High resolution computed tomography in asthma

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Abstract
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 phantoms for verification.