Filters in radiology

1. FILTERS &
GRIDS IN RADIOGRAPHY

2. FILTERS
• Filtration - process of shaping x-ray beam to increase the ratio of photons
useful for imaging to those photons that increase patient dose or decrease
image contrast.
• First few centimeters of tissue - more radiation than the rest of the
patient,hence can be protected by absorbing the lower energy photons
• Interposing a filter material between the patient and the x-ray tube

3. • Beginning at the x-ray source, these are as follows:
1.The x-ray tube and its housing (inherent filtration)
2. Sheets of metal placed in the path of the beam (added filtration)
3.The patient

4. INHERENT FILTRATION
• Filtration resulting from the absorption of x rays as they pass through the x-
ray tube and its housing is called inherent filtration.
• By glass envelope enclosing the anode and cathode, the insulating oil
surrounding the tube, and the window in the tube housing.
• Measured in aluminum equivalents (varies between 0.5 and 1.0 mm)
• Beryllium window tubes - produce an essentially unfiltered beam when loss
of contrast is detrimental to image quality

5. ADDED FILTRATION
• Added filtration results from absorbers placed in the path of the x-ray beam
• Aluminium and copper are the used.
• Aluminium (13) –m/c used & excellent filter material for low energy radiation
,good general purpose filter. Copper (29) is a better filter for high energy
radiation.
• A compound filter - two or more layers of different metals.
• Most filtration occur in the copper, aluminum serves to absorb the
characteristic radiation from copper

6. FILTERTHICKNESS
• Two millimeters of aluminum absorb
nearly all photons with energies less
than 20 keV, so most of the
advantages of filtration are achieved
by this thickness.

7. • The intensity on the low energy side
of the curve (left) is reduced
considerably more than the
intensity on the high energy side of
the curve (right), and the highest
point in the curve is shifted from 25
to 35 keV.

8. Effect of Filters on Patient Exposure

9. Effect on Exposure Factors
• Major disadvantage of filtration - reduction in intensity of x-ray beam.
• Compensated by increasing exposure factors (mAs)
• x-ray tube puts out more photons ; filter absorbs many of them eventually
total number reaching patient actually decreases.

10. Wedge Filters
• Used to obtain films of more
uniform density when part being
examined varies greatly in
thickness from one side of field to
other.
• Shaped like a wedge.
• Less radiation is absorbed by the
thinner part of the filter, so more is
available to penetrate the thicker
part of the patient.
• used in lower-limb angiography

11. HEAVY METAL FILTERS (K-Edge Filters)
• These filters make use of K-absorption edge of elements with At no. greater
than 60, when imaging barium or iodine.
• This maximum contrast is obtained when photon energy of x-ray beam is
close to, but slightly above, the K absorption edge of the absorber
• attenuation has a relative maximum immediately above the K edge.
• HMF - produce x-ray beam that has a high number of photons in diagnostic
energy range
• Eg gadolinium , holmium

13. • The "window" in the 33 to 55
keV range.
• In this window high
transmission by holmium filter
overlaps the region of high
attenuation by iodine
• In this "overlap window" of
about 20 keV that affords
improved contrast when
imaging iodine or barium when
x-ray beam filtered by a heavy
metal filter is compared to that
produced by an aluminum filter.

14. molybdenum filters
• A special application of K edge filters
use of molybdenum filters with molybdenum target x-ray tubes for mammography

15. GRIDS
• Grid has series of lead foil strips separated by x-ray-transparent spacers.
• Invented - Dr. Gustave Bucky in 1913
• Use -removing scatter radiation from large radiographic fields.
• Scatter radiation arises from many points within the patient so is
multidirectional - most of it is absorbed by lead strips and only a small
amount passes between them.
• Interspace grids made of Aluminum

17. Grid ratio
• Ratio between the height of the lead strips and the distance between them.
• Used to express a grid's ability to remove scatter radiation.
• Range from 4:1 to 16: l.
• Higher the ratio, the better the grid functions.

