Influence of CT Reconstruction Settings on Extremely Low Attenuation Values for Specific Gas Volume Calculation in Severe Emphysema

Rationale and Objectives

Emphysema is characterized by lung tissue destruction and trapped gas. On computed tomographic (CT) images, this may be expressed by widespread areas with high specific gas volume (SV g ). SV g is highly sensitive to very low attenuation values, which frequently occur in the CT images of patients with severe emphysema. The purpose of the present work was to study if and how different reconstruction settings and different scanners significantly influence SV g distribution, particularly in the very low attenuation range.

Materials and Methods

Two sets of CT images taken from two different CT scanners at two different lung volumes in 10 healthy volunteers and 18 subjects with severe emphysema were analyzed. Images were reconstructed using two different settings of reconstruction parameters: (1) thin slice thickness and sharp filter and (2) thick slice thickness and smooth filter. For each set of images, average values of SV g and their variation (ΔSV g ) from total lung capacity to residual volume were calculated in the whole lung.

Results

Very low attenuation values are always present in CT images when reconstructed with thin slice thickness and a sharp filter and in very large numbers in patients with severe emphysema. SV g values were in general significantly higher in patients with emphysema than in healthy subjects, at both total lung capacity and residual volume ( P < .001), and were significantly influenced by the reconstruction filter ( P < .001) and CT scanner ( P < .001). ΔSV g was lower in patients with emphysema than in healthy subjects ( P < .001) and was significantly affected by the reconstruction setting but not by the CT scanner.

Conclusions

The disproportionate effect of low-attenuation pixels on SV g likely causes overestimation of the severity of emphysema and trapped gas. This can be significantly reduced, however, by using thick slices and a smooth filter for image reconstruction. ΔSV g is generally robust for quantifying the functional impairment of the lung in severe emphysema.

Emphysema is characterized by the destruction of lung tissue in the distal airspaces. This is visualized by areas of abnormally low attenuation on computed tomographic (CT) images , whose pattern and distribution vary widely from one patient to another.

Coxson et al proposed a simple method to convert lung density into specific gas volume (SV g ), which is the volume of gas per gram of lung tissue. SV g can be suitably used to study emphysematous lungs on a regional basis and has a significant advantage in identifying trapped gas. Recently, we showed in a pig model of regional airway obstruction that SV g determined at different lung volumes provides a useful method for clearly identifying and quantifying the extent and severity of trapped gas . These methods and findings have important potential clinical implications for the assessment of human subjects undergoing lung volume reduction surgery or minimally invasive interventions such as transbronchial stents or endobronchial lung volume reduction .

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Figure 1

Relationship between Hounsfield units (HU) and corresponding value of specific gas volume (SV g ) for very low attenuation values, where −1024 HU represents zero x-ray attenuation corresponding to an infinite value of SV g (division by zero in equation (2), see text). Clearly, low-attenuation voxels have disproportionately high SV g .

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Materials and methods

Subjects and Specimens

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CT Imaging

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CT Image Analysis

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SVg=specificvolume(tissue&gas)−specificvolume(tissue), SV

specific

volume

(

tissue

gas

)

−

specific

volume

(

tissue

)

where specific volume (milliliters per gram) is the inverse of density (grams per milliliter). The specific volume of the lung (tissue and gas) was measured from the CT image as

SV(tissue&gas)(mL/g)=1024HU(mg/mL)+1024. SV

(

tissue

gas

)

(

)

1024

(

)

1024

On the basis of existing literature , the specific volume of tissue was assumed to be

SVtissue=1/1.065mL/g. SV

tissue

1.065

In equation (2) , HU is the CT raw data rescaled into HUs according to the linear transformation

HU=rawpixelvalue×rescaleslope+rescaleintercept. HU

raw

pixel

value

rescale

slope

rescale

intercept

In the CT scanners used in the present study (Siemens and GE), rescale intercept is equal to −1024, so this value represents zero density.

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Statistical Analysis

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Results

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Table 1

CT Volumes and Emphysema Index Values in Healthy Subjects

Subject CT Volume (L) RV/TLC (%) Emphysema Index (%) TLC ∗ RV ∗ TLC † RV † TLC ‡ RV ‡ 1 6.13 1.76 28.74 25 2 45 1 2 5.83 1.11 19.02 23 1 43 1 3 4.33 1.63 37.68 13 1 8 1 4 5.48 2.90 53.01 23 5 38 3 5 9.20 2.51 27.33 27 3 41 1 6 5.44 1.27 23.27 20 1 34 1 7 3.70 0.86 23.16 14 0 6 1 8 4.65 1.65 35.54 17 0 12 1 9 5.46 1.23 22.52 24 0 33 1 10 4.41 1.14 25.88 25 0 42 0 Mean 5.46 1.61 29.62 21 1 30 1 SD 1.52 0.65 10.06 5 2 15 1 SE 0.48 0.21 3.18 2 0 5 0

CT, computed tomographic; RV, residual volume; SD, standard deviation; SE, standard error; TLC, total lung capacity.

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Table 2

CT Volumes and Emphysema Index Values in Patients with Emphysema

Patient Scanner CT Volume (L) RV/TLC (%) Emphysema Index (%) TLC ∗ RV ∗ TLC † RV † TLC ‡ RV ‡ 1 Siemens 7.81 6.67 85.38 57 53 76 70 2 Siemens 5.69 4.58 80.51 53 49 70 63 3 Siemens 8.00 7.12 88.93 55 54 71 69 4 GE 5.46 4.74 86.87 38 32 61 52 5 GE 4.88 3.88 79.56 33 24 53 42 6 Siemens 7.39 6.39 86.42 44 40 54 43 7 GE 4.73 3.51 74.07 19 12 37 18 8 Siemens 7.56 5.73 75.76 47 42 60 46 9 Siemens 7.33 5.29 72.15 47 40 63 44 10 GE 5.73 4.50 78.46 39 30 55 43 11 GE 5.87 3.73 63.64 40 27 60 39 12 GE 4.75 3.72 78.39 37 30 58 47 13 Siemens 8.85 7.34 82.94 49 44 64 55 14 Siemens 5.26 5.14 97.66 42 41 45 50 15 Siemens 7.13 4.77 66.88 44 37 57 34 16 Siemens 7.89 6.47 82.01 46 42 59 48 17 GE 8.94 6.95 77.79 37 27 61 45 18 GE 5.44 3.56 65.48 34 19 59 31 Mean 6.60 5.23 79.05 42 36 59 47 SD 1.42 1.32 8.68 9 11 9 13 SE 0.33 0.30 1.99 2 3 2 3

CT, computed tomographic; RV, residual volume; SD, standard deviation; SE, standard error; TLC, total lung capacity.

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Discussion

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Conclusions

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References

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Influence of CT Reconstruction Settings on Extremely Low Attenuation Values for Specific Gas Volume Calculation in Severe Emphysema

Rationale and Objectives

Materials and Methods

Results

Conclusions

Materials and methods

Subjects and Specimens

CT Imaging

CT Image Analysis

Statistical Analysis

Results

Discussion

Conclusions

References

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