Elsevier

European Journal of Radiology

Volume 80, Issue 3, December 2011, Pages e524-e530
European Journal of Radiology

How accurate is CT morphometry of airway? Phantom and clinical validation study

https://doi.org/10.1016/j.ejrad.2010.12.035Get rights and content

Abstract

Objective

To assess the reliability of CT airway measurement using phantom and to evaluate its reproducibility using clinical chest CT images.

Materials and methods

The phantom consisted of six tubes with different diameters and wall thicknesses was used. The wall area ratio (%WA) and wall thickness ratio (%WT) were calculated and the difference from the actual value (error ratio) was assessed. In vivo validation was performed with MDCT data of consecutive 10 patients and inter- and intra-rater agreements of the measurement were evaluated.

Results

The error ratio of %WA and %WT increased for the phantom tube with a 1 mm or thinner wall thickness. The FOV size has an influence on the airway measurement especially for the tube with 1 mm wall thickness. Inter-rater reliabilities between two observers for %WA and %WT were excellent and good intra-class correlation coefficient for %WA and %WT were obtained as 0.825 and 0.811, respectively. Intra-rater reliabilities of measurement also showed good intra-class correlation coefficient for %WA and %WT as 0.822 and 0.800, respectively.

Conclusion

Although the phantom study showed there is a tendency to overestimate the wall area of an airway with a thickness 1 mm or less, the software has a potential to be used in assessing longitudinal observation of the same patient, as well as, comparison among different patients due to good intra- and inter-rater reliability.

Introduction

It has recently become widely recognized that air flow limitation in COPD (chronic obstructive pulmonary disease) is caused by a combination of parenchymal and airway changes [1], [2], [3], [4], [5] and a large number of the latest publications have dealt with measurement of the airway wall dimension [6], [7], [8], [9], [10], [11]. It has been reported that the airway wall dimension is well correlated with air flow limitation on a pulmonary function test [6], [7], [9], [10], [12] and therefore it has the potential to be used in clinical practice for assessment of severity and longitudinal follow up of COPD.

Although several researchers have noted that CT morphometry is reliable even in the oblique or distal bronchus [5], [7], [10], [12], [13], reports which analyzed the reliability and reproducibility with reference to various technical factors of CT are few [14], [15]. It is necessary to recognize those when measurement software is applied to CT data obtained from daily clinical practice, otherwise the airway measurement cannot be used correctly in the management of COPD.

This study has two purposes:

  • 1.

    To evaluate the influences of various technical factors, including size of the airway, size of the FOV, oblique angle of the airway, beam width and helical pitch on the calculated airway dimension using CT scan data of an airway phantom with the identical spatial resolution of a clinical chest CT.

  • 2.

    To assess the inter- and intra-rater reliability of CT morphometry using clinical chest CT images.

Section snippets

The process for measuring airway dimension

Newly developed software, specialized for analyzing morphometry in CT images of COPD patients (Thoracic VCAR version:vtxl9.3.23, GE healthcare, Milwaukee, United States) installed on the image processing work station (Advantage Workstation version4.5, GE healthcare, Milwaukee, United States) was used. Among the various measurement modes, the airway analysis mode was used in this study (Fig. 1). After selecting the image file on the workstation, a three dimensional image of the tracheobronchial

Validation study using the airway phantom

The error ratio of %WA and %WT were less than 5% for the phantom tube with 2 and 1.5 mm wall thickness (Fig. 3). However, the ratio increased for the phantom tube with a 1 mm or thinner wall thickness. It was most significant when the second smallest tube was analyzed in which the error ratio of %WA and %WT were −39.2% and −57.2%.

The influence of the FOV size is not significant on the measurement of %WA for the tube with a 2 mm wall thickness (Fig. 4). However, the large FOV, larger than 400 mm,

Discussion

The results of the validation study using airway phantom are summarized as follows:

  • 1.

    Airway wall area tended to be overestimated if the wall thickness was 1 mm or less.

  • 2.

    FOV has a significant influence on the measurement of airway diameter especially in tubes with a 1 mm or less wall thickness.

  • 3.

    Obliquity of the tube has a significant influence on the measurement of the airway, except in tubes with a 1.5 mm or thicker wall.

It is extremely important to realize that a significant measurement error cannot

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