Artefacts.
Pressure marks (crimp marks).
Produced by careless film handling – if the film is crimped or buckled either
before or after exposure crescent-shaped images in the processed radiograph
will result. Light marks indicate crimping before exposure, dark marks crimping
after exposure but before film processing.
It is usually possible to identify crimp marks by viewing the film in reflected
light. They should appear as indentations in the surface of the film. Lead
screens which have been crimped should be discarded.
Scratches: On the film
Radiographic film emulsion is delicate; it is easily damaged if handled carelessly
at any stage. Areas used for film handling must be free from dust and films
must be handled carefully at all times. Depending upon how severe and when
or how formed film scratches may produce either light or dark images, and can
usually be identified using reflected light.
Scratches: On lead intensifying screens
May appear as either light or dark images which cannot be seen in reflected
light. If the intensifying screens used to make the radiograph can be positively
identified then it may be possible to trace the shape and position of such an
image to a scratch on the screens. Even so it will probably be necessary to
reshoot the radiograph. Scratched lead screens should be discarded.
Dirt: On the film or screens
Dirt which finds its way between the film and the screens will generally produce
a light image on the resultant radiograph not visible in reflected light. Greasy
fingers will produce dark marks on a finished radiograph which can easily be
seen in reflected light, but light marks before development.
Streakiness or mottling: Poor development
Usually caused by insufficient agitation in the early stages of development and
is due to a process known as bromide streaming. Reaction products from the
chemical interaction of the developer with the silver halides in the film emulsion
tend to build up around high film density zones. These reaction products slow
down the action of the developer, and since they are relatively heavy they tend
to flow down the surface of the film leading to a light coloured streak in the
finished radiograph. Under- or over-development usually leads to a mottled
effect on the finished radiograph. A similar effect will be produced by developer
which has passed its service life. In less severe cases such artefacts may not be
a cause for rejection of the radiograph but darkroom procedures should be
reviewed to prevent a recurrence or a further deterioration in radiographic
quality.
Developer splashes
Appear as dark spots on the film and indicate poor dark room practice and are
usually visible in reflected light.
Fixer splashes
Appear as light spots on the film and again indicate poor dark room practice.
Such marks are usually visible in reflected light.
Water splashes
Appear as either light or dark images on a radiograph. Water splashes before
exposure tend to cause light marks, after exposure tend to cause dark marks.
Such marks are usually visible in reflected light.
Water marks
Easily seen on the radiograph in both transmitted and reflected light and are
due to uneven drying. They commonly occur where a dry or partially dry film is
wetted locally either by splashing or by excess water running down from a film
clip. The appearance of water marks can be reduced or eliminated by using a
squeegee to remove excess water or a final wash that contains a small amount
of detergent (ie a wetting agent).
Air bells
Light marks caused by air bubbles adhering to the film in the early stages of
development and will not occur if the film is properly agitated.
Diffraction mottling
Can be a problem when X-rays are used to radiograph large grained material,
for example being austenitic steels. Diffraction is an apparent bending of a
beam of radiation due to interference. It occurs when radiation passes through
a grating that has spacing approximately equal to one wavelength. The spacing
of atoms in a metallic crystal is about 0.1nanometres. This corresponds to X-ray
radiation with photon energy in the region of 10keV. If low energy components
are removed from the X-ray beam by filtration the problem with diffraction
mottling will disappear. Diffraction mottling does not occur in gamma
radiography because of the absence of low energy beam components.
Diffraction can be used to advantage and it is the basis for the study of metal
crystals by X-ray crystallography.
Static marks
Penetrating radiation is by definition ionising. It always causes the build-up of
an electric charge on the film during exposure but under normal circumstances
this is not a problem because the charge quickly flows to earth. In dry climates,
however, a static charge may remain on the film until it is unloaded in the
darkroom, whereupon it flows to earth suddenly in a manner which could be
painful for the radiographer. Such a sudden dissipation of electrical energy
leads to the emission of a sudden burst of light which produces dark tree-like
marks on the finished radiograph. Static marks can be avoided by careful film
handling.
Dichroic fogging
Radiographs affected by dichroic fog will appear reddish when viewed using
transmitted light and greenish in reflected light. Dichroic means two-coloured.
It is caused when the development process continues during the fixing process,
when the fixer solution has become insufficiently acidic to stop the development
process. The use of an acidic stop bath between the development and fixing
processes will generally prevent the occurrence of this seldom seen artefact.
Reticulation
Appears on the radiograph as an orange peel-like mottling effect. It is caused
when the film emulsion is subjected to a temperature shock at any stage during
the film processing as the sudden change in temperature causes the film
emulsion to wrinkle. It will not generally occur as long as the sudden change in
temperature is less than 10°C.
Film fogging by X- or gamma rays
If radiographic film is not stored well away from sources of ionising radiation
then it is likely to become fogged. Films which have been fogged in this way will
produce reduced radiographic contrast (fogging has much the same effect as
scattered radiation which is explained in a section above). If it is suspected that
the film is fogged then the level can be checked by processing a piece of
unexposed film. Film which has a density due to fogging of 0.3 or more is not
suitable for use in high quality industrial radiography.
Light fogging
Exposure to light other than from darkroom safe lamps (and even prolonged
exposure to safe lamps) will cause fogging at any stage prior to fixing the film.
Such fogging may be localised or general – localised is not a problem unless it
encroaches onto the diagnostic film area but general light has the same effect
as fogging due to exposure to ionising radiation.
Film fogging due to inadequate storage conditions
Film stored at too high a temperature or exposed to chemical fumes may
become fogged. The fog level of all film increases with age, even under ideal
storage conditions, therefore all film boxes are marked with an expiry date.
High speed films deteriorate more quickly than slower ones.
Solarisation
Image reversal due to extreme over exposure to X or gamma rays or caused by
exposure to light during film development.
A final word on artefacts
It should be stressed again that artefacts are cause for rejecting the film only if
they interfere with interpretation. A large number of artefacts present on
radiographs indicate poor practice and the interpreter should take time to
inspect the radiographic facilities and review darkroom procedures.
