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