1. Radiation Exposures Unit 2
Chapters 10, 15, 16, and 18

2. Objectives
Define filtration, inherent filtration, added filtration , compound filtration, compensating filtration and total filtration.
Explain concept of half-value layer equivalency measurements of filtration.
Appraise various types of filters for specific clinical situations.
Describe effect of filtration on entire x-ray beam.
Identify the factors that affect scatter.
Discuss types of scatter control used by radiographers
Explain the purpose, construction, advantages and disadvantages of beam-restricting devices.
Describe the effect of beam restriction on image quality and patient dose.
Explain the process of attenuation
Describe the basic composition of the human body and how they attenuate the x-ray beam
Explain the relationship of the patient to density/image receptor exposure, contrast, recorded detail and distortion of the recorded image.
Describe the purpose of a grid and its construction
Explain: material, ratio, frequency and lead content
Differentiate various grid patterns.
Describe: parallel vs focused grids and stationary vs moving grids.
Explain grid and technique selection based on : patient , procedure, dose and IR exposure.
Discuss proper use and errors with grids and how both effect the image.

3. Chapter 10
Filtration

4. Filtration
The process of eliminating undesirable low-energy photons by inserting absorbing material into the primary beam
Shape the photon emissions into a more useful beam
“hardens” the beam
Removes low energy (soft ) photons
Main Purpose
Decreases radiation dose to the patient by removing photons that would not enhance the image.

5. Filter Any material designed to selectively absorb photons
Placed between the tube( source) and the patient
Aluminum
Most common / standard filter material used
All other materials are measured against it ability to filter- Aluminum equivalency
(Al/Eq)
Alternated Materials
glass, oil , copper and tin

6. Measurement of Filtration
Express in half-value layer ( HVL ) – amount of absorbing material to reduce the intensity of the primary beam by ½ the original value
Also expressed in Al/Eq example : HVL = 2.0 mm Al/Eq
Federal govt. specifies min HVL for diagnostic tubes:
2.5 mm Al/Eq

7. Types of Filtration 2 types: Inherent – in the design of the tube
Added- between tube and image receptor

8. Inherent filtration Result of composition of tube and housing
Glass envelope
Oil surrounding tube
Glass window – where most of the filtration comes from
Total inherent is
Increases as tube ages because of vaporization of tungsten
HVL testing for QC tests for reduced tube efficiency because of the added filtration

9. Added Filtration
Any filtration that occurs outside the tube but before the IR
Absorb as many low-energy photons as possible while allowing max amount of high-energy photons through.
Aluminum used as a low energy absorber
Collimators are considered added_ 1.0 mm Al/Eq
Most comes from the silver on the mirror that reflects the positioning light.

10. Types of Added Filtration
Compound filtration ( AKA: K-Edge)
Uses two or more material that work together well to absorb
High atomic number material sandwiched with a lower atomic number material
Cooper ( 29) with Al ( 13)
Built to absorb characteristic photons ( K shell photons) created by the previous layer
The higher atomic number will absorb higher energies but Al is needed to absorb the lower energy photons created.
Examples: Thoreaus filter- used in radiation therapy
Some QC testing uses copper + Al filters

11. Types of Added Filtration
Compensation Filters
Designed to solve a problem of unequal subject densities
Produces a more uniform IR exposure
Components:
AL, Lead, leaded plastic or plastic – saline solution bag
Common types
Wedge- thicker portion is placed over thinner part- foot, t-spine
Trough ( double wedge) – chest to even out mediastinum

12. Total Filtration
Equal to the sum of inherent + added filtration but DOES not include compensating filters
Thickness depends on use of equipment
Above 70 kVp 2.5 mm Al

13. Effect on Output
As filtration increases- technical factors must increase to maintain IR exposure
Even though there is an increase in technique necessary for compensation- the ESE is decreased as compared to no filtration in place at lower techniques
See table 10-4 page 176

14. Beam Restriction
Chapter 15

15. Scatter
Product of Compton interactions ( scatter that contributes to fog and tech dose)
Not part of the useful beam
Impair image quality by adding exposure to the IR unrelated to patient anatomy
Best way to control scatter reaching the IR is to use a grid.
Best way to control scatter to the patient is use beam restriction.
Principle factors affecting scatter
1. kilovoltage
2. irradiated material

