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This definition is less useful in terms of estimating geometric magnification than a measurement to a defined point within the Imaging Subject, but accounts for what is realistically measurable in an automated fashion in a clinical setting.

This measurement does not take into account any air gap between the Imaging Subject and the "front" of the table or bucky.

If the detector is not mounted in a table or bucky, then the actual position relative to the patient is implementation or operator defined.

This value is traditionally referred to as Source Object Distance (SOD).