Image Production · Geometry and detail2 min read5-question check
Magnification Factor and OID
MF = SID ÷ SOD. Divide the image size by MF to find true object size; keep OID small and SID long.
The rule
Every radiographic image is at least slightly larger than the part itself. The magnification factor, or MF, tells you how much larger. MF equals SID divided by SOD, where SOD, the source-to-object distance, is SID minus OID. An MF of 1.0 would mean no magnification at all, which would require zero OID. Even a part resting on the detector has some depth, so MF is always slightly above 1.
Magnification Factor
Why it happens
X-rays spread out from a small focal spot in a diverging beam. The farther the part sits from the detector, the more the beam spreads after passing through it, so the shadow grows. Moving the tube farther away makes the rays that pass the part more nearly parallel, so the shadow grows less. That is why increasing OID raises magnification and increasing SID lowers it.
The numbers
Worked example: SID is 40 inches, about 102 cm, and OID is 4 inches. SOD is 40 minus 4, or 36 inches. MF is 40 divided by 36, or 1.11, so the image is 11% larger than the part. If a vessel measures 10 cm on the image, its true size is 10 divided by 1.11, or 9 cm. Now use 72 inches, about 183 cm, with the same 4-inch OID: SOD is 68, MF is 1.06, and magnification drops to 6%.
On the job
Place the part as close to the detector as the exam allows. When OID cannot be avoided, as with a patient on a stretcher or a lateral hip, a longer SID offsets the loss. PA chest at 72 inches keeps the heart close to its true size. Measurement work, such as leg length or implant templating, often uses a calibration marker of known size.
Clinical Example
A cross-table lateral knee is done with the detector 3 inches from the joint because of a splint. The technologist increases SID from 40 to 48 inches to limit magnification and keep the edges sharp. The surgeon's measurements are then closer to true size.
Common Mistakes
- Dividing SID by OID instead of by SOD.
- Multiplying the image size by MF when solving for object size instead of dividing.
- Forgetting that a longer SID needs more mAs to keep receptor exposure the same.
- Calling magnification shape distortion; it changes size only.
Why It Matters
Magnification blurs edges and misstates size, which matters for measurements and surgical planning. The registry regularly asks you to calculate MF or true object size.
Key Relationships
- MF = SID ÷ SOD, and SOD = SID − OID
- Object size = image size ÷ MF
- Increase OID → more magnification
- Increase SID → less magnification