GRIDS IS PRESENTATION IN RADIO PHYSICS MUST IMPORTANT

Grids

1. Dr.Archana Koshy

2. •The radiographic grid consists of a series of Lead foil strips
separated by X-ray transparent spacers .

•Invented by Dr.Gustave Bucky in 1913

•Most effective way of removing scatter radiation from large
radiographic fields .

•The interspaces of grids are filled either with aluminium or some
organic compound .

4. GRID RATIO : Ratio between the height of the Lead strips and
the distance between them .

5. •GRID PATTERN : Orientation of the Lead strips in their
longitudinal axis .
•Pattern of the grid that we see from as a view from the top

I.LINEAR GRID

II.CROSSED GRID

III.PARALLEL GRID

IV.FOCUSSED GRID

6. LINEAR GRID

The Lead strips are parallel to each other in their longitudinal axis
Major advantage is that they allow us to angle the X-ray tube
along the length of the Grid without loss of primary radiation
from grid ‘cutoff ‘

CROSSED GRID
Made of two superimposed linear grids that have the same
focusing distance .
Grid ratio of two crossed grids is equal to the sum of the ratio of
the two linear grids .
Cannot be used with oblique techniques requiring angulation of
the Xray tube .

9. FOCUSSED GRID

Grid made up of lead strips that are slightly angled so that they
focus in space .
May be either linear or crossed .

Line focused grids converge at a line in space called CONVERGENT
LINE .

Crossed Grids converge in a point in space called CONVERGENT
POINT .

FOCAL DISTANCE : Perpendicular distance between the grid and
the convergent line or point .

10. PARALLEL GRID

Lead strips are parallel when viewed in cross section .
Focused at infinity so they do not have a convergent line .
Can only be used effectively with either very small Xray fields
or long target-grid distances .

11. LINES PER INCH : Number of Lead strips per inch of the grid .

Calculated by adding the thickness of the Lead strips and
interspaces and dividing this sum into 1 .

12. •Three main methods :

I.PRIMARY TRANSMISSION (Tp)

II.BUCKY FACTOR (B)

III.CONTRAST IMPROVEMENT FACTOR (K)

13. •Measurement of the percentage of primary radiation
transmitted through a grid .
1.Made with the grid in place to determine the intensity of the
radiation through the grid .

2.Made after removal of the grid to determine the intensity of
the radiation directed at the grid .

Tp = Ip/Ip’ x 100

14. •Ratio of the incident radiation on the grid to the transmitted
radiation passing through the grid .

•Indicates the amount of exposure factors that should be
increased when the change from grid to a non grid technique is
made .

•Measure of the Grid’s ability to absorb scatter radiation

•Unlike primary transmission , Bucky factor indicates the
absorption of both primary and secondary radiation .

B= INCIDENT RADIATION / TRANSMITTED RADIATION

15. •Ratio of the contrast with a grid to the contrast without a grid .
•Measure of the grid’s ability to improve contrast which is its
primary function .
•It depends on :
1.kVp
2.Field size
3.Phantom thickness

•More closely related to the Lead content of the grid than any
other factor . (g/cm²)

16. •Loss of primary radiation that occurs when the images of the
Lead strips are projected wider than they would be with
ordinary magnification .

•Result of a poor geometeric relationship between the primary
beam and the Lead foil strips of the grid .

•Cut off is complete and no primary radiation reaches the film
when the projected images of the Lead strips are thicker than
the width of the interspaces.

•Amount of cut off is always greatest with high ratio grids and
short grid focus distances .

18. •4 SITUATIONS THAT PRODUCE GRID CUT OFF :



1.FOCUSED GRIDS USED UPSIDE DOWN

2.LATERAL DECENTERING (GRID ANGULATION )

3.FOCUS GRID DISTANCE DECENTERING

4.COMBINED LATERAL AND FOCUS-GRID DISTANCE
DECENTERING .

19. •When a focused grid is used upside down, there is severe
peripheral cut off with a dark band of exposure in the center
of the film with no exposure at the periphery .

•The higher the grid ratio,narrower the exposed area .

21. •When the X-ray tube is positioned lateral to the convergent line
but at the correct focal distance.

•Uniform loss of radiation over the entire surface of the grid,
producing a uniformly light radiograph .

•3 factors affect the magnitude of cut off from lateral
decentering :

-Grid ratio
-Focal distance
-Amount of decentering

23. OFF LEVEL GRIDS
When a linear grid is tilted , there is uniform loss
of primary radiation across the entire surface of
the grid .
Effect on the film being same as that of lateral
decentering .

25. •The target of the X-ray tube is correctly centered to the grid ,
but it is positioned above or below the convergent line .

•Cut off is greater with near than far focus –grid distance
decentering

•The central portion of the film isnt affected but the periphery is
light .


•The loss of primary radiation is directly proportional to the grid
ratio and the distance from the center line.

28. •Causes an uneven exposure resulting in a film that is light on
one side and dark on the other side.

•Directly proportional to the grid ratio and decentering distance
and inversely proportional to the focal distance of the grid .

30. •Invented by Dr.Hollis.E.Potter in 1920 .
•Also called a Bucky grid
•Moved to blur out the shadows cast by the lead strips .
•Continuously moves 1 to 3 cm back and forth throughout the
exposure .

•Precautions :
1.The grid must move fast enough to blue the lead strips
2.The transverse motion of the grid should be synchronous with
the pulses of the Xray generator .

31. DISADVANTAGES :
•Costly
•Subject to failure
•May vibrate the Xray table
•Places a limit on the minimum exposure time because they move
slowly .
•INCREASES THE PATIENT RADIATION DOSE.

32. •The price of increased “cleanup “ with high ratio grids is that
patient exposure is considerably increased and that Xray tube
centering becomes critical .

•8:1 grids will give adequate results below 90 kVp

•Above 90 kvp,12:1 grids are preferred
•In cases of Biplane cerebral angiography, crossed grids are
preferred as there is a great deal of scatter radiation.

34. •Scatter radiation arising from the patient from Compton
reactions disperses in all directions , so the patient acts like a
large light bulb .

•The closer the patient is to the film, the greater the
concentration of scatter per unit area .

•With an air gap, the concentration decreases due to more
photons missing the film in the gap .
•Used in Magnification radiography and Chest radiography .

36. 1.Thicker the part,the more advantageous a larger air gap

2.The first inch of any air gap improves contrast more than any
subsequent inch

3.Image sharpness deteriorates with increasing gap width unless
the focal film distance is increased to compensate for the
greater magnification.

4.If the gap is widened by moving the patient away from the
film with a fixed focal film distance the patient is closer to the
X-ray tube and his exposure increases.