If SID is 100 inches and OID is 20 inches, what is the source-to-object distance (SOD)?

If SID is 100 inches and OID is 20 inches, what is the source-to-object distance (SOD)?

• A. 120 inches
• B. 20 inches
• C. 80 inches
• D. 0 inches

Answer: (C)

SOD is the distance from the X-ray source to the object, and it is found by subtracting the object-to-detector distance (OID) from the source-to-image distance (SID). So SOD = SID − OID. With SID = 100 inches and OID = 20 inches, the SOD is 80 inches. This also gives a magnification factor M = SID / SOD = 100 / 80 = 1.25 (25% magnification). The other numbers don’t fit because 120 would be adding OID to SID, 20 would be the OID itself, and 0 would imply no separation from the source to the object, which isn’t possible with the given distances. The correct distance is 80 inches.

SOD is the distance from the X-ray source to the object, and it is found by subtracting the object-to-detector distance (OID) from the source-to-image distance (SID). So SOD = SID − OID. With SID = 100 inches and OID = 20 inches, the SOD is 80 inches. This also gives a magnification factor M = SID / SOD = 100 / 80 = 1.25 (25% magnification). The other numbers don’t fit because 120 would be adding OID to SID, 20 would be the OID itself, and 0 would imply no separation from the source to the object, which isn’t possible with the given distances. The correct distance is 80 inches.