Postgraduate Medical Journal (March 1984) 60, 208-212

Computed tomography of the thorax

BASIL STRICKLANDF.R.C.P., F.R.C.R.

Brompton Hospital, London SW3

Computed tomography (CT) was developed forclinical use by Godfrey Hounsfield of EMI Ltd inEngland in 1969. It has contributed to the investiga-tion of almost every system in the body. Its applica-tion to the thorax was relatively late in comingbecause the exposure times required were too long inrelation to respiration, but now its value in this fieldis established and increases with each new generationof CT scanner, so that published comment is soonoutdated by electronic advance.Computed tomography is carried out by a narrow

beam of X-rays traversing the body in a transverseplane through an arc of 180°. The emergent rays arereceived by a series of sensitive detectors andrecorded as numbers by a computer. These CTnumbers are displayed as various shades of grey on acathode ray tube (or monitor) and the image tran-sposed to photographic film for clinical viewing.Since each tissue absorbs different amounts ofradiation, each type of tissue will be represented by ashort series of consecutive numbers which can beread by a cursor from the monitor picture; the lowestnumber will be air, the highest will be bone, withother tissues being represented by numbers betweenthe high and the low. The facility is very valuable forassessing the pathological nature of mediastinalmasses, although in practice it is less reliable in thethorax than elsewhere in the body. Because of thewide range ofCT numbers required in the chest (verylow numbers for the lung pattern and high values forthe mediastinum), the window level and windowwidth must be varied to examine lung parenchymaand mediastinum separately, since the two cannot beassessed with the one setting. The window leveldetermines the point on the number scale at whichthe window lies. Window width is the range ofnumbers incorporated within a grey scale display.Modern CT scanners use exposure times between 1

and 5 seconds, a short exposure time being animportant factor in the examination of the cardiovas-cular and respiratory systems. By giving an intrave-nous injection of contrast medium (i.e. Hexabrix 100ml) the major vascular structures may be opacified,

and much greater accuracy of diagnosis is therebyachieved in relation to mediastinal disease. Theintelligent use of contrast media, the image quality,the scan speed and the experience of the viewer areall important factors for successful diagnosis. Lesionsmay also be seen in the sagittal plane by computerreconstruction from the cross section scans. This hasnot been fully exploited in the thorax and machinesvary in their capability of sagittal image reconstruc-tion. Whilst the patient is lying on the CT table, it is arelatively simple matter to extend the scan to includethe liver or the brain if a more distant lesion (i.e.metastasis) is a clinical possibility. Time, expense,anxiety and clinical doubt may all be curtailed by theone examination, and more invasive and hazardousinvestigations may be obviated (Fig. 1).

The chest wallChest wall lesions are clearly demonstrated. Tu-

mours of the soft tissues, cartilage or bone are clearlyseen. Even severe thoracic cage deformities no longerobscure the underlying lung. Neither severe scoliosisor previous thoracoplasty prevent radiographic as-sessment of the lung fields (Fig. 2). CT is not valuablein demonstrating rib erosion or atrophy, but willdemonstrate tumours penetrating the chest wall ineither direction.

The pleuraCT is unique in the clarity with which it can

demonstrate pleural plaques, calcification and thick-ening and is therefore especially valuable in assessingchanges due to asbestos exposure (Fig. 3). It not onlyshows the anatomical deployment of the abnormalpleura but also the degree and extent of pleuralthickening, including involvement of the fissures andthe lung parenchyma. If free fluid is present, it can bedemonstrated by turning the patient from supine toprone whence the fluid moves to the dependent partof the pleural cavity.

Small collections of air in the pleural cavity aremore accurately shown by CT than by any other

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Computed tomography of the thorax 209

FIG. 1. (a) Carcinoma of the bronchus, right midzone.

4.a4

FIG. 1. (b) The urogram was suspicious of a renal neoplasm on theright. The CT demonstrates a cyst of the lower pole of the kidney but

no evidence of neoDlasm.

FIG. 1. (c) CT ofthe brain shows a metastasis in the left parietal lobe.

method, and it may be possible to distinguish aloculated pneumothorax from a peripheral bullawhere this is difficult from the standard radiographs.

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FIG. 2. Previous thoracoplasty on the right. A good view of theunderlying structures is obtained. Clacification and pleural thicken-ing is present on the right (-..), the ribs have been partially removed.CT is valuable in all types of thoracic cage deformity whereas the

standard radiograph is usually uninformative.

FIG. 3. Mesothelioma on the right. The gross pleural thickeningsurrounds, invaginates and deforms the lung. The mediastinum is

pulled to the affected side.

The lung parenchyma

Although fine detail can only be seen on the mostmodem scanners (Fig. 4), an excellent picture isobtained of the vascular pattern of the lung. Proxi-mally arteries are distinguishable from veins, and anaccurate assessment of the larger pulmonary arteriesand veins is possible. The fissures can be localized(Fig. 5) and the lumina of the intermediate bronchiare clearly visible. Cavitation, calcification and fluidlevels within cavities are all well demonstrated andmycetomata can be shown to move within a cavity byaltering the patient's position (Fig. 6).

Benign and malignant lesions are localized withaccuracy and their relationship to adjacent structures(chest wall, pleura, fissures, vessels, heart and air-ways) assessed with confidence. In bronchogeniccarcinoma the lesion itself can be assessed forextension, invasion and spread. Hilar nodal invasioncan be seen although this is no more accurate than

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210 B. Strickland

FIG. 4. Interstitial fibrosis is visible in the peripheral parts of the leftlower lobe and posteriorly in the right lower lobe (arrowed). The

chest radiograph showed no abnormality.

