Operating Manual

51
50
6
Radiation images, filters and
intensifying screens
To influence the effects of radiation on an image, filters and intensifying screens are used to :
• filter / harden the radiation to influence contrast and/or
• to intensify the effect of radiation to improve contrast
6.1 Radiation images
The intensity of a beam of X-rays or gamma-rays undergoes local attenuation as it passes
through an object, due to absorption and scattering of the radiation. On a uniform object
attenuation of the primary beam will also be uniform and the film evenly exposed. If the
object contains defects or is of variable thickness, the surface of the film will be unevenly
exposed resulting in a shadow image of the object and the defects in it. When the film is
processed the variations in radiation intensity show up as varying film densities; higher
radiation intensity producing higher film density resulting in a negative X-ray image as
shown in figure 1-6.
When the primary beam is partly absorbed in the object, some radiation,
as shown in figure 2-6, will be scattered and reach the film as secondary radiation by an
indirect path. The quality of the radiograph is reduced by this scattered radiation, and it is
important to keep its effects to a minimum.
At any point P on the film, therefore, the total radiation reaching that point is made up of
some transmitted primary radiation forming the image of cavity (N), the “image forming”-
or direct radiation intensity I
p
, and some secondary “ non-image forming” , scattered radi-
ation, intensity I
s
. Hence, the total radiation intensity at P is (I
p
+ I
s
).
The ratio I
s
/I
p
is called the “scattered radiation factor” and can be as high as 10 for great
wall thicknesses, which means that the scattered radiation is ten times higher than the
image-forming radiation. The ratio (I
p
+I
s
)/Ip = 1+I
s
/I
p
is called the “build-up factor” and
is of considerable importance for the detectability of defects. It usually has a value between
2 and 20, depending on radiation energy and object thickness.
It must also be appreciated that any object in the neighbourhood of the object being exam-
ined (table, walls, ground and so on) which is struck by the gamma- or X-rays will partial-
ly reflect these rays in the form of “backscatter” which is liable to fog the film.
Backscatter coming from the object under examination is less hard than the primary radi-
ation that has caused it and can be intercepted by a metal filter between object and film.
Radiation scattered by objects nearby the film can be intercepted by means of a protective
sheet of lead at the rear face of the film cassette.
defect
object
object
film
A
A
C
N
D
E
B
F
P
source (S)
primary radiation
Fig. 1-6. The negative X-ray image
film
negative
(shadow) image
on the film
radiation intensity
after passing
through the object
Fig. 2-6. Image forming and non-image
forming radiation
Only the radiation from source (S) that
reaches the film in straight lines via beam
section DE, produces an image of cavity
N at P. The remainder, not reaching P
directly, is scattered radiation, no defect
image forming thus reducing the image
quality.