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FILTERS,
COLLIMATORS
AND GRIDS
Presented by: Dr Georgina George
Moderator: Dr Kartik Dattani
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
Filters grids and collimators
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.
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.
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.
FILTER THICKNESS
FILTERED VS UNFILTERED RADIATION
EFFECT OF FILTERS ON PATIENT EXPOSURE
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)
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.
HEAVY METAL FILTER IN IMAGING IODINE
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
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
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
Filters grids and collimators
Filters grids and collimators
Filters grids and collimators
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.
FUNCTIONS OF COLLIMATORS
I. PATIENT PROTECTION
Smaller the x-ray field, smaller the volume of the patient that is irradiated.
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.
GRIDS
Radiographic grids consists of a series of lead foil strips separated by x-ray
transparent spacers.
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)
GRID FREQUENCY
● Number of lead strips per inch/ cms of the grid
● Usually 24-45 lines per cm
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
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.
3. FOCUSED GRID
Made of lead strips that are angled slightly so that they focus in space.
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.
4. PARALLEL GRID
The lead strips are placed parallel when viewed in cross section
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.
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.
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.
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
Filters grids and collimators
LEAD CONTENT
The amount of lead in a grid is a good indicator of its ability to improve
contrast.
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.
SITUATIONS THAT CAUSE CUT OFF
1. UPSIDE DOWN FOCUSED GRID
The grid lines go opposite to the angle of divergence of x-ray beam.
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=
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.
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.
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
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.
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.
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.
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
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
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.
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
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.
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.
THANK YOU

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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.
  • 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.

Editor's Notes

  • #3: When exposure is done, both high and low energy photons are produced High energy penetrate while low energy get absorbed in the body increasing patient dose which means the first few cms receive much more radiation than the rest. This tissue can be protected by absorbing the lower energy photons from the beam by using FILTERS
  • #5: AQ: thickness of aluminium that would produce the same degree of attenuation as thickness of material in question Varies between 0.5 to 1.0 mm Disadv: reduces image contrast
  • #6: Beryllium has atomic number of 4 and is more transparent to low energy radiation than glass
  • #7: Ideally a filter material should absorb all low energy photons and transmit all high energy photons. Unfortunately no such material exists. What can be done? Attenuation is most intense when photoelectric effect > compton reactions. It is inconvenient to change filters between examinations, hence radiologists mostly use aluminium filters. The layers are arranged in such a way that, higher atomic number element faces the x-ray tube and lower number faces the patient. Most filtration occurs with copper, aluminium absorbs the characteristic radiation from copper. (8 keV) Its own radiation (1.5 keV) is absorbed by the air gap between patient and filter.
  • #8: More the thickness of filters, more the attenuation of x-rays by it. Ie attenuation ∝ filter thickess In the table, all high energy photons are attenuated by 10mm because the low energy are absorbed by the thinner filters. An aluminium filter >3mm offers no advantage, Quality of beam is not altered, but intensity is diminished, hence the time required for exposure increases.
  • #9: Effect of aluminum filtration on 90 kvp beam The unfiltered beam has energies from 10-20 kev range. The highest point occurs at 25 kev Filtration reduces the total number of photons (area under curve), mostly it removes a large number of low energy photons. the hIghest point is shifted from 25 to 25 kev Over all effect is increase in mean energy of xray beam.
  • #10: multiple radiographs of a 18cm thick pelvic phantom exposure times were adjusted to produce euqal density and the radiation exposure to skin was measured upto 80% DECREASE IN EXPOSURW with 3 mm of aluminum filtration
  • #11: Thin part is placed under the thick body part and vice versa. Less radiation is absorbed by thinner part of filter, so more is available to penetrate the thicker part of patient Used in lower limb angiography when one images from lower abdomen to the ankles with a single exposure.
