details regarding xray grids

1. X-RAY GRIDS
Dr. VISHAL
JR1
DNB RADIOLOGY

2. INTRODUCTION
• The radiographic grid are the devices that
reduce the amount of scatterred
radiation reaching the image receptor
• It consists of a series of lead foil strips
separated by X-RAY transparent spacers.
• Most effective way of removing scatter
radiation from large radiographic fields
• function is to improve image contrast

3. INTRODUCTION
• The grid is an extremely effective device for
reducing the level of scattered radiation that
reaches to the image receptor (IR)
• A carefully fabricated series of section of
radio-opaque material (grid stripes)
alternating with the section of radiolucent
materials (interspace materials)
• the grid is positioned between the patient
and image receptor
• the grid is designed to transmit only those
xrays whose direction is on straight line from
source to IR.
• Scattered radiations absorbed in the grid
materials.

4. HISTORY
• The grid was first invented
in 1913 by DR. GUSTAV
BUCKY.
• consisted of wide strips of
lead approx. 2cm apart in
crisscross pattern

5. HISTORY
• In 1920 the grid was improved by DR. HOLLIS POTTER.
• He re-alligned the lead strips so they would run in only one direction
• He made the lead strips thinner and therefore less obvious on th
image and design a device that allowed the grid to move during the
exposure (now known as potter bucky grid)

6. PRINCIPLE
• When the primary beam interact with the
patients body, the scattered radiation are
produced that may or may not be absorbed
depending on angle of incidence and
physical characteristics of the grid.
• If the angle of scattered beam is greater
enough to interact with lead strips , it is
absorbed.
• If the angle is small, the scattered beam will
be transmitted as the primary beam.
• Grid removes the scattered radiation before
it reaches to the film, therefore it improves
contrast.

7. IDEAL GRID
• Should pass all the primary photons i.e photons coming from focal
spot
• Should block all the secondary photons i.e photons not coming from
focal spot

8. GRID CONSTRUCTION
• 3 Different aspects
1. GRID RATIO
2. GRID FREQUENCY
3. GRID MATERIALS
 INTERSPACE MATERIALS
 LEAD STRIPS

9. GRID RATIO
• A grid has 3 dimensions
1. Thickness of lead stripe (T)
2. Width of interspace materials
(D)
3. Height of grid (H)
• SO GRID RATIO = H/D

10. GRID RATIO
• Higher the grid ratio the more clean of scatter radiation.
• Grid Ratio is expressed as X:1
• If H is kept constant and D is decreased the grid ratio will increase.
• TYPICAL VALUES:
1. 5:1 TO 16:1 - in general use
2. 3:1 TO 5:1 -- in mammography

11. GRID FREQUENCY
• Defined as number of lead strips per inch or
centimeter
• usually ranges from 60-200 lines / inch
• most commonly we use 85 - 100 lines / inch
• mammographic grids- 200 lines/ inch
• HIGH FREQUENCY GRIDS
more and thinner strips
high radiographic techniques
higher patient dose
no signifcant grid lines on the image

12. GRID MATERIALS
• A series of radioopaque lead stripes with alternate radiolucent
materials
1. INTERSPACE MATERIALS
• Maintains the precise separation between strips
• Generally constructed from aluminium or plastic fiber
• Aluminium does not absorb moisture as plastic fibre does
• Produce less visible grid lines on the radiograph
• But it increases the absorption of primary xrays
• so we require higher mAs -- higher patient dose
• FIBER MATERIAL are preferred where their application can
contribute to lower patient dose-- mammography

13. GRID STRIPS
• Should be thick enough to absorb scatter radiation and thin enough
to allow primary radiation
• height varies from 2-5 mm
LEAD IS WIDELY USED BECAUSE
1. Easy to shape
2. Relatively inexpensive
3. High atomic number and high mass density
Tungsten , Platinum, Gold, Uranium can also be used but none of
them has desirable characters as that of lead

14. GRID PERFORMANCE
• Principle function of grid is to improve contrast
• Therre are 3 methods of evaluating perforemance
1. Contrast improvement factor
2. Bucky factor
3. Selectivity

15. CONTRAST IMPROVEMENT FACTOR
• Measures improvement in image quality when grid is used
• It compares contrast improvement with a grid to that without a grid
• represented by letter K
• k= 1 means noimprovement
• most grids have k= 1.5-2.5
• K= contrast with grid/ contrast without grid
• It is complex function of-- 1)XRAY emission spectrum 2) Patient
thickness 3)Tissue irradiated

16. BUCKY FACTOR
• Also called as grid factor
• B= Patient dose with grid/without grid
• Higher the grid ratio higher will be the bucky factor
• Means higher the H and/ lower the D (higher grid ratio) higher will be
the B and higher will be the patient dose

