Infographics / Image Quality

Air-gap scatter rejection

An X-ray focus projects a beam through a patient onto a detector placed a large distance below. Primary photons travel in straight lines from the focus and reach the detector. Scattered photons are produced in the patient and travel at wider angles, so across the air gap they diverge and pass beyond the edges of the detector, missing it. The gap increases the object-to-detector distance, raising magnification and geometric unsharpness unless the source-to-image distance is increased.

Air-gap technique for scatter reduction An X-ray focus projects a beam through a patient onto a detector placed a large distance below. Primary photons travel in straight lines from the focus and reach the detector. Scattered photons are produced in the patient and travel at wider angles, so across the air gap they diverge and pass beyond the edges of the detector, missing it. The gap increases the object-to-detector distance, raising magnification and geometric unsharpness unless the source-to-image distance is increased. X-ray focus Patient Detector SOD OID air gap (≥ 30 cm) primary photons — reach the detector scattered photons — diverge and miss the detector (not removed) A larger air gap lets more divergent scatter miss the detector — a geometric alternative to a grid, but magnification and geometric unsharpness increase unless the SID is raised. SID = SOD + OID
Practise questionsDownload SVG

More in Image Quality

Practice the Physics paper

FRCRX is a Part 1 Physics question bank: true/false items with explanations, timed mocks and topic analytics.

Start question practiceSee plans