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High resolution computed tomography in asthma
  1. Alan N Mcleana,
  2. Michael W Sprouleb,
  3. Michael D Cowanb,
  4. Neil C Thomsona
  1. aDepartment of Respiratory Medicine, bDepartment of Radiology, cWest Glasgow Hospitals University NHS Trust, Glasgow G12 0YN, UK
  1. Dr A N Mclean.

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The plain chest radiograph, although useful in the assessment of acute asthma, mainly to exclude unsuspected pneumothorax, collapse or consolidation,1 ,2 is unable to demonstrate more subtle changes within the lungs of asthmatic patients. The ability of high resolution computed tomography (HRCT), with a theoretical resolution of 100 μm, to examine small structures within the chest is now being exploited in the investigation of asthma. Recent studies have focused on the visualisation of acute airway responses to bronchoconstrictor and bronchodilator stimuli as well as airway and lung parenchymal changes in chronic asthma.

Measurement techniques using HRCT

High resolution computed tomography has evolved from a combination of improvements in scanner hardware and the software used to reconstruct the images. The single most important feature in improving the spatial resolution is the slice thickness or collimation. In practice this is usually 1–1.5 mm. The second important feature of HRCT is to reconstruct the image using a high spatial frequency algorithm. This decreases contrast resolution and increases the visibility of image noise, but it significantly improves spatial resolution.3 Other features of HRCT are quick scan times to reduce motion artefact, and the use of targeted reconstruction when necessary. These techniques allow selected areas of the lungs to be viewed at close to the inherent maximal spatial resolution of the scan system. The data are manipulated in digital form by sophisticated software to produce the final image. Lung slices can then be examined for evidence of airway and parenchymal lung disease according to recognised criteria. HRCT scanning now has an established role in the investigation of diffuse parenchymal lung disease and bronchiectasis.4 The main disadvantages in the application of HRCT scanning in humans are radiation dose, the complexity of HRCT data analysis for the more complex edge finding algorithms,5 and technical problems over selection of …

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