Which parameter, when increased, contributes to both decreased spatial resolution and increased penumbra?

Which parameter, when increased, contributes to both decreased spatial resolution and increased penumbra?

• A. Source-to-image distance (SID)
• B. Focal spot size (FSS)
• C. Object-to-image distance (OID)
• D. Tube voltage

Answer: (C)

Spatial resolution in radiography is governed by geometric unsharpness, which arises from the finite focal spot and the way the beam projects the object onto the image receptor. The amount of edge blur, or penumbra, increases as the geometry becomes more divergent. A helpful way to think about it is that geometric unsharpness is proportional to the focal spot size times the ratio of the object-to-image distance to the source-to-image distance: U ∝ FSS × (OID / SID). If you increase the object-to-image distance while keeping the source-to-image distance fixed, this ratio grows, so the blur increases. That means the image becomes less sharp (lower spatial resolution) and the penumbra around edges becomes larger. Other options either would reduce blur (increasing SID) or mainly affect image contrast and dose rather than the geometry of blur (tube voltage), though increasing focal spot size would also increase blur and penumbra in general.

Spatial resolution in radiography is governed by geometric unsharpness, which arises from the finite focal spot and the way the beam projects the object onto the image receptor. The amount of edge blur, or penumbra, increases as the geometry becomes more divergent. A helpful way to think about it is that geometric unsharpness is proportional to the focal spot size times the ratio of the object-to-image distance to the source-to-image distance: U ∝ FSS × (OID / SID). If you increase the object-to-image distance while keeping the source-to-image distance fixed, this ratio grows, so the blur increases. That means the image becomes less sharp (lower spatial resolution) and the penumbra around edges becomes larger. Other options either would reduce blur (increasing SID) or mainly affect image contrast and dose rather than the geometry of blur (tube voltage), though increasing focal spot size would also increase blur and penumbra in general.