Chapter 19 & 20 Image Quality & Techniques

1
Chapter 19 20 Image Quality Techniques

• There are three geometric factors that affect
  radiographic quality.
• Magnification
• Distortion
• Focal Spot Blur
• We have explored these factors in the laboratory.

2
Magnification

• All objects on the radiograph are larger that
  their actual size. This is called magnification.
• The magnification factor is the image size
  divided by the object size.
• At 40 (100 cm) factor is 1.1
• At 72 (180 cm) factor is 1.05

3
Magnification

• Usually we do not know the size of the object so
  we must determine the magnification factor
  another way.
• Image size S0D
• MF ----------------
  ------------
• Object Size SID

4
Minimizing Magnification

• Large SID Use the less divergent beam.
• Chest X-rays are done at 72 SID to minimize
  magnification.
• Lateral C-spine done at 72
• Small OID Get patient as close to the film as
  possible.
• Basic principle for positioning.

5
Distortion

• Distortion is the misrepresentation of the true
  size and shape of the object being radiographed.
• The amount of distortion depends upon the
  thickness, position and shape.

6
Thickness

• Thick objects are more distorted than thin
  objects because of the greater change in Object
  Image Distance.

7
Thickness

• The position of the object relative to the
  central axis will cause greater distortion with
  thick and/or irregular shaped objects.

8
Object Position

• If the object plane and image plane are parallel
  the image will not be distorted.
• If the object plane and image plane are not
  parallel, distortion will occur.

9
Spatial Distortion

• When multiple objects at different OIDs occur,
  we get spatial distortion due to unequal
  magnification.
• Two arrows appear as one.
• When shifted laterally more distortion occurs

10
Object Shape Distortion

• When the object plane is not parallel to the
  image plane as when inclined, shape distortion
  occurs.
• This will result in foreshortening.

11
Focal Spot Blur

• Focal spot blur is caused by the effective size
  of the focal spot, which is larger at the cathode
  side.
• Focal spot blur is the most important factor in
  determining spatial resolution.

12
Focal Spot Blur

• Focal spot blur is impacted by the Object to
  Image Distance.

13
Focal Spot Blur Heel Effect

• There is more to the heel affect than just the
  attenuation of the beam by the anode.
• The focal spot blur is smaller at the anode side
  and larger at the cathode side.

14
Taking advantage of the Anode Heel Effect
15
Did you see a problem ?

• If the tube is mounted correctly for the AP Full
  spine, Chest and A-P Thoracic Spine, the patient
  must stand on his head for the lateral thoracic
  spine!!!!
• For erect radiography, the use of the anode heel
  affect is limited.

16
Object Factors that Affect Quality

• Subject Contrast
• Patient or part thickness
• Tissue mass density
• Affective atomic number
• Object shape
• kVp

17
Radiographic Contrast

• Radiographic Contrast is how the film looks.
• It is the combination of receptor contrast and
  subject contrast.
• Contrast occurs between structures of different
  densities.

18
Thickness

• The thicker the body part or body section, the
  greater attenuation of the beam. Contrast is
  relative to the number of x-rays leaving the
  body.
• Remember that x-rays are merely shadows of the
  anatomy based upon attentation of the beam.

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Thickness

• Radiographs of thin patients will have more
  contrast than those of large patients.
• Thicker object absorb more rays and will appear
  lighter than thin objects.

20
Tissue Mass Density

• Different sections of the body have may equal
  thickness yet different mass density.
• Two wrist may be the same thickness but the one
  that is swollen will have greater mass density
  due to water in the tissues.

21
Effective Atomic Number

• While Compton interactions with tissues are not
  impacted by the relative atomic number of
  tissues, the photoelectric effect vary with the
  cube of the atomic number.
• When the effective atomic number of adjacent
  tissues is very different, subject contrast is
  very high.

22
Object shape

• The object shape not only influences the geometry
  but also through its contribution to subject
  contrast.
• A vessel on end has high contrast while other
  have lower contrast.

23
Varying tissue densities

• Bone absorbs most x-rays leaving a white shadow.
• Water absorbs less x-rays leaving a light gray
  shadow
• Fat absorbs fewer x-rays leaving a dark gray
  shadow.
• Air absorbs little x-rays and is black on the
  film.

