Showing posts with label Image Production. Show all posts
Showing posts with label Image Production. Show all posts
Control of Scatter Radiation
Labels:
Image Production
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Effect of Scatter Radiation on Image Contrast
One of the most important characteristics of image quality is contrast, the visible difference between the light and dark areas of an image. Contrast is the degree of difference in OD between areas of a radiographic image. Contrast resolution is the ability to image and distinguish soft tissues.
Even under the most favorable conditions, most remnant x-rays are scattered.
When primary x-rays interact with the patient, x-rays are scattered from the patient in all directions.
Two types of devices reduce the amount of scatter radiation that reaches the image receptor: beam restrictors and grids.
Beam Restrictors
Basically, three types of beam-restricting devices are used: the aperture diaphragm, cones or cylinders, and the variable-aperture collimator (Figure 14-11).
Aperture Diaphragm
An aperture is the simplest of all beam-restricting devices. It is basically a lead or lead-lined metal diaphragm that is attached to the x-ray tube head. The opening in the diaphragm usually is designed to cover just less than the size of the image receptor used. Figure 14-12 shows how the x-ray tube, the aperture diaphragm, and the image receptor are related.
FIGURE 14-11 Three types of beam-restricting devices.
Dental radiography represents another application of aperture diaphragms. Dental radiographs are customarily obtained at 20 or 40 cm SID. Most dental imaging systems are supplied with rectangular collimation, which requires that the dental radiologic technologist precisely align and position the x-ray tube head, the patient, and the image receptor.
Cones and Cylinders
Radiographic extension cones and cylinders are considered modifications of the aperture diaphragm.In both, an extended metal structure restricts the useful beam to the required size. The position and size of the distal end act as an aperture and determine field size.
Radiographic cones and cylinders produce restricted useful x-ray beams of circular shape.
In contrast to the beam produced by an aperture diaphragm, the useful beam produced by an extension cone or cylinder is usually circular. Both of these beam restrictors are routinely called cones, even though the most commonly used type is actually a cylinder.
One difficulty with using cones is alignment. If the x-ray source, cone, and image receptor are not aligned on the same axis, one side of the radiograph may not be exposed because the edge of the cone may interfere with the x-ray beam. Such interference is called cone cutting.
Variable Aperture Collimator
The light-localizing variable-aperture collimator is the most commonly used beam-restricting device in radiography.
Automatic variable-aperture collimator. (Courtesy Huestis Medical.)
Simplified schematic of a variable-aperture light-localizing collimator.
Scatter Radiation
Labels:
Image Production
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As scatter radiation increases, the radiograph loses contrast and appears gray and dull. Three primary factors influence the relative intensity of scatter radiation that reaches the image receptor: kVp, field size, and patient thickness.
Some x-rays interact with the patient and are scattered away from the image receptor (a). Others interact with the patient and are absorbed (b). X-rays that arrive at the image receptor are those transmitted through the patient without interacting (c)and those scattered in the patient (d). X-rays of types c and d are called image-forming x-rays.
Three primary factors influence the relative intensity of scatter radiation that reaches the image receptor: kVp, field size, and patient thickness.
kVp
As x-ray energy is increased, the absolute number of Compton interactions decreases, but the number of photoelectric interactions decreases much more rapidly. Therefore, the relative number of x-rays that undergo Compton interaction increases.
Also, fewer x-rays reach the image receptor at low kVp—a phenomenon that is usually compensated for by increasing the mAs. The result is still higher patient dose.
With large patients, kVp must be high to ensure adequate penetration of the portion of the body that is being radiographed. If, for example, the normal technique factors for an AP examination of the abdomen are inadequate, the technologist has the choice of increasing mAs or kVp.
Increasing the mAs usually generates enough x-rays to provide a satisfactory image but may result in an unacceptably high patient dose. On the other hand, a much smaller increase in kVp is usually sufficient to provide enough x-rays, and this can be done at a much lower patient dose. Unfortunately, when kVp is increased, the level of scatter radiation also increases, leading to decreased image contrast.
Collimators and grids are used to reduce the level of scatter radiation.
Field Size
Another factor that affects the level of scatter radiation and is controlled by the radiologic technologist is x-ray beam field size. As field size is increased, scatter radiation also increases.
Collimation of the x-ray beam results in less scatter radiation, reduced dose, and improved contrast resolution.
