c. The major factors in the application of this principle are:
Position of the source-point of radiation and focal-spot size.
Alignment of the CR.
Point on object under consideration.
Alignment of divergent rays in relation to CR.
d. Neither SID nor the OFD exert any significant influence on the outcome of the
end result insofar as a specific dimension is concerned. Divergent rays falling in a
plane in line with the CR and at right angles to the long axis of the object have the same
dimensional validity as the CR. However, it is a good rule to keep the OFD (object film
distance) to a practical minimum and the SID to a feasible maximum in order to obviate
the penumbra effects due to focal-spot size and magnified distortion inherent in short
a. General. The principles of orthoradiography may be incorporated in
numerous methods, depending upon the nature of the problem and the facilities
available. The method most generally used to spot-scanography. The following
methods are rarely used today, having been replaced by other modalities. They can still
be encountered in some facilities.
b. Spot-Scanography. In spot-scanography, two "spot" exposures are made at
specific points on the object. The film-object plan is parallel and the CR is
perpendicular to the film plane. Film identification is "burned-in" after completion of the
spot exposures. If the object under consideration is of such length that is overrides the
film, the film may be placed diagonally with the long axis of the object. Measurements
are made after the film is processed.
c. Spot-Scanography in Combination with Calibrated Rule. The method is
identical to that described above, with the exception that a radiopaque calibrated rule is
placed on the film alongside the object. This rule may consist of a strip of lead 3/4 inch
wide and of sufficient length. One edge of the strip should be notched at 1-cm intervals.
The lead strip should be mounted on a flat piece of wood or aluminum, to provide
rigidity and ensure the maintenance of accurate length. Commercially produced rulers
are available for this purpose. These measure 100 cm long, and are calibrated in 1-cm
increments. Calibration markings and numbers are painted with lead-impregnated paint
to provide visibility on the radiography. By using two separate films, this method