Operating Manual

1716
• By means of radiation detectors, e.g.: crystals, photodiodes or semiconductors in a
linear array by which in a series of measurements an image is built up of a moving
object. This method is applied in systems for luggage checks on airports.
The source of radiation should be physically small (a few millimetres in diameter), and as
X-rays travel in straight lines from the source through the specimen to the film, a sharp
“image” is formed of the specimen and discontinuities. This geometric image formation is
identical to the shadow image with a visible light source. The sharpness of the image
depends, in the same way, on the radiation source diameter and its distance away from
the surface on which the image is formed.
The “classic” film in its light-tight cassette (plastic or paper) is usually placed close behind
the specimen and the X-rays are switched on for an appropriate time (the exposure time)
after which the film is taken away and processed photographically, i.e. developed, fixed,
washed and dried. In direct radiography (DR), a coherent image is formed directly by
means of an computerised developing station.
The two methods have a negative image in common. Areas where less material (less
absorption) allows more X-rays to be transmitted to the film or detector will cause incre-
ased density. Although there is a difference how the images are formed, the interpreta-
tion of the images can be done in exactly the same way. As a result, the DR- technique is
readily accepted.
The “classic” film can be viewed after photochemical treatment (wet process) on a film
viewing screen. Defects or irregularities in the object cause variations in film density
(brightness or transparency). The parts of the films which have received more radiation
during exposure – the regions under cavities, for example – appear darker, that is, the film
density is higher. Digital radiography gives the same shades of black and white images,
but viewing and interpretation is done on a computer screen (VDU).
The quality of the image on the film can be assessed by three factors, namely :
1. Contrast
2. Sharpness
3. Graininess
As an example, consider a specimen having a series of grooves of different depths machi-
ned in the surface. The density difference between the image of a groove and the back-
ground density on the radiograph is called the image contrast. A certain minimum image
contrast is required for the groove to become discernible.
With increased contrast:
a. the image of a groove becomes more easily visible
b. the image of shallower grooves will gradually also become discernible
Assuming the grooves have sharp-machined edges, the images of the grooves could
still be either sharp or blurred; this is the second factor: image blurring, called image
unsharpness.
At the limits of image detection it can be shown that contrast and unsharpness are inter-
related and detectability depends on both factors.
As an image on a photographic film is made up of grains of silver, it has a grainy appea-
rance, dependent on the size and distribution of these silver particles. This granular appe-
arance of the image, called film graininess, can also mask fine details in the image.
Similarly, in all other image forming systems these three factors are fundamental para-
meters. In electronic image formation, e.g. digital radiography or scanning systems with
CCTV and screens, the factors contrast, sharpness and noise are a measure for the image
quality; pixel size and noise being the (electronic) equivalent of graininess .
The three factors: contrast, sharpness and graininess or noise are the fundamental para-
meters that determine the radiographic image quality. Much of the technique in making
a satisfactory radiograph is related to them and they have an effect on the detectability of
defects in a specimen.
The ability of a radiograph to show detail in the image is called “radiographic sensitivity”.
If very small defects can be shown, the radiographic image is said to have a high (good)
sensitivity. Usually this sensitivity is measured with artificial “defects” such as wires or
drilled holes. These image quality indicators (IQIs) are described in chapter 13.