Understanding HRCT of the lungs.

327 High-Resolution Computerized Tomography (HRCT) of the lung is now a well-established and widely used technique in the management of respiratory disease. It is the only imaging technique currently available that allows the in vivo visualization of the secondary pulmonary lobule. 1 This unique ability has enabled HRCT to revolutionize not only the diagnosis, but also the understanding of interstitial lung disease. The technique is relatively simple to implement. Virtually any CT scanner, of any age, is capable of performing a basic HRCT of the lung. Attention to detail is important. The minimum standards have been set out by a number of professional organizations. The most widely used are the American College of Radiology guidelines. 2 The basic technique involves obtaining very thin (1-2 mm) axial sections of the chest. The sections are spaced 10-15 mm apart. In this way, approximately 20-25 percent of the lung is sampled. The sections are processed using a sharp or 'bone' algorithm to enhance the detection of edges. The images are viewed on suitable lung windows (Level-500 to-750, width 1000 to 1500). Although the exact windowing is not important, it is important to standardize the values for a particular institution. The same window setting used each time a HRCT is performed make evaluation of serial examinations much more effective as subtle changes in lung attenuation are not lost in the technical factors. Another important factor to note is that administration of intravenous contrast also changes lung parenchymal attenuation. Contrast should not be given for HRCT examinations. If the patient requires both i.e. a HRCT as well as a contrast enhanced scan, the HRCT should be performed first. The initial description of HRCT was for axial images only. These axial images remain the mainstay of the test, however, recent studies on multi-detector row CT (MDCT) has shown a role for reformatting and viewing the scan in coronal and saggital modes. 3 The indications for the HRCT of the lung are well-established. 4 These include all the diffuse lung diseases such as idiopathic interstitial pneumonia, emphysema etc. HRCT is also used to investigate respiratory symptoms when the chest X-rays are normal. In a country like Pakistan, where tuberculosis is endemic, another important indication of HRCT is differentiation between TB and other causes of interstitial infiltration and nodules. The HRCT should not be interpreted in isolation. The consideration of the clinical history and comparison with any previous …