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How the eye works?
The human eye gathers light, focuses the light, converts it to nervous impulse and transits it to the brain for processing
Fovea centralis?
gathers light to nerve impulse
Optic nerve?
Transmits nerve impulses to the brain
Parts of the eye-
Aqueous humor
Cornea
Iris
Lens
Phototopic
daylight vision- vision is controlled by the CONES they require relatively bright light to function they can see long=yellow
Medium= green
Short= violet
-good focus-good contrast perception
Scotopic
-night vision- vision is controlled by the RODS very sensitive to low levels and unable to see bring lights
Can not seen colors—but come best see green if any
-good peripheral vision
What is perception?
recognition and interpretation of what we see
Threshold detection?
Perception of extremely small or faint detail
Is affected by background noise
Noise= fog(Compton scatter)
Artifacts=jewelry
What is boundary effect?
Hard to see the difference in colors with colors are not close together
The longer the boundary the easier to tell the difference between shades
Mach Effect?
Nerve impulse and what our brain perceives with bright light or a big change
Edge enhancement?
See a thin white strip b4 a big change in contrast
Scanning motions can help to reduce this or when gradually change contrast
Veil glare?
Bright light shining out sides of view box
Can’t judge contrast
How does change in distance effect light? And what goes on?
Distance makes light diverge. It also changes blind spots. Humans have a blind spot at 9 inches. It also changes the cell so you don’t overuse a cell
Patter recognition
Faster when techs hang radiographs correct
Localization process
2 radiographs 90 degrees apart from one another
How projection tells what is superimposed?
the name tells what is being superimposed
What type of interactions produce scatter
-
Incedent photons and outer shell electrons
compton interactions (high kvp)
What happens to film when scatter strikes?
Density increase and Contrast decrease
Increase kvp? Interactions? Affects on contrast and scatter?
As kvp increase there is more comptons interactions which increase scatter and decrease contrast.
Changes in volume affecting scatter?
As volume increases scatter increases
What affects volume? Patient / tube?
Field size
Patient thickness
How does atomic number affect scatter
The higher the atomic number the less scatter
Bones. Contrast media
Field size
Large field size= more scatter produced
Small field size= less scatter produced
3 beam restricters
- Aperature diaphragms
- Cones and cylinders
- Collimaters
Aperature Diaphrams Reduce penumbra, off focus, or stem?
they increase penumbra
dont decrease off focus
dont decrease stem
What do cones and cylinders reduce
Reduce penumbra
Reduce off focus
PBL?
Senses size and orientation of the cassette
What is penumbra? Off focus?
Penumbra- fuzzy borders-reduces detail -
Off focus- xray photons being created at tube and not at focal spot
Purpose of grid
Keep the scatter from hitting the cassette
What components control scatter
Kvp
Irradiated material
beam restrictors
Attenuation?
Reduction in the total number of x-ray photons and the lower energy of the beam
Types of attenuation
Absorption
scatter
3 possibilites at x-rays pass through matter
Continue on their way
Be absorbed
Change direction
How do x-rays attenuate
Exponentially
Beam that exits the patient
Through transmission
How thickness affects attenuation
Thicker the patient the more attenuation
How atomic number affects attenuation
higher the atomic number the more attenuation *photoelectric absorbtion
How density affects attenuation
More dense the more the attenuation
More tissue density the less radiographic density
Number of electrons affect attenuation
Higher the electrons the more attenuation
Calcium-bones=more attenuation
4 substances that account for variations of absorption
air
fat
muscle
bone
Air, fat,muscle and bones? How they compare in attenuation
Air- least
Fat-second least
Muscle-second most
Bone-most attenuation
Define subject density and how it works?
More subject density less radiograph density
less subject density more radiograph density
Subject contrast
Amount of differential absorption
Chest- high subject contrast
Mammography- low subject contrast
Age and size is what affects contrast
Subject detail (resolution)
How patient affects Sharpness of structure as it appears on film
Highest detail when have the part is closest to film
We rely on where part lies on body and where patient is to the film
Less detail larger patients
Subject distortion
Misrepresentation of size and shape of structure
Magnification-size
Angle-sharp
Larger the patient-less detail- more distortion – has to do with subject size
Greatest variable?
patient
Additive condition?
Kvp- 5- 15 %
A place where there used to be or is supposed to be air turns into something more dense
As additive disease gets worse radiographic density gets less (whiter)
Destrictive condition
mAs- 25- 50 %
a place that used to be a dense tissue turns to air or a less dense tissue
as destructive disease gets worse radiograph density gets greater (blacker)
Patient history
good patient history
observe patient
clinical assessment
luenectomy
old exams
How does kvp affect scatter
higher the kvp the more scatter= contrast then decrease
basic construction of grid and when to use a grid?
Thin flat rectangular device- in contains radiopaque (absorb) lead strips sepaorated by radiolucent( pass through) space material
Used body part greater than 10 cm
Used when kvp higher than 60 =kvp penetrating power
Grid ratio
Higher the grid ratio= closer the strips = more scatter clean up= higher the contrast
Beam has to be pretty straight
Grid frequency
Number of grids lines per inch
Higher grid frequency the thinner the strips= less scatter cleanup= lower contrast
what do you want when dealing with grid ratio and frequency
high grid ratio (close) – and less grid frequency (thick)
Grid patterns
linear- can angle longitudinal
Criss cross (cross hatched)- can not angle
Grid types
Parallel- all grids run up and down = will never touch Use with longer SID
Focused-lead strips angle to match the beam Tube is supposed to be aligned to the convergence line o Have to be in the correct focal range
Not too much room for error
high or low grid ratio makes more room for error?
Lower grid ratio makes more room for error
How does bucky work (reciprocating grid)
Grid strips move perpendicular to the way they run
Strips run length of bucky and table
GCF compared to Ratio and mAs
When use grid you have to increase mAs
Higher the grid ratio the higher the GCF
Higher ratio higher the mAs
Grid errors
Off level- one but not the other is angled—parallel and focused
Off center- tube is not aligned to grid -- focused
Off focus- center good the rest bad-- focused
Upside down- center good the rest bad -- focused
Author
acmolt
ID
8705
Card Set
Exposures
Description
test 2
Updated
2010-03-02T16:01:32Z
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