Thorax 1991;46:77-84 77 THORAX Thorax: first published as 10.1136/thx.46.2.77 on 1 February 1991. Downloaded from Review The role of high resolution computed tomography in the diagnosis of interstitial lung disease David M Hansell, Ian H Kerr Until recently the chest radiograph has been study. This protocol is widely used when a the only imaging technique used in the assess- comprehensive examination of the lungs is ment of patients with suspected diffuse lung required-for example, in the search for disease. In this context the chest radiograph is metastases. The volume averaging that occurs less than ideal. Problems arise with both false within the 1 cm thickness of the scan, negative and false positive results. It is well however, substantially reduces the ability of established that the chest radiograph may conventional computed tomography to resolve appear entirely normal in up to 10% of small structures. For high resolution com- patients with biopsy proved diffuse lung dis- puted tomography the section thickness is ease of various causes,1 and a poor quality reduced to 1-3 mm and a different software chest radiograph, especially of an obese reconstruction of the image is used to improve patient, may misleadingly raise the spectre of spatial resolution (figs la and lb). These scans diffuse lung disease. The two dimensional are interspaced by at least 1 cm. Where 3 mm nature of a chest radiograph dictates that there sections are taken 1 cm apart the radiation is superimposition of structures over the lungs dose to the breast is reduced to about half that and it has been estimated that up to 40% of of conventional computed tomography. None the lungs is obscured in normal subjects. This the less, the radiation burden inherent in high obscuration will be further compounded by resolution computed tomography is con- http://thorax.bmj.com/ the presence of pleural thickening or an siderable and in a complete study using 3 mm effusion. Even when the chest radiograph sections every 1 cm the dose is roughly 40 shows definite evidence of diffuse lung disease times that received from a single chest radio- McCloud has pointed out that "the chest graph.6 radiograph is often non-specific. Various Because high resolution computed tomo- radiographic patterns, together with their graphy includes only very short segments of predominant location, correlate statistically pulmonary vessels these narrow sections may with their pathologic entities but in individual be misinterpreted as showing a nodular pat- on October 1, 2021 by guest. Protected copyright. cases the chest roentgenogram is rarely diag- tern. For this reason some radiologists nostic. "' advocate conventional computed tomography Until recently the main use for computed before high resolution computed tomography tomography has been detection of pulmonary but this obviously adds to the radiation dose. nodules and, despite some pioneering work by Because, by definition, diffuse lung disease is Kreel in the early 1980s,3 it has had a limited widespread many radiologists now perform a role in the investigation of diffuse lung dis- limited number of high resolution computed ease. Improvements in computed tomography tomography scans, perhaps as few as six, in scanner technology, notably in terms of spatial such patients. In this way not only is the resolution and shorter scan times, have led to radiation dose restricted but the cost of the renewed interest in the application of com- examination is not much more than that of a puted tomography to show the fine morpho- posteroanterior and lateral chest radiograph. logical detail of the lung. Research in this area It has been estimated in the United States that is particularly active and is reflected in the high resolution computed tomography used in large volume of descriptive work on the com- this way costs $180, compared with $90 for a puted tomographic appearances of many dif- posteroanterior and lateral chest radiograph. fuse lung diseases that has appeared over the In the same comparison a transbronchial last five years4; there are no signs that this biopsy with bronchoalveolar lavage costs Department of period of research is over. The technique of $1500 and an open lung biopsy $5000 (R A Radiology, Royal Brompton National high resolution computed tomography, used Webb, personal communication). Computed Heart and Lung to show the lung parenchyma,' is sufficiently tomography is becoming more widely avail- Hospital, London different from conventional computed tomo- able throughout the United Kingdom and SW3 6HP D M Hansell graphy to warrant a brief description. most computed tomography machines instal- I H Kerr Conventional computed tomography of the led over the last three years are able to provide Reprint requests to: thorax uses contiguous 1 cm sections; in this high resolution images. Dr Hansell way the entire lungs are included in one The spatial resolution of the latest gener- 7X8 Hansell, Kerr Figure 1 ( a (Conventional 10 nini width co-mputed tomographic section through the upper lobes Thorax: first published as 