The following tables illustrate the main factors and how these affect radiographic definition and contrast.
| Abbreviations: |
ffd |
focus-to-film distance |
| |
ofd |
object-to-film distance |
| |
s or f |
focal spot or source size |
| |
Ug |
definition |
Factors affecting definition
| Factor | Effect | Explanation |
|---|
| Film speed (grain size) |
Slower film (finer grain) = better definition |
Boundaries are defined by grains: smaller dots = sharper boundary |
| Energy (kV, kVp, keV, MeV) |
Lower energy = better definition |
Lower energy = smaller area affected by a photon/electron strike |
| Screens |
Close contact = better definition |
Affected area is enlarged by any gap between screens and emulsion |
| Vibration |
Any vibration reduces definition |
Applicable to any radiograph |
| Geometry of image formation |
Longer ffd, smaller s or f, smaller ofd = better definition |
Interaction of ffd, f or s, ofd in determining Ug = (f x ofd) / (ffd-ofd) |
| Development |
Optimum time is required |
Under-development is uneven, developing only part of a boundary; over-development leads to irregular growth along a boundary of black metallic silver into the less dense area |
Factors affecting contrast
| Factor | Effect | Explanation |
|---|
| Photographic density |
Higher density = higher contrast |
Derived from the slope of the characteristic (sensitometric) curve: minimum density determined from contrast, maximum from adequate transparency |
| Film speed (grain size) |
Slower film (finer grain) = better contrast |
Making grain size smaller greatly slows the film: to reduce the slowing, more silver salt is added to the emulsion giving higher contrast |
| Energy (kV, kVp, keV, MeV) |
Lower energy = better contrast |
Energy affects subject (object) contrast: at lower energies, a given thickness increase (or physical density increase) results in greater absorption |
| Screens |
Lead screens = less scatter |
Scatter has a longer path length through the lead, and scatter is more readily absorbed per mm of path length, than the primary beam |
| Variations in object thickness or physical density |
Large difference in thickness or physical density = higher contrast |
Subject (object) contrast is the difference in the amount of radiation penetrating adjacent areas of the object |
| Development |
Optimum time is required |
Under-development gives low density and unevenness of density; over-development results in the preferential darkening of areas that should be light |
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