24
Varying densities in the Body

• Air, oil, water and metal (natural) absorb
  different degrees of the x-rays and produce
  contrast.
• Heavy metals are used as contrast media to
  enhance contrast in the body in medical
  radiology. Principle ones are Barium and Iodine.

25
kVp

• We have little control over the previous factors
  controlling subject contrast.
• BUT!!!!!
• The absolute magnitude of subject contrast is
  greatly influenced by the kVp of operation.

26
kVp

• kVp also influences film contrast but not to the
  extend it controls subject contrast.
• Low kVp high contrast short scale
• High kVp low contrast broad scale

27
Motion Blur

• If any element of the x-ray moves during
  exposure, we get motion blur.
• Patient motion is the most common cause of motion
  blur.
• Motion blur is more common in erect radiography.

28
Ways to Control Motion Blur

• Use the shortest possible exposure time
• Restrict patient motion by instructions or
  restraining device.
• Use a large SID
• Use a small OID

29
Tools to Improve Quality

• Patient Positioning
• Get the patient close to the film reduce OID.
• Center the beam to the area of interest.
• Get the area of interest parallel to the beam or
  film.
• Restrain motion and communicate with the patient.
• Use short exposure times.

30
Tools to Improve Quality

• Image receptors
• Use the correct film screen combination for the
  examination. Extremity for wrist. Regular for
  spines.
• Intensifying screens reduce patient exposure at
  least 20 times.
• As the speed of the image receptor increases,
  radiographic noise and contrast resolution
  decreases.

31
Tools to Improve Quality

• Image receptors
• Low contrast imaging procedures have wider
  latitude and a larger margin of error in
  producing acceptable radiographs.
• Use the highest speed system that will provide
  adequate contrast and density over the entire
  spectrum of examinations.

32
Tools to Improve Quality

• Technique selection
• We must select the optimum technical factors.
• We must get the exposure time as low as possible
  so high frequency machine are important.
• Contrast controlled by the kVp used
• Density controlled by the mAs used.

33
Tools to Improve Quality

• kVp has a greater influence than mAs.
• Technique selection is a balancing act. Balancing
  contrast, density and exposure.

34
Chapter 20 Radiographic Technique

• Several factors influence the selection of
  technical factors. The primary factors that
  impact exposure and image quality are
• SID
• mAs
• kVp

35
Patient Factors

• The anatomic thickness and body composition
  greatly impact the technical factors.
• The technique chart is based upon the Sthenic
  Body Type.

36
Patient Factors

• Sthenic is strong active
• Hyposthenic is thin but healthy
• Hyperstenic is obese
• Astenic is small, frail sometime emaciated, and
  often elderly

37
Patient Thickness

• The thickness of the patient should not be
  guessed.
• It should be measured with calipers.
• Patient thickness is measured in cm.

38
Body composition

• The type of tissue in the area of exposure will
  impact the technical factors.
• The tissue types in the chest are different from
  the abdomen.
• Disease processes will also impact the exposure
  factors. Obtaining a good clinical history is
  important. History must be communicated to the
  radiographer.

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Classifying Pathology

• Radiolucent (Destructive)
• Active TB
• Atrophy
• Bowel obstruction
• Cancer
• Degenerative arthritis
• Emphysema
• Osteoporosis
• Pneumothorax

• Radiopaque Constructive)
• Aortic aneurysm
• Ascites
• Atelectasis
• Cirrosis
• Hypertrophy
• Metastasis
• Pleural Effusion
• Pneumonia
• Sclerosis

40
Image Quality Factors

• Image quality factors include
• OD
• Contrast
• Image Detail
• Image Distortion
• OD is the optical density or radiographic
  density. OD is controlled by the mAs and SID.

41
Optical Density

• Numerically low OD is a low number like 0.25.
• Dark is a high number like 2.20 to 4.0
• Light is underexposed
• Dark is over exposed
• If density is the only factor that needs to be
  changed, change the mAs.

42
Optical Density

• A 30 change in mAs is needed to make a
  perceptible change in optical density.
• Usually when a change in optical density is
  needed, the mAs is either doubled or halved.
• kVp must be changed by 4 to produce the same
  change in optical density.
• Changing kVp will also impact penetration and
  contrast.

