Is CT a new research tool for COPD?

Chronic obstructive pulmonary disease (COPD) is composed of two diseases: chronic bronchitis and emphysema.

The essence of emphysema is loss of alveolar walls. Computed tomography (CT), which is based on the attenuation of x-rays, provides precise anatomical information on the density of tissues. Therefore, CT, especially HRCT, is the in vivo method of choice for detection and quantification of emphysema. Reduction of radiation dose has little influence on lung density measurements. This opens up opportunities for CT as a screen for emphysema. Furthermore, repeat scans can be used to follow the progress of emphysema. The depth of inspiration is an important source of variation of lung density measurements, but inspiratory level has no influence on the total weight of the lung derived from CT scans, making it easy to adjust lung densities to a standardised volume such as predicted Total Lung Capacity (TLC) by the following formula: adjusted lung density = observed lung density x observed TLC/predicted TLC.

Chronic bronchitis represents the airway component of COPD. During recent years, improved resolution of HRCT with faster scans, smaller detectors and multi-detector-row technology has made it possible to visualise airways down to a range of 2 mm diameter (6th generation), which represents the upper-size limit of the pathophysiologically important 'small airways'.

In the future, CT may replace pulmonary function measurements (FEV(1)) as the gold standard for assessing the response to treatment in clinical trials of COPD, provided that the lung community does the same painstaking work of standardising and validating CT that it did for spirometry.