18. Grid pattern
• It is the orientation of the lead strips in their longitudinal axis.
• More precisely the pattern of grid that we see from top view.
• The two basic grid patterns - linear and crossed.
• LINEAR GRID - lead strips are parallel to each other in their longitudinal axis.
Most x-ray tables are equipped with linear grids.
• CROSSED GRID - made up of two superimposed linear grids that have the
same focusing distance
• The grid ratio of crossed grids is equal to the sum of the ratios of the two
linear grids.

19. • FOCUSED GRID -a grid made of lead strips that are angled slightly so that
they focus in space
• May be either linear or crossed.Most grids are focused.
• Linear focused grids converge at a line in space - convergent line.
• Crossed grids converge at a point in space -convergent point.
• The focal distance is perpendicular distance between grid and convergent
line or point.
• It indicates distance within which grid can be used without a significant loss
of primary radiation. It is wide for a low-ratio grid and narrow for a high-
ratio grid

21. • PARALLEL GRID - the lead strips are parallel when viewed in cross section.
They are focused at infinity hence no convergent line.
• Used in fluoroscopic spot film devices.

22. Lines per inch
• Lines per inch is the number of lead strips per inch of grid.

23. EVALUATION OF GRID PERFORMANCE
• Grids - improve contrast by absorbing secondary radiation before it reaches
the film.
• The ideal grid does not exist
• Give maximum film contrast without an unnecessary increase in patient
exposure.
• Grids with high ratios give maximum contrast.
• Performance evaluation by:
l. primary transmission (Tp)
2. Bucky factor (B)
3. contrast improvement factor (K)

24. Primary transmission
• Primary transmission is a measurement of %
of primary radiation transmitted through a
grid.
• Ideal grid should transmit 100% of the
primary radiation
• The equipment for measuring primary
transmission
• The x-ray beam is collimated to a narrow
pencil of radiation, and the phantom is
placed a great distance from the grid.With
this arrangement, no scatter radiation
reaches the grid.

25. • There is a significant loss of primary radiation with grids
• The equation of anticipated primary transmission is given below. It was
generally lower for cross grids
• The equation for fractional transmission is below.
• The measured primary transmission is always less than calculated primary
transmission.

26. BUCKY FACTOR
• It is the ratio of incident radiation falling on grid to transmitted radiation
passing through the grid
• indicates how much we must increase exposure factors and how much the
patient's exposure dose is increased - when we change from a nongrid to a
grid technique.
• Indicates the absorption of both primary and sec radiations

27. • B= incident radiation with grid
removed /transmitted radiation with
grid in place
• High-ratio grids absorb more scatter
radiation & have larger Bucky factors
than low-ratio grids.
• Higher the Bucky factor, greater the
exposure factors & radiation dosage
to the patient

28. Contrast Improvement Factor
• It is the ultimate test of grid performance as it is measure of a grid's ability
to improve contrast, which is its primary function.
• It depends on kV p, field size, and phantom thicknes
• It is related to lead content of grid.

29. • Larger the quantity of scatter radiation- poorer contrast - lower the
contrast improvement factor
• Higher the grid ratio, higher the CIF

30. LEAD CONTENT
• The lead content of a grid is expressed in g/cm2
• Is a good indicator of its ability to improve contrast,
• If the grid ratio remains constant and number of lines per inch is increased,
the lead content must decrease.
• The only ways to increase number of lines per inch is by decreasing the
thickness of either the lead strips or interspaces.

31. GRID CUTOFF
• The loss of primary radiation that occurs when images of lead strips are
projected wider than they would be with ordinary magnification
• disadvantage of grids - increase the amount of radiation that the patient
receives & grid cut off.
• REASON - poor geometric relationship between the primary beam and the
lead foil strips of the grid
• The resultant radiograph will be light in the area in which the cutoff occurs.

32. • The amount of cutoff is greatest with high-ratio
grids and short grid-focus distances
• There are four situations that produce grid cutoff:
 focused grids used upside down
lateral decentering (grid angulation)
focus-grid distance decentering
combined lateral and focus-grid distance
decentering

33. Upside Down Focused Grid
• All focused grids have a tube side, which is side of focus of the lead strips.
• When a focused grid is used upside down- there is severe peripheral cutoff
with a dark band of exposure in center of film & no exposure at periphery.
• .The higher grid ratio- narrower the exposed area.

