Pitfalls in medical imaging in thoracic injury

336

Injury (1986) 17,336-337

Symposium

Printedin

Great Britain

paper

Pitfalls in medical imaging in thoracic injury F. M. J. Heystraten Department

of Radiology,

University

Hospital

St Radboud,

INTRODUCTION MANY problems can be encountered in the interpretation of chest X-ray films of injured patients. Some of these are related to the fact that these radiographs are taken in a supine position. The evaluation of a pneumothorax and of the mediastinal shadow can be especially difficult on a supine radiograph of the chest. PNEUMOTHORAX The aetiology of pneumothorax and pneumomediastinum is tabulated in Table 1. Generally, a tension pneumothorax is not difficult to recognize (even on a supine radiograph) because of a shift of the mediastinal shadow and of the diaphragm. On the other hand, collapse of the lung is mostly not marked because of the stiffness of the lung due to consolidation or pulmonary contusion. A pneumothorax without tension can be difficult to detect because in a supine position air will accumulate anteriorly and basally and the typical pleural line is absent. However, the following signs can be seen and are often helpful (Table 11). Generally the air in the mediastinum (pneumomediastinum) escapes .to the subcutaneous tissues (subcutaneous emphysema) and not to the pleural space; conversely, air from the pleural space can escape into the mediastinum. In the presence of subcutaneous emphysema a pneumothorax can be difficult to detect; one always has to look for it carefully. Sometimes an unusual encapsulated collection of air can be seen within the mediastinal shadow in injured patients, which represents air in the pleural ligament. This will not cause subcutaneous emphysema and needs no drainage. It is called a ‘paramediastinal pneumatocele’ and disappears spontaneously. When a combination of haemothorax and pneumothorax is present, the pneumothorax can be difficult to detect, because when much fluid is present the X-ray is not tangential to the pleural interface and no pleural line will be seen. EVALUATION OF THE MEDIASTINAL SHADOW Evaluation of the mediastinal shadow on a chest radiograph of an injured patient is difficult because of the supine position of the patient, bad inspiration, short focus-film distance and sometimes rotation of the patient. Mediastinal widening can result from bleeding of small arteries and veins or can originate from bleeding caused by sternal fracture.

Nijmegen,

The Netherlands

We evaluated the widened mediastinal shadow in 125 patients with chest injuries; 63 with aortic rupture, 62 without aortic rupture. The most important signs that differentiated the patients with aortic rupture from those without rupture are listed in Table Ill. A combination of these signs is strong evidence of aortic rupture. Some pitfalls need special mention. Plain chest radiography When a large haemothorax is present the proper evaluation of mediastinal changes is impossible. Evacuation for evaluation of the mediastinal shadow is imperative. Furthermore the pleural haematoma will usually have a different aetiology. Concomitant heart injury or diaphragmatic rupture occurs not infrequently in these patients. Conversely, aortic rupture can be present without external evidence of chest injury and without Table 1. Aetiology mediastinum

of pneumothorax in injured patients

Tear of the thoracic wall PleuraVparenchymal tears Alveolar rupture Tracheal/bronchial rupture Perforation of the oesophagus

Table II. Signs of pneumothorax graph

and

pneumo-

(rare)

on a supine chest radio-

Deep sulcus sign Sharp hemidiaphragmatic border Sharp mediastinal contours Hyperlucency above liver and spleen Hyperlucency of entire hemithorax Additional signs Pneumomediastinum Displaced mediastinal shadow

Tab/e 111.Most important aortic rupture

mediastinal

Displaced nasogastric tube Displaced tracheal shadow Widened paratracheal stripe Widened paraspinal line Mediastinal widening >8 cm Opacified pulmonary window

changes indicating

Heystraten:

Interpreting

chest radiographs

337

any other pathological finding on the chest radiograph. Although the left-sided mediastinal haematoma will often displace the trachea and the nasogastric tube to the right this is not always present. A normal position of the tracheal shadow and nasogastric tube should be no reason to reject the diagnosis of aortic rupture if other signs of mediastinal haematoma are present. Although mediastinal widening due to aortic rupture is most often present on the initial chest radiograph, sometimes progression of the widening can be seen on subsequent radiographs.

of a ductus diverticulum (remnant of the ductus arteriosus) can mimic a false aneurysm. In case of a ductus diverticulum no intimal flap will be seen in the protrusion and no retention of contrast takes place. Intravenous digital subtraction angiography (DSA) can be useful in diagnosing aortic rupture; however, the patient has to be cooperative or intubated with artificial respiratory arrest during the recording, otherwise the angiogram may be useless due to respiratory movement. In most cases intra-arterial DSA or conventional angiography is the method of choice.

Angiography Because the heart rate is generally increased in injured patients care must be taken to administer a large bolus of contrast medium (e.g. 30 cc/set for 2 seconds), otherwise opacification of the aorta will be insufficient. A left anterior oblique (LAO) position is needed to unfold the aortic arch and isthmus where most ruptures are located. If an anteroposterior projection only is used the false aneurysm can be missed. The appearance

CONCLUSION Interpretation of chest films of injured patients can be difficult, especially regarding the recognition of pneumothorax, haemothorax or a combination of both. In addition, the evaluation of the mediastinal shadow is more difficult than normal because of the supine position of the patient. Close collaboration with the radiologist is mandatory.

Requests for repin& should be addressed to: Dr F. M. J. Heystraten, The Netherlands.

Department

of Radiology,

University

Hospital

St Radbovd,

Nijmegen,