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Chapter 19 & 20 Image Quality & Techniques

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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 OID’s 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.

19 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.

39 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= 24*2+30=78kVp 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 This type of chart should be avoided.
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 90 70 110+

71 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

79 Return to Lecture Index Return to Physics Homepage
End of Lecture Return to Lecture Index Return to Physics Homepage

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