• INTRODUCTION
INDEX
• FUNCTIONS
• COMPOSITION
• ACTION OF GRIDS
• PRINCIPLE OF GRIDS
• VARIOUS SIZES OF GRIDS
• GRID CHARACTERISTICS
• TYPES OF GRIDS
• MOVING GRIDS
• OSCILLATING AND RECIPROCATING MOVEMENTS
• The anti-scatter grid (usually called as grid) is
composed of large number of long parallel strips of
lead and parallel to each other by an x-ray
transparent interspace material
• The thickness of grid is about 6 mm
• The image quality can be dramatically influenced
by the choice of grid
• A radiograph can be ruined using the wrong grid or
using the grid in an incorrect manner
FUNCTIONS:
• The function of a grid is to reduce the amount of
scattered radiation exiting a subject that reaches
the film
• The grid, which is placed between the subject and
the film, preferentially removes the scattered
radiation and spares primary photons
• It reduces
- Non-imaging exposure (fog)
- Increases subject contrast
COMPOSITION:
• composed of alternating strips of a radiopaque material (usually
lead) & strips of radiolucent material (often plastic)
• The strips of the grids should be
- As thin as possible to minimize blockage of unscattered light
- Opaque to scattered radiation
- Non-fluorescent, i.e. should not release characteristic x-rays
as scattered x-ray photons are absorbed
• Lead foil of 0.05 mm (50 µm) thickness is used to
build strips for most radiographic grids
• The interspace material between grid strips may
be
- FIBRE: Transparent to x-ray
- PLASTIC: Transparent to x-ray
- ALUMINUM: Sturdy
VARIOUS SIZES OF GRIDS
•
Grids are manufactured to accommodate all standard film sizes
from 9x12 to 45x45 mm or 8"x10" to 18"x18“
•
Customized grids to specifications are available upon request
•
Grids for use
– inside cassettes are 3 mm (1/8") smaller than the corresponding
film size, whereas
– for use outside the cassettes are approximately 20 mm (1")
larger than the corresponding film size
•
Extra large grids are made from two or more smaller sized grids
that can be fitted together up to a maximum size of 49 cm x 49
cm (19"x19")
INTERACTION OF THE GRID WITH AN X-RAY BEAM
IN LINEAR GRID
• When secondary photons generated in the subject
are scattered toward the film, they usually are
absorbed by the radiopaque material in the grid
• This occurs because the direction of the scattered
photons deviates from that of the primary beam,
and remnant beam consequently they cannot pass
through the parallel plates of the grid
IN FOCUSED GRIDS: (usually used)
• The strips of radiopaque material are all directed
towards
common
point-
the
focal
spot,
some
distance away
• This is because a focused has the lead strips
angulated towards the focal spot, their path
coincides with the direction of the paths of
diverging photons in the primary x-ray beam,
there by accomodating their passage through the
grids
• Lead strips absorb the scattered photons as their paths
diverge from those of the primary photons
• A focused grid can be used only within a range of
distances from the focal spot where the alignment of lead
strips closely coincides the path diverging x-ray beam
• The range of distance is specified on the grid
• The presence of a grid between an object and the film
causes the images of the radiopaque absorbing material to
be projected onto film.
• The closer together the grid lines are on the film, however
less objectionable they become
• Grids are manufactured with a varying number of line pairs of
absorbers and radiolucent spaces per inch
• Grids with 80 or more line pairs per inch do not show
objectionable grid lines on the image
• The ratio of grid thickness to the width of the radiolucent
spacer is known as the GRID RATIO
• The higher the grid ratio, the more effectively scattered
radiation is removed from the x-ray beam
• Grids with a ratio of 8 or 10 are preferred
• The image of radiolucent grid lines can be deleted mechanically
by moving the grid in a direction of 900
THE PRINCIPLE OF GRIDS
ACTION OF THE GRIDS:
1. The scattered radiation will be moving obliquely
because of change in direction which occurs in
scattering process. The majority will be stopped
by the lead of the grid. The
primary
majority of the
beam passes through the interspace and
reach the film
2. Still some times the scattered radiation reaches
the film but is negligible
3. A reasonable grid is able to remove as much as
80-90% of scattered radiation present and this
removal of scatter produces a worthwhile and large
increase in contrast
- This measurement is measured by a quantity
known as the CONTRAST IMPROVEMENT FACTOR
(K) where
K =
X-ray contrast with grid
X-ray contrast without grid
• For grid lo be good one ‘K' should be large
• Practical grids have values varying from 1.5 to about 3.5
depending upon the exact construction of grid and exposure
conditions
• These values refer to situations where a reasonable amount
of scatter is present
• Even an ideal grid cannot produce any improved contrast if
scatter is not present e.g. In hand radiographs, scatter is
small and hence grids are not used
• Although grid is able to remove majority of unwanted scatter
quite successfully, but it removes some of useful primary
radiation
• The proportion of primary radiation removed usually is about
10-15%
• This primary absorption creates on the film an overall
pattern of thin parallel white, (clear) lines which are infact
the shadows of lead strips, these are known as GRID LINES
which might disturb viewing anatomic details or same fine
details
• It
is
usually
arranged
either
for
this
pattern
to
be
sufficiently fine lines as to be not noticed, or for the lines
to be blurred by using a moving grid
• All
this mean
that removal
of
some primary
radiation requires the exposure to be increased if
film density is to be maintained
• In the absence of grid, the scattered radiation
would, also contribute to film density, so the tube
exposure should be further increased
• The increase in exposure is by increasing the Kv,
which sacrifices little extra contrast by use of
grid, but minimized the increase in patient dose
