What is GRIDS.
Grids are advice which use to remove the scattered radiation from the x-ray film.it was invented by Dr. Gustave bucket in 1913. The grid is most usefully method to removing scattered radiation from the x-ray film. It is made of lead foil strip separated by x-ray transparent spaces.
The interspace material is support the thin lead foil strip which is made of aluminum, cardboard, carbon fiber. The grid is covered by the aluminum. It is located between the patient and the cassette. It is long axis in the direction of x-ray beam. The primary x-ray beam come out the patient, passes through the inter space so it is parallel to the grid. The scattered x-ray beam which is not a parallel direction so it strike the grid and absorbs.
What Is Grid ratio ?
GRID RATIO:- Grid ratio is the ratio of height of lead strip (h) and distance (d) between lead strip in grid.
Grid radio= h/d
The ratio is the parameters which define the ability to removing the scattered radiation from the x-ray film. The range of grid radio is 4:1 to 16:1. The grid ratio is depends on the KVp. Low grid ratio used for low KVp and high grid ratio used for high KVp. The lead strips are about 0.05 mm thick. The range of grid frequency most is 60 to 110 lines/ inches or 25 to 45 lines/cm.
In mammography the low grid ratio used, the range is 2:1 to 4:1. Because it work in low KVp technique.
What Are The Types Of Grid ?
There are two types of grid.
1) Stationary grid.
2) Moving grid.
1) Stationary grid :- There are three types of stationary grid.
A) Parallel grid.
B) Crossed grid.
c) Focused grid.
A) Parallel grid :- In parallel grid, the lead strips are designed parallel to each other. It types of grid are used in manly mobile unit x-ray machine. It is very easy to design but it has cutoff may be estimated from the ratio of source to image distance to grid ratio.
2) Moving grid:- A movie grid is also called focused grid. The focused grids are placed in a mechanism so the moving grid is moved. The moving grid invented by Hollis S. Potter in 1920. In moving grid, the device moves grid during exposure is called potter- bucky grid.
There are two types of moving grids.
A) Reciprocating grid.
B) Oscillating grid.
A) Reciprocating grid :- The reciprocating grid is driven by a motor and the grid moves back and forth several times, during exposure. The distance traveled may be 2 cm.
B) Oscillating grid :- The grid is kept in a frame, which has 2-3 cm clearance on all sides. An electromagnet pulls and releases the grid before the exposure. The oscillating grid oscilates in circular path about the frame and comes to rest after 20-30 second.
Disadvantage of Using Moving Grids.
1. Moving grid contain thick lead strip.
2. Long exposure time.
3. It requires bulky so that distance between the patient and film is increase which create image blur.
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