Computed Tomography of the Thorax 1

Computed Tomography

R. Gilbert Jost, M.D., 2 Stuart S. Sagel, M.D., Robert J. Stanley, M.D., and Robert G. Levitt, M.D.2 Computed tomography (CT) provides a valuable new perspective in assessing abnormalities of the thorax. In the patient with a mediastinal mass or widening detected by plain chest radiography, a definitive diagnosis is sometimes possible which would not be obtainable by conventional noninvasive radiological techniques. In certain clinical situations, CT is a highly sensitive modality for examining the lungs. The proved and potential clinical uses of CT in solving diagnostic problems of the thorax are discussed. INDEX TERMS: Aorta, diseases. Computed tomography, thoracic, 6[0].1211 • Lung neoplasms, diagnosis • Mediastinum, anatomy. Mediastinum, neoplasms

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the past two years, computed tomography (CT) of the body has become established as an important new radiologic procedure (1-4). The ability of CT scanning to better distinguish specific tissue densities in the thorax and to display the lungs and mediastinum in a transverse plane at times provides unique diagnostic information unobtainable with conventional radiographic techniques. During the first fifteen months of our clinical experience with computed body tomography 144 patients with problems involving the thorax have been studied, representing just under 10% of our total series. The clinical impact of CT has been less dramatic in the thorax than it has been in the abdomen (4); nevertheless, certain clinical situations have been identified in which CT can provide useful diagnostic information.

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The CT examinations in this series were performed with an EMI Prototype Body Scanner, whose principles of operation have already been described (2). When feasible, a thoracic CT study should be performed in suspended full inspiration. Of the 144 patients studied, 90% were able to suspend respiration for the 18 seconds required to complete each section. Each procedure is monitored by a radiologist, with appropriate scan levels selected depending upon the problem under analysis. A preliminary supine scout radiograph of the thorax with lead markers at 2-cm intervals is obtained to localize the area of interest. In cases where a possible vascular lesion is being studied, or where it is important

Fig. 1. Normal anatomy. A. Level of lung apices: 1. trachea; 2. esophagus; 3. left subclavian artery; 4. left carotid artery; 5. left brachiocephalic vein; 6. right bracheocephalic vein; 7. right brachiocephalic artery. B. Level of the bronchial bifurcation: 1. descending aorta; 2. ascending aorta; 3. hilar vessels; 4. left mainstem bronchus; 5. right mainstem bronchus; 6. azygous vein; 7. esophagus. 1 From theMallinckrodtlnstituteofRadiology.WashingtonUniversitySchoolofMedicine.St. Louis, Missouri. Presented at the Sixty-second Scientific Assembly and Annual Meeting of the Radiological Society of North America, Chicago, 111., Nov. 14-19, 1976. 2 James Picker Scholar. ss

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Fig. 2. A. Mediastinal widening (arrows) is simulated on a CT scan performed during expiration. B. A scan at the same level during inspiration is normal.

to separate vascular and nonvascular lesions, intravenous contrast is commonly administered. Sustained enhancement can be obtained with a rapid intravenous infusion of 300 ml of Conray 30. The infusion generally requires approximately 7 minutes through an 18-gauge needle, and at least 5 well-enhanced scans can be obtained beginning 4 minutes after the commencement of the infusion. NORMAL ANATOMY

Cross-sectional CT scans provide a unique perspective of the mediastinum, and it is often possible to identify structures which cannot be separated out by conventional radiographic techniques (Fig. 1). The aorta is usually well seen. It is commonly separated from other mediastinal structures by a thin band of fat, and is often outlined in the elderly patient by a rim of calcium. Other vessels may be similarly profiled. The esophagus is difficult to evaluate,

as it blends in with other mediastinal structures. A small amount of air may be seen in the esophagus in approximately 40 % of normal patients. In the hilar areas, the pulmonary arteries, veins, and airways can be identified. In order to see the lungs, a different window width and level is necessary. At the appropriate setting, it is possible to demonstrate the pulmonary parenchyma, the intrapulmonary vessels, and even some normal bronchi. As in a standard chest radiograph, there is preferential pulmonary blood flow to the dependent portion of the lung in a CT scan (the dorsal portion of the lung in the supine position). Also, as in conventional radiography, the degree of inspiration at the time of a scan affects the normal transverse anatomy of the thorax. During expiration the lung volume is smaller, the mediastinum appears wider, and the attenuation values in the lung are considerably higher due to the resulting compression of parenchymal

Fig. 3. Asymptomatic 47-year-old woman. A. Postero-anterior chest radiograph demonstrates a left cardiophrenic angle mass (arrows). B. CT scan shows the mass (M) with an attenuation value approximating water, intermediate in density between fat and soft tissue, separated from the heart by a band of fat (arrows). The CT findings represent a classic pericardial cyst.

