Grids

Radiation physics-by <MUHAMMED ASLAM BS It is easy to Medical imaging technology students

1. GRIDS
MUHAMMED ASLAM BS
BSC MIT

2. OBJECTIVES
1.DEFINITION
2. CONSTRUCTION OF GRIDS
3. PATTERNS OF GRIDS
4.TYPES OF GRIDS
5.EFFICIENCY OF GRID PERFORMANCE
6.GRID CUT-OFF

3. DEFINITION
•A radiographic grid is a device that is
placed between the anatomic area of
interest and the image receptor to absorb
scatter radiation exiting the patient.
•Invented in 1913 by Gustave Bucky
•Most effective means for limiting the
amount of scatter radiation

4. GRIDS

5. •Scatter radiation is detrimental to image
quality and decreases radiographic
contrast.

•Grids are used only when the anatomic
part is 10 cm or greater in thickness and
more than 60 kVp is needed for
examination.

6. GRID CONSTRUCTION
•Grids contain thin lead strips or lines
separated by x-ray transparent spacers.
•Interspace material is made up of
aluminium.
•Lead lines and interspace material are
covered by an aluminium front and back
panel.
•Main purpose of inerspace material is to
support the thin lead strips.

7. WHY ALUMINIUM?
•Aluminium :1.structurally stronger

2. absorbs both primary
radiation, thus increasing patient dose,
however it also absorbs more secondary
radiation thereby improving contrast.

8. •GRID RATIO:
it is the ratio of height of the lead strips
to the distance between the strips.
r = h/D
Where r is the grid ratio
h is the height of the lead strips
D is the width of the spaces
between the strips.

Example :if the lead strips are 2mm high and
the space separating them is 0.4 mm,grid
ratio is 2/0.4=5:1

9. GRID RATIO

11. •Grid ratio range from 4:1 to 16:1,where the
first number is the actual ratio and second
number is always 1.

•Higher the grid ratio straighter the rays
have to be to get through the interspaces,
better is the absorption of scattered
radiation and better is the radiographic
contrast.

12. •GRID FREQUENCY:
expresses the number of
lead lines per unit length, in inches ,
centimetres or both.
•Can range in value from 25-80lines/cm.
•Lower grid frequency (thicker strips) will
be more efficient in removing scattered
radiation and improving contrast , because
of higher lead content.

13. •But more primary radiation is also
absorbed and hence it requires larger
exposure for the same film density.

•As the grid frequency increases ,the grid
ratio must also be increased to maintain
same efficiency.

•Lead content = indicator of performance of
the grid.

14. GRID PATTERN
•Refers to orientation of the lead strips in
the longitudinal axis.


•Two types:
1.Linear grid
2.Crossed or cross-hatched grid

15. LINEAR GRIDS
•Lead lines run in only one direction. They are
parallel to each other in their longitudinal axis.

•They allow angulation of the x-ray tube along
the length of lead lines

16. LINEAR GRID

17. CROSSED GRID
•Lead lines run at right angle to one another.

•Crossed grids remove more scattered
photons than linear grids because they
contain more lead strips, oriented in two
directions.

Disadvantage: x-ray tube cannot be angled in
any direction without producing grid cut-off

18. CROSSED GRID

19. GRID FOCUS
•Refers to orientation of lead lines to one
another.

•Two types-
1.Parallel grid(non-focussed)
2.Focussed grid

20. PARALLEL GRIDS
•Lead lines run parallel to one another.

•Used primarily in fluoroscopy and mobile
imaging

22. FOCUSSED GRIDS
•A focussed grid has lead lines that are
angled, or canted, to match approximately
the angle of divergence of the primary
beam.

•These allow more transmitted photons to
reach the image receptor than parallel
grids

23. FOCUSSED GRIDS

25. CONVERGENT POINT:
If imaginary lines were drawn from each
of the lead lines in a focussed grid, these
lines would meet to form an imaginary
point called convergent point.

CONVERGENT LINE :
•If points were connected along the length
of the grid , they would form an imaginary
line called convergent line.

26. •FOCAL DISTANCE (GRID RADIUS):
It is the distance between the grid and the
convergent line or point.

•FOCAL RANGE:
Recommended range of SIDs that can be
used with focussed grid.
•In practice , grids have a focussing range
that indicates the distance within which the
grid can be used without significant loss
of primary radiation.

27. CONVEREGRNT POINT AND LINE

28. FOCAL RANGE

29. •The convergent line or point always falls in
the focal range.

•The focussing range is fairly wide for a low
grid ratio and narrow for a high grid ratio.

•Focal range is indicated on top of the grids
by manufacturers.

30. TYPES
STATIONARY MOVING

31. STATIONARY GRIDS

1.Wafer grids: matches the size of the
cassette and is used by placing it on top
of the image receptor.
2.Grid cassette : image receptor that has a
grid permanently mounted to its front
surface.
3.Grid cap: permanently mounted grid and
allows the image receptor to slide in
behind it.

32. MOVING GRIDS
•Invented by Dr. Hollis E . Potter in 1920.
•Also known as reciprocating grids or
Potter-Bucky diaphragm.
•Located directly below the radiographic
tabletop , the grid is found just above the
tray that holds the image receptor.
•These grids continuously move 1-3 cm
back and forth throughout the exposure.

33. DIRECTION OF GRID TRAVEL


•While moving parallel to its surface , the grid
must move perpendicular to the long axis of
its lead strips.

34. •LONG DIMENSION GRIDS
Linear grids having lead strips running
parallel to the long axis of the grid.