16. 1. Kilovoltage As kVp increases
More photons pass through the patient to interact with the image receptor
Image quality is increased with less scatter
Photons that do not pass through the matter interact as Compton and photoelectric interactions
Higher kVp will increase Compton( scatter) and decrease photoelectric ( pt dose)
Since photoelectric contribute to patient dose, dose will be decreased
kVp should be based on part/anatomy thickness /size and the amount of contrast desired
Increased kVp with NO change in mAs : increased scatter
Increased kVp and reduction in mAs: decreased scatter

17. 2. Irradiated Material
Scatter is affected by the volume and atomic number of the material being x-rayed.
Volume is controlled by field size and patient thickness
Increased volume increases scatter ( more interactions)
Volume increases as part increases and as field size increase
14 x17 has more volume than 10 x 12 – increased scatter , so smallest field size should be used
Large patients should have better collimation to decrease interaction ( decrease scatter)
When collimation is used- more technique is needed ( 14x17 to 10x12- aprox increase is 25% , 14x17 to 8 x 10 aprox increase of 40%)
Higher atomic numbers will have less scatter- they absorb more through photoelectric absorption- more contrast ( black and white)
Bone absorbs more than soft tissue- less scatter
Iodine, Ba and Pb absorbs more- less scatter

18. Types of Beam Restrictors
1. Collimators 2. Aperture diaphragms and cones / cylinders 3 . Ancillary devices

19. 1. Collimator
Most common beam restrictor – results in filtration of the beam – aprox 1 mm of Al
Multiple size configurations- but should always be minimized to the part being radiographed
Lead shutters at right angles and move in opposing pairs
3 purposes:
Regulate field size
Reduce penumbra
Act as a light field

20. Field Size Various square or rectangle shapes
PBL device - positive beam limitation device- automatic collimation of the beam size- works with a sensor in the Bucky tray to determine film size being used.
Collimator accuracy must be checked yearly – Within 2 % of the distance

21. Penumbra Geometric unsharpness around the edge of the image
Result of off focus radiation – photons emerge at different angles from the tube and interact with the tissue at various angles and create a “shadow “ image outside the exposed field of radiation
Reducing penumbra will increase recorded detail

22. Light Field Provides an exposure field with crosshair
Some units will have an AEC chamber outline
Lightbulb is placed at a to a mirror – improper placement of the mirror can cause a mis-projection of the light and misrepresent the field size

23. 2. Aperture diaphragms and cones / cylinders
Aperture diaphragm- flat sheet of metal( Pb) with opening cut in the middle
Attached to the bottom of the collimator box
Simplest of all beam limiting devices
Different sizes for different receptor sizes at different distances
Mostly seen in mammography because they don’t use collimators
Cones and cylinders
Circular aperture diaphragm with metal extensions
Can flare or diverges- small at top than bottom
Most effective use of scatter control

24. 3. Ancillary Devices
Designed for a specific need and are tailored for specific use during a given procedure
Lead blockers or lead masks
Blockers
Lead impregnated rubber
Used to absorb scatter created from soft tissue- used a lot on larger patients
May be placed behind a patient’s back on an lateral L-spine
Masks
cut to a specific shape and attached to collimator

25. The patient as a beam emitter
Chapter 16
The patient as a beam emitter

26. Attenuation
The reduction in the total number of x-ray photons remaining in the beam after passing through an object
Result of :
x-rays interacting with matter and being absorbed or scattered
The thicker the body part, the more attenuation
Photoelectric absorption( provides information) and Compton scattering ( no useful info- only dose to tech )
Determined by the amount or type of irradiated material
High atomic number = higher attenuation – bone , barium, lead
Low atomic number= low attenuation- hydrogen, carbon, oxygen
Lowes to highest attenuators in body by density: Air, fat, water, muscle, bone

27. The Patient The greatest variable
Atomic makeup : hydrogen (atomic # 1) , carbon(6), nitrogen(7) and oxygen(8) calcium (20)
4 Major substances account for most variations in absorption:
Air
Fat
Muscle
Bone

28. 1. Air
Higher atomic number than fat or muscle but lower tissue density
Absorbs fewer photons as it passes through allowing more photons to reach IR
Present in : ?????
Lungs
Sinuses
GI tract

29. 2. Fat
Similar to muscle- both soft tissue structures- fat has less tissue density
Effective atomic number similar to water- water is used to simulate fat in experiments
Varies from patient to patient
Fat sometimes allow for better visualization of certain organs during x-raying
kidneys

