Geometrical fallacies.

Below is a review of the main geometrical points made in this paper. 1) Distortion is caused by: a. angling the beam against any spherical anatomy. b. leaving the beam perpendicular to the film when the part is tilted. c. angling the beam perpendicular to the part when it is tilted. 2) Distortion is negligible or absent when: a. angling the beam against a part that is parallel to the film. b. angling the beam one-half of the tilted-part-film angle. c. not angling when the part is parallel to the film. d. tomographing. 3) Magnification is not caused by: a. angling the beam, regardless of tube-to-tabletop distance compensation. b. tomography. 4) To evaluate a radiograph for positioning rotation, tilt, or flexion/extension, the pair of anatomical parts observed should be: a. stable and immovable. b. with their long axes nearly perpendicular to the direction of shift being evaluated. c. as close to the CR or center of the film as possible. d. as far from each other and from the midline as possible. Many of the geometrical factors involved in radiography are more complex than we often appreciate. They are sometimes taught in the form of "blanket" statements or rules that are oversimplified. Nonspecific, vague, or "blanket" questions in this area should be avoided in constructing the registry examinations. Finally, teaching in radiography should be based upon objective, repeatable, and verifiable experimentation rather than on appeal to authority. Such an approach will not only make radiography training more accurate, but also more exciting and fun.