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Evaluation of mediastinal width on a supine chest film is now a standard part of the initial assessment of the trauma patient. 1 - 3 In some cases of mediastinal widening, it is clear that the mechanism of injury and the condition of the patient should lead to further investigation. This may include contrast computed tomography (CT), transoesophageal echocardiography and definitive arch aortography. 4 - 6 Interpretation is a diagnostic challenge when the mediastinum appears wide, the patient is well and the mechanism of injury is unlikely to have caused a traumatic rupture of the thoracic aorta.
Studies in the 1970s suggested an upper limit for mediastinal width of 8 cm-8.8 cm. 7 , 8 Since then trauma management has changed and therefore radiographic techniques have had to adapt.
It is our intention to discover whether these historical upper limits still apply and if not, to define new upper limits.
Methods
To ascertain the width of the normal mediastinum and its position within the supine chest, we conducted an 18 month retrospective review of chest CT scans of white adults. Scans where pathology distorted the mediastinum were excluded. The remaining scans were then examined at the level of the maximum diameter of the aortic arch to determine:
The composition and transverse diameter of the mediastinum at this level.
The maximum width of the aortic arch.
The distance from the anterior surface of the aortic arch to the skin of the posterior chest wall. (The position to which the aortic arch gravitates in the supine chest).
The images were obtained on an Elscint Twin II Helical scanner and stored on optical disk. The measurements were taken directly from the screen using an electronic cursor (fig 1). Two perpendicular lines from the edges of the object enabled measurement to be confirmed at two points. At the completion of the series, accuracy was confirmed by re-measuring every fifth scan.
CT scan of the chest at the level of the aortic arch.
Having obtained these figures, we were able to look at magnification in various simulated clinical settings.
Magnification is dependent on the:
Focus to film distance (FFD). The distance from the x ray source (focus) to the x ray film (fig 2).
Object to film distance (OFD). The...