The object and its influence on image characteristics

Tissue contrast and total contrast

After X-rays have gone through the object and lead grid the quality of radiation (X -ray distribution in Fig. 2) cannot be further influenced. Local variations in the intensity of radiation, in other words tissue contrast must be conferred as effectively as possible to the image.

Tissue contrast is visualised in a radiograph when two (nearby) areas can be separated from each other due to their different optical densities or darkness of film. Reasons for this can be 1) that tissues with equal thickness have different coefficients of attenuation, or, 2) that their thickness is not the same (Figs. 9 and 10).

Tissue contrast is therefore a property of an object, and it depends on the inner structure and tissue or elemental composition of the object. The visualisation of tissue contrast depends in addition on kVp and radiation spectrum as well as on the amount of scattered radiation. Fig. 9 shows how attenuation of tissues, based on attenuation of elements, changes as a function of energy.

Tissue contrast is caused mainly by differences in the coefficients of tissue attenuation, which depend on energy. An X-ray image is produced by an X-ray spectrum, in other words by a huge number of polychromatic quanta with different energies (Fig. 5). The probability of each photon ending up in a photoelectric absorption or Compton scattering depends on its energy. Photons with medium energy are most abundant in a spectrum, but a radiograph is the result of all interactions of photons in tissues. The low-energy photons cause the biggest contrast, but their ability to penetrate an object is the lowest.

With radiation detectors such as a screen-film combination (radiography) or image intensifier (fluoroscopy) or with a digital subtraction device (DSA) it is possible to increase total contrast (which is tissue contrast amplified by detector contrast). The amplification factor for film (gamma value) is usually between 2 and 3. An image intensifier does not generally reinforce contrast, but with DSA one can very substantially emphasize contrast between soft tissues and blood when a contrast media bolus is used (e.g. in angiography).

When the current (mA) of an X-ray tube, exposure time (s) or their product (mAs) is increased, the spectrum also changes, its total area or intensity increases in the same proportion. The image becomes darker, as blackness or optical density on the film increases. On the other hand, when voltage (kVp) is increased, this also increases photon energy and the radiation beam becomes more penetrating (resulting in a smaller dose), but contrast is reduced.

In an X -ray image or radiograph the shadows of bones are demonstrated white or light, because bone efficiently stops radiation quanta, especially at low X -ray energies. Soft tissues are seen in grey tones and organs containing gas in dark tones. In digital image manipulation this grey shade scale is easy to turn upside down. In DSA strongly absorbing objects, like veins filled with contrast media are normally displayed black, etc.

Photographically speaking, an X-ray film is a negative. Normal X-ray imaging without contrast media (plain radiography) is suitable for the examination of bones and organs containing gas (like the lungs), but soft tissues cannot be separated from one another. Liver and kidney for instance, as well as brain and cerebrospinal fluid are equally grey in a radiograph. For the visualization of soft tissues contrast media and/or digital methods with a computer must be used.

Influence of scattering

After Compton scattering the photon continues with reduced energy in a new direction (see Interactions of X -ray and gamma quantum with matter). All scattering angles have nearly the same probability, but at higher energies scattering in small, forward directed angles is more probable. This is regrettable, because film is positioned in the direction of the primary photons and these small angle scattered photons. Primary photons make the image, but scattered photons only reduce contrast.

Scattered radiation is present in all X-ray and nuclear medicine imaging. Its influence is smallest in thin objects imaged with small field size and at low energy. When examining large and thick objects (body) the number of scattered photons in the exit field, in other words at the film, can be 5 or even 10 times bigger than the number of primary photons.

The following ways are efficient in diminishing the adverse influence of scattered radiation in X-ray examinations:
1. Keep the field size as small as possible. In other words, collimation of radiation, e.g. with a blade-type diaphragm must be used.
2. Use a grid against scattered radiation.
3. The space between an object and the film can be used to reduce scatter (so-called air gap technique).
4. The object can be compressed
5. Low voltages reduce scattering (but this is against the main principle of radiation protection as it increases the patient dose).

A lead grid allows primary photons from the focus to go through to the film like a Venetian blind allows light to go through. The grid consists of thin non transparent lead lamellae placed side by side with transparent aluminium or carbon fibre lamellae. It lets only merely parallel or almost parallel photons pass through (Fig. 2). The relation between the height of a lamella (a few mm) and the distance from a non-transparent lamella to the next one (0.1-0.5 mm) is called the grid ratio. It is generally between 5 and 15. Both parallel and focused grids are in use. The

Figure 11. Distortions arise in an image due to imaging geometry and thecharacteristics of an object.

grid can also be set in motion during the exposure so that the lamellae can not be seen in the image.

Imaging geometry

In conventional X-ray examinations tissues can be divided into four main groups: skeletal structures (seen as white or in light tones in the image), soft tissues (grey), fat (somewhat darker than soft tissues) and gas (dark). The basic X-ray examination is well suited to skeleton and thorax examinations, because the boundaries between tissues (with the exception of soft tissues and fat) can clearly be seen. To separate muscles, inner soft tissue organs etc. from one another, contrast media or newer examination methods like CT or magnetic resonance imaging must be used.

Electromagnetic radiation travels in straight lines. Without scattering, the understanding of the formation of an X-ray image would demand only appreciation of laws of geometry, in the same manner as articles between a light source and a screen cause shadows. Fig. 11 shows the geometrical enlargement in exaggeration, as well as different distorted shadows of object details on a film surface.

There is always enlargement in a radiograph. It is biggest on the edges of an image and from those objects which are most distant from the film surface. Enlargement is smallest in the middle of the image field and from objects nearest to the film surface. A shadow on an image is caused by a real object, i.e. a lesion in tissue with different absorption properties to its surroundings, or it can be a sum of shadows of two or more objects on each other in the direction of the radiation beam. It can happen that a small or rather poorly absorbing object lying behind a bigger, more strongly absorbing object, can not be distinguished at all (for instance a small tumour lesion behind a rib).

The understanding of geometrical facts is of primary importance for an image interpreter and user of an X-ray device. Long exposure time when an object moves causes, for instance, unsharp images of sharp bone edges. Short exposure times are always recommended; they require large amounts of current and demand a big load from the X-ray tube.

An important factor is the shape of an object. Many organs are cylinders or they have curved surfaces. More radiation is absorbed in the middle of spherical objects and therefore a radiograph tends to be darker at its edges. This absorption unsharpness (for instance in a lung tumour) can be seen in an image.

Aaro Kiuru