Computed Tomography

Fig. 4. Asymptomatic obese 43-year-old woman. A. Postero-anterior chest radiograph demonstrates a right cardiophrenic angle mass (arrows). B. CT scan shows a typical pericardial fat pad of no clinical significance (arrows). Also, notice the extra-pleural fat in the posterior thorax (arrowheads).

Fig. 5. A 27-year-old woman with low grade fever and malaise. A. and B. Postero-anterior and lateral radiographs demonstrate a right cardiophrenic angle mass (arrows). C. CT scan shows the mass (M) to have an attenuation value of soft tissue, similar to that of the inferior vena cava (I) and the heart (H), suggesting solid neoplasm rather than cyst or fat pad. A subsequent work-up revealed Hodgkin's disease.

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A

Fig. 6. A 68-year-old man without chest symptoms. A. and B. Postero-anterior and lateral chest radiographs demonstrate mediastinal widening with filling in of the retrosternal clear space. C. CT scan shows abundant fat (arrows) surrounding the great vessels, accounting for the mediastinal widening, and obviating the need for further evaluation.

tissue (Fig. 2). These changes need to be considered in the evaluation of normal and pathologic thoracic anatomy. At the present time, little information can be obtained about the anatomy of the heart due to cardiac motion during the scanning cycle, although gating techniques show promise for useful cardiac studies at some time in the future (5). While anecdotal cases of the CT demonstration of pericardial effusion or valvular or coronary artery calcification can be presented, in our experience these pathologic entities have been more easily shown by other simpler noninvasive techniques such as ultrasonography and fluoroscopy. CLINICAL USES OF THORACIC CT SCANNING

Fig. 7. CT scan in this 65-year-old man with mediastinal widening on plain chest radiographs shows aneurysmal dilatation of the aortic arch. Note the characteristic rim of calcification (arrows) surrounding the dilated aorta.

In the course of investigating its potential value, CT has been used to study a wide variety of problems involving the thorax, with the mediastinum the primary area of interest in half of the patients studied. Common clinical problems in which CT has provided a useful solution are outlined in TABLE I.

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Mediastinum Attenuation Value Determination of Mass: A common diagnostic problem in the thorax involves the etiologic determination of a mediastinal mass detected on a plain chest radiograph. Recognition of the precise attenuation value of a mediastinal mass by CT may permit a definitive, noninvasive diagnosis (Fig. 3). Lesions which present with attenuation values characteristic of fatty tissue [-40 Hounsfield (H) or EMI units] include a Morgagni hernia, Bokdalek hernia (Fig. 22 in Ref. 2), and pericardial fat pad (Fig. 4). In these cases, the diagnosis is conclusive, and no further diagnostic work-up is indicated. Should the mediastinal mass demonstrate an attenuation value higher than 10-15 EMI units, characteristic of soft tissue, neoplasm cannot be excluded and further work-up is necessary (Fig. 5). We wish to emphasize that attenuation values and histology are not synonymous. Cystic lesions, for example, are sometimes difficult to identify with certainty, because their attenuation values are not always equivalent to water. In 3 of the 6 bronchogenic cysts studied, the attenuation values were close to that of soft tissue. At surgery, these cysts were not filled with a thin serous fluid, but contained thick viscid secretions, which accounted for their higher attenuation values. Evaluation of Mediastinal Widening: When mediastinal widening is identified on the plain chest radiograph, the finding may be caused by a normal variant such as fat, aneurysm of the aorta, or solid neoplasm. With CT scanning it is often possible to determine the cause for the widening, obviating the need for more invasive procedures such as aortography. Widening of the mediastinum is not always related to vessels or tumor, but may represent excessive fat deposition in the mediastinum, a diagnosis which can be readily made by CT (Fig. 6). This may occur in patients who are obese (6) or on steroid medication (7). When neoplasm is the suspected cause of mediastinal widening, CT generally plays its most important role in excluding the diagnosis of an aneurysm (Fig. 7). While CT

Fig. 8. A 28-year-old man with mediastinal widening discovered by chest radiography. A tumoral mass (1). subsequently proved to be due to Hodgkin's disease, is separated from a normal aortic arch (2) by a thin band of fat.

