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Linear tomography geometry

The X-ray tube and film travel linearly in opposite directions about a fixed fulcrum. A point in the focal (fulcrum) plane images onto the centre of the film in both tube positions, so it stays sharp; a point above the plane images off-centre by opposite amounts, so it smears. The tube sweep (swing angle, about 30 to 40 degrees) and pivot height set the section that stays in focus.

Linear tomography geometry The X-ray tube and film travel linearly in opposite directions about a fixed fulcrum. A point in the focal (fulcrum) plane images onto the centre of the film in both tube positions, so it stays sharp; a point above the plane images off-centre by opposite amounts, so it smears. The tube sweep (swing angle, about 30 to 40 degrees) and pivot height set the section that stays in focus. Linear tomography: tube and film move oppositely about a fixed fulcrum X-ray tube (sweeps one way) swing angle θ ≈ 30–40° B focal (fulcrum) plane A / fulcrum pivot height film travels film (receptor) shown at two instants — travels opposite to the tube centre 1 centre 2 B B A: point in the focal plane B: point off the focal plane in the film’s frame the images become: sharp blurred Larger swing (tomographic) angle → more blur of off-plane structures → thinner section. Blur increases with distance from the focal plane; the focal plane itself stays sharp. Raising or lowering the pivot shifts the in-focus plane; the patient stays still. Typical section thickness: several mm (up to ~1 cm) — not sub-mm as in CT.
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