•SHORT DIMENSION GRIDS
Linear grids having strips running
perpendicular to the long axis of the grid.
useful for examination where it is difficult
to centre the central ray correctly for long
dimension grid

36. GRID PERFORMNCE
1.Primary transmission(T
p).

2.Bucky factor or Grid Conversion factor(B)

3. Contrast Improvement Factor(K)

37. PRIMARY TRANSMISSION
•Primary transmission is the measurement
of the percentage of primary radiation
transmitted through the grid.

T
p = I
p/I’
p X 100
Where T
p is the primary transmission
I
p is the intensity with grid
I’
pis the intensity without grid

38. APPARATUS TO MEASURE T
p

39. BUCKY FACTOR
•It is the ratio of incident radiation falling on
the grid to the transmitted radiation
passing through the grid.

•Bucky factor can be used to determine the
adjustment in mAs needed when changing
from using a grid to non grid (or vice
versa)

40. •GCF or Bucky Factor = mAs with grid
mAs without grid

•Generally ,high ratio grids absorb more
scatter radiation and have larger Bucky
Factors than low ratio grids.

41. APPARATUS TO MEASURE BUCKY
FACTOR

42. BUCKY FACTOR
GRID RATIO GCF
NO GRID
1
5:1
2
6:1
3
8:1
4
12:1
5
16:1
6

43. CONTRAST IMPROVEMENT
FACTOR
•It is the ratio of the contrast with the grid to
the contrast without the grid.

•This is the measure of grid’s ability to
improve contrast , which is its primary
function.

44. •K factor depends on kVp,field size and
part thickness

•To permit comparison of different grids ,the
contrast improvement factor is usually
determined at 100kVp with a large field
and a phantom of 20 cm thick.

45. SELECTIVITY
•Selectivity = ratio of primary radiation
transmitted through the grid to the
scattered radiation transmitted through the
grid.

•Larger the ratio , greater will be the
selectivity of the grid.

46. GRID CUT-OFF
•Grid cut-off refers to decrease in the
number of transmitted photons that reach
the image receptor because of some
misallignment of the grid.

•Higher grid ratio and short grid –focus
distance results in more potential grid cut-
off.

47. •Grid cut-off is a result of poor geometric
relationship between the primary beam
and lead foil strips of the grid.

•When the projected images of the lead
strips are thicker than the width of the
interspaces , cut-off is complete and no
primary radiation reaches the film.
•The resultant radiograph will be light in the
area in which cut-off occurs.

48. TYPES OF GRID CUT-OFF
ERRORS
1.Upside –down focussed grid
2. Lateral decentering(off-centre)
3.Focus-grid distance decentering(off-
focus)
4.Combined lateral and focus-grid distance
decentering.

49. UPSIDDE DOWN FOCUSSED
GRID
•This occurs when a focussed grid is
placed upside down on the image recptor ,
resulting in the grid lines going opposite
the angle of divergence of the x-ray beam.
•Radiographically , there is significant loss
of exposure along the edges of the image.

50. •Photons easily pass through the centre of
the grid because the lead lines are
perpendicular to the image receptor
surface.
•The tilted strips near the edges of the grid
will absorb progressively more radiation,
causing severe peripheral cut-off.
•However with parallel grids either side
may face the tube or film.

52. LATERAL DECENTERING
•It occurs when the central ray of x-ray
beam is not aligned from side to side with
the centre of focussed grid.
•All the lead strips cut off the same amount
of primary radiation.
•Hence, there is uniform loss of radiation
over the entire surface of the grid
,producing a uniformly light radiograph.

53. •FACTORS AFFECTING MAGNITUDE OF
CUT –OFF:
1. Grid ratio
2.Focal distance
3. Amount of decentering

L = rb/f
oX100
where r is the grid ratio
b is the lateral decentering
distance
f
o is the focal distance of grid

54. •Amount of cut-off increases as the grid
ratio and decentering distance increases,
and cut-off decreases as the focal
distance increases.


•In portable radiography, low ratio grids and
long focal distances should be used .

55. OFF-LEVEL GRIDS
•Off level grid cut off results when the x-ray beam is
angled across the lead strips.

•Occur from either the tube or the grid being angled.

•Often seen with portable radiograph or horizontal
beam examinations.

•Only type that occurs with bth focussed and parallel
grids

56. OFF-LEVEL GRID

57. •IMPORTANT RELATIONSHIP :
Angling the x-ray tube across the
grid lines or angling the grid itself during
exposure produces an overall decrease in
exposure on the radiograph.

59. FOCUS-GRID DISTANCE DECENTERING
•Target of the x-ray tube is correctly centered
to the grid,but it is positioned above or below
the convergent line.

•Far focus-grid distance decentering-target is
above
•Near focus-grid distance decentering-target is
below

60. NEAR FOCUS GRID DISTANCE
DECENTERING

61. FAR FOCUS GRID DISTANCE
DECENTERING

62. •The loss of primary radiation increases as the
grid ratio increases and as the distance from
the centre of the grid increases.

•Loss is greater with near than far-focus grid
distance.

64. COMBINED
LATERAL AND FOCUS-GRID DISTANCE
DECENTERING
•Easy to recognise

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

67. PATIENT PROTECTION ALERT
•GRID SELECTION:
In order to keep the patient exposure as
low as possible , grids should be used only
when appropriate , and the grid ratio
should be lowest that would provide
sufficient contrast improvement .
• below 90 kVp-8:1,above 90 kVp 12:1

68. CONCLUSION
•Construction of the grid-lead strips separated by
x-ray transparent spacers.
•They are used to absorb scatter radiation and
improve radiographic image contrast
•Generally high ratio grids with high lead content
have the highest contrast improvement factors.
•The amount of grid cut-off with a particular
decentering error is greatest with high ratio grids
and short focussing distance .
•Grid selection involves compromise between film
quality and patient exposure.

69. THANK YOU