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Chapter 19 & 20 Image Quality & Techniques There are three geometric factors that affect radiographic quality. There are three geometric factors that affect.

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Presentation on theme: "Chapter 19 & 20 Image Quality & Techniques There are three geometric factors that affect radiographic quality. There are three geometric factors that affect."— Presentation transcript:

1 Chapter 19 & 20 Image Quality & Techniques There are three geometric factors that affect radiographic quality. There are three geometric factors that affect radiographic quality. Magnification Magnification Distortion Distortion Focal Spot Blur Focal Spot Blur We have explored these factors in the laboratory. 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. 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. The magnification factor is the image size divided by the object size. At 40” (100 cm) factor is 1.1 At 40” (100 cm) factor is 1.1 At 72” (180 cm) factor is 1.05 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. Usually we do not know the size of the object so we must determine the magnification factor another way. Image size S0D Image size S0D MF = ----------------= ------------ MF = ----------------= ------------ Object Size SID Object Size SID

4 Minimizing Magnification Large SID: Use the less divergent beam. 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. 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. 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. 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. 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. 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 parallel the image will not be distorted. If the object plane and image plane are not parallel, distortion will occur. 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. When multiple objects at different OID’s occur, we get spatial distortion due to unequal magnification. Two arrows appear as one. Two arrows appear as one. When shifted laterally more distortion occurs 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. When the object plane is not parallel to the image plane as when inclined, shape distortion occurs. This will result in foreshortening. 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 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. 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. 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. 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. 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!!!! 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. For erect radiography, the use of the anode heel affect is limited.

16 Object Factors that Affect Quality Subject Contrast Subject Contrast Patient or part thickness Patient or part thickness Tissue mass density Tissue mass density Affective atomic number Affective atomic number Object shape Object shape kVp kVp

17 Radiographic Contrast Radiographic Contrast is how the film looks. Radiographic Contrast is how the film looks. It is the combination of receptor contrast and subject contrast. It is the combination of receptor contrast and subject contrast. Contrast occurs between structures of different densities. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. Bone absorbs most x-rays leaving a white shadow. Water absorbs less x-rays leaving a light gray shadow Water absorbs less x-rays leaving a light gray shadow Fat absorbs fewer x-rays leaving a dark gray shadow. Fat absorbs fewer x-rays leaving a dark gray shadow. Air absorbs little x-rays and is black on the film. 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. 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. 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. We have little control over the previous factors controlling subject contrast. BUT!!!!! BUT!!!!! The absolute magnitude of subject contrast is greatly influenced by the kVp of operation. 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. kVp also influences film contrast but not to the extend it controls subject contrast. Low kVp = high contrast = short scale Low kVp = high contrast = short scale High kVp = low contrast = broad scale High kVp = low contrast = broad scale

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

28 Ways to Control Motion Blur Use the shortest possible exposure time Use the shortest possible exposure time Restrict patient motion by instructions or restraining device. Restrict patient motion by instructions or restraining device. Use a large SID Use a large SID Use a small OID Use a small OID

29 Tools to Improve Quality Patient Positioning 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 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 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 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. kVp has a greater influence than mAs. Technique selection is a balancing act. Balancing contrast, density and exposure. 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: Several factors influence the selection of technical factors. The primary factors that impact exposure and image quality are: SID SID mAs mAs kVp kVp

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

36 Patient Factors Sthenic is strong & active Sthenic is strong & active Hyposthenic is thin but healthy Hyposthenic is thin but healthy Hyperstenic is obese Hyperstenic is obese Astenic is small, frail sometime emaciated, and often elderly Astenic is small, frail sometime emaciated, and often elderly

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

38 Body composition The type of tissue in the area of exposure will impact the technical factors. 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) Radiolucent (Destructive) Active TB Active TB Atrophy Atrophy Bowel obstruction Bowel obstruction Cancer Cancer Degenerative arthritis Degenerative arthritis Emphysema Emphysema Osteoporosis Osteoporosis Pneumothorax Pneumothorax Radiopaque Constructive) Radiopaque Constructive) Aortic aneurysm Aortic aneurysm Ascites Ascites Atelectasis Atelectasis Cirrosis Cirrosis Hypertrophy Hypertrophy Metastasis Metastasis Pleural Effusion Pleural Effusion Pneumonia Pneumonia Sclerosis Sclerosis

40 Image Quality Factors Image quality factors include 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. 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. Numerically low OD is a low number like 0.25. Dark is a high number like 2.20 to 4.0 Dark is a high number like 2.20 to 4.0 Light is underexposed Light is underexposed Dark is over exposed Dark is over exposed If density is the only factor that needs to be changed, change the mAs. 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. 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. 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. 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 under exposed, double the mAs. If the film is over exposed, cut the mAs in half. 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 underexposed, increase the mAs 30%. If the film is slightly overexposed, reduce 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 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. 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. 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. Increase of 15% in kVp is equal to cutting the mAs in half. Decrease of 15% in kVp is equal to doubling the mAs. Decrease of 15% in kVp is equal to doubling the mAs. If the film is underexposed, increase kVp 15%. If the film is underexposed, increase kVp 15%. If the film is overexposed, decrease kVp 15%. If the film is overexposed, decrease kVp 15%.

