Penumbra decreases as

• A. Focal spot decreases
• B. OID decreases
• C. SID increases
• D. All of the above

Answer: (D)

Penumbra, or the edge blur in radiographic imaging, comes from geometric factors such as focal spot size, object-to-image distance (OID), and source-to-image distance (SID). A smaller focal spot reduces the geometric unsharpness at edges, so the blur is minimized. Decreasing the distance between the object and the image receptor (OID) also lowers the amount of blur magnification, sharpening the edge. Increasing the distance from the source to the image (SID) further reduces the penumbra by spreading the divergent rays over a larger area before they reach the receptor. Because penumbra decreases when any of these changes are made—smaller focal spot, smaller OID, or larger SID—the overall effect is reduced edge blur with all of the above adjustments. All of the above.