35. Lateral Decentering
• CAUSE- xray tube being positioned lateral to the convergent line but at
correct focal distance
• All lead strips cut off the same amount of primary rdtn - uniform loss of
radiation over entire surface of grid ---- producing a uniformly light
radiograph.
• This is probably the most common kind of grid cutoff

36. • As LD increases, films become progressively lighter but exposure is uniform
(center and edges of film equally exposed)
• 3 factors affecting magnitude of cutoff from LD: grid ratio, focal distance
&the amount of decentering
• The loss of primary radiation due to LD can be minimized with low-ratio
grids & a long focal distance.

38. • Off-Level Grids.
When a linear grid is tilted (in portable radiography) there is a uniform loss of
primary radiation across the entire surface of the grid
The effect on the film is the same as the effect of lateral decentermg.

39. Focus-Grid Distance Decentering
• The target of the x-ray tube is correctly centered to the grid, but it is
positioned above or below the convergent line.
• If the target is above the convergent line, it is called far focus-grid distance
decentering;
• If the target is below the convergent line, it is called near focus-grid distance
decentering.
• The cutoff is greater with near than with far focus-grid distance
decentering.

40. • The central portion of the film is not affected, but the periphery is light.
• The loss of primary radiation is directly proportional to the grid ratio and
distance from the center line.
• Loss of primary radn inc as the grid ratio inc and as distance from center of
the grid inc.
• Parallel grids are focused at infinity - they are always used with near focus-
grid distance decentering & have low grid ratio to dec cutoff.

42. Combined Lateral and Focus-Grid Distance
Decentering
• The most commonly grid cutoff is from combined lateral and focus-grid
distance decentering.
• Results in a film that is light on one side and dark on the other side.
• 2 types- tube target is above or below convergent line.
• The amnt of cutoff is prop to the grid ratio & decentering distance inv prop
to focal distance of grid.
• The projected images of lead strips - broader on side opp tube target &
film is light on the far side.

44. MOVING GRIDS
• Grids are moved-1 to 3 cm back and forth to blur out the shadows cast by
lead strips.
• In moving grid to avoid grid lines 1) grid must move fast enough to blur its
lead strips. 2)Transverse motion of grid should be synchronous with pulses
of x-ray generator.
• Disadvantages - costly, subject to failure,vibrate x-ray table ,put limit on
minimum exposure time & incr patient's radn dose

45. GRID SELECTION
• 8:1 grids give good results below 90 kVp , above 90 kVp 12:1 grids are
preferred.
• In Crossed grids there is a great deal of scatter radiation, as in biplane
cerebral angiography.

46. AIR GAPTECHNIQUES
• The closer the patient is to the film, the greater the concentration of scatter
per unit area.
• With an air gap, the concentration decreases because more scattered
photons miss the film .
• Scatter radiation decreases not from filtration but from scattered photons
missing the film.
• Many scattered photons from input surface are absorbed during their long
journey through pt , whereas those originating near exit surface have only a
short escape distance.

47. • A large air gap removes a larger percentage of scatter radiation
• Used in magnification radiography chest radiography
• With magnification techniques , object-film distance is optimized for the
screen-focal spot combination & air gap reduces scatter radiation to
acceptable levels
• The air gap loses less primary radiation, so the patient's exposure is less

48. • The following four guidelines should be used to select a gap width:
• The thicker part, more advantageous a larger gap
• The first inch of air gap improves contrast more than any subsequent inch.
• Image sharpness deteriorates with increasing gap width unless focal-film
distance is inc to compensate for greater magnification.
• If the gap is widened by moving the patient away from the film, the patient
is closer to x-ray tube and his exposure increases. - corrected by inc in FFD

50. • Magnification with Air Gaps
• Magnification is greatest with a short focal-film distance and a long object-
film distance.
• Image sharpness deteriorates with magnification.
• The objective is to preserve image sharpness by lengthening the focal-film
distance until magnification returns to pre-air-gap levels.