30. 3. Muscle
Higher atomic number and density than fat- cells are packed closer together
Greater attenuator
Can often see psoas muscles on abdomen radiograph

31. 4. Bone Calcium level in bones makes them higher attenuators
Highest effective atomic number and tissue density of the four basic substances of the body
Results in few photons reaching the IR

32. Patients Relationship to Image Quality
Patient has affect on all properties of image quality
Density/image receptor exposure ( subject density)
Contrast ( subject contrast)
Recorded detail ( subject detail )
Distortion ( subject distortion)

33. 1. Subject Density
The impact the subject has on the resultant radiographic density or IR exposure
Varies depending on the amount or type of tissue being irradiated
Thicker the part/ more dense the tissue = less IR exposure

34. 2. Subject Contrast The difference in densities of a recorded image
The degree of differential absorption resulting from the differing absorption characteristics of the tissue in the body
Dependent on patient’s tissues
Less difference in adjacent densities- fat and muscle- less contrast
More difference in adjacent densities- bone and air- more contrast

35. 3. Subject Detail
Primary factor affecting sharpness in distance from structure to IR
Dependent on where structure is in the body
Even size of body- larger patients the part is further away from IR
How the patient is positioned
PA vs AP, RT vs LT lateral

36. 4. Subject Distortion
The misrepresentation of size or shape of the structure of interest
Varies with patient position

37. Chapter 18
The grid

38. Invention of the Grid
1913- Gustav Bucky, an American radiologist, designed the first grid
Designed in a cross-hatched pattern using lead strips
Grid showed on image but decrease scatter and improved contrast
1920- Hollis Potter, a Chicago radiologist, improved Bucky’s design
Redesigned lead to run in one direction
Made lead thinner and less obvious on image
Potter also designed the Potter-Bucky diaphragm- allowed for the grid move during exposure
Lead strips became blurred and were no longer visible

39. Purpose Improves contrast of the image
Absorbs scatter before reaching the IR
Used with thicker body parts and higher kVp since scatter increases as both of these increase
Greater than 10cm
kVp above 60

40. Construction
Series of radiopaque strips alternated with interspace strips
Radiopaque strips absorb scatter and are made of high atomic number material
Interspace is radiolucent – allows radiation to pass through without interaction
Construction Involves:
materials
grid ratio
grid frequency

41. 1. Materials Interspace- aluminum and plastic fiber
Should not absorb any radiation
However, Al has a high atomic number and actually absorbs primary photons- disadvantage at low-kVp techniques- fiber is better for low kVp techniques

42. 2. Grid Ratio
Major influence on the ability of the grid to improve contrast
Defined as the “ ratio of the height of the lead strips to the distance between the strips”
Expressed as a formula:
Grid ratio = h h=lead strip height mm height and .25 mm interspace =
D D= interspace width / .25 = 12:1 grid
Height and distance are inversely proportional:
If height is constant but we decrease the interspace distance, we increase ratio
3 mm height and .25 = 12:1 grid
If height is constant but we increase the interspace distance, we decrease ratio
3 mm height and .50 = 6:1 grid ratio

43. Comparing grid ratios
Higher grid ratios allow less scatter to pass through- more effective at removing scatter
Disadvantage to high grid ratio: must be more precise positioning of grid so primary isn’t absorbed – higher grid error

44. 3. Grid Frequency The number of grid lines per inch or cm
Higher grid frequencies have thinner lead strips
Range from lines/inch
Most common lines / inch
Very high frequencies lines/inch are common for digital to minimize grid lines on image( because digital is more sensitive)

45. Grid Ratio and Grid Frequency
The combination of the two variable will determine the total quantity of lead in the grid.
Lead content is the most important in determining grid efficiency
Lead content is greater in a grid with high radio and lower frequency – because the strips will be thicker in lower frequency

46. Grid Patterns: 2 types Linear grids- lead runs in one direction
Most common- used in x-ray table
Allow for tube angle along the direction of the lines ( long axis) – head to feet
Can have short axis grids- helpful for Port CXR where gird is place crosswise.
Criss-crossed or cross hatched- lead strips run at right angles
Do not allow for tube angle
Limited applications
Grid Cut-off- can occur with ANY grid
when the lead strips “cut-off” or absorb the primary beam- not allowing for it to reach IR- no image in that area of the IR

