This document discusses grids used in radiography. Grids are placed between the patient and film to reduce scattered radiation and improve image contrast. They are made of alternating strips of lead and plastic. Grids absorb scattered radiation traveling at oblique angles while allowing primary radiation straight through. Different grid types include stationary parallel, stationary focused, and moving grids. Grid ratio and density affect absorption of scattered rays. While grids improve image quality, they also increase dose and cost.

1. A SEMINAR ON
“GRID”
”
PRESENTED BY- NEELOTPAL MITRA
BDS FINAL YEAR

2. CONTENTS
 INTRODUCTION
 DEFINITION
 CONTENT OF GRID
 FUNCTION OF GRID
 TYPES OF GRIDS
GRID RATIO
 GRID CASSETTE
 GRID PATTERN
 GRID ASSESSMENT
 GRID DENSITY
 DISADVANTAGES
 CONCLUSION

3. INTRODUCTION
 Grids are placed between the patient and the X-ray film
to reduce the scattered radiation(produced mainly by
compton effect) and thus improve image contrast.
 When the primary radiation passes through the subject,
scatter radiation reaches the film, it sensitizes even areas
which should not be sensitized, which produces
radiographic fog on the film.

4. DEFINITION
 Grid is a device used to remove scattered radiation
from large radiographic field. It is made up of series of
lead strips separated by X-ray transparent spacer .

5. CONTENT OF GRID
 A grid consists of series of lead strips separated by X-
ray transparent spacer.
 It consists of a large number of long , parallel strips of
lead which is a radiopaque material , interspersed with
radiolucent interspace material (plastic).

6. CONTENT OF GRID

7. FUNCTION OF GRID
 Many scatter radiations arise within the patient and is
multidirectional.
 Only those rays which are travelling in the direction of
transparent spacer can reach the film, whereas those that are
scattered at different angle are absorbed by the grid.
 They are placed between object and the film.

8. EFFECT OF GRID ON SCATTER RADIATION

10. TYPES OF GRID
GRID
STATIONARY
PARALLEL FOCUSED
MOVING

11. STATIONARY GRID:
 PARALLEL GRID :
In parallel grid, the secondary radiation
are absorbed by the parallelly placed
radiopaque lead strips.

12.  FOCUSED GRID :
In this strips of radiopaque material are directed
towards the direction of the paths of the diverging
secondary radiation, thereby eliminating the
absorption of more secondary radiation than
parallel grid.

13. MOVING GRID :
 It was invented by Dr Hollis E Potter in
1920 and is known as Potter-Bucky grid.
The moving grid is placed under the table
where X-ray film cassette is also placed.

14. Mostly they move 3-5 cm back and forth
throughout the exposure. They start moving
when anode begins to rotate.
Advantage of this grid is that they eliminate
grid lines.

15. MOVING GRIDS

16. GRID RATIO (r) :
It is defined as the ratio between the height of the
lead strip and the distance between them.
r=h/d
h-: is the thickness of lead strip
d-: is their separation
The lead strips are 0.05 mm thick. Interspaces are
much thicker than the lead strips. Grid ratio ranges
from 4.1 to 16.1 .

17. GRID CASSETTE
 A grid cassette contains an incorporated
grid at the front of it to minimize the scattered
x-ray photons from reaching the film.
 Stationary grid is built in the tube side of
cassette.

18. GRID CASSETTES

19. GRID PATTERN
GRID
PATTERN
LINEAR
GRID
CROSSED
GRID
There are two basic patterns:

20.  LINEAR GRID :
o In linear grid the lead strips are parallel to each
other in longitudinal axis.
o This grid allows the angle of the x-ray tube along
the length of the grid without loss of primary
radiation from grid cutoff.
o These grids can only be used effectively with very
small x-ray fields or long target grid distance.

21. LINEAR GRID

22.  CROSSED GRID :
o A crossed grid is made up of two superimposed
linear grids at right angle to each other that have
same focusing distance.
oThe disadvantage of crossed grid is that it cannot
be used with oblique technique requiring
angulation of the X-ray tube.

23. CROSSED GRID

24. GRID ASSESSMENT
Following factors must be taken into
consideration while evaluation of grid to be done:
Primary transmission: It is the
measurement of percentage of primary
radiation transmitted through the grid.
BUCKY FACTOR (B): It is the ratio of
incident radiation falling on the grid to the
transmitted radiation passing through the grid.

25.  It indicates absorption of both primary and
secondary radiation.
 High grid ratio absorbs more scatter
radiations and has larger Bucky than low grid
ratio.
B = Incident radiation
Transmitted radiation
CONTRAST IMPROVING FACTOR (K): It is the
ratio of the contrast with grid to the contrast
without grid.
K= Contrast with grid
Contrast without grid

26. GRID DENSITY
 Grid density refers to the number of X-ray
absorbing strips per 1cm.
X-ray grids are available in densities of 34,
40, 60, and 80 lines per 1 cm.
The higher the grid density, the better the
film image, with less visible grid lines.

27. DISADVANTAGES
 They are costly.
 They put a limit on the minimum exposure
time.
They move slowly.
They increase patient’s radiation dose.

28. CONCLUSION
 Grids are thus used to prevent the
scattered radiations caused by various
substances in the subject, from reaching
the film.
An ideal grid should be capable of
removing 80-90% of the scattered
radiation. The resultant image thus has a
better contrast.