Compared with a large field size, radiographic exposure factors may have to be increased for the purpose of maintaining the same OD when the exposure is made with a smaller field size. Reduced scatter radiation results in lower radiographic OD, which must be raised by increasing technique.
Patient Thickness
Imaging thick parts of the body results in more scatter radiation than imaging thin parts does. Compare a radiograph of the bony structures in an extremity with a radiograph of the bony structures of the chest or pelvis. Even when the two are taken with the same screen-film combination, the extremity radiograph will be much sharper because of the reduced amount of scatter radiation.
Patient Thickness
Types of Film
Labels:
Film,
Image Production
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Screen-Film
As was previously stated, screen-film is the most widely used IR in radiology. Several characteristics must be considered when one is selecting screen-film: contrast, speed, spectral matching, anticrossover/antihalation dyes, and requirement for a safelight.
Table 11-1 Types of Film Used in Medical Imaging
Type
|
Emulsions
|
Characteristics
|
Applications
|
|---|---|---|---|
Intensifying screen
|
Two
|
Blue or green sensitive
|
General radiography
|
Laser printing
|
Single with antihalation backing
|
Matches laser used (about 630 nm)
|
Laser printers attached to CT, MRI, ultrasound, etc.
|
Copy or duplicating
|
Single with antihalation backing
|
Pre-exposed to Dmax
|
Duplicating radiographs
|
Dental
|
Two packed in sealed envelope
|
Has lead foil to reduce back scatter
|
Dentistry
|
Radiation monitoring
|
Two packed in sealed envelope
|
One emulsion can be sloughed off to increase OD scale
|
Radiation monitoring
|
Dry transfer
|
One
|
Thermally sensitive
|
“Dry” printers
|
Non- Screen Film
Non screen film also known as direct exposure film is film that does not require the application of intensifying screen. This is most commonly used in mammography.
Radiographic Film
Labels:
Film,
Image Production
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Construction of the Radiographic Film
1.) Base
-
Supports the emulsion
150-300 mm thick, semi rigid lucent and made of polyester, the foundation of
the Radiographic Film. Its primary purpose is to provide a rigid structure unto
which the emulsion can be coated. It is flexible and fracture-resistant. It
maintains the film shape and size during use and processing to limit extortion.
It is flexible. It provides region structure.
Ø Dye
-
Is added to tint the film
blue to reduce eye strain.
Ø Dimensional Stability
-
is the property of the base
to maintain its size and shape.
Four Types of Base
1.) Glass Plate –
the original film base
2.) Cellulose Nitrate –
becomes a substitute of the glass plate and highly flammable.
3.) Cellulose Triacetate –
not flammable and safety base
4.) Polyester –
more resistant to warping from age and stronger and thinner than cellulose
triacetate.
Ø Emulsion
– the heart and the most important part of the x-ray film. This is where the
x-rays and light photons from IS interacting and transfer information. Consist
of homogenous mixture of gelatin and silver halide crystals and coated evenly
in 3-5 mm
thick.
Ø Gelatin –
provides mechanical support for the silver halide crystals.
Ø Silver Halide Crystals –
the active ingredient of the emulsion. 98% of the crystal is Ag halide crystal
and 2% is Ag iodide.
Atomic Number of the following:
Bromine = 35, Silver = 47, Iodine = 53, Gelatin and Base
= 7
Ø Contaminant –
is the imperfection of the arrangement of crystals
3.) Adhesive Layer – a
thin coating of material between the emulsion and base, it ensures the uniform
adhesion between them. It allows the emulsion and base to maintain proper
contact during use and processing.
4.) Overcoat Layer –
the protective covering of the gelatin. It protects the emulsion from
scratches, pressure, and contamination during handling, processing and storage.
It also allows a rough manipulation of film before exposure.
Grid
Labels:
Image Production
·
Grid- reduces the amount of scattered radiation that reaches the IR, thus producing a short scale contrast. This is placed between the patient and the IR. Its principal action is to improve contrast by minimizing and reducing the amount of scattered radiation.-used only when the thickness of the body part to be examined is above 12 cm.
a. Grid Ratio- height of the grid divide by the thickness of the interspace material (h/D)
High ratio grids are more effective in cleaning up scatter radiation but at the same time, contributes to high patient dose and very expensive.
b. Grid Frequency- number of grid strips or grid lines per inch or per cm.
