An external calibration marker is a radiopaque object with a known size placed within the radiographic field to aid in orthopedic templating. It is important that they are not mistaken for a different type of foreign body.
Clinical use
The calibration marker, often a ball bearing or washer, has a known size and is used in templating software when planning orthopedic procedures such as total hip arthroplasty 1.
Manufacturers provide specific instructions regarding the placement of these markers. Some involve a series of radiopaque lines (or, in some cases, a ruler) placed on the digital detector to ensure consistency throughout examinations.
Practical points
The placement of the external calibration marker should be as close to the joint of interest as possible. Small deviations in technique can lead to inaccurate measurements, which, in turn, can result in the incorrect selection of equipment for procedures.
There is little education regarding the effects of source-to-image distance, object-to-image distance, and beam divergence on templating.
For the most accurate measurements, the calibration marker should be placed at the center of the image (unless otherwise specified by the manufacturer), with a consistent source-to-image distance maintained between exposures. An example of this would be placing the marker between the patient's legs 2.
The placement of the marker between the patients legs can cause some confusion and may be mistaken for a foreign body if the reader is not aware.
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