10.1136/thx.46.2.77 on 1 February 1991. Downloaded from nlormtial). (b High resolution 3 mmil width computed tomographic sectiotn at the santie level. Note the different appearanice of the blood zvssels anid brotichi. L 41 Lk http://thorax.bmj.com/ on October 1, 2021 by guest. Protected copyright. ation of computed tomography scanners from the pattern on a chest radiograph have cnables structures as small as 200 pm to be largely been discredited."M This is partly resolved.7 As a result the outline of the basic because the abnormal shadowing on a radio- unit of the lung, the secondary pulmonary graph results from summation of numerous lobule, can often be identified. The inter- foci; the chest radiograph has been fancifully lobular septum measures 100 pm across in described as "the two-dimensional projectional normal individuals and is therefore more often sum of several thousand histologic slices.""' identified in pathological states when there is Another compelling reason is the over- thickening of the interstitium (fig 2). These whelming frequency with which interstitial septa, analogous to the Kerley B septal lines and airspace lesions coexist in any given disease seen in a different plane on the chest (see fig 2). Nevertheless, high resolution com- radiograph, are seen most frequently in the puted tomography does show interstitial thick- periphery of the anterolateral parts of the lung ening precisely in the few diseases in which the and over the surface of the diaphragm. The pathological process is largely confined to the secondary pulmonary lobule itself is an interstitium. An example is shown in figure 4; irregular polyhedron and contains a connec- the chest radiograph of this patient showed tive tissue sheath that surrounds the non-specific shadowing in the right upper bronchovascular bundle. This core may be zone. Despite extensive investigation a diag- visible as a small dot in the centre of the nosis was not reached. High resolution com- lobule (fig 3). puted tomography through the upper lobes Attempts to discriminate so called "air- shows unilateral thickening of the bronchovas- space" from "interstitial" disease by inferences cular bundles and interlobular septa, features The role of high resolution computed tomography in the diagnosis of interstitial lung disease 79 Figure 2 a) Polygonal outlines representitng thickened interlobular septa (arrowheads) in silicosis. The background Thorax: first published as 10.1136/thx.46.2.77 on 1 February 1991. Downloaded from nodularity is more obvious on sections through the upper zones. (b) Thickening of the interlobular septa in areas of airspace consolidation due to alveolar proteinosis. * @}.R }0 t- ,w';w e q1 I mm.a http://thorax.bmj.com/ !vN\ ;fi i *Xi-SV t b Ji # on October 1, 2021 by guest. Protected copyright. 6E characteristic of lymphangitis carcinomatosa." ance. The crescentic subpleural distribution From the pathological descriptions of lym- concentrated in the posterobasal segments of phangitis the high resolution computed the lower lobes is virtually pathognomonic'4 15 tomography appearances can be predicted and (fig 7). Not surprisingly, given this basal dis- the same applies to pulmonary sarcoidosis.' tribution, high resolution computed tomo- There are subtle but definite features that graphy will show disease despite a normal chest differentiate the high resolution computed radiograph.'" Scans taken in the prone position tomography appearances of sarcidosis from may be helpful in distinguishing parenchymal lymphangitis carcinomatosa: in sarcoidosis the opacification due to increased perfusion of the nodular thickening of the bronchovascular dependent parts of the lung from minimal bundles is more pronounced and thickening of fibrosing alveolitis. As the fibrosis progresses a the interlobular septa less obvious (fig 5). widespread, coarse reticular pattern super- Nevertheless, as with chest radiography, pul- venes, resulting in a non-specific appearance of monary sarcoidosis has many guises and a wide end stage pulmonary fibrosis. In the clinical range of high resolution computed tomography context of unexplained deterioration of a appearances has been reported'3 (fig 6). Fibros- patient with fibrosing alveolitis high resolution ing alveolitis, whether "lone" or associated computed tomography may be of value as it with other diseases, has been shown to have a may reveal other disease-for example, a bron- characteristic computed tomographic appear- chogenic carcinoma. High resolution com- 80 Hansell, Kerr Figure 3 Intralobular bronchovascular bundles ..-. visible as minute central dots (arrows) in this case ofsubacute extrinsic Thorax: first published as 10.1136/thx.46.2.77 on 1 February 1991. Downloaded from allergic alveolitis. 4.. ..disk& .. I.. cooe.... 0 .%... %""'..'...'.,.'."' A W... I,, .. I L. Figure 4 Lymphangitis carcinomatosa: in the right lung there is thickening of the bronchovascular http://thorax.bmj.com/ bundles and prominent subpleural interlobular septa. There is a pleural plaque posteriorly in the left chest. on October 1, 2021 by guest.