17. GRID SELECTIVITY
• Although grids are allowed to absorb the scatter radiation, they also
asorb some primary radiation called as grid cutoff
• Grids that absorb greater percentage of scatter radiation than primary
are descibed to have greatter degree of selectivity
• grids with higher lead content will have higher selectivity.
• SIGMA (E)= Primary radiation transmitted / scatter radiation
transmitted
• HEAVY GRID- MORE LEAD - HIGHER SELECTIVITY - HIGH CONTRAST
IMPROVEMENT- MORE EFFICIENT

18. GRID TYPES
1. LINEAR
• Parallel
• Crossed
1. FOCUSSED
2. MOVING
• Single stroke
• Reciprocating
• Oscillating

19. LINEAR GRIDS
1) PARLLEL
• Lead and interspace material parallel
• only reduces scatter in the direction of the grid lines
2) CROSSED
• Two linear grids at right angle
• more efficient than parallel
• DR. Bucky originally used it
• grid cutoff is primary disadvantage
• xray tube and table should be perfectly alligned
• central ray must be perfectly alligned with centre of
grid

20. FOCUSSED GRIDS
• Designed to minimize grid cutoff
• Lead strips lie on the imaginary radial lines
of a circle centered at the focal spot so
they can coincide with the divergence of
the xray beam
• if positioned properly-- no grid cutoff
• more difficult to manufacture than parallel
• Must be positioned at a particular distance
called SOURCE TO IMAGE DISTANCE (SID).
• normally SID = 100cm in table and 180 cm
in chest radiography

21. MOVING GRIDS
• GriD lines are visible if primary x rays are absorbed even grid strips
are very small
• invented by Hollis E Potter in 1920 with simple idea of moving the
grid while exposure is made
• usually focussed grids are used
• motion blurs out the grid strips.
Disadavntages-
• mechanical problems may occur
• costly
• increase the patient radiation dose

22. MAMMOGRAPHIC GRID
• Moving grid with ratio of 4:1 to 5:1
• grid frequency - 30-40 lines/cm
• bucky factor= 2-3
• Does not compromise resolution but increase pt. dose
• HTC grid is the choice

23. HIGH TRANSMISSION CELLULAR GRID(HTC)
• Reduces scatter radiation in 2
dimension
• Grid strips are made of copper
• interspace material is air
• physical dimensions= 3.8:1

24. GRID PROBLEMS
• Most frequent error- improper
positioning
• Off-level
• Off-center
• Off-focus
• Upside down
• Off-center,Off focus

25. 1)OFF LEVEL
• This occurs when central ray
is angled across the axis of
the grid strips
• Therefore central rays are
incident on the grid at an
angle
• Grid cut-off will occur
• Arises due to improper tube
position

26. 2) OFF CENTER
• If the central ray is not properly
centered on the centermost
interspace of the grid i.e lateral
shifting of grid
• Common problem with mobile
XRAY tables or ceiling suspended
tubes
• Also called lateral decentering
• Arises due to improper tube
position

27. 3) OFF FOCUSED
• Due to unspecified SID selection
in case of focused grid
• if SID increases the grid cutoff
increases
• Grid cutoff is more severe at the
edge than center
• This problem will not occur if all
chest radiographs are taken at
180 cmSID and all table
radiographs at 100 cm SID

28. 4)UPSIDE DOWN
• Major grid problem but will not be noticed easily
• Maximum cutoff on either side of central ray
• Occurs when grid is used upside-down
• So side be checked prio to use

29. 5) COMBINED OFF CENTER & OFF FOCUSED
• Most common
improper grid
position
• mostly occurs
during mobile
radiography
• Resultant
radiograph with
dark on one side
and light on the
other side i.e
uneven exposure

30. GRID SELECTION
• Do not use grid for children
• Grid can be used if thickness is > 10cm
• Moving grid is always better than stationary
• focused > parallel
• Upto 8:1 grid ratio is good for <90kvp and more tha 8:1 for > 90 kvp
• Pt. dose increases with increase in grid ratio
• High ratio grids are used for high kvp examination
• Pt. dose at high kvp is more than at low kvp

31. AIR GAP TECHNIQUE
• Alternative to use of grid
• Involves placing the patient at a
greater object-image receptor
distance
• Image receptor is moved 10-15
cm away from the patient
• So the amount of scatter
reaching the image receptor will
be reduced
• Result is improved contrast
• But disadvantage is loss of
sharpness

32. THANK YOU
• REFERENCES-
Stewart C. Bushong
Chesney equipment
Website articles