Operating Manual

177176
The previous chapter (16) dealt with techniques that would be impossible without the aid
of computers. These techniques share a common feature, whereby the processing, interpre-
tation and storage of data is done by a central computer and monitor, also called the work
station. In the current chapter (17) computers also play an everincreasing important role in
some of the techniques discussed. Computer tomography (CT) and the Compton backscatter
technique for example would not exist without them.
17.1 Image magnification techniques
17.1.1 Common image magnification technique
By positioning an object between an X-ray tube and film or detector, as illustrated in
figure 1-17, a magnified image is obtained. As a consequence any defect will be magnified
as well. The sharpness of the image is dependent of focal spot size, the smaller the spot size
the better the sharpness.
Any unsharpness, as illustrated in figure 1-17, is determined by the relationship between F
1
and F
2
, and the size of U
f
.
The effective unsharpness is calculated as follows: U
g
= U
f
(F
2
- F
1
) / F
1
For example: An X-ray tube with a focal spot size of 20 microns with focus-to-object
distance (F
1
) of 50 mm and focus-to-film distance (F
2
) of 550 mm will have a
geometric unsharpness of: 0.02 (550 - 50) : 50 = 0.20 mm
The magnification factor is: F
2
: F
1
= 550 : 50 = 11
The magnification technique is mainly used in combination with a radiation-sensitive
device such as fluorescent screen, image intensifier or flat panel detector. A computer
workstation may be used for image processing and/or enhancement prior to interpretation
on the screen.
17
Special
radiographic techniques
focal spot
film,
detector,
image intensifier
object
density
brightness
geometric
unsharp-
ness
Fig. 1-17. Image magnification