A patient has a nail protruding from the plantar surface of the foot, creating an unavoidable large OID on an AP foot radiograph. To minimize magnification, what should the technologist do?

A patient has a nail protruding from the plantar surface of the foot, creating an unavoidable large OID on an AP foot radiograph. To minimize magnification, what should the technologist do?

• A. remove the nail so the OID can be reduced
• B. use a shorter source-to-object distance (SOD)
• C. reduce the usual 40-inch (102 cm) SID
• D. increase the SID beyond the standard 40 inches (102 cm) to reduce the magnification

Answer: (D)

Magnification on a radiograph is governed by how far the image receptor is from the object relative to how far the X-ray source is from the object. When an object sits farther from the receptor (a large object-to-image distance), the image of that object tends to be larger on the film. If the object-to-image distance is already large due to the nail protruding from the plantar surface, the way to reduce magnification is to increase the distance from the X-ray source to the image receptor (maximize SID). By moving the receptor farther away while the object remains at the same distance from the source, the projection size on the image becomes closer to the true size. So, increasing the SID beyond the standard 40 inches reduces magnification in this situation, even though the OID cannot be reduced. Removing the nail is not a practical option for imaging purposes, and decreasing the SID or shortening the SOD would actually increase magnification rather than decrease it.

Magnification on a radiograph is governed by how far the image receptor is from the object relative to how far the X-ray source is from the object. When an object sits farther from the receptor (a large object-to-image distance), the image of that object tends to be larger on the film. If the object-to-image distance is already large due to the nail protruding from the plantar surface, the way to reduce magnification is to increase the distance from the X-ray source to the image receptor (maximize SID). By moving the receptor farther away while the object remains at the same distance from the source, the projection size on the image becomes closer to the true size.

So, increasing the SID beyond the standard 40 inches reduces magnification in this situation, even though the OID cannot be reduced. Removing the nail is not a practical option for imaging purposes, and decreasing the SID or shortening the SOD would actually increase magnification rather than decrease it.