TABLE

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CLINICAL USES OF THORACIC CT SCANNING

Mediastinum Attenuation value determination of mass Evaluation of mediastinal widening Evaluation of paraspinal widening Lung Occult disease (metastases, interstitial. obscured) Pulmonary nodule ("calcification") New perspective in extent of bronchogenic carcinoma

may be helpful in defining the extent of a mediastinal neoplasm, a precise histologic diagnosis cannot be established. The aorta is generally separable from the otherstructures in the mediastinum and is easily identified. A fine rim of calcification (Fig. 8) has been identified surrounding all but one of the thoracic aortic aneurysms studied. The exception was a newly developed mycotic aneurysm. If there is any question about identifying the aorta, intravenous contrast, preferably a high dose infusion, will opacify the vessel lumen. With such techniques, even intramural thrombi can be identified (8). Eight patients were evaluated by CT when the possibility of a thoracic aortic aneurysm was suggested on the plain chest radiograph. An aneurysm was correctly diagnosed in 6, correctly excluded in 1, and one scan was indeterminate, requiring aortography. An unsuspected thoracic aortic aneurysm was discovered in 2 additional patients. In only one patient was a mediastinal abnormality detected on CT that was not previously seen on the plain chest radiograph (Fig. 9). While CT may rarely prove helpful in evaluating hilar abnormalities, we have generally found conventional tomography [often in the 55 0 posterior oblique projection (9)] quite adequate for evaluating the hilus. Evaluation of Paraspinal Widening: Paraspinal widening may present a diagnostic dilemma which can be difficult

Fig. 9. A 37-year-Old patient with right hilar lymph node enlargement demonstrated on plain chest radiograph. The CT scan demonstrates the hilar mass (1) plus an additional mass (2) just lateral to the azygoesophageal recess. This latter mass was not detectable, even in retrospect, on the plain chest radiographs or on conventional tomograms.

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Fig. 10. A. Normal retrocrural anatomy: 1. aorta; 2. diaphragmatic crura; 3. retrocrural fat containing small densities which may represent either the azygous or hemiazygous veins or normal lymph nodes. B. Normal lymph nodes, enhanced by Iymphangiographic contrast media, in retrocrural space.

to resolve with conventional techniques. In selected clinical situations, computed tomography is an excellent tool for clarifying the cause of the abnormal widening. When paraspinal widening is due to adenopathy, the abnormal nodes can be clearly identified (Figs. 10-11). Other unexplained causes of widening in the parasplnal region may also bedlaqnosed by CT, including aneurysms of the descending aorta (Fig. 12), herniation of omental fat through the esophageal hiatus (Fig. 13), and direct extension of skeletal infection or tumor (Fig. 14). Again, it should be emphasized that in many cases of paraspinal widening, CT is not necessary to make the diagnosis. When vertebral body involvement or hiatus hernia is suspected, detailed spine radiographs or a barium study, respectively, should

first be obtained as the diagnostic procedure of choice. CT should be reserved for those cases of paraspinal widening which cannot be resolved by conventional techniques. Lungs Occult Disease: Computed tomography may make it possible to evaluate certain lung diseases which are undetectable or poorly seen by conventional plain chest radiography and tomography. An example would be the lung recesses near the diaphragm posteriorly (Fig. 15). Also, CT can be useful in evaluatlnq the patient with parenchymal or mediastinal disease obscured by a surrounding pleural effusion (Figs. 16 and 17).

Fig. 11. A 45-year-old woman with a previous history of carcinoma of the cervix. A. Postero-anterior chest radiograph demonstrates paraspinal widening (arrows), not present on a radiograph obtained 6 months previously. B. CT scan shows that paraspinal widening is due to enlarged retrocrural lymph nodes (arrow) displacing the diaphragmatic crura and paraspinal line laterally.

