Alderney: Colophon, 1986. 2 Eisenberg RL. Radiology: an illustrated history. St Louis: Mosby-Year Book, 1992. 3 Kotzur I
dimensional computer reconstruction of images obtained with x rays and other modalities, and the use of computer systems to enhance, store, and display images. Techniques such as ultrasonography and magnetic resonance imaging that do not use ionising radiation will continue to replace some x ray procedures. New imaging techniques may include infrared imaging and microwave tomography. In radiotherapy, technological improvements will enable the radiation beam to be confined more precisely to the tumour. The benefits of R6ntgen's discovery have been enormous, but there has been a down side. Initially, the dangers associated with the use of x rays were not known, and some of the pioneers received doses of radiation that proved fatal. Tremendous advances in protection against radiation and in reducing doses have taken place over the years and have considerably reduced the dangers from ionising radiation. Although the risks are slight, there should always be a valid clinical indication for any request for an x ray examination. The Rontgen centenary congress will be held at the
International Convention Centre and National Indoor Arena in Birmingham on 12-16 June. It will comprise a scientific programme, technical exhibition, and historical exhibition. DANIEL J NOLAN Consultant radiologist
Radiology Clinical Centre, John Radcliffe Hospital, Oxford OX3 9DU 1 2 3 4 5 6 7 8 9
Burrows EH. Pioneers and earlyyears: a history of British radiology. Alderney: Colophon, 1986. Eisenberg RL. Radiology: an illustrated history. St Louis: Mosby-Year Book, 1992. Kotzur IM. W C Rontgen: a new type of ray. Radiology 1994;193:329-32. Schuster A. On the new kind of radiation. BMJ 1896;i: 172-3. The new photography [editorial]. Lancet 1896;i: 179. Stanton A. English translation. On a new kind of rays. Nature 1896;53:274-6. The new photography [editorial]. BMJ 1896;i:289-90. Lodge 0. Photograph showing bullet deeply embedded in wrist. BMJ 1896;i:497. Rowland S. Report on the application of the new photography to medicine and surgery. BMJ 1896;i:361-4. 10 Rowland S. A series of collotype illustrations with descriptive text, illustrating applications of the new photography to medicine and surgery. Archives ofClinical Skiagraphy 1896;1:5-20. 11 Seldinger SI. Catheter replacement of the needle in percutaneous arteriography. A new technique. Acta Radiol 1953;39:368-76. 12 AmbroseJ, Hounsfield G. Computerized transverse axial tomography. BrJRadiol 1973;46:148-9.
High resolution computed tomography of the lungs Better than chest radiography for some conditions What you see on high resolution computed tomography of the lungs is as detailed as what you see when you look at a gross pathological specimen.' It is the most accurate non-invasive method of evaluating lung parenchyma2 and has improved our understanding of the patterns and pathology of many pulmonary diseases. In conventional computed tomography slices are 10 mm thick and scans are obtained at 10 mm intervals; in high resolution computed tomography slices are 1-2 mm thick and scans are obtained at 10-40 mm intervals. The images therefore represent only one tenth or less of the volume of the lung. This sampling is suited to diffuse processes affecting extensive areas of the lung but will clearly miss small lesions lying between slices. High resolution computed tomography can resolve an object of 0 5 mm diameter and is ideally suited for use in the lungs, where a high contrast between tissue and air exists.2 High resolution computed tomography comes into its own in showing the secondary pulmonary lobule, which is the site of characteristic changes in many lung diseases. Although chest radiography remains the preferred imaging technique in the assessment of patients with lung disease, two features limit its sensitivity and specificity for detecting subtle lung disease. Firstly, small differences in attenuation between normal and abnormal lung tissue are difficult to observe in a chest radiograph so minor parenchymal abnormalities are not readily detected. Secondly, many structures, both normal and abnormal, are superimposed in a radiograph, making diagnosis difficult.3 By contrast, high resolution computed tomography provides more accurate information about the anatomical site of abnormalities which is less subject to technical variations and variations in interpretation than information in plain chest radiographs. Several studies have shown that high resolution computed tomography is better than conventional computed tomography in assessing patients with chronic diffuse infiltrative lung disease.45 One study found that, although both modalities were equivalent in recognising nodules, masses, nodular BMJ VOLUME 3 1 0