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FIG. 5. The azygos vein fissure is clearly seen on the right. It isnormal but thicker than usual. The azygos vein is seen as a small

triangular density at the anterior end of the fissure.

with standard tomography. Large arteries and veinsmay simulate nodes at the hila even when contrastinjections are used.

Metastatic or direct extension into the medias-tinum is usually easily identified although falsepositives are not uncommon. Claims that a benignlesion in the lung parenchyma may be distinguishedfrom malignant lesions are not substantiated byexperience, and at present such claims should bediscounted.CT is more accurate in demonstrating metastases

than any other method, especially in the sub-pleuralregion (where metastases are common). Occasionallyhowever lesions may be demonstrated by standardtomography which cannot be demonstrated by CT.No method can equal CT in demonstrating em-

physematous bullae (Fig. 7). A clear and accuratepicture is given in regard to the number, size anddeployment. Films taken in both phases of respira-tion demonstrate whether bullae are ventilating aswell as the degree of mediastinal shift and lungherniation. The extent of lung expansion and relaxa-tion during respiration may be assessed by densitychanges in the lung fields.

Diffuse interstitial and fine parenchymal disease isclearly outlined and localized by CT. Sometimeschanges are apparent on the CT scan when the chestradiograph shows no evidence of disease. Should amore localized lesion develop within the diffuseprocess (i.e. neoplasm) it will be apparent on CT wellbefore it can be identified by other means.

The lung apex

This can be a very difficult area of the lung field todemonstrate by standard radiography or tomographyespecially in thick-set or obese individuals. CT is veryvaluable in demonstrating apical lesions and indifferentiating between tuberculous and neoplastic

FIG. 6. (a) supine; (b) prone. Bullous emphysema on the left. Posteriorly an old tuberculous cavity contains a rounded mass. It moves from theposterior wall to the anterior wall when the patient turns from supine to prone position. It is an aspergilloma.

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Computed tomography of the thorax 211

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FIG. 7. Large basal bullae; the smaller bulla on the right was barelyvisible on the chest radiograph.

disease (Fig. 8). In the latter the degree of pleuralthickening, extra-pleural spread and the relationshipof the lesion to the thoracic inlet and brachial plexusmay all be demonstrated with clarity. Rib involve-ment is rarely shown satisfactorily by CT because theribs are cut tangentially by the beam. Vertebral bodyerosion is well demonstrated.

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FIGS. 8. On the chest radiograph an ill-defined opacity was presentat the left apex. CT shows this apex to be occupied by a substantialhomogeneous opacity which extends between the ribs beyond theconfines of the thoracic cavity. Carcinoma invading the chest wall.

The mediastinum

Computed tomography is perhaps most valuablein assessing mediastinal disease and will demonstratefat and calcium with great accuracy (Fig. 9). Whenthe mediastinum is examined by CT a film series withand without contrast medium must be carried out.The latter involves an intravenous injection of a largebolus of contrast medium (i.e. 100 ml of Hexabrix)followed by a rapid series of CT exposures. In thisway the cardiac chambers and large arteries and

veins are clearly demonstrated and will not beconfused with non-vascular abnormalities (i.e. nodesand tumours). Aortic aneurysms (Fig. 10) andpulmonary artery lesions are clearly visible andparacardiac tumours of all types are defined sepa-rately from the heart (Fig. 11, 12). If they contain fator fluid, the CT number indicates the likely patho-logical cause.

FIG. 9. A large rounded retrosternal opacity shows circumferentiallinear calcification with a low density centre. The CT number of the

central portion suggested the presence of fat. Dermoid cyst.

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FIG. 10. Dissecting aneurysm of the descending thoracic aorta. Thenormal ascending aorta is seen just behind the sternum (whitearrow). The aneurysmal descending aorta (black arrow) lies on theleft adjacent to the spine; contrast medium outlines the lumenposteriorly. The false lumen is full of clot and does not opacify.

Reference to the identification of enlarged medias-tinal nodes has already been made. CT assessment ofmediastinal neoplastic disease is the most accuratenon-invasive investigation available, and is widelyused for staging lymphoma and carcinoma and in thepreoperative assessment of bronchogenic neoplasms.False positives are not uncommon although falsenegatives are unusual. If in bronchogenic carcinomaa good quality CT examination shows no evidence of

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B. Strickland

(a)

(b)

FIG. 11. (a) Chest radiograph 3 days following coronary bypasssurgery. The large opacity at the left hilum was thought to be anaortic aneurysm. (b) The CT scan shows a normal ascending anddescending aorta (white arrows) and pulmonary artery. The opacity(black arrow) lies to the left and is unrelated to these major vessels. It

proved to be a false aneurysm of the left coronary artery.

mediastinal involvement, thoracotomy would usuallybe undertaken without resort to prior mediastino-scopy.

FIG. 12. On the left a low density opacity at the cardiac apex istypical of a benign pericardial cyst clearly demarcated from the

heart.

Comment

The value of CT is not confined to clinicaldiagnosis. CT guided needle biopsy is widely usedelsewhere in the body, and its potential has not beenfully explored in the chest, especially in the media-stinum. Dynamic studies of the circulation throughthe heart, coronary circulation and the pulmonaryvascular tree complement echocardiography andfurther work on the lung density change in emphy-sema is likely to yield valuable information. CTshould not be used in isolation, but always inconjunction with chest radiography in two planesand sometimes with standard tomography. CT wid-ens the scope of radiographic diagnosis, renders itmore accurate and shortens assessment time. Fromthe educational aspect it 'opens up' the chest X-rayand often explains puzzling features present on thestandard radiograph.The capital outlay for CT is high, maintenance

costs are considerable and the irradiation dose ishigher than in standard radiography. If carried outproperly it is very time consuming for the radiologistand radiographic staff, but with good prior clinicalassessment and an experienced 'imaging radiologist',the clinical return can be very great and will increasein the future.

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