  • #12: The K-absorption edge (K-edge) refers to the abrupt increase in the photoelectric absorption of x-ray photons observed at an energy level just beyond the binding energy of the k-shell electrons of the absorbing atom As atomic number increases, so does K edge. 2mm Al filter transmits a broad spectrum of bremsstrahlung Gadolinium filter 0.25mm thick, transmits increasing number of photons from 25 to 50.2 kev range The heavy metal filter transmits a narrower spectrum of energy (decreased high and low photons) Decrease in low energy photons decrease patient exposure and decrease in high will improve image contrast ie photoelectric>> compton Patient dose reduction upto a factor of 2 and mAs increases by a factor of 2 or more Improved contrast is maximal for thin bodies The need for more mAs is minimized by use of low kVp techniques. (more useful for pediatric patients)
  • #13: Mass attenuation coeff of iodine is compared to that of holmium. The mass attenuation coefficient (also known as the mass absorption coefficient) is a constant describing the fraction of photons removed from a monochromatic x-ray beam by a homogenous absorber per unit mass. The ac of I increases at 33.17kev and then decreases with an increase in photon energy Conversely, the ac of Holmium decreases steadily from 33 to 55.6kev at which attenuation by holmium increases dramatically 33 to 55 kev = window= high transmission by the holmium filter overlaps the region of high attenuation by iodine This overlap window of about 20 kev affords improved contrast when imaging iodine or barium
  • #14: The center of the x-ray field is exposed by the entire focal spot, but the periphery of the field sees only a portion of the focal spot, this is known as penumbra Reduced by positioning diaphragm as far away from target as possible with the help of a cone.
  • #17: 2 sets of shutters(s1 and s2) control the beam dimensions. They move together as a unit so that s2 aligns with s1 to clean up its penumbra Each shutter consists of 4 or more lead plates, which move as independent pairs. One pair can be adjusted without moving the other= infinite variety of square and rectangular fields.
  • #18: The x-ray field is illuminated by a light beam from a light bulb which is deflected by a mirror mounted in the path of the x-ray beam at an angle of 45 degrees. The target of xray tube and light bulb should be exactly the same distance from the center of the mirror, otherwise one might be smaller in field compared to the other.
  • #19: Collimator can also identify the center of the xray field. This is accomplished by painting a cross line on a thin sheet of plexiglass mounted t the end of the collimator They also have a backup system for identifying dield size in case the light bulb burns out. The xray field for various target-film distances is indicated by a calibrated scale in the front.
  • #20: When a cassette is loaded into the film holder, sensors in the tray identify the size and alignment of the cassette, and position the shutters to exactly match the size of film being used.
  • #21: If a 20x20 cm field is collimated to 10x10 cm, the area of patient that is irradiated, decreases from 400 to 100cm2 Round fields expose portions of the patient that are not even included on the film, so x-ray beam restrictors were made square. But some parts are better examined with round fields, such as gallbladders and paranasal sinuses.
  • #22: The exact contribution from scatter depends on the thickness of the part being examined and on the energy of the x-ray beam. The amount of primary radiation at the plane of the film is independent of field size, because the number of transmitted photons per unit area is the same no matter the field area. (PRIMARY LINE IN THE GRAPH) As field size is enlarged, the amount of scatter increases rapidly at first and then tapers off with larger fields.( reaches maximum around 30 x 30 cm) As the x-ray field is decreased, the exposure factors must be increased to maintain film density.
  • #23: Invented by dr gustav bucky in 1913, still the most effective way of removing scatter radiation from large radiographic fields. Used when anatomy >10cm= abdomen, skull, spine, breast (4:1) Primary radiation is oriented in the same axis as the lead strips and passes between them, to reach the film unaffected by the grid. Scatter radiation arises from many points within the patient and is multidirectional, most of it is absorbed by the lead strips. The interspaces are filled with aluminum or some organic compound to support the thin lead foil strips. Aluminum helps absorb scattered rays not absorbed by lead strips, but they also absorb more primary radiation increasing patient exposure.
  • #24: Here the ratio is 2/0.25= 8 The interspaces are much thicker that the lead strips. Ratios generally range from 4:1 to 16:1 Higher the ratio, better the grid functions as high ratio grids allow lesser angle of scatter but it increases the patient dose.
  • #26: In a linear grid the lead strips are parallel to each other in their longitudinal axis. Most xray tables are equipped with linear grids. Their major advantage is that they allow us to angle the x-ray tube along the length of the grid without loss of primary radiation from grid “cutoff”.
