 ± 10 5 mm 39 ± 18 40 ± 18 40 ± 18 44 ± 15 44 ± 13 10 mm 35 ± 19 35 ± 19 36 ± 18 39 ± 16 40 ± 14 −910 HU 1 mm 57 ± 16 57 ± 15 58 ± 14 57 ± 10 56 ± 8 2 mm 56 ± 16 56 ± 16 57 ± 15 57 ± 12 55 ± 10 5 mm 53 ± 18 53 ± 17 53 ± 17 55 ± 14 54 ± 13 10 mm 49 ± 19 49 ± 18 49 ± 18 51 ± 16 51 ± 15 −890 HU 1 mm 68 ± 13 67 ± 13 67 ± 12 64 ± 10 62 ± 10 2 mm 67 ± 14 67 ± 13 67 ± 13 65 ± 11 62 ± 10 5 mm 64 ± 15 65 ± 15 65 ± 14 64 ± 13 63 ± 12 10 mm 61 ± 16 61 ± 16 61 ± 16 61 ± 14 60 ± 14

HU, Hounsfield unit.

Index values are mean index in % ± SD. Differences were statistically significant ( P < .001) for section thickness at all attenuation thresholds (including those not shown in table), and for reconstruction kernel at all attenuation thresholds (including those not shown in table) except -910 HU.

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Influence of Emphysema Index Magnitude

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Quantitative CT and Histology

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Table 3

Correlation Coefficients Comparing Lm Values in 4 Lobes and 11 Lungs with Computed Tomography Emphysema Index Values at Each Combination of section Thickness, Reconstruction Kernel, and Emphysema Index HU Threshold

1 mm B20f B30f B40f B50f B60f −960 HU .67 .67 .66 .63 .60 −950 HU .67 .67 .66 .62 .59 −940 HU .67 .67 .66 .61 .59 −930 HU .66 .66 .65 .60 .58 −920 HU .65 .64 .64 .59 .57 −910 HU .63 .63 .62 .59 .57 −900 HU .60 .60 .60 .58 .56 −890 HU .57 .57 .58 .57 .56

2 mm B20f B30f B40f B50f B60f −960 HU .67 .67 .67 .64 .63 −950 HU .68 .68 .67 .64 .62 −940 HU .68 .67 .67 .63 .61 −930 HU .67 .67 .66 .62 .60 −920 HU .65 .65 .65 .61 .60 −910 HU .63 .63 .63 .59 .59 −900 HU .60 .60 .60 .58 .58 −890 HU .57 .57 .57 .58 .57

5 mm B20f B30f B40f B50f B60f −960 HU .64 .65 .66 .61 .64 −950 HU .66 .67 .67 .60 .64 −940 HU .66 .67 .67 .60 .63 −930 HU .66 .67 .66 .60 .62 −920 HU .64 .65 .65 .59 .61 −910 HU .63 .64 .63 .58 .60 −900 HU .60 .61 .61 .57 .59 −890 HU .56 .58 .58 .55 .58

10 mm B20f B30f B40f B50f B60f −960 HU .61 .62 .62 .63 .63 −950 HU .64 .64 .64 .63 .63 −940 HU .65 .65 .65 .63 .62 −930 HU .65 .65 .65 .63 .61 −920 HU .64 .64 .64 .62 .61 −910 HU .62 .62 .62 .60 .59 −900 HU .60 .59 .59 .58 .58 −890 HU .56 .56 .56 .56 .56

Lm, mean linear intercept; HU, Hounsfield unit.

P < .05 for all r values.

Reconstruction kernels ordered from smoothest (B20f) to sharpest (B60f).

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Discussion

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References

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Effects of CT Section Thickness and Reconstruction Kernel on Emphysema Quantification

Rationale and Objectives

Materials and Methods

Results

Conclusion

Materials and methods

Subjects

Imaging and Image Analysis

Lung Fixation and Quantitative Histology

Statistical Analysis

Results

Effects of Section Thickness and Reconstruction Kernel

Influence of Emphysema Index Magnitude

Quantitative CT and Histology

Discussion

References

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