Grids are used in radiography to reduce scattered radiation and improve image contrast. They consist of alternating strips of lead and low-density material. Parallel grids have strips parallel to the x-ray beam, while crossed grids have perpendicular strips for better scatter reduction. Moving grids oscillate during exposure to avoid grid lines on the image. Higher grid ratios provide more scatter reduction but also increase patient dose. Proper grid selection and positioning are important for diagnostic image quality while minimizing radiation exposure.

1. RADIOGRAPHIC GRIDS

2. GRIDS
• When a beam of X-ray passes through the
patient, the beam is absorbed and scattered.
The absorbed primary beam gives a useful
shadow while the scattered radiation will tend
to spoil the shadow.

3. GRIDS
• Scattered radiation contributes a constant
background fog to the film image. This will
increase the noise in the image. The ratio
between the amounts of scattered radiation
energy to the amount of primary radiation
energy at a point is called as scatter to primary
ratio (SPR).

4. GRIDS
• The SPR increases with thicker patient and
larger field sizes. For example, in a abdomen
radiography, only 20% of the photons
contributes to the image formation and the
other 80% energy goes as scattered radiation.
Hence, scattered radiation must be removed,
in order to increase the image contrast.

5. GRIDS
• The scattered radiation can be removed by a
grid, placed in between the film and the
patient. The grid consists of a series of parallel
lead or tantalum strips of thickness ‘c’ (50
mm) and of height ‘h’ separated by spacers of
low attenuating material of width ‘b’ (350
mm) . Aluminum or plastic fibers are used as
low attenuating spacers.

6. GRIDS
• The grid is positioned between the patient
and the detector, so that its long axis is
pointed towards the X-ray beam. The primary
X-rays coming out of the patient, passes
through the inter space, since it is parallel in
direction.

7. GRIDS
• The scattered X-rays, which are in non parallel
direction, strike the grid bars and get
absorbed. The ratio of the primary
transmission to the scatter transmission of a
grid is called the selectivity.

9. GRID RATIO
• The ability of the grid to discriminate against
scattered radiation is measured by the grid
ratio, which is defined as the ratio of the
height (h) to the width of the spacer (b)
between the lead strips. Grid ratio = h/b. As
the grid ratio increases, the grid removes
more scatter radiations.

10. GRID RATIO
• Typical grid ratio ranges from 4:1 to 16:1 and
strip line densities are 25–60 lines per cm. The
performance of a grid can be understood by
contrast improvement factor. It is the ratio
between image contrast with grid and image
contrast without grid at 100 kVp.

11. GRID RATIO
• Higher grid ratio provides higher contrast
improvement factor. However, it increases
patient dose, as it employs higher exposure
techniques. Bucky factor (Gustave Bucky,1913)
is another parameter which relates to the
patient dose. It is the ratio between the
patient dose with grid and patient dose with
out grid. Bucky factor increases with increase
of kVp and grid ratio.

12. TYPES OF GRID
• Grids may be classified as (i) parallel grid, (ii)
crossed grid, (iii) focused grid, and (iv) moving
grid (Potter-Bucky). In a parallel grid, the lead
strips are parallel to each other in their
longitudinal axis.

13. LINEAR GRID
• X-ray tables are provided with linear grids. It
is easy to design, but has the property of grid
cutoff. This means that the attenuation of
primary radiation is greater at the edges and it
can be partial or complete cutoff. The distance
of grid cutoff may be estimated from the ratio
of source to image distance (SID) to grid ratio.

16. Crossed grid
• made up of lead strips that are parallel to the
long axis and short axis of the grid. Usually, it
is designed with two parallel grids, that are
perpendicular each other. The grid ratio of
crossed grids is equal to the sum of the ratios
of the two parallel grids.

18. Crossed grids
• efficient in removing scatter radiations and
has higher contrast improvement factor and
high grid ratio. It is useful at high kVp and tilt-
table exposure techniques. The disadvantage
includes difficulty in positioning, proper
alignment of tube and table, and higher
patient dose. Crossed grid also suffers from
grid cutoff.

19. Focused grid
• made mainly to reduce grid cutoff. The lead
strip lies in imaginary radial lines of a circle,
whose centre is the focal spot. The strips are
parallel to the divergence of the X-ray beam.
The grids are marked with focal distance and
the side facing the target. If it is reversed, grid
cutoff may occur, hence enough care is
needed to position focused grid.

21. MOVING GRID
• When a focused or parallel grid is used, each
lead strip will appear on the radiograph as
very fine line. These lines may spoil the
information in the film. However, these lines
may be removed by moving the grid during
the radiographic exposure. This is the principle
of PotterBucky grid (Hollis E. Potter,1920).

22. MOVING GRID
• Generally, focused grids are used as moving
grids. The grid may be made to move
continuously in one direction. The grid motion
is timed by the exposure control of the X-ray
machine. It starts moving just before the X-
rays are turned on and continues to move
even after the exposure is off. The traveling
period of the grid should be greater then the
exposure time.

23. MOVING GRID
• There are two types of moving grids, namely,
(i) reciprocating grid and (ii) oscillating grid.
The reciprocating grid is driven by a motor and
the grid moves back and forth several times,
during exposure. The distance traveled may be
2 cm.

24. MOVING GRID
• In the oscillating type, the grid is kept in a
frame, which has 2–3 cm clearance on all
sides. An electromagnet pulls and releases the
grid before the exposure. The grid oscillates in
circular path about the frame and comes to
rest after 20–30 seconds.

25. MOVING GRID
• Moving grids increases the distance between
patient and film, resulting in magnification,
cassette motion and image blur. However, the
motion blur is undetectable and hence used
widely.

26. • The use of grid will always increase the
exposure, because it will absorb some of the
primary radiation. In order to reduce the
exposures, grids with smaller ratios are
preferred.

27. • Low ratio grids such as 8:1 is used with
energies up to 90 kVp. High-ratio grids such as
12:1 are preferred for high energy radiation. In
mammography, grid ratio of 4:1 or 5:1 is used.

28. • These grids produce films with better contrast
with increased patient dose. Grids are
generally used for body parts > 12 cm thick or
techniques > 70 kVp. Grid can produce
artifacts when improperly aligned.

29. References
• Christensen’s physics of diagnostic
radiology.4th ed.
• The physics of radiology and imaging.
(Thayalan)