  • #27: A crossed grid made of two 5:1 linear grids has a ratio of 10:1 They cannot be used with oblique techniques requiring angulation of the x-ray tube
  • #28: Can be linear or crossed Linear focused grids converge at a line= CONVERGENT LINE Crossed grids converge at a point in space= CONVERGENT POINT FOCAL DISTANCE= perpendicular distance between the grid and the convergence line/point.
  • #30: Focused at infinity, hence no convergent line Can only be used effectively with either very small x-ray fields or long target-grid distances. Frequently used in fluoroscopic spot film devices
  • #31: The ideal grid wound absorb all secondary radiation with no absorption of primary radiation, giving out maximum contrast without unnecessary increase in patient exposure. This does not exist The price of better film contrast is increased exposure. We must always compromise one with the other. Grids with high ratios have maximum contrast but with increased exposure. The phantom is placed a great distance from the grid, with this arrangement no scatter radiation reaches the grid.
  • #33: incident= grid removed transmitted= grid in place High ratio grids absorb more scatter radiation and have larger bucky factors High energy beams generate more scatter radiation that low energy radiation It also tells us how much the patients exposure is increased by the use of a grid.
  • #34: This is the ultimate test, because its a measure of a grids ability to improve contrast which is its primary function. Factor of 1 says no improvement Most grids have 1.5-2.5 factors.
  • #36: There is a definite relationship between grid ratio, lead content and number of lines per inch. If the lead strips are made thinner, the numb of lines per inch increases without affecting grid ratio (at the cost of a large decrease in lead content) If the numb of lines is increased then the lead strips must be made shorter to keep grid ratio constant (again with decrease of lead content) In practice when grids are constructed with many lines per inch, they improve contrast lesser than grids of comparable ratios with fewer lines per inch.
  • #37: It is the result of a poor geometric relationship between the primary beam and the lead foil strips of the grid. Cutoff is complete and no primary radiation reaches the film when the projected images of the lead strips are thicker than width of interspaces. The resultant radiograph will be light in the area in which cutoff occurs. Amount of cutoff is greatest with high ratio grids and short grid focus distances.
  • #38: There is a severe peripheral cutoff with a dark band of exposure in the center of the film and no exposure at the films periphery. Higher the grid ratio, narrower the exposed area
  • #39: Most common kind. A series of film strips that were all taken with the same exposure factors but with increasing amount of lateral decentering. With the above formula, we understand that the amount of cutoff increases as the grid ratio and decentering distance increases, and cut off decreases as the focal distance increases The loss can be reduced with low ratio grids and long focal distances. This is more applicable in portable radiography, when exact centering is not possible. It takes 8 inches of lateral decentering for 100% cut off.
  • #41: CUTOFF IS GREATER WITH NEAR THAN FAR FOCUS GRID DISTANCE CENTERING The central portion is not affected but the periphery is light. The loss of primary radiation is directly proportional to grid ratio and distance from central line
  • #42: The projected images of the lead strips directly below the tube target are broader than those on the opposite side, and the film is light on the near side.
  • #44: To avoid grid lines- 2 precautions- Grid must move fast enough to blur its lead strips Transverse motion of grid should be synchronous with pulses of x-ray generator. Reciprocating starts moving when the anode starts rotating.
  • #45: Why is there an increase in exposure? Due to lateral decentering, as the grid moves, the tube is not centered directly over the center of the grid during most of the exposure. Advantages outweigh the disadvantages.
  • #46: Studies show that there is little decrease in transmitted scatter beyond 8:1 ratio and almost no change between 12:1 and 16:1 grids. The improvement is not worth the greater patient exposure.
  • #47: Grids are used in large radiographic fields, but an alternate method has similar results with less patient exposure Used in magnification radiology and CXR Why closer greater scatter? Because further parts of body have a natural gap which seperates the film from the scattering site. Many scattered photons are absorbed during their long journey Greater angle of scatter!!!
  • #48: In chest radiography, air gap is used and to restore sharpness focal-film distance is lengthened from 6 to 10 ft