-as grid frequency increases there is a relatively more grid to absorb x-rays and therefore the patient dose is higher. To compensate this, use high speed IS to decrease the exposure time.
Interspace Material- its purpose is to maintain a precise separation between the delicate lead strips of grid. The interspace material is usually aluminum or plastic fiber.
Grid Strip- this should be infinitely thin and have high x-ray absorption property. The most widely used strip is the Lead.
Grid Casing- grid should be encased by a thin cover of aluminum. This provides rigidity and helps to seal out moisture.
d. Grid Performance
1. Contrast Improvement Factor (K)- the ratio of the contrast of the radiograph made with a grid to the contrast made without grid. Most grids have a K of 1.5 to 2.5.
K= radiographic Contrast with Grid/Radiographic contrast without Grid
2. Bucky Factor- measures how much an increase in technique will be required compared with non-grid exposure.
-as Bucky Factor is increases, radiographic technique and patient dose increase proportionately.
-sometimes called as grid factor
3. Selectivity- related to the grid ratio but primarily influenced by the lead content.
-the heavier the grid, the more lead in contains, the higher its selectivity and the more efficient it is in cleaning up scatter radiation.
General Rule Regarding the Grid
1. High ratio grids have higher contrast improvement factor.
2. As the grid absorbs scatter radiation, contrast is improved on the radiograph.
3. High Frequency grid has thin strips of interspace material.
4. Heavy grids have high selectivity and have high contrast improvement factors.
5. The heavier the grid, the more lead it contains, the higher its selectivity and the more efficient it is in cleaning up scatter radiation.
Types of Grid According to Construction
1. Linear Parallel Grid- the simplest type of grid.
- all lead grid strips are parallel
- it is the easiest to manufacture
- results to grid cut- off; the undesirable absorption of the primary x-rays, which maybe partial or complete and can result to reduced OD or total absence of film exposure
2. Crossed Grid- has grid strips running to both the long and short axis of the grid. This is made to overcome the deficiency of the Linear Parallel grid. This is made up by sandwiching two linear grids together with their grid strips perpendicular to one another.
-not difficult to manufacture, therefore not expensive.
2 serious disadvantages
a. Positioning he grid is critical
b. Tilt table technique is possible only when the x-ray tube and the table are properly aligned.
3. Focused Grid- designed to minimize the deficiency of the linear and crossed grid which is the grid cut- off. Its lead strips run only along one axis and are tilted so that they lie on imaginary lines of the divergent x-ray beam.
- its advantages are; difficult to manufacture and therefore expensive and there is an SID limitation.
Two Types of Grid According to Movement
1. Stationary Grid- place in a stationary position or it does not move during exposure and are used usually in a mobile radiography.
2. Moving Grid- grid that moves during the exposure and also known as Potter Bucky
Diaphragm or Bucky Grid.
Grid lines- are the images made when the primary x-rays are absorbed in the grid lines.
Types of Moving Grid
1. Reciprocating Grid- it is motor driven and therefore does not need any resetting after each exposure.
2. Oscillating Grid- positioned in a frame 2-3 cm tolerance on all sides between frame and grid. Powered by electromagnet, fashioned a circular pattern of motion.
The only difference between the reciprocating and the oscillating grid is their pattern of motion; reciprocating grid is to and fro and the latter is in circular motion.
Disadvantages of Using Moving Grid
1. Early moving grid contained large, thick lead strips and therefore produced objectionable gridlines.
2. It requires bulky mechanism that is subject to failure, the distance between the patient and the film is increased which creates image magnification and image blur.
3. If not properly designed, it can create a stroboscopic effect.
4. Long exposure time.
Grid Problems
1. Off- Level Error- caused by improper positioning of the x-ray tube and grid.
2. Off- Center Error- caused by lateral decentering of the grid and this caused partial grid cut- off over the entire film.
3. Off- Focus Error- caused by improper positioning of the grid at a specified focal distance. This is the most common and major problem in grid.
4. Upside Down Grid Error- caused by improper usage of grid, the tube side and the film that it must be labeled appropriately.
Grid Selection Factors
1. Patient dose increases with increasing grid ratio.
2. High- ratio grids are usually used for high kVp examinations.
3. Patient dose at high kVp is less than at low kVp.
Air- gap Technique- a clever technique used as an alternative to the use grid. This reduces scatter radiation, thereby enhancing image contrast. One disadvantage of this technique is the image magnification with associated focal spot blur.
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