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At times, CT provides a sensitive technique for demonstrating the presence of interstitial disease undetectable by conventional radiographic techniques. In patients with sarcoidosis, for example, occult parenchymal involvement can be demonstrated. However, the definite clinical utility of this observation is not apparent, as abnormal pulmonary function studies in these patients generally indicate accurately the presence of parenchymal lung disease. CT scanning has been reported to provide a more sensitive method for detecting pulmonary metastases than conventional radiographic techniques (2, 4, 10). In a rather uncontrolled fashion at our Institute, the thoraces of 60 patients with a history of malignant tumor were studied by CT. Five patients were shown to have pulmonary nodules which were undetected on plain chest radiographs or conventional tomograms (Fig. 18). These nodules were located in sub-pleural regions or in the lung apices or the recesses near the diaphragm. Four of these patients pre-

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viously had only one known lung lesion, and the demonstration of additional pulmonary nodules markedly affected clinical management. In the other patient, no change in size of two "occult" pulmonary nodules over a nine-month period suggested that the nodules represented healed granulomas rather than neoplastic disease. The importance of discovering occult nodules in the lung must be put in perspective, however. It is certainly of no clinical utility to demonstrate additional pulmonary nodules in a patient who has already been shown to have two or more lung metastases. CT will affect patient management only in the case where a lung tumor is discovered for the first time, or where a second malignant lung nodule is discovered in the patient who was felt to have only one lung lesion. The eventual role of computed tomography in screening for occult metastases is still undetermined. There is little question that this technique provides a highly sensitive tool

Fig. 12. A. and B. Postero-anterior and lateral chest radiographs in an asymptomatic 80-year-old woman show an abnormal paraspinal density near the diaphragm (arrows). C. A CT scan following contrast infusion demonstrates that the mass represents aneurysmal dilatation of the descending aorta.

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Fig. 13. A. Radiograph from an upper gastrointestinal series in a 59-year-old man shows a paraspinaJ mass (arrows) which was associated with a small esophageal hiatus hernia. B. CT scan shows the mass to be of fat density (arrows), consistent with herniation of omentum through the esophageal hiatus. No further work-up was considered necessary.

for detecting small nodules in the lung. Nevertheless it would be impractical to scan every preoperative patient with neoplastic disease. An average adult patient whose thorax measures 30 cm (12 inches) in length would require at least 30 scans to completely examine the lungs, and many of the nodules detected would not be neoplastic, especially in areas of endemic granulomatous disease. Differentiation might require rescanning the thorax at an appropriate time interval (e.g., 6 weeks) to discern whether

Fig. 14. A 50-year-old man with low thoracic back pain and low grade fever. A chest radiograph at another hospital demonstrated paraspinal widening, and the patient was referred for further evaluation. The CT scan shows destruction of the T-11 vertebral body, with a large adjacent paraspinal mass (arrow). Scans at higher levels showed two additional destroyed vertebral bodies, strongly suggesting inflammatory disease. A needle aspirate of the paraspinal soft tissue mass disclosed Mycobacterium tuberculosis. (In retrospect, this patient did not require a CT scan, as a later review of the outside chest radiographs disclosed vertebral body destruction.)

the nodules were growing. Therefore, routine preoperative screening is not performed at the present time. While further data is being accumulated, our indications for this type of screening examination are limited to the indications used for traditional whole lung tomography. This would include: (a) the patient for whom extensive surgery is planned because of a known primary neoplasm that has a propensity for lung metastases (e.g., osteosarcoma or testicular neoplasm); (b) the patient with an apparent solitary lung metastasis for whom resection is planned; in such cases, the demonstration of otherwise occult pulmonary metastases would clearly alter the planned surgical management; and (c) the patient with a positive sputum cytology and no lesion identifiable with conventional radiographic techniques or fiberoptic bronchoscopy. Pulmonary Nodule ("Calcification '): The definitive evaluation of a solitary pulmonary nodule detected on a plain chest radiograph is a common radiological problem. Low kV spot films or conventional tomograms are often employed to discern calcification and its pattern within a nodule. Cases have been described, however, where calcifications demonstrated by a CT scan (Fig. 19) could not be detected with conventional tomograms (2). This situation has occurred in 3 of the 42 pulmonary nodules studied. Thus the detection of previously unsuspected calcification within a nodule by CT has affected patient management in only a small percentage of cases. Clearly the role of CT in evaluating solitary pulmonary nodules awaits further investigation. A few high attenuation values in a pulmonary nodule may simply represent statistical variation. Furthermore, it is usually impossible to determine an accurate attenuation value for a small pulmonary nodule because of partial volume averaging with

