Radiographic Image Formation (PDF)

1 The radiographic image Image formation The X-rays used in medical diagnosis are produced from a small area within the X-ray tube when an exposure is made. They diverge outwards from this area, travel in straight lines, and can be detected by a variety of devices used for medical imaging. As t...

1 The radiographic image Image formation The X-rays used in medical diagnosis are produced from a small area within the X-ray tube when an exposure is made. They diverge outwards from this area, travel in straight lines, and can be detected by a variety of devices used for medical imaging. As the X-rays pass through the body, some will be absorbed by the organs and structures within the body whilst others will pass through to the equipment used to form the image. In summary, the term ‘density’ can be used in the following ways: Patient or physical density: relates to the mass per unit volume of the structures within the patient and their absorption characteristics. Image density: the amount of signal detected in the image receptor or, put crudely, ‘blackening’ within the image. If measured on film using a densitometer, this will be optical density. In diagnosis: refers to a small defined area of pathology. Projection and view: a radiographic image is a projection of the object. View Projection 1 Density and contrast Definition of Density: 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. Photographic film: If the image is captured on a photographic emulsion, then the term ‘photographic density’ or ‘optical density’ should be used. Digital image capture: If the image was captured by a digital system such as computed radiography (CR) or direct radiography (DR). Definition of Contrast: 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. Lower contrast Higher 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. Radiographic contrast is the difference in optical density on different parts of the processed film Subjective contrast is the personal appreciation of the differences in optical density. The radiographic image Density and contrast (contd) Subject contrast 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. 2 Scatter reduction in lateral hip radiography using an air-gap technique and a secondary radiation grid (Radiograph showing the effect of high subject contrast between the bodies and spinous processes of the lumbar spine) Factors that influence subject contrast include the following: The region of the body under examination: Contrast media: Pathology: Kilovoltage: Subjective contrast: The personal appreciation of the contrast in the image is called subjective contrast. Subjective contrast depends on: the observer: visual perception, fatigue, etc.; viewing conditions: e.g. ambient lighting. Radiographic contrast: 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: Image-acquisition device: Film fog: Exposure: Development: Subjective contrast depends upon the following: Radiographic contrast. The observer: poor eyesight, fatigue. Viewing box: brightness, evenness and colour of illumination. Computer monitor: Ambient lighting: 3 The radiographic image Magnification and distortion Magnification: focus-to-film distance (FFD), FOD is the focus-to-object distance. Image distortion: If the object and film are not parallel to each other, then there is a difference in magnification of different parts of the object, leading to a distorted image. Image sharpness: In radiography, the aim is to produce an image that is as sharp as possible in order to resolve fine detail within the image. Unfortunately, there are several factors that lead to image unsharpness. These are unsharpness due to: geometry (Ug); movement (Um); absorption (inherent factors) (Ua); photographic/acquisition factors (Up). Geometric unsharpness: If X-rays originated from a point source, then a perfectly sharp image would always be obtained. The degree of geometric unsharpness increases with an increased focal spot size and increased object-to-film distance: 4 Movement unsharpness: This type of unsharpness is due to patient, equipment or film movement during the exposure. Sharpness can be increased by using a shorter exposure time (achieved by a lower mAs with higher kVp The radiographic image Image acquisition and display Images can be acquired in several different ways depending on the equipment used by any particular imaging department. These are: conventional film/screen technology; fluoroscopy/fluorography; digital imaging: – computed radiography (CR); – direct digital radiography (DDR). Fluoroscopy systems Luminos dRF Max Fluoroscopy Machine Direct Radiography Equipment Mobile Digital X Ray AGFA Computed Radiography 5 Exposure factors Each time a radiograph is to be produced, a set of exposure factors has to be chosen to give the type of image required. The choice of these factors will depend on the region being examined, including its thickness, density, pathology, etc. The exposure factors to be selected are: the milliampere seconds (mAs); the kilovoltage; the FFD. 1- Milliampere seconds This indicates the intensity or, put simply, the amount of radiation being used. mAs is a product of the X-ray tube current (mA) and exposure time (seconds). 2- Kilovoltage This indicates how the X-ray beam will penetrate the body. The range of kilovoltages used in diagnostic radiography is normally between 50 kVp and 120 kVp, although a kilovoltage as low as 25 kVp may be used for certain soft-tissue examinations, such as mammography. High-kVp techniques, such as those used in chest radiography, employ a kilovoltage in excess of 120 kVp. The kilovoltage will have a profound effect on the image density. Another reason for increasing the kVp is to allow the mAs,and therefore the exposure time, to be reduced. Effect of changing mAs on film density for the same kV Effect of changing kV on film density for the same mAs 3- Focus-to-film distance Most radiographic examinations are carried out with an FFD of 100 cm, which gives acceptable focus-to-skin distance and geometrical unsharpness but does not put unnecessary thermal stress on the X-ray tube. If this is the customary FFD used, then the department will require grids focused at 100 cm. following formula can be used: 6 For example, if at 100 cm FFD an exposure of 65 kVp and 20 mAs produced a satisfactory result, then at 200 cm FFD, the new mAs would be calculated as follows: 4- Intensifying screens Intensifying screens used in conjunction with photographic film are usually in pairs, with the film sandwiched between them and contained in a rigid, light-tight container, i.e. a cassette. 5- Digital image capture If a digital method of image acquisition is being used, such as CR, then a wide range of exposures will produce an image. 6- Secondary radiation grid Grids are used when the thicker or denser parts of the body are being examined, Grids are usually focused (e.g. at 100 cm), which means that the X-ray tube should be at this distance (e.g. 100 cm) above the grid for maximum transmission of primary radiation through all parts of the grid. If the grid has a lattice of 50 lines or more per centimetre, then it can be used tationary without the grid lines being obvious on the radiograph. One cause of grid cut-off: beam angulation across grid lines Grid ratio Choice of exposure factors Kilovoltage is selected to give the required penetration and subject contrast. mAs is selected to give the correct image density. Its value depends on: the type of image-acquisition device, e.g. the relative speed of intensifying screens; the FFD; the grid factor (if a grid is used). 7