Grid

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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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