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Idiopathic pulmonary fibrosis. High resolution computed tomography through the lung bases in a 64 year old man shows honeycombing in a typical subpleural distribution and interlobular septal thickening
irregularities of the interfaces, large cystic air spaces, and architectural distortion, high resolution computed tomography showed fine bronchial and parenchymal lesions better and allowed the assessment of changes in ground glass attenuation (important in identifying the early treatable stage of fibrosis).6 Concern has been expressed over the relatively high doses of radiation used in computed tomography: with conventional imaging of the thorax patients receive a radiation dose 100 times that received in standard chest radiography7; high resolution computed tomography of the thorax delivers a radiation dose 10-20 times that received in standard chest radiography,8 and further reduction in radiation dose is possible without substantially reducing the resolution of the image.9 In patients with chronic diffuse lung disease a limited number of high resolution computed tomographic sections 615
can be combined with a low dose technique, providing more diagnostic accuracy than a chest radiograph with no increase in the radiation dose. '0 High resolution computed tomography is particularly valuable in assessing patients with a normal chest radiograph but clinical symptoms and abnormalities of lung function indicating diffuse infiltrative lung disease." Comparative studies in patients with disease proved by biopsy have shown that high resolution computed tomography has a sensitivity of about 94% for the detection of chronic infiltrative lung disease, compared with 80% for chest radiography.'2 It has a valuable role in patients with questionable radiographic abnormalities because confidently identifying normal subjects is easier than with chest radiography."3 High resolution computed tomography defines the pattern and anatomical location of lung parenchymal abnormalities, which in many cases are sufficiently characteristic to provide a specific diagnosis. These include lymphangitis carcinomatosa, sarcoidosis, lymphangioleiomyomatosis, and pulmonary fibrosis.'4 1'
High resolution computed tomography is now the preferred diagnostic test for bronchiectasis, having replaced bronchography. Its sensitivity in detecting emphysema approaches that of gross pathological examination.'6 In patients with fibrosing alveolitis it can distinguish active, potentially reversible or treatable disease from irreversible fibrosis. Two distinct patterns of abnormality are seen: ground glass opacity (corresponding to a cellular histological appearance at biopsy) and a reticular pattern (corresponding to fibrotic areas on histological examination).'7 18 Muller et al compared findings on high resolution computed tomography with pathological determinants of disease activity in 12 patients with cryptogenic fibrosing alveolitis and found that high resolution computed tomography correctly identified all five patients with appreciable disease activity and five out of seven patients with mild disease activity.'7 In patients with fibrosing alveolitis changes in appearance have also been correlated with improvement in pulmonary function after steroid treatment.'8 High resolution computed tomography provides a more accurate assessment of disease activity and the probable prognosis in patients with fibrosing alveolitis and frequently obviates more invasive procedures such as open lung biopsy, thereby reducing the risks to the patient and the costs of investigation.
Parenchymal disease The new technique is also proving increasingly useful in acute lung disease. It may show parenchymal disease in the presence of normal or questionable radiographic findings, particularly in immunocompromised patients. For example, about 10% of patients with pneumocystis pneumonia have a normal chest radiograph at presentation,'9 and in this condition high resolution computed tomography can show bilateral perihilar ground glass opacity. In patients with AIDS this is sufficiently characteristic to allow a
