Filters grids and collimators

1. FILTERS,
COLLIMATORS
AND GRIDS
Presented by: Dr Georgina George
Moderator: Dr Kartik Dattani

2. FILTERS
Filtration is the process of shaping the x-ray beam to increase the amount of
high energy photons (useful) and decrease the low energy photons,
resulting in improved contrast and decreased patient radiation.
TYPES
1. Inherent filtration
2. Added filtration
3. The patient

4. 1. INHERENT FILTRATION
Filtration performed by the x-ray tube and its housing is called as inherent
filtration.
Materials responsible :
● Glass envelope enclosing anode and cathode
● Insulating oil surrounding the tube
● Window in the tube housing
Measured in ALUMINIUM EQUIVALENTS.

5. When can unfiltered radiation be useful?
● Filtration reduces the mean energy of
an x-ray beam which in turn reduces
tissue contrast.
● The decrease is insignificant in the higher
energy range but with lower energy
radiation, under 30 kVp it interferes
with image quality.
● BERYLLIUM WINDOW TUBES (exit portal)
are used to produce an unfiltered beam.

6. 2. ADDED FILTRATION
● Added filtration results from absorbers
placed in the path of the x-ray beam.
● Aluminium is an excellent filter for low energy
photons.
● Copper is a better filter for high energy
radiation
● Compound filter = Aluminium + Copper =
decreased thickness.

7. FILTER THICKNESS

8. FILTERED VS UNFILTERED RADIATION

9. EFFECT OF FILTERS ON PATIENT EXPOSURE

10. WEDGE FILTERS
Used at places where the body part to be radiographed varies greatly in
densities. (thick on one side and thin on the other)

11. HEAVY METAL FILTERS
Purpose is to produce a beam that has a high
number of photons in the specific energy range
that will be most useful in diagnostic imaging.
These filters use the K-EDGE ABSORPTION of the
element.
The k-edge properties of certain materials can be
specifically chosen for its use in contrast media,
and beam filters.

12. HEAVY METAL FILTER IN IMAGING IODINE

13. X-RAY BEAM RESTRICTORS
A device that is attached to the opening in the x-ray tube housing to regulate
the size and shape of an x-ray beam.
1. APERTURE DIAPHRAGMS
● A sheet of lead with a hole in the center.
● Size of the hole determines the size and shape of the
x-ray beam
● Aperture can be altered to any size and shape.
DISADVANTAGE:
● Large penumbra

14. 2. CONES AND CYLINDERS
They reduce the unexposed area at the
edges by reducing penumbra.
Flared shape of the cone is similar to the
geometric shape of x-ray beam
DISADVANTAGES:
● Limited number of field sizes
● Changing them is inconvenient
● Cone cutting- when x-ray beam and
cone and receptor are not aligned

15. COLLIMATORS
The best x-ray restrictor.
Advantages :
● Provides an infinite variety of rectangular x-ray fields
● A light beam shows the centre and exact configuration of the x-ray field

19. POSITIVE BEAM LIMITING DEVICES
They are automatic motor driver collimators
Also known as automatic light-localized variable-aperture collimators
A perfectly aligned collimator leaves unexposed borders on all sides of the developed film.
It is required that total misalignment of the edges of the light field with the respective edges of the x-ray
field shall not be greater than 2% of the distance from focal spot of the x-ray tube to the center of the field.

20. FUNCTIONS OF COLLIMATORS
I. PATIENT PROTECTION
Smaller the x-ray field, smaller the volume of the patient that is irradiated.

21. II. DECREASE SCATTER RADIATION
The quantity of scatter radiation reaching an x-ray film depends on field size,
ie, the larger the field the more the scatter radiation.

22. GRIDS
Radiographic grids consists of a series of lead foil strips separated by x-ray
transparent spacers.

23. GRID RATIO
● It is defined as the ratio
between the height of the
lead strips and the distance
between them.
● Ratio is always expressed as,
for eg: 8:1 (where 8 is the
actual ratio and the second
number is always 1)

24. GRID FREQUENCY
● Number of lead strips per inch/ cms of the grid
● Usually 24-45 lines per cm

25. GRID PATTERN
Grid pattern refers to the orientation of the lead strips in their longitudinal
axis
It is the pattern of the grid that we see from a top view.
1. LINEAR GRID

26. 2. CROSSED GRID
Made of two superimposed grids that have the same focusing distance.
The grid ratio is equal to the sum of the ratios of the two linear grids.

27. 3. FOCUSED GRID
Made of lead strips that are angled slightly so that they focus in space.

28. In practice grids have a focusing range that
indicates the distance within which the grid can
be used without a significant loss of primary
radiation.
Its fairly wide for a low-ratio grid and narrow
for a high-ratio grid.
For eg:
● A 5:1 grid focused at 40 inches has a
focusing range of approximately 28-72
inches.
● A 16:1 grid focused at 40 inches has a range
of only 38-42 inches.

29. 4. PARALLEL GRID
The lead strips are placed parallel when viewed in cross section

30. EVALUATION OF GRID PERFORMANCE
To help in grid selection and design, several tests have
been devised based of three methods.
1. PRIMARY TRANSMISSION (Tp)
● Is the measurement of the percentage of
primary radiation transmitted through a
grid.
● Requires a special equipment to measure
primary transmission of grid.

31. Two measurements must be taken to determine Tp.
1. Intensity of radiation transmitted through the grid (with grid in place)
2. Intensity of radiation directed at the grid (after removal of grid)
The measured primary transmission is always lesser than the calculated
primary transmission.
The difference is largely due to absorption by the interspace material or
manufacturing defects in the focusing of the lead strips.

