Which factors influence penumbra in radiographic imaging?

• A. SID only
• B. OID only
• C. Both SID and OID
• D. Neither distance influences penumbra

Answer: (C)

Penumbra is the geometric unsharpness that appears around a radiographic image because a finite focal spot projects onto the detector. How large that unsharpness is depends on the focal spot size and the geometry of the setup: the distance from the X-ray source to the image receptor (SID) and the distance from the object to the image receptor (OID). As SID increases, the beam becomes more parallel and penumbra decreases. As OID increases, the projection of the focal spot onto the receptor spreads out more, increasing penumbra. A practical way to think about it is that geometric unsharpness is proportional to the focal spot size multiplied by the ratio of OID to SID (U ≈ f × (OID / SID)). Because both distances influence penumbra, both SID and OID matter. To minimize penumbra, use a small focal spot, a large SID, and a small OID.