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presumptive diagnosis without further invasive procedures.20 In patients with AIDS pulmonary Kaposi's sarcoma produces a characteristic distribution of nodules on high resolution computed tomography.2' Invasive aspergillosis may be recognised by the presence of nodules with a halo of ground glass attenuation.22 Abnormal high resolution computed tomographic scans in patients with normal chest radiographs have also been described in drug induced lung disease and radiation pneumonitis.'2 In acute lung diseases several patterns of abnormality are seen that are not sufficiently specific to permit a diagnosis without further investigation. Rapid advances are coming from studies of correlations between tomographic and pathological appearances.23 As access to computed tomographic scanners becomes more widely available it is imperative that high resolution computed tomography of the lungs is used to its full potential. It may enable earlier diagnosis and more accurate follow up and can guide diagnostic procedures such as bronchoscopy and open lung biopsy to the most auspicious sites. It may also obviate invasive procedures. In some situations high resolution computed tomography of the chest will probably replace conventional imaging techniques. SYLVIA WORTHY Senior registrar in radiology Royal Victoria Infirmary, Newcastle upon Tyne NEI 4LP 1 Meziane MA, Hruban RH, Zerhouni EA, Wheeler PS, Khouri NF, Fishman EK, et al. HRCT of the lung parenchyma with pathologic correlation. Radiographics 1988;8:27-54. 2 Mayo JR. HRCT-technical aspects. Radiol Clin North Am 1991;29:1043-9. 3 Webb WR, Muller NL, Naidich DP. High-resolution computed tomography of the lungs. New York: Raven Press, 1992:1. 4 Zerhouni EA, Naidich DP, Stitik FP, Khouri N F, Siegelman SS. Computed tomography of the pulmonary parenchyma. Part 2. Interstitial disease.J Thorac Imaging 1985;1:54-64. 5 Mathieson JR, Mayo JR, Staples CA, Muller NL. Chronic diffuse infiltrative lung disease: comparison of diagnostic accuracy of CT and chest radiography. Radiology 1989;171:111-6. 6 Remy-Jardin M, Remy J, Deffontaines C, Duhamel A. Assessment of diffuse infiltrative lung disease: comparison of conventional CT and HRCT. Radiology 1991;181:157-62. 7 Di Marco AF, Briones B. Is chest CT performed too often? Chest 1993;103:985-6. 8 Mayo JR, Jackson SA, Muller NL. HRCT of the chest: radiation dose. Am J Roentgenol
1993;160:479-81. 9 Zwirewich CV, Mayo JR, Muller NL. Low-dose HRCT of lung parenchyma. Radiology
1991;t8O:413-7. 10 Lee KS, Primack SL, Staples CA, Mayo JR, Aldrich JE, Muller N L. Chronic infiltrative lung disease: comparison of diagnostic accuracies of radiography and low and conventional-dose thin-section CT. Radiology 1994;191:669-73. 11 Muller NL. Clinical value of HRCT in chronic diffuse lung disease. Am J Roentgenol 1991;157:1 163-70. 12 Padley SPG, Adler B, Muller NL. HRCT of the chest: current indications. J Thorac Imaging 1993;8: 189-99. 13 Padley SPG, Hansell DM, Flower CDR, Jennings P. Comparative accuracy of HRCT and chest radiography in the diagnosis of chronic diffuse infiltrative lung disease. Clin Radiol 1991;44: 222-6. 14 Stein MG, Mayo JR, Muller NL, Abele DR, Webb WR, Gamsu G; Pulmonary lymphangitic spread of carcinoma: appearances on CT scans. Radiology 1987;162:371-5. 15 Muller NL, Miller RR. CT of chronic infiltrative lung disease: state of the art. Am Rev Respir Dis
1990;142:1440-8. 16 Sanders C. The radiographic diagnosis of emphysema. Radiol Clin NorthAm 1991;29:1019-30. 17 Muller NL, Staples CA, Miller RR, Vedal S, Thurlbeck WM, Ostrow DN. Disease activity in
idiopathic pulmonary fibrosis: CT and pathological correlation. Radiology 1987;165:731-4. 18 Lee JS, Im JG, Ahn JM, Kim YM, Han MC. Fibrosing alveolitis: prognostic implication of ground-glass attenuation at HRCT. Radiology 1992;184:451-4. 19 Murray JF, Mills J. Pulmonary infectious complications of human immunodeficiency virus
infection. Am Rev RespirDis 1990;141:1356-72. 20 Hartman TE, Primack SL, Muller NL Staples CA. Diagnosis of thoracic complications of AIDS: accuracy of CT. AmJRoentgenol 1994;162:547-53. 21 Naidich DP, Tarras M, Garay SM, Bimbaum B, Rybak BJ, Schinella K. Kaposi's sarcoma: CT-radiographic correlation. Chest 1989;96:723-8. 22 Primack SL, Hartman TB, Lee KS, Muller NL. Pulmonary nodules and the CT halo sign.
Radiology 1990;190:513-5. 23 Brown MJ, Miller RR, Muller NL. Acute lung disease in the immunocompromised host: CT and
pathologic examination findings. Radiology 1994;190:247-54.
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