Let’s start this post- HOW TO USE GRID IN RADIOGRAPHY
INTRODUCTION
Grids invented by DR.GUSTAVE BUCKY in 1913, is the most effective method of removing scatter radiation from the Radiographic field.
Grids consist of a series of lead foil strips separated by X-ray transparent spaces, the whole strips are cover by outer aluminum covering.
When Radiographer exposure the X-ray, scattering radiation arises from many points within the patient and most it 80-90 % absorbed by Grid and only 1% X-ray cross the body of entire parts of a patient, that increase the image contrast.
Parameter of Grid –
Grid ratio is the ratio of the height of the lead strip and the distance between them
Grid ratio is a parameter used to express grid ability to remove scattered radiation.
Grid ratio =h/H
Grid ratio 4:1 ,16:1 are available
A low grid ratio used for low KVp work.
A high grid ratio is used to high KVp work or where the production of a large amount of scatter radiation.
A higher grid ratio is higher patient dose.
Most grids have frequencies in the range of 60 to 110 lines/ inches
Grid frequencies can be calculated by dividing the thickness of one line pair ( thickness of grid material plus the thickness of interspace material )
Grid frequency in lines/inches =25.4/D+d
Where d =is thickness of lead strip ( in mm)
D=Thickness of interspace (in cm )
Grids used for mammography the ratio is 2:1 to 4:1 these low grid ratios have grid frequencies of approximately 200 lines/inch – 80 lines/inch.
TWO TYPES OF GRIDS
- Stationary Grid
- Moving Grid
1.Stationary Grid three types
- a.Parallel grid
- b.Focussed Grid
- c.Cross Grid
Note-Cross Grid has higher contrast improvement factor than parallel and focused Grid
The cross grid ratio is 6:1
Cross Grid formed by two parallel or Focused Grids placed at the right angle to one another.
2. Moving Grid – a major improvement in the grid is developed by 1920, Hollis S. Potter on a very simple idea moves the grid when X-ray exposure is being made.
A device moves Grid during exposure called Potter bucky grid.
Moving grid usually Focussed grid, they are placed in a mechanism that is moved.
- The moving grids have several disadvantages they are required a bulky mechanism that is reason to failure.
- Costly
- May vibrate x-ray tube
- The distance between the patient and film has increased that reason the extra distance creates an unwanted increase in magnification and image blur.
AT PRESENT Cross stationary grid is used which easy to handle & durable for working and good result for radiography
IMPORTANT QUESTION ARISE –
Grid made of lead strip
Cover lead strips foil with Aluminium
Grid ratio 4:1, 16:1 ,6:1, 12:1 ,2:1, 4:1
Cross grid ratio generally 6:1
Grid ratio = h/D , where h= height , d= thickness
Grid frequency lines/ inch = 25.4/d+D
Grid is used for absorbed scattering radiation and improves contrast in the image of the film.
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