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Figure 1: Axial images from enhanced computerised tomography of the neck at the levels of the first tracheal ring (a), and the thoracic inlet (b). At each level, the anteroposterior and lateral dimensions of the trachea were measured, and the cross-sectional area was measured by freehand region of interest

Figure 1: Axial images from enhanced computerised tomography of the neck at the levels of the first tracheal ring (a), and the thoracic inlet (b). At each level, the anteroposterior and lateral dimensions of the trachea were measured, and the cross-sectional area was measured by freehand region of interest