Which equation correctly expresses the source-to-object distance (SOD) in terms of SID and OID?

Which equation correctly expresses the source-to-object distance (SOD) in terms of SID and OID?

• A. SOD = SID + OID
• B. SOD = SID - OID
• C. SOD = OID - SID
• D. SOD = SID * OID

Answer: (B)

In radiographic geometry, the distances along the beam path add up along the line from source to receptor. The distance from the source to the receptor (SID) is made up of the distance from the source to the object (SOD) plus the distance from the object to the receptor (OID). So SID = SOD + OID. Solve for the source-to-object distance: SOD = SID − OID. For example, if SID is 100 cm and OID is 10 cm, SOD is 90 cm. The other forms would violate the actual path length or mix units, so they don’t fit the geometry.

In radiographic geometry, the distances along the beam path add up along the line from source to receptor. The distance from the source to the receptor (SID) is made up of the distance from the source to the object (SOD) plus the distance from the object to the receptor (OID). So SID = SOD + OID. Solve for the source-to-object distance: SOD = SID − OID. For example, if SID is 100 cm and OID is 10 cm, SOD is 90 cm. The other forms would violate the actual path length or mix units, so they don’t fit the geometry.