GRID CHARACTERISTICS
1. THE NUMBER OF STRIPS OF LEAD PER CM (N):
• The
length of each strip and the total width of grid can vary
between about 20 cm upto 43 cms, depending upon the film size
with which it is used
• The greater the no. (N) of strips the less will be grid lines
visible to the radiologist. What is more important is that these
lines must be straight and parallel
• The manufacture of uniform grid with large no. of lines/cm is
costly
• Commonly used grids have between 20 to 28 lines although good
grids have as many as 40 lines per cm
2. GRID RATIO:
• Grid ratio is defined as the height of the lead strip
compared to the distance between the strips
• The grid ratio measures the narrowness of the slit
through which primary radiation passes and also
determines the extent to which oblique moving scatter
is absorbed
• Grid ratio (r) is defined by
R =
h is height of lead strips
h
D
D is their separation or interspace
• Higher ratio leads to greater restriction in the
volume from which scattered radiation can reach
the film, so high grid ratio is preferred
• Usually value of about 5-8 to 10-12 are used and
in high Kv work grids with ratio of 16 is also used
• Grids are manufactured with the following ratios
as standard:
- Parallel and Focused N30 r 6, 8, 10
- Parallel and Focused N40 r 6, 8, 10, 12
- Parallel and Focused N70 r 6, 8, 10, 12, 16
- Other ratios can be manufactured on special
request
3. LEAD CONTENT:
• SELECTIVITY =
Transmitted primary radiation
Transmitted scatter radiation
• An efficient grid should have high selectivity
• Recent
work
content/unit
has
area
shown
that
(measured
as
actual
gm/cm2)
lead
is
determining factor for efficiency of grid
• A heavy grid is more efficient than light one
• Values of lead content in commonly used grids
range from 0.2 gm/cm2 to 0.9 gm/cm2 and should
be properly arranged
4. GRID FACTOR:
• The grid factor represents the extent to which
the x-ray tube exposure must be increased
to
compensate for the loss of primary and scattered
radiation reaching the film
• Typical values of grid factor range between about
2 to 6, the value depending on type of grid and
how much scatter there is to remove
5. INTER SPACE MATERIAL:
• Usually plastic is used betweent the lead strips
and the whole things is covered in a sealed
aluminium container to protect it from the humidity
or mechanical damage
6. THE THICKNESS OF LEAD STRIP:
• The usual thickness that is employed is about
0.005-0.008 cms
• More thickness - coarse grid lines
• Less thickness - Scattered photons cannot be
stopped
• Hence the compromise between the two
7. FOCAL RANGE:
• Focal range is the distance interval in which
an X-ray tube can be used with a certain
grid. With lower ratio grids, the focal
range is considerably wider, with a higher
ratio, the range is narrower
TYPES
OF
GRIDS
STATIONARY GRIDS
1. Linear grids
2. Focused grids
3. Pseudo-focused grids
4. Crossed grids
MOVING GRIDS
1. Single stoke
2. Oscillating and reciprocating
LINEAR OR PARALLEL GRIDS
• Lead strips arranged strictly parallel to
each other
• The overall size of grid may be quite large
and has completely to cover the whole film
area
DISADVANTAGE:
• Cut of the beam at the border as primary radiation
encounter the strips obliquely, so the grid should be
placed close to x-ray source
• At a large distance primary beam can be completely cut
off
• High ratio grid may be used only with small films or
with large film focus distance
• Cut off can occur in centre of the film if the grid
is not exactly perpendicular to the beam central
axis
• To compensate this FOCUSSED grid is used
FOCUSSED GRID
DISADVANTAGE:
• Should be used at a distance at which it is
designed
• The central ray must pass through the grid centre
1 cm error can be included
• Correct the face of the grid towards the tube
PSEUDO FOCUSSED GRIDS
• Manufacturing is extremely difficult, as failure to
achieve uniform rid pattern may mar the visual
appearance and usefulness of radiograph
• This is constructed by reducing the height of
strips progressively from centre to edge
CROSSED GRIDS
• Two grids of any type (mainly parallel) are used, placed
on top of each other act at mutually right angle to
each other
• Such combination is effective in removing scattered
radiation
• In these circumstances a high grid ratio is required and
often better to use crossed grid of smaller ratio e.g.
Two crossed grid of ratio 7 is better than single grid
of 15
• When
using
crossed
grid,
x-ray
beam
must
be
perpendicular and no large amount of angulation to any
direction
MOVING (Bucky) GRIDS
• To remove the fine lines when wing stationary
grids, one solution is to move the grid side ways
across the film during the exposure, then the
shadow of grid strips are blurred out
• For lines to be effectively blurred out grid must
move distance equal to 3-4 interspaces during the
time of exposure
• If not, there will be incomplete blurring of lines
• The required minimum speed is determined by
exposure time and no. of grid lines/cm
• The good average installation would have a grid
movement and grid with the characteristics in the
range of
- Ratio: 8-10
- Focussing Distance: 90 cms
- Movement: 0.6-1.2 cm either side of the of the
centre
- Lines/cm: 30-34
• Moving grids were introduced by DR. POTTER
in
1920 and since then many design of moving mechanism
have been suggested and used mainly to overcome
stoboscopic effect
• SINGLE STROKE:
- Spring is used to pull the grid at uniform speed
across film
- Speed of movement controlled by adjustable oil
dash pot
- Overall movement is about 2.5 cms for about
0.2 to 1.5 seconds
DISADVANTAGE:
- This has to be set manually before each exposure
OSCILLATING AND RECIPROCATING
MOVEMENTS
• Here the necessity of resetting the grid before every
exposure
is
avoided
as
the
grid
is
always
moving
at
sufficiently high speed
• The grid moves to and fro across the film repeatedly during
exposure
DISADVANTAGE:
• There is space of 5 cms required between patient and film to
accommodate the movement thus causing magnification and
geometric unsharpness