Density and contrast
Density
The various uses of the term ‘density’ were discussed briefly in the previous section. When considering the radiographic image, the term ‘density’, as stated previously, can be defined crudely as the degree of ‘blackening’ within the image. The greater the amount of radiation that is incident upon the image detector,
the greater will be the density within the image.
The general term ‘density’ can be defined more accurately when
The general term ‘density’ can be defined more accurately when
the type of image receptor is considered:
Photographic film
If the image is captured on a photographic emulsion, then the term ‘photographic density’ or ‘optical density’ should be used. Higher densities will be produced by greater exposures of radiation, which in turn leads to a form of silver being liberated from the photographic emulsion. This remains on the film after processing and produces the ‘blackening’ within the image. Photographic or optical density can be measured by determining the degree of opacity, i.e. the proportion of light absorbed by the processed film.
If the image was captured by a digital system such as computed radiography (CR) or direct radiography (DR), then the term ‘image density’ refers to the greyscale displayed on the monitor
used to display the image. Put simply, it is the computer screen brightness.
The image-processing software will analyse the range of exposures that were captured by the image receptor (e.g. the CR phosphor screen). It will then assign the highest computer
screen brightness to areas that have received relatively low exposures (low image density). Conversely, the lowest computer screen brightness (darkest areas) will be assigned to areas that have received a relatively high radiation exposure (high image density).
In order to detect pathology, an imaging system must be able to detect the differences in the density (patient density) of the pathology compared with that of the surrounding tissues. This must then be translated into differences in density within the final image (image or film density) that are visible to the observer. Contrast is the difference in density between structures of interest within the image. A low-contrast image will show little difference in density between structures of interest, whereas a high-contrast image will show a larger difference in density between structures.
The contrast seen on a radiograph is built up in three main stages:
• Subject contrast is a feature of the object (subject) under examination. The differences in radiation intensities emerging from the object result from the spatial distribution of linear attenuation coefficients within the object. At a given beam energy, the degree of beam attenuation between anatomical structures is
determined by the physical density and atomic number of those structures. Subject contrast will change if the beam energy (kVp) is varied or via the use of a contrast agent, which will change atomic number within an area of the object.
• Radiographic contrast is the difference in optical density on different parts of the processed film or differences in computer screen brightness recorded as a result of the range of emergent beam intensities.
• Subjective contrast is the personal appreciation of the differences in optical density or computer screen brightness when the image is viewed.
Some of the factors that influence each of the above will now be considered.
X-Radiation passing through the body is attenuated by different amounts by the different thicknesses, densities and atomic numbers of the structures in the body. The beam emerging from
the patient varies in intensity: more will emerge if the beam encounters only a small thickness of soft tissue. The difference in intensities in the emergent beam is called subject contrast or radiation contrast.
Factors that influence subject contrast include the following:
• The region of the body under examination: there is less subject contrast if all parts of the region have a similar linear attenuation coefficient. Soft-tissue structures such as the breast have a low subject contrast, whereas the subject contrast increases if the region includes bone or large differences in the thickness of tissue. A good example of an area of the body that demonstrates high subject contrast is the body and
spinous process of a lumbar vertebra on a lateral projection of the spine and the lateral cervicothoracic junction.
• Contrast media: if high or low density/atomic number substances are introduced into cavities in a region, then there will be a greater difference in absorption of X-rays by different parts of that region and thus an increase in subject contrast.
• Pathology: if the density of a structure is changed due to pathology, then there will be a change in subject contrast; for instance, it will be reduced if the bone density reduces, as in osteoporosis.
• the observer: visual perception, fatigue, etc.;
• viewing conditions: e.g. ambient lighting.
 | ||
| Radiograph produced on film, showing three different densities. The highest density is on the right of the image |
Digital image capture
used to display the image. Put simply, it is the computer screen brightness.
The image-processing software will analyse the range of exposures that were captured by the image receptor (e.g. the CR phosphor screen). It will then assign the highest computer
screen brightness to areas that have received relatively low exposures (low image density). Conversely, the lowest computer screen brightness (darkest areas) will be assigned to areas that have received a relatively high radiation exposure (high image density).
Contrast
The contrast seen on a radiograph is built up in three main stages:
• Subject contrast is a feature of the object (subject) under examination. The differences in radiation intensities emerging from the object result from the spatial distribution of linear attenuation coefficients within the object. At a given beam energy, the degree of beam attenuation between anatomical structures is
determined by the physical density and atomic number of those structures. Subject contrast will change if the beam energy (kVp) is varied or via the use of a contrast agent, which will change atomic number within an area of the object.
• Radiographic contrast is the difference in optical density on different parts of the processed film or differences in computer screen brightness recorded as a result of the range of emergent beam intensities.
• Subjective contrast is the personal appreciation of the differences in optical density or computer screen brightness when the image is viewed.
Some of the factors that influence each of the above will now be considered.
Subject contrast
the patient varies in intensity: more will emerge if the beam encounters only a small thickness of soft tissue. The difference in intensities in the emergent beam is called subject contrast or radiation contrast.
Factors that influence subject contrast include the following:
• The region of the body under examination: there is less subject contrast if all parts of the region have a similar linear attenuation coefficient. Soft-tissue structures such as the breast have a low subject contrast, whereas the subject contrast increases if the region includes bone or large differences in the thickness of tissue. A good example of an area of the body that demonstrates high subject contrast is the body and
spinous process of a lumbar vertebra on a lateral projection of the spine and the lateral cervicothoracic junction.
• Contrast media: if high or low density/atomic number substances are introduced into cavities in a region, then there will be a greater difference in absorption of X-rays by different parts of that region and thus an increase in subject contrast.
