Fluke Biomedical 07-553 - Manual (Page 7)

General Information
Magnification Geometric Effect Determination
1
1-3
1.3 Magnification Geometric Effect Determination
1. Place the test plate in the position of the part to be radiographed. Align the test plate so that it is at
right angles to the anode-cathode direction of the tube.
2. To eliminate film-screen effects, a non-screen film should be used in the normal film location.
3. Place the x-ray tube in the position normally used for magnification and center it over the test plate.
Use the kV settings appropriate to the clinical study. Adjust the mAs to give a gross optical density
of about 1.5 on a portion of the film not covered by the test plate.
4. Determine the limiting resolution by inspecting the finished radiograph with a magnifying glass. It is
very important to scan in the direction of increasing resolution and to define the limiting resolution
as the lowest frequency at which the contrast becomes zero.
5. Repeat with the test plate parallel to the anode-cathode axis of the x-ray tube. It is not unusual for
these two measurements to disagree. This effect is due to the asymmetric nature of the focal spot.
Figure 1-3 is a study of the resolution, under magnified conditions, of a large (1.3 mm) and a small (0.3
mm) focal spot. The technical factors are: Magnification – 3.7, 60 kVp, 3-phase; type RP/M film, 100 cm
FFD; 100 mA; small focus; 100 mAs; large focus; 80 mAs; test plate perpendicular to the tube.
Figure 1-3
The limiting resolution is seen to be 3.5 line pairs per millimeter for the small focal spot and 0.6 line pairs
per millimeter for the large focal spot.