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the surrounding lung. In practice, it is often necessary to simply compare the attenuation of a small nodule with nearby blood vessels of similar size. In the future, the use of thinner collimators may overcome the averaging phenomenon. Nevertheless, it is currently difficult to conclusively determine whether a pulmonary nodule is benign simply on the basis of a CT scan. Until more data are available, we have adopted a very cautious approach to ruling out malignant disease on the basis of whether "calcification" is detected in a pulmonary nodule. More invasive procedures, such as needle aspiration biopsy or surgical resection, unfortunately, are still usually required to differentiate a benign from a malignant nodule. New Perspective in Extent of Bronchogenic Carcinoma: Computed tomography offers a new perspective in evaluating the extent of disease in a patient with bronchogenic carcinoma. Currently, a comprehensive study of the efficacy of CT in evaluating proved lung neoplasms is being conducted. Some preliminary observations are available.

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Twenty-two patients with bronchogenic carcinoma have been studied. Two were shown to have additional pulmonary metastases not previously suspected. In 8 patients, direct extension into the mediastinum or evidence of lymph node metastases was diagnosed on CT scans (Fig. 20). In 4 cases, surgical confirmation of mediastinal involvement is available. In some recent cases, however, mediastinal involvement on CT scan has become an additional factor in influencing the referring surgeon to decide against a thoracotomy, thus precluding direct pathologic correlation. The ultimate utility of CT in this clinical context also awaits further investigation. In all likelihood, the actual impact of CT in the evaluation of bronchogenic carcinoma will depend greatly on the attitude of physicians in a particular hospital. In some centers, resection of bronchogenic carcinoma is attempted even when mediastinal adenopathy is demonstrated on the plain chest radiograph. Nevertheless, CT has the potential for adding new infor-

Fig. 15. A 50-year-old man with cough and low grade fever. A. and B. Postero-anterior and lateral chest radiographs suggest but do not exactly define a density in the right lung base. Conventional tomograms in the supine position provided little additional information. C. CT scan clearly defines a cavitary lesion containing air and fluid within the right lower lobe (arrows) posteriorly, with preservation of the surrounding mediastinal fat. Abundant pericardial fat is noted incidentally. Surgery disclosed an intralobar sequestration.

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mation which may help in the management of the patient with bronchogenic carcinoma. DISCUSSION

The use of computed tomography of the thorax has had considerably less impact in patient management than similar studies of the head or abdomen. This is largely because the conventional chest radiograph is a much more sensitive and accurate examination than plain film studies of the head and abdomen. Most of the thoracic lesions

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studied with CT have first been detected on a chest radiograph. It is clear that the clinical role of CT in the thorax is still evolving. There is no doubt that in certain clinical situations CT provides information which is unobtainable with conventional techniques. When masses of fat density are identified, when a classic cystic structure is demonstrated, or when a mediastinal mass is shown to be vascular, a dramatic and definitive diagnosis is often possible, obviating the need for further invasive procedures.

Fig. 16. A 72-year-old man with weight loss. A. Postero-anterior chest radiograph demonstrates a right pleural effusion and paramediastinal density (arrow) which might represent either a mass or loculated pleural fluid. B. Decubitus view confirms the right pleural effusion and persistence of the paraspinal mass. C. and D. Wide and narrow window width CT scans document the presence of a large paraspinal mass (M), different in attenuation from the associated right pleural effusion (E). Also, some enlarged lymph nodes posterior to the descending aorta and a small left pleural effusion are seen (arrow). (In our experience, small pleural effusions seen on the CTscan have not been completely gravity-dependent). Subsequent biopsy disclosed a lymphocytic lymphoma.