• Pathology: if the density of a structure is changed due to pathology, then there will be a change in subject contrast; for instance, it will be reduced if the bone density reduces, as in osteoporosis.
|
Subjective contrast
When a radiograph is viewed, the observer sees an image made up of different densities or brightnesses. However, different observers might have a different appreciation of the image contrast. The personal appreciation of the contrast in the image is called subjective contrast. Subjective contrast depends not only on the person but also on the viewing conditions. For example, if an image is viewed on a computer monitor and that monitor is placed near a window, then the sunlight incident upon the screen will severely impair the observer’s ability to appreciate the density differences within the image. There may be good radiographic contrast but the observer cannot appreciate this because of the sunlight on the screen, so the subjective contrast will be low.
Subjective contrast depends on:• the observer: visual perception, fatigue, etc.;
• viewing conditions: e.g. ambient lighting.
Radiographic contrast
After leaving the patient, the X-radiation passes to an image-capture device. As it passes through the body, some of the radiation will be scattered. Scatter reduces the differences in X-ray intensity emerging from different areas of the body and thus reduces contrast. The production of scattered radiation can be reduced by collimating the beam or by the use of compression devices. In each of these cases, this reduces the volume of tissue irradiated. In a large proportion of examinations, a secondary radiation grid is placed between the patient and the image-capture device to intercept a large proportion of the scattered radiation, which, if it were to reach the image detector, would reduce image contrast. Once the image has been captured, it can be viewed either on photographic film or by some electronic means such as a computer monitor. The different patient densities are recorded either as varying photographic densities or as
differences in computer screen brightness. These different densities can be measured either using a densitometer or image-analysis software to give an objective measurement of contrast. Thus, differences in measured image density between specified parts of the radiographic image are known as radiographic or objective contrast.
Radiographic (objective contrast) depends upon the following:
• Subject contrast.
• Scattered radiation reaching the image receptor: the use of a secondary radiation grid between the patient and the cassette to reduce the scatter reaching image receptor improves radiographic contrast. Lead-backed cassettes or lead rubber under cassettes may reduce back-scatter, which may also improve radiographic contrast. If the cassette is some distance away from the patient, then scatter crossing the intervening
gap might not reach the image receptor.
• Image-acquisition device: the design and function of the device used to acquire the image can have a profound effect on contrast. For example, certain types of film emulsion, intensifying screen and phosphor plate may be designed to
give inherently greater contrast. In digital systems, the contrast is also influenced profoundly by the software used to process the initial image captured by the device.
• Film fog: if the image is viewed using a photographic-based system, then film fogging due to incorrect film handling or storage may reduce radiographic contrast.
• Exposure: if too much or too little radiation is used, then the image-acquisition device may be unable to respond or may be saturated to the point that it is unable to function properly. In these examples, there may be a reduced range of densities or no difference in density visible on the image, thus radiographic contrast will be reduced or non-existent.
• Development: if a photographic emulsion is used to capture the image, then optimum radiographic contrast can be attained only if the film is developed to the correct film contrast. This is achieved by careful control of factors such as developer temperature, development time and processing chemical activity. To ensure this, the film processor must be subject to a rigid quality-control regime.
Subjective contrast depends upon the following:
• Radiographic contrast.
• The observer: poor eyesight,fatigue.
• Viewing box: brightness, evenness and colour of illumination.
• Computer monitor: many factors related to the quality of construction and design of the monitor will influence the contrast visible to the observer.
• Ambient lighting: if the room lighting is low and there is a reduction in extraneous light reaching the eye, then subjective contrast will improve. Radiographs are often viewed under poor conditions in hospital, especially in the ward environment. Radiographers have an important role in educating all hospital staff as to the benefits of viewing radiographs under proper lighting conditions.
A
Radiographic (objective contrast) depends upon the following:
• Subject contrast.
• Scattered radiation reaching the image receptor: the use of a secondary radiation grid between the patient and the cassette to reduce the scatter reaching image receptor improves radiographic contrast. Lead-backed cassettes or lead rubber under cassettes may reduce back-scatter, which may also improve radiographic contrast. If the cassette is some distance away from the patient, then scatter crossing the intervening
gap might not reach the image receptor.
• Image-acquisition device: the design and function of the device used to acquire the image can have a profound effect on contrast. For example, certain types of film emulsion, intensifying screen and phosphor plate may be designed to
give inherently greater contrast. In digital systems, the contrast is also influenced profoundly by the software used to process the initial image captured by the device.
• Film fog: if the image is viewed using a photographic-based system, then film fogging due to incorrect film handling or storage may reduce radiographic contrast.
• Exposure: if too much or too little radiation is used, then the image-acquisition device may be unable to respond or may be saturated to the point that it is unable to function properly. In these examples, there may be a reduced range of densities or no difference in density visible on the image, thus radiographic contrast will be reduced or non-existent.
• Development: if a photographic emulsion is used to capture the image, then optimum radiographic contrast can be attained only if the film is developed to the correct film contrast. This is achieved by careful control of factors such as developer temperature, development time and processing chemical activity. To ensure this, the film processor must be subject to a rigid quality-control regime.
Subjective contrast depends upon the following:
• Radiographic contrast.
• The observer: poor eyesight,fatigue.
• Viewing box: brightness, evenness and colour of illumination.
• Computer monitor: many factors related to the quality of construction and design of the monitor will influence the contrast visible to the observer.
• Ambient lighting: if the room lighting is low and there is a reduction in extraneous light reaching the eye, then subjective contrast will improve. Radiographs are often viewed under poor conditions in hospital, especially in the ward environment. Radiographers have an important role in educating all hospital staff as to the benefits of viewing radiographs under proper lighting conditions.
A