47. Grid types
1. Parallel – least common 2. Focused

48. 1. Parallel grids Lead and interspace run parallel to each other
will never intersect
Strips do not coincide with divergence of the beam
Absorb primary beam as a result-more along lateral edges
Worse at short SIDs
Better at long SIDs because beam is more straight

49. 2. Focused Grid
Lead and interspace are still parallel but are angle to match the divergence of the beam at different distances
Would eventually intersect at a convergence line
The distance from the face of the grid to the point of convergence is called grid radius
Tube must be located across the convergence line
Focused grids are grouped by focal ranges ( distances) inch focus, inch focus – grid will be marked
Lower grid ratios allow for greater latitude in tube alignment
Higher grid ratios proper tube alignment is critical

50. Grid Uses Stationary Mounted moving- AKA Bucky
Come in various sizes to match cassette size
Grid lines are more noticeable
Low frequency grids( thicker strips) worse than high frequency ( thinner strips)
Used with portable or in rooms when patient is not on x-ray table- gurney or hosp bed
Grid cassettes- for film/screen- grid was build into cassette
Mounted moving- AKA Bucky
Mounted below table top ( or upright holder ) and hold cassette in place below (behind) the grid
Moves during exposure so grid lines will be blurred and not seen
Larger than largest cassette size used in machine
Grid lines run along the axis of table ( head to foot)
Movement moves the grid at right angles to the grid- side to side
Two movement types
Reciprocating- side to side- motor driven
Oscillating- circular movement- electromagnet driven

51. Grid Selection
Made in equipment purchase- not normally interchangeable
Stationary grids will be selected by size/ focus/ grid ratio

52. Grid Conversion Formula:
Compensation of technique (mAs) must be made between type of grid selected
Same technique can’t be used for no grid vs 16:1
Formula:
mAs = GCF 1
mAs GCF 2
Image 1 = 8:1 grid 35 mAs
Image 2 = 12:1 grid ?? mAs
= 4 X
4x= x= 52.5 mAs
Grid Ratio
Conversion Factor
No grid 1
5:1 2
6:1 3
8:1 4
10:1 or 12:1 5
16:1 6
This chart is different than in text- eliminates need to figure in kVp ( easier to apply and remember! )

53. Grid Performance Evaluation
The measuring of a grid in cleanup or removing scatter
Determined by two factors:
Selectivity
Contrast improvement ability

54. 1. Selectivity
The ratio between the quantity (%) of the primary photons transmitted through the grid to the quantity (%) of scatter photons transmitted
- The better the grid is at removing scatter ; the greater the selectivity- higher grid ratios have higher selectivity
Formula:
selectivity = % of primary radiation transmitted
% of scatter radiation transmitted

55. 2. Contrast Improvement Ability
Best way to determine grid function
Contrast improvement factor ( K) – dependent on the amount of scatter produced
As scatter increases , contrast decrease, thus lowering the improvement factor
The higher the K factor the greater the contrast improvement
Formula:
K= Radiographic contrast with the grid
Radiographic contrast w/out the grid

56. Grid Errors Result of improper use
More frequent with focused grids- tube must be centered and at correct SID
Also has tube side and IR side
Types of Errors
Off- Level
Off-Center
Off-Focus
Upside-down
Moiré Effect

57. Off-Level
Occurs when tube is angled across the long axis of the grid strips
Result of improper tube or grid position
Most common with stationary grid- portable procedures
Result
Absorption of primary beam & decrease in overall exposure – light image

58. Off-Center When tube is not centered to grid
Does not line up with divergence of beam
The greater degree of lateral off-centering the greater the cut-off
Result :
Decreased exposure across entire image ( light image)

59. Off-Focus When a grid is used at incorrect SID
Higher grid ratios require greater accuracy to prevent cutoff
Result:
Grid cutoff along the peripheral edges of the image

60. Upside-Down Focused grids have an identified tube side Result:
If used upside-down- severe peripheral grid cut- off will occur

61. The Moire Effect
Occurs with digital image receptors when grid lines are captured and scanned parallel to the scan lines of the imaging plate reader
Grid lines run parallel with the laser
Happens with stationary grids- ran across short axis
Corrected by high-frequency grids w/ 103 lines per inch or higher

62. Alternate to Grid Restricting beam size
Compress body part- compression band
Air-Gap Technique
Increased OID – same amount of scatter will leave patient , but won’t reach patient
10” air gap has similar clean-up of 15:1 grid
References sheet for all documents

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