43
30 - 50 Rule

• If the film is under exposed, double the mAs.
• If the film is over exposed, cut the mAs in half.
• If the film is slightly underexposed, increase
  the mAs 30.
• If the film is slightly overexposed, reduce the
  mAs 30.

44
30 Density Change

• The lower image was the first image taken. It was
  dark but normally would be acceptable.
• The top image was the mAs reduced 30. The air
  fluid levels in the sinus is easier to see.

45
15 Rule

• The OD can be changed with kVp but it will also
  impact exposure and contrast also.
• Increase of 15 in kVp is equal to cutting the
  mAs in half.
• Decrease of 15 in kVp is equal to doubling the
  mAs.
• If the film is underexposed, increase kVp 15.
• If the film is overexposed, decrease kVp 15.

46
Contrast

• The function of contrast is to make the anatomy
  more visible.
• Contrast is the difference in density of adjacent
  structures.
• The relative penetrability of the x-ray through
  different tissues determines the image contrast.

47
Contrast

• Contrast can be measured as the Gray Scale of
  Contrast. It is the range of optical density from
  white to black on the image.
• Contrast is controlled by kVp.

48
Adjusting Contrast with 15 Rule

• An 15 increase in kVp and a reduction of mAs by
  50 will produce the same OD but lower contrast.
• Used to reduce exposure or reduce exposure time/
• An 15 decrease in kVp and doubling the mAs will
  produce the same OD but higher contrast.

49
Image Detail

• The sharpness of image detail refers to the
  ability to see structural lines or borders of
  tissue in the image.
• The visibility of image detail is best measured
  by the contrast resolution.
• The geometric factors of focal spot selection,
  SID and OID will impact sharpness.

50
Image Detail

• Visibility of image detail is impacted by
  factors such as image fog.
• Scatter radiation reduces the ability to
  visualize lines of detail.
• Light fog or processing can impact the visibility
  of structures.
• Collimation, screen combination and the use of a
  grid are other factors that impact image detail.

51
Distortion

• The position of the x-ray tube greatly impacts
  distortion of the image. The image may be
  elongated or foreshortened.
• The proper Positioning of the tube, anatomic part
  and image receptor greatly impacts distortion.

52
Types of Technique Charts

• There are four primary means to establish
  techniques.
• Variable kVp Fixed mAs
• Fixed kVp with varying mAs.
• High kVp with varying mAs
• Automatic Exposure Charts when AEC is used.

53
Variable kVp Charts

• The mAs is fixed and the kVp is varied based upon
  patient thickness.
• Usually by a formula such as 2 x thickness 30
  kVp for single phase
• 24 cm patient 2423078kVp
• For high frequency use 23 and for three phase
  use 25.
• Small patient used low kVp high contrast
• Large patient used high kVp low contrast

54
Variable kVp Charts

• Contrast was very inconsistent.
• Very little latitude on smaller patients.
• Higher radiation exposure
• This type of chart should be avoided.

55
Fixed kVp Technique

• kVp is fixed and mAs varies by patient thickness.
  Usually 30 per two cm.
• Uses Optimum kVp for the body part
• Contrast is constant.
• Wider latitude
• Lower exposure

56
Fixed kVp Technique Variations

• High kVp technique uses over 100 kVp
• No longer used for bone.
• Long ago used for spine but images are too gray.
  Low contrast
• Mostly used for chest and barium contrast studies.

57
Fixed kVp Technique Variations

• Automatic Exposure Technique Charts
• Uses optimum kVp and high backup mAs.
• Ion chamber or photo cell determines when correct
  density is achieved on film and terminates
  exposure.
• Proper positioning is critical to get the area of
  interest over the ion chamber.

58
Using the Technique Chart

• The chart is not the Bible but is a guide.
• Works about 85 of the time so it is a great
  starting point.
• Lists factors used for each view based upon
  measurement of the patient.
• Can include as much as you want to include.

