An image is acquired with a 2 mm focal spot, 5 cm object-to-image distance (OID), and 102 cm source-to-image distance (SID). What is the resulting focal spot blur?

• A. 0.05mm
• B. 0.106mm
• C. 38mm
• D. 40mm

Answer: (B)

Geometric unsharpness comes from the finite size of the focal spot and how that size projects onto the image through the imaging geometry. The blur on the image, called focal spot blur, is given by U = F × (OID / SOD), where F is the focal spot size and SOD is the source-to-object distance (SID minus OID).

Here, F = 2 mm, OID = 5 cm, SID = 102 cm, so SOD = SID − OID = 97 cm. Convert to consistent units: U = 2 mm × (5 cm / 97 cm) = 2 × 0.05155 ≈ 0.103 mm. That’s about 0.10 mm, which is very close to 0.106 mm, the value shown among the choices.

So the focal spot blur is roughly 0.10 mm. To reduce blur, you’d want a smaller OID or a larger SID (which increases SOD).