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87 Cards in this Set
- Front
- Back
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Three factors affecting quality?
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Magnification
Distortion Geometric Blur |
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What is magnification?
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The degree of image enlargement
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What is magnification influenced by?
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SID & OID
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What is magnification expressed and measured by?
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Magnification factor (MF)
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Formulas for Magnification Factor?
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MF= Image Size/ Object Size
MF=SID/SOD SOD= SID-OID |
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Magnification decreases with what 2 things?
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Increasing SID
Decreasing OID |
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When an object is placed equidistance between the source and film, how much will the image be magnified?
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It will be two time the object size.
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For a fixed SID, magnification is (increased or decreased) by bringing the ocject nearer to the xray tube.
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Increased
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In order to maintain the same relative size of an image, if the OID is increased what should you do to the SID?
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Increase SID to minimize magnification
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Two rules to minimize magnification
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Large SID
Small OID |
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Percent of Magnification Formula
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(Image Size - Object Size)/ Object Size
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What is Distortion?
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Unequal magnification of different portions of the same object. Variation in the true size and shape of a body part.
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Two conditions that cause distortion
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Thickness (size and shape) of the object
Position of the object |
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The object plane and the image plane must be (parallel or perp.) to prevent distortion?
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Parallel
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Three factors that influence distortion
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Alignment of tube to film
Alignment of part to film OID |
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Employing tube angulations will (increase or decrease) the amount of distortion on a film
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Increase
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Tube angulations creates elongation or foreshortening?
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Elongation
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Part angulations creates elongation or foreshortening?
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Foreshortening
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Foreshortening of an image will most likely result from?
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Improper tube-part-film alignment
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Types of Distortion?
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Size Distortion
Shape Distortion |
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What is size distortion?
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Objects are magnified will also be distorted.
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What will increasing SID do to distortion and magnification?
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It will decrease distortion because magnification is decreased.
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What will increasing OID do to distortion and magnification?
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It will increase distortion because magnification is increased.
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Increasing the tube angle will create what?
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Elongation
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Increasing the part angle will create what?
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Foreshortening
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Improper tube-object and film alignment with no change in SID will result in what?
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Shape distortion
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What two things help to minimize image distortion?
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Increased SID and decreased OID
Proper positioning |
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What is geometric blur?
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Known as "focal-spot blur"
It is the blurred region or area of unsharpness due to the fact that the effective foal spot is not a point. |
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Where is focal-spot or geometric blur greater?
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Greater on the cathode side
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What does focal-spot blur do?
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It reduces resolution or detail
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Three conditions resulting in focal-spot blur
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Large effective focal spot
Short SID Long OID (The last two govern magnification and influence focal spot blur) |
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As magnification increases, what happens to focal-spot blur?
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It will increase
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Four factors that affect unsharp borders:
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OID
SID Focal-spot Blur Degree of Collimation |
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What is the Heel Effect?
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Varying intensity across the xray field caused by attenuation of xrays in the heel of the anode
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How is the focal spot of the anode side?
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Small on the anode side and large of the cathode side
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What is size distortion?
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Objects that are magnified will also be distorted
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What will increasing SID do to distortion and magnification?
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It will decrease distortion because magnification is decreased
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What will increasing OID do to distortion and magnification?
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It will increase distortion because magnification is increased
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Increasing the tube angle will create what?
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Elongation
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Increasing the part angle will create what?
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Foreshortening
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Improper tube-object and film alignment with no change in SID with result in what?
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Shape distortion
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Two ways to reduce image distortion:
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Increased SID and decreased OID
Proper positioning |
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What is geometric blur?
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Known as "focal-spot blur"
Blurred region of unsharpness due to the fact that the effective focal spot is not a point |
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Where is focal spot or geometric blur greater?
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Cathode side
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What does focal spot blur do to the film?
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Reduces resolution
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Three conditions resulting in focal spot blur:
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Large effective focal spot
Short SID Long OID (The last two govern magnification and influence focal spot blur) |
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Four factors affecting unsharp borders:
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OID
SID Focal spot size Degree of Collimation |
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How do you minimize focal spot blur?