46 Contrast The function of contrast is to make the anatomy more visible. The function of contrast is to make the anatomy more visible. Contrast is the difference in density of adjacent structures. Contrast is the difference in density of adjacent structures. The relative penetrability of the x-ray through different tissues determines the image contrast. 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 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. 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. 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. 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 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 visibility of image detail is best measured by the contrast resolution. The geometric factors of focal spot selection, SID and OID will impact sharpness. 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. 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 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. 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. There are four primary means to establish techniques. Variable kVp Fixed mAs Variable kVp Fixed mAs Fixed kVp with varying mAs. Fixed kVp with varying mAs. High kVp with varying mAs High kVp with varying mAs Automatic Exposure Charts when AEC is used. Automatic Exposure Charts when AEC is used.

53 Variable kVp Charts The mAs is fixed and the kVp is varied based upon patient thickness. 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 Usually by a formula such as 2 x thickness +30 = kVp for single phase 24 cm patient= 24*2+30=78kVp 24 cm patient= 24*2+30=78kVp For high frequency use 23 and for three phase use 25. For high frequency use 23 and for three phase use 25. Small patient used low kVp= high contrast Small patient used low kVp= high contrast Large patient used high kVp= low contrast Large patient used high kVp= low contrast

54 Variable kVp Charts Contrast was very inconsistent. Contrast was very inconsistent. Very little latitude on smaller patients. Very little latitude on smaller patients. Higher radiation exposure Higher radiation exposure This type of chart should be avoided. 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. kVp is fixed and mAs varies by patient thickness. Usually 30% per two cm. Uses Optimum kVp for the body part Uses Optimum kVp for the body part Contrast is constant. Contrast is constant. Wider latitude Wider latitude Lower exposure Lower exposure

56 Fixed kVp Technique Variations High kVp technique uses over 100 kVp 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 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. The chart is not the Bible but is a guide. Works about 85% of the time so it is a great starting point. 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. Lists factors used for each view based upon measurement of the patient. Can include as much as you want to include. Can include as much as you want to include.

59 Using the Technique Chart Recommended factors for chart: Recommended factors for chart: optimum kVp for view optimum kVp for view mAs based upon cm measurement mAs based upon cm measurement filters used filters used SID & tube angle used SID & tube angle used Bucky or non-Bucky Bucky or non-Bucky Cassette & film type Cassette & film type

60 Using the Technique Chart Charts should be: Charts should be: –accessible –easy to read –not hand written –based upon the type of machine and machine controls. mAs or mA and time mAs or mA and time

61 Technique Variables Variable machine & electrical output 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% Single phase to High Frequency reduce mAs 50% High Frequency to Single phase double mAs High Frequency to Single phase double mAs –Grid ratio –Non-Bucky Holder

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

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

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

66 kVp Rule kVp will also change density. kVp will also change density. A light film from low kVp is called under exposed or under penetrated. 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. 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) 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. Density is very sensitive to changes in kVp. A 2 kVp (HF) to 4 kVp change is noticeable. About 4%. A 2 kVp (HF) to 4 kVp change is noticeable. About 4%. The 15% rule works with density adjustment. The 15% rule works with density adjustment.

68 kVp Rule 15% increase in kVp will double density. 15% increase in kVp will double density. 15% decrease in kVp will reduce density 50%. 15% decrease in kVp will reduce density 50%. 15% increase in kVp = doubling mAs 15% increase in kVp = doubling mAs 15% decrease in kVp = half the mAs 15% decrease in kVp = half the mAs 10 kVp = 15% change in the 60 to 90 kVp range. 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. 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. If using the optimum kVp you should not need to adjust kVp. kVp can be changed based upon body habitus and disease. kVp can be changed based upon body habitus and disease.

70 Optimum kVp Small Extremity Small Extremity Large Extremity Large Extremity Cervical Spine AP or Lat Cervical Spine AP or Lat APOM APOM Thoracic AP Thoracic AP Thoracic Lat Thoracic Lat Lumbar AP Lumbar AP Lumbar Oblique Lumbar Oblique Lumbar Lateral Lumbar Lateral Pelvis Pelvis Abdomen Abdomen Ribs Ribs Chest Chest 55-65 55-65 65-70 65-70 70-74 70-74 75-78 75-78 75 75 80 80 74 74 80 80 90 90 80 80 70 70 110+ 110+

71 Dark film Dark & No contrast = reduce kVp no change to mAs Dark & No contrast = reduce kVp no change to mAs Black no structures seen reduce both. Black no structures seen reduce both. Dark = look at the optimum kVp range. 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. If film is so light that no structures are seen then it is under penetrated so increase kVp. kVp controls penetration. kVp controls penetration. If structures seen but lacks density increase double mAs. If structures seen but lacks density increase double mAs.

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

74 Patient Factors Osteoporotic patient : over 60 years old decrease mAs 30% to 50% Osteoporotic patient : over 60 years old decrease mAs 30% to 50% 6 to 12 years old reduce mAs 30 to 50% 6 to 12 years old reduce mAs 30 to 50% Infants to 6 years old decrease mAs 75% Infants to 6 years old decrease mAs 75%

75 Item that Affect Detail Spatial Resolution controlled by focal spot size and image receptor. Spatial Resolution controlled by focal spot size and image receptor. Detail influenced by: Detail influenced by: –SID –OID –Motion Blur –Density & Contrast of Image

76 Items that Affect OD Optical density is controlled by mAs Optical density is controlled by mAs OD influenced by: OD influenced by: –kVp –SID –Thickness –Density –Collimation

77 Items that Affect OD OD influenced by: OD influenced by: –Grid Ratio –Development time and temperature –Image receptor speed

78 Items that Affect Contrast Contrast controlled by kVp Contrast controlled by kVp Contrast influenced by: Contrast influenced by: –mAs –Development Time & Temperature –Collimation –Grid ratio –Image receptor

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


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