**Saia Unit 4 Flashcards
Energy deposited into the image receptor
Density
The controlling factors of density are
mA
Time
mAs
mAs
The number of X-rays in polyenergetic primary beam
mAs has the primary control of
Image density
Density is placed in one of three categories
Acceptable
Underexposed (too light)
Overexposed (too dark)
30% rule
The human eye requires a change in density of at least 30% before it can be visualized
Fuji exposure indicator
S number
Range 180-220
Indirect relationship
Kodak exposure number
EI number
Range 1800-2200
Direct relationship
Agfa exposure indicator
LGM number
Range - 1.9-2.5
Direct relationship
A perfect exposure indicator value is
1mR of exposure to imaging plate/panel
Increasing the KVP will increase the quality or energy energy of the beam giving the beam more
Penetration ability
As KVP is increased, density…
Will increase
Energy deposited into the image receptor is
Density
15% rule
Used to alter KVP settings to change density and contrast
A 15% increase in KVP = a 50% decrease in mAs
A 15% decrease in KVP = a doubling in mAs
As KVP goes up, image contrast
Goes down
As KVP goes down, image contrast
Goes up
If SID is increased with no other change, density is
Decreased
If SID is decreased with no other change density is
Increased
SID and density have an ? relationship
Inverse
Density
Overall blackening of a film/image
Inverse square law
The intensity of the beam is inversely proportional to the square of the distance.
I1 (D2)2/I2 (D1)2
Density maintenance law
Formula that will compensate for changes in SID to maintain density
mAs1 (D1)2/mAs2 (D2)2
If you increase the OID less scatter reaches the film resulting in a decrease in
Density
Less OID will result in a greater density due to
More radiation reaching the film
OID and density have a ? relationship
Inverse
Imaging system speed
As speed increases the efficiency of placing energy into the IR increases, and density increases also.
Grids are used to
Absorb scatter to prevent it from reaching the film.
Formula to maintain density with grids
Mas1 GCF1 / mAs 2 GCF2
If collimation is increased (smaller light field) then
Less radiation leaves the tube
Less scatter produced
Less energy reaches IR
LESS DENSITY IS THE RESULT
Radiolucent tissue
Tissue that let’s xray energy through
Radiopaque
Tissue that will absorb ray energy
Disease
Physical and chemical changes in tissue
Anode heel effect
Variation in the beams intensity across its longitudinal axis,
More X-rays on cathode side than anode side
As filtration is increased,
More photons are absorbed resulting in fewer X-rays and less xray energy in the primary beam
Contrast
The difference in adjacent shades across the radiographic image
Contrasts primary function
To make recorded detail visible
Contrast is primarily controlled by
KVP
Energy deposited into the IR is
Density
The more energy values = more shades of density
High contrast
Few shades of gray
Increased contrast
Lower KVP
“Short scale” contrast
Low contrast
Many shades of gray
Decreased contrast
High KVP
“Long scale” contrast
Differential absorption
Differing materials absorb xray at differing degrees
As KVP decreases patient dose ?
Increases (weaker xray, more absorbed in patient)
As KVP increases patient dose ?
Decreases
Stronger xray beam, more passes through patient
Grid ratio
Height of lead strips to distance between
Grid frequency
Number of lead strips per unit distance
Increasing beam restriction - smaller field
Less tissue irradiated
Less scatter produced
- “short scale”, higher contrast
An increase in OID results in
Less scatter reaching the film - higher contrast
As beam filtration increases
Shorter scale
Higher contrast
Positive contrast media
(Iodine/barium) - temporarily make anatomy more dense than normal
Negative contrast media
Oxygen, carbon dioxide, room air
Allow more X-rays to penetrate due to the low atomic number resulting in an increase in density
Barium atomic number
56
Iodine atomic number
53
High contrast
Big difference in shades, short scale, low KVP, very black and white
Think of the top of shade triangle (small peak)
Low contrast
Little difference between shades, long scale, high KVP, many grays
Think of the bottom of shade triangle (large base)
Photographic factors
Density
Contrast
Density
Overall image blackness
Contrast
Visibility of adjacent structures
Geometric factors
Detail
Distortion
Detail
Visibility of fine anatomical structures
Distortion
Misrepresentation of an objects size/shape
Recorded detail aka
Resolution, sharpness, definition, detail
Measured in lp/mm
Line pair
A line and it’s adjacent space, as recorded in an image of the tool (resolution tool)
Human visual acuity is
5 lp/mm
Unsharpness
Degree of loss of detail of the anatomical structure by the imaging system
Penumbra
Geometric unsharpness around the periphery of the structure of the image
As focal spot size increases
Penumbra increases
Effective focal spot
focal spot projected toward the patient
Usually .5-2.0mm
Line focus principal
The effective focal spot is smaller than the true/actual focal spot
True/actual focal spot
Area where electron stream strikes the anode target
Small focal spot
Maximum image detail
Umbra
Objects true size
Why not always use a small focal spot
Small focal spot uses small filament which are mA limited, large focal sometimes needed
Increased SID will result in
Less penumbra
Increased OID will result in
More penumbra
Macro radiography
Magnification radiography
Intentionally magnifying to exaggerate small structures
Most common in mammography
IR speed
Faster speed is used to save patient dose, however resolution not good
Quantum mottle
Not enough X-rays in the primary beam to adequately form image
Distortion
Misrepresentation of an objects true size and shape
Size distortion is always
Magnification
Magnification
The misrepresentation of the actual size of an object
As SID increases, magnification?
Decreases
As OID increases, magnification ?
Increases
Magnification factor
SID/SOD (SID divided by SOD)
SID/SOD
Image width/object width = SID/SOD
Shape distortion
Misrepresentation of an anatomical part due to misalignment of source, IR, and/or part
Results in elongation or foreshortening
When the xray beam or IR is misaligned =
Elongation
When the body part is misaligned=
Foreshortening
Three points to perfect alignment
CR perpendicular to body part (most important)
CR perpendicular to IR
Body part parallel to IR
Magnification formula
Mag=SID/SOD
Percent magnification =
Image size - object size / object size X 100 = % magnification
Intensifying screens
Used to convert xray energy into visible light energy.
- 90-99% of film exposure is due to visible light
- 1-10% of film exposure is due to direct xray
Xray film is much more sensitive to
Visible light energy than it is to xray energy