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A Chest X-ray as an Example - 2
In digitising this picture some contrast is lost, but
you can notice the difference between the air outside the body and air in the
lungs – lungs are soft tissue filled with air!
Note the shape of the heart. The thick muscle wall and
the blood that fills the heart create a white image – in places whiter than
bone.
Structures in the hilum of the lung can be seen with
variable clarity (A). A blood vessel “end-on” is more opaque than one “across”
the beam.
Cervical and upper thoracic vertebrae can be seen in the
oval B. Lower thoracic vertebrae, along with the descending aorta, are seen as
a band running down the heart.
A
B
Key Points :
Air containing structures ‘darken’ other superimposed structures.
Thickness makes the heart as opaque as bone!