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The air-gap technique is a scatter-reduction method in radiography that uses increased distance between the patient and the image receptor (film or detector) to reduce the amount of scattered radiation reaching the detector — without using an anti-scatter grid.

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Essentially, a layer of air acts as a natural scatter absorber, because scattered X-ray photons travel at oblique angles and diverge away from the detector surface before they can reach it.

Principle

• Scattered radiation emerges from the patient in random, oblique directions.

• By increasing the object-to-image distance (OID), much of this scattered radiation misses the image receptor.

• The primary (useful) radiation, which travels nearly straight, still reaches the detector.

  → Result: Improved image contrast similar to that achieved using a grid.

The air gap functions as a “virtual grid.”

Technique and Setup

Mechanism of Scatter Reduction

• Scatter photons diverge at wide angles from the patient.
• As OID increases, fewer scattered photons strike the image receptor.
• The larger the gap, the greater the proportion of primary-to-scatter radiation at the detector.
• Thus, contrast improves while overall fog decreases.

Advantages