59
Using the Technique Chart

• Recommended factors for chart
• optimum kVp for view
• mAs based upon cm measurement
• filters used
• SID tube angle used
• Bucky or non-Bucky
• Cassette film type

60
Using the Technique Chart

• Charts should be
• accessible
• easy to read
• not hand written
• based upon the type of machine and machine
  controls.
• mAs or mA and time

61
Technique Variables

• Variable machine electrical output
• Incoming power and ability of machine to
  compensate for variations in incoming power.
• Type of High-voltage Power
• Single phase to High Frequency reduce mAs 50
• High Frequency to Single phase double mAs
• Grid ratio
• Non-Bucky Holder

62
Technique Variables

• Variable machine electrical output
• Relative Speed Value of cassettes film
  combination.
• 400 speed to 200 speed double mAs
• 200 speed to 400 speed reduce mAs 50

63
mAs kVp Relationship

• There are some basic rules for mAs and kVp that
  are used to adjust the technical factors.
• Remember x-rays are like toast.
• Dark is too dense
• Light has inadequate density
• This tells you which was to go.

64
mAs Rules

• Since mAs controls density, it is usually used to
  adjust density.
• 30 increase needed to make a noticeable change
  in density.
• 50 mAs reduction will reduce density 50
• Doubling mAs will doubles density.

65
mAs Rules

• If image is too dark reduce mAs 50.
• If image is too light double mAs.
• Doubling mAs can be done by doubling mA or time.
• Doubling time increases chance for motion blur.

66
kVp Rule

• kVp will also change density.
• A light film from low kVp is called under exposed
  or under penetrated.
• Very white image because no x-rays reached the
  film.
• Too dark is over exposed, some say over
  penetrated. They are different.

67
kVp Rule

• Over penetrated will result only if the kVp used
  is too high for the view. It will be dark and
  very flat (lacking contrast)
• Density is very sensitive to changes in kVp.
• A 2 kVp (HF) to 4 kVp change is noticeable. About
  4.
• The 15 rule works with density adjustment.

68
kVp Rule

• 15 increase in kVp will double density.
• 15 decrease in kVp will reduce density 50.
• 15 increase in kVp doubling mAs
• 15 decrease in kVp half the mAs
• 10 kVp 15 change in the 60 to 90 kVp range.

69
Optimum kVp

• Optimum kVp will provide the best contrast with
  the least amount of radiation.
• If using the optimum kVp you should not need to
  adjust kVp.
• kVp can be changed based upon body habitus and
  disease.

70
Optimum kVp

• Small Extremity
• Large Extremity
• Cervical Spine AP or Lat
• APOM
• Thoracic AP
• Thoracic Lat
• Lumbar AP
• Lumbar Oblique
• Lumbar Lateral
• Pelvis
• Abdomen
• Ribs
• Chest

• 55-65
• 65-70
• 70-74
• 75-78
• 75
• 80
• 74
• 80
• 90
• 80
• 70
• 70
• 110

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Dark film

• Dark No contrast reduce kVp no change to mAs
• Black no structures seen reduce both.
• Dark look at the optimum kVp range.
• If reducing kVp goes beyond optimum kVp
• reduce mAs 50

72
Light Film

• If film is so light that no structures are seen
  then it is under penetrated so increase kVp.
• kVp controls penetration.
• If structures seen but lacks density increase
  double mAs.

73
Patient Factors

• Very muscular or large boned increase mAs by 50
• Very muscular and large boned increase mAs 50
  and kVp 4 to 6 kVp.
• Obese increase mAs 50
• Edema increase mAs 30
• Frail decrease kVp 5 to 15

74
Patient Factors

• Osteoporotic patient over 60 years old decrease
  mAs 30 to 50
• 6 to 12 years old reduce mAs 30 to 50
• Infants to 6 years old decrease mAs 75

75
Item that Affect Detail

• Spatial Resolution controlled by focal spot size
  and image receptor.
• Detail influenced by
• SID
• OID
• Motion Blur
• Density Contrast of Image

76
Items that Affect OD

• Optical density is controlled by mAs
• OD influenced by
• kVp
• SID
• Thickness
• Density
• Collimation

77
Items that Affect OD

• OD influenced by
• Grid Ratio
• Development time and temperature
• Image receptor speed

78
Items that Affect Contrast

• Contrast controlled by kVp
• Contrast influenced by
• mAs
• Development Time Temperature
• Collimation
• Grid ratio
• Image receptor

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End of Lecture