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Short OID
Long SID Use the smallest focal spot possible Collimate as much as possible CR should pass thru the center of the part and be perp. to the plane of the film No movement of part and short exposure time Intensifying screens should be in good contact with film Pt should be correctly positioned |
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What is the Heel Effect?
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Varying intensity across the xray field caused by attenuation of xrays in the heel of the anode
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Where is focal spot blur small?
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On the anode side
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How will image look towards the cathode side of the tube?
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They will have a higher blur and poorer resolution than those of the anode side
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When is the Heel Effect most likely to occur?
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Most likely to occur with the use of a short SID and a film size larger than a 10x12
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What is the formula for geometric unsharpness?
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Geometric Unsharpness = FSS x OID / SID
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What are subject factors?
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Contrast
Motion |
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Definition of Subject Factors
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Those associated not so much with the positioning of the patient as with the selection of a radiographic technique that compensates for patient size, shape, and composition
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What is radiographic contrast?
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The contrast you see on a radiograph
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Formula for radiographic contrast
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RC = Film contrast x Subject contrast
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Five factors affecting subject contrast:
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Patient thickness
Tissue mass density Effective atomic number Shape Kilovoltage |
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How does patient thickness affect the radiograph?
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A thick body section will attenuate more xrays than a thin body section.
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How is the subject contrast related to the relative number of xrays exiting adjoining sections of the body?
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It is directly proportional
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What is tissue mass density?
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Adjoining sections of the body may have equal thicknesses yet greatly different mass densities.
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The density of a tissue affects subject contrast because of what?
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The number of eletrons per cm cubed
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What is effective atomic number?
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Subject contrast is greatly influenced by the atomic number of the tissue being radiographed
Important due to photoelectric effect |
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What is the Effective atomic number of bone, muscle, and fat?
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Bone = 13.8 (highest)
Muscle = 7.4 (middle) Fat = 6.3 (Lowest, except for gas) |
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What does shape do to an image?
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It reduces radiographic quality during imaging
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Why does absorption unsharpness happen?
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It results because a patient is not made up of objects that have nice sharp edges
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Where is absorption unsharpness greatest?
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Greatest with round objects
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How can you reduce absorption unsharpness?
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By patient compression
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What is most closely related to absorption unsharpness?
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Geometric unsharpness
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What is the most important influence on subject contrast?
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kVp
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What is the primary exposure factor that is used for regulating radiographic contrast?
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kilovoltage
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As kVp decreases, what happens to scatter and contrast?
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Contrast increases and scatter decreases
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kVp has the greatest effect on what three things?
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beam QUALITY
Attenuation Exposure latitude (number of useful densities) |
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Scatter absorption happens where?
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grid
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Scatter production happens where?
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patient, kVp
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Disadvantage to low kVp radiographhy?
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high patient dose
loss of exposure latitude |
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What is the principle controlling factor for scale of contrast?
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kVp
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Extremity exposures result in short scale contrast due to what?
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Low kVp (50-60)
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Abdominal exposures result in long scale contrast due to what?
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Low contrast (70-80 range)
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Subject contrast can be greatly enhance by the use of what?
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Contrast media
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Barium and iodine results in what kind of subject contrast?
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high subject contrast
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To insure proper quality when barium is used for UGI, what do you do? Very important!
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An increase in kVp is necessary to penetrate (usually 120 kVp)
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What will motion do to an image?
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Movement of patient or xray tube during the exposure will result in blurring of the image.
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Motion blur will result in what?
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Loss of quality
Patient motion is usually the cause of motion blur |
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Four factors to reduce motion blur:
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Use the shortest possible exposure time
Restrict patient motion by proper instruction or restraining devices Use a large SID Use a small OID |
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Considerations for Improved radiographic quality
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Patient positioning
Imaging devices Selection of technique factors |
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How should the patient be positioned?
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Place atomic structure as close to film as possible and make sure that is is parallel with film.
CR should be in center of structure Effectively restrain patient to minimize motion blur |