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Fig. 17. A 68-year-old man with mild dyspnea. A. Chest radiograph demonstrates a left pleural effusion and a suggestion of mediastinal widening. Thoracentesis and Cope needle biopsy on two occasions failed to yield a diagnosis. B. CT scan demonstrates thickening of the pleura around the left lung, most marked around the brachiocephalic vessels (arrow) (compare to Figure 1,A), suggesting a malignant mesothelioma. Limited thoracotomy with open biopsy confirmed this diagnostic impression.

plications to lung densitometry (e.g., demonstration and characterization of early interstitial disease or emphysema) may become a useful possibility.

In other areas the role of CT scanning is not so obvious, and further investigatidn is necessary before definitive guidelines can be set forward. Preliminary studies suggest that CT provides a more sensitive technique for detecting pulmonary nodules than standard radiographic procedures, but it is not currently feasible to use CT as a screening test in the routine preoperative evaluation of every patient with known or highly suspected cancer. Further investigation hopefully will clarify how CT should be used pragmatically in the detection of occult metastases, in the evaluation of the solitary pulmonary nodule, and in the determination of the extent of known bronchogenic carcinoma. With the development of significantly shorter scanning times, ap-

1. Alfidi RJ, Haaga JR: Computed body tomography. Radiol Clin North Am 14:563-570, Dec 1976 2. Sagel SS, Stanley RJ, Evans RG: Early clinical experience with motionless whole-body computed tomography. Radiology 119: 321-330, May 1976 3. Sheedy PF, Stephens DH, Hattery RR, et al: Computed tomography of the body: initial clinical trail with the EMI prototype. Am J RoentgenoI127:23-51, Jul 1976

Fig. 18. A 67-year-old woman with left upper lobe atelectasis secondaryto metastatic endobronchial melanoma. No other lesion was demonstrable on conventional full chest laminograms. A CT scan demonstrates the left upper lobe atelectasis and pleural-based metastases in the right lung (arrow).

Fig. 19. A 42-year-old asymptomatic woman with a nodule in right middle lobe (arrow) discovered incidentally on plain chest radiographs. Neither low kV spot films nor standard tomograms showed calcification within the nodule. CT scan demonstrates high attenuation values within the nodule compatible with calcification. A presumptive diagnosis of granuloma was made and a conservative course eJected. No change in the size of the nodule has occurred within 1 year.

REFERENCES

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Fig. 20. A. and B. Postero-anterior and lateral chest radiographs in a 65-year-old man with bronchogenic carcinoma of the left upper lobe, previously diagnosed by bronchoscopy. C. The CT scan shows obliteration of fat planes (arrow) about the aortic arch. Also note air indicative of necrosis within the tumor. D. A lower-level scan demonstrates obliteration of the pericardia' fat plane and left hilar lymph node enlargement (arrow). Thoracotomy confirmed all of the CT findings, and the neoplasm was unresectable.

4. Stanley RJ, Sagel SS, Levitt RG: Computed tomography of the body: early trends in application and accuracy of the method. Am J RoentgenoI127:53-67, Jul 1976 5. SageI SS, Weiss ES, Gillard RG, et al: Gated computed tomography of the human heart. Presented at the 25th Annual Meeting of the Association of University Radiologists, Kansas City, Missouri, 2 May 1977 6. Lee WJ, Fattal G: Mediastinal lipomatosis in simple obesity. Chest 70:308-309, Aug 1976 7. Koerner HJ, Sun 0: Mediastinal lipomatosis secondary to steroid therapy. Am J Roentgenol 98:461-464, Oct 1966 8. Korobkin M: Indications for contrast enhancement. Presented at Course on Computed Tomography, Body/Head, Washington Uni-

versity School of Medicine, Mallinckrodt Institute of Radiology, 29-31 Oct, 1976 9. McLeod RA, Brown LR, Miller WE, et al: Evaluation of the pulmonary hila by tomography. Radiol Clin North Am 14:51-84, Apr 1976 10. Muhm JR, Brown LR, Crowe JK: Detection of pulmonary nodules by computed tomography. Am J RoentgenoI128:267-270, Feb 1977 Mallinckrodt Institute of Radiology Washington University School of Medicine 510 S. Kingshighway St. Louis, Missouri 63110