32. 2. BUCKY FACTOR (B)
Ratio of incident radiation falling on the grid to the transmitted radiation
passing through the grid.
The Bucky factor is similar to Tp except that Tp indicates only the amount of
primary radiation absorbed by a grid, whereas the Bucky factor indicates
the absorption of both primary and secondary radiation.
Measures ability to absorb scatter radiation.
If the Bucky factor for a particular grid-energy
combination is 5, the exposure factors and
patient exposure both increase 5 times over
what they would be for the same examination
without a grid.

33. 3. CONTRAST IMPROVEMENT FACTOR (K)
Ratio of the contrast with a grid to the contrast without a grid.
Unfortunately , K factor depends on :
❖ kVp
❖ Field size
❖ Lead content (most important)
Higher the grid ratio, higher is the K factor

35. LEAD CONTENT
The amount of lead in a grid is a good indicator of its ability to improve
contrast.

36. GRID CUTOFF
Grid cutoff is the loss of primary radiation that occurs when the images of the
lead strips are projected wider than they would be with ordinary
magnification.

37. SITUATIONS THAT CAUSE CUT OFF
1. UPSIDE DOWN FOCUSED GRID
The grid lines go opposite to the angle of divergence of x-ray beam.

38. 2. LATERAL DECENTERING
When the x-ray tube is positioned lateral to the
convergent line but at the correct focal distance.
There is a uniform loss of radiation over the entire
surface of the grid, producing a uniformly light
radiograph (all strips cut off same amount of primary)
Equation for calculation the loss of primary radiation=

39. 3. OFF LEVEL GRIDS
When a linear grid is tilted, there is a
uniform loss of primary radiation
across the entire surface of the grid.
The effect on film is the same as lateral
decentering.

40. 4. FOCUS-GRID DISTANCE DECENTERING
The target of the x-ray is correctly centered, but is positioned above or below the
convergent line.
1. If the target is above the convergent line, it's called far focus-grid distance
decentering.
2. If the target is below the convergent line, it’s called near focus-grid distance
decentering.

41. 5. COMBINED LATERAL AND FOCUS GRID DISTANCE DECENTERING
It causes an uneven exposure,
resulting in a film that is light on one
side and dark on the other side
● The amount of cutoff is directly
proportional to the grid ratio
and decentering distance, and
● inversely proportional to the
focal distance of the grid.
CUT OFF IS GREATEST ON THE SIDE
DIRECTLY UNDER THE X-RAY TUBE
IN FAR FOCUS DECENTERING

42. CUTOFF IS LEAST ON THE
SIDE UNDER THE X-RAY TUBE.
With equal decentering errors,
the amount of cutoff is
greater with combined
decentering below the
convergent line than with
above the convergent line.

43. MOVING GRIDS (POTTER-BUCKY GRID)
● Grids are moved to blur out the shadows
cast by the lead strips (eliminate grid lines)
● Grid lines are produced due to absorbed x-
rays by the strips, which can be reduced by
moving the grid.
● Cannot be used in portable machines.
Two types:
1. Reciprocating- move 1-3cm back and forth.
2. Oscillating- several times about 2-3cm in circular
pattern.

44. DISADVANTAGES
1. Costly
2. Subject to failure
3. May cause vibration of the x-ray table
4. Increased exposure time as they move slowly
5. Increased distance between patient and receptor causing unwanted
blur and magnifications.

45. GRID SELECTION
● Grids with smaller ratios are preferred to reduce exposure.
● High ratio grids have increased patient exposure and x-ray
tube centering becomes critical.
● Below 90 kVp = 8:1 grids
● Above 90 kVp = 12:1 grids
● Crossed grids are used only when there is great deal of
scattered radiation
Eg: biplane cerebral angiography

46. AIR GAP TECHNIQUE
● Scattered radiation from the patient due to
Compton reactions disperses in all directions,
so the patient acts like a LARGE LIGHT BULB.
● The closer the patient to the film, 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
● Scatter radiation decreases not from
filtration but from the scattered photons
missing the film

47. OPTIMUM GAP WIDTH
Thicker the part to be imaged, larger the air gap
1. The first inch of any air gap improves contrast more than any
subsequent inch
2. Image sharpness deteriorates with increasing gap width,
unless the focal distance is increased to compensate for
greater magnification
3. If the gap is widened by moving the patient away from the film
with a fixed focal distance, the patient is closer to the x-ray
tube and his exposure increases.

48. SUMMARY
FILTERS are sheets of metal placed in the path of the x-ray beam near the
tube to absorb low energy radiation.
● Aluminum is most commonly used
● FILTER THICKNESS ∝ ATTENUATION
● 2.5mm of aluminum filtration is used for x-ray beam energies greater
than 70 kVp
● Heavy metal filters/ K-edge filters are used to remove higher energy
photons from the x-ray beam

49. X-RAY BEAM RESTRICTORS
They regulate the size and shape of x-ray beam to
● Reduce area of exposure
● Reduce the amount of scatter radiation
COLLIMATORS are the best general purpose beam restrictors
● Illuminate the x-ray field
● Field can be adjusted to an infinite variety of rectangular
shapes and sizes.

50. GRIDS
Consist of lead foil strips separated by x-ray transparent spacers.
● They absorb scattered radiation and improve image contrast.
● High ratio grids with higher lead content have the highest contrast
improvement factors.
● Grid cutoff is the loss of primary radiation that occurs when the images
of the lead strips are produced wider than they would be with ordinary
magnification
● Cutoff is greatest with high ratio grids and short focusing distances.
● Moving grids eliminate the image of lead strips from the film.
● Air gaps are an alternative to grids.

51. THANK YOU