Upgrade to remove ads
Only $35.99/year

Radiology Ch. 4: Factors effecting radiographic quality

Terms in this set (67)

What are the 3 basic requirements for an acceptable radiograph?
1. All structures as close to their natural shapes and sizes with minimum distortion and superimposition
2. image area examined completely, with enough surrounding tissue to distinguish between the structures
3. free of errors and show proper density, contrast, and definition
Radiolucent
1. Portion of the image that is dark or black
How do structures appear radiolucent?
they permit the passage of x-rays with little or no resistance
Examples of radiolucent structures
pulp chamber and periodontal ligament spaces
radiopaque
portion of the image that is light or white
How do structures appear radiopaque?
they are dense and absorb or resist the passage of x-rays
examples of radiopaque structures
enamel, dentin, and bone
density
degree of darkness or image blackening
What does a light radiograph mean?
little density
What does a dark radiograph mean?
more density
What does an increase in mA/exposure time going to result in?
increase in density
How does blackness result?
when x-rays strike sensitive crystals/pixels in the film emulsion/sensor
Contrast
refers to the many shades of gray that separate the dark and light areas
What is good contrast?
black, white, and enough shades of gray to differentiate between structures and their conditions
High contrast between adjacent areas are _______.
Large
Another term for high contrast
Short-scale contrast
What do gray tones indicate?
the differences in absorption of the x-ray photons
Where are radiographs radiolucent?
where the tissues are soft or thin
Where are radiographs radiopaque?
where the tissues are hard or thick
Another term for low contrast
long-scale contrast
Low contrast means the density differences between adjacent areas are _____.
Small
Sharpness
Geometric factor that refers to detail and clarity of the outline of structures shown on a radiograph
What can sharpness be affected by?
pixel size and distribution
What is unsharpness generally caused by?
movement of the patient, image receptor, or tube head
What are shadow cast rules often referred to as?
geometric factors that contribute to the quality of a radiograph
What are the 5 shadow cast rules/geometric factors that contribute to the quality of a radiograph?
1. small focal spot
2. long target-object distance
3. short object-film distance
4. parallel relationship between object and film
5. perpendicular relationship between central ray of x-ray beam and the object and film
How does a small focal spot contribute to the quality of a radiograph?
reduces size of penumbra resulting in a sharper image and slightly less magnification
How does a long target-object distance contribute to the quality of a radiograph?
Reduces penumbra and magnification
How does a short object-film distance contribute to the quality of a radiograph?
reduces penumbra and magnification
how does a perpendicular relationship between the central ray of the x-ray beam and the object and film contribute to the quality of a radiograph?
prevents distortion of the image
what are 4 factors affecting the radiographic image?
1. radiographic contrast
2. sharpness/definition
3. magnification/enlargement
4. distortion
What 6 things affect radiographic contrast?
1. subject
2. kV
3. scatter radiation
4. film/digital sensor type
5. exposure
6. processing (film)
How does the subject affect the radiographic contrast?
1. No contrast if uniform thickness and composition
2. human tissues vary in size, thickness, and density
How does the kV affect the radiographic contrast?
1. higher kV produces lower contrast
2. Lower kV produces higher contrast
How does scatter radiation affect radiographic contrast?
1. scattered x-rays add a uniform exposure to a radiograph that adversely decreases contrast
2. collimator and grid affect contrast by narrowing beam diameter
How does the film/digital sensor type affect radiographic contrast?
1. each film has its own inherent contrast that may vary
2. pixel size affect contrast
How does exposure affect radiographic contrast?
1. underexposed = too light
2. overexposed = too dark
3. fogged radiograph will result in diminished or poor contrast (stray radiation, heat and humidity)
How does film processing affect radiographic contrast?
improper development time or temperature will not have ideal contrast
What are 7 factors that affect sharpness/definition
1. small focal spot
2. long target-image receptor distance
3. short object-image receptor distance
4. motion
5. screen thickness
6. screen-film contact
7. small crystal/pixel size of intraoral image receptors
How does a small focal spot affect the sharpness/definition
1. less penumbra
2. tube head must remain still
How does motion affect the sharpness/definition?
movement of the patient and or image receptor in addition to the tube head results in a loss of image sharpness
How does screen thickness affect sharpness/definition?
thicker the screen the less sharp radiographic image
how does screen-film contact affect sharpness/definition?
Poor screen-film contact results in less sharp radiographic image
What 2 factors affect magnification/enlargement?
1. target-object distance
2. object-image receptor distance
how does object-image receptor distance affect magnification/enlargement?
large object-image receptor distance = more magnification/enlargement
What 2 things result in distortion?
1. unequal magnification of different parts of the same object
2. when the image receptor is not parallel to the object and or when the central ray of the x-ray beam is not perpendicular to the object and the plane of the image receptor
Which 2 shadow casting rules minimize distortion?
4 and 5
What are 4 varying exposure factors that affect radiographs?
1. mA
2. exposure time
3. mAs (miliampere-seconds)
4. kVp
What is affected by varying the mA?
Density (increasing mA increases density)
What factor is affected by varying the exposure time?
darkness/lightness (increasing exposure time darkens a radiograph)
What is the easiest exposure factor to change?
exposure time
When mA is increased, exposure time must be _____.
decreased
What factor does varying the kVp affect?
contrast (increase kVp = decrease contrast) and density (increase kVp = increase density)
What is the only exposure factor that directly influences contrast?
kVp
If the kV is increased what must you do to maintain constant density?
reduce mAs or exposure time
What are the 4 effects of variations in distances?
1. target-surface distance
2. object-image receptor distance
3. target-object distance
4. target-image receptor distance
What is the length of the target-surface distance determined by?
lenght of the PID
What do all intraoral techniques require?
the open end of PID be positioned to almost touch the patient's skin
Which technique would you use a longer PID for?
paralelling technique
What is the target-image receptor distance a sum of?
target-object and object-image receptor distance
When does the target-image receptor distance improve the image quality?
when the distance is increased
What can affect the target-image receptor distance?
location of the x-ray tube housing
inverse square law
O intensity/New intensity = new distance^2/O distance^2
What does the inverse square law state?
intensity of radiation varies inversely as the square of the distance from its source
Where are exposure charts available?
commercially or custom made by the practice
Where should the exposure charts be posted?
at the x-ray unit control panel for easy reference
What have some dental x-ray machine manufacturers incorporated and where?
commonly used exposure factors into the dial of the control panel
Students also viewed