Final Review: Prime Factors Flashcards
Prime exposure factors
kvp
mAs
mA
exposure time
Distance(d)
SID
Beam penetrability
how many photons will penetrate anatomy
numerically represented by HVL
Quality
intensity of beam
how many photons are within the beam (determined by mAs , kvp, distance filtration)
measured in Roentgens
Quantity
Factors affecting xray emission
Quantitative and qualitative
Quantitative:
mAs
kilovoltage
distance
filtration
Qualitative:
Kilovoltage
Filtrations
determines beam quantity or intensity
mA
The unit for current:
Ampere
when you change the current to the filament it changes:
it changes number of electrons released throw thermionic emission
when you change mA station on equipment it changes:
changes current to filament
-controls duration of exposure
-directly proportional to intensity of beam
short exposure times useful in reducing motion artifacts
Exposure time
which factor is seen as a whole in which is demonstrated as individual
kvp - as a whole
kev- is individual
smallest OID as possible and long SOD does what
decreases magnification
change from 200mA to 300mA is a change in what
50% increase in number of electrons emitted from filament
50% increase in number of photons
**if not change made to exposure time, patient dose also increases
mA * seconds =
mAs
Controls:
quantity
Image receptor exposure
patient dose
mAs
Same image receptor exposure will result from different mA and time selections (intensity and duration), provided that mAs totals are equal
mAs Reciprocity
when you increase our distance what do you do to your technique
increase technique to reach IR
what is the relationship of mA and time when total mAs maintained:
inverse
with mAs and digital imaging, mAs still controls the intensity and the number of photons reaching the patient and the IR. However what happens when mAs is too high or too low.
-if mAs is too high, computer automatically rescales the image to appear with adequate density
-if mAs too low, quantum mottle will still occur
-Quantified as a numerical value to indicate radiation intensity to a digital image
-Two common terms: exposure index(EI#) and S#
Image Receptor Exposure
-Traditional term for infrared (IR) exposure to film
-Amount of silver deposited on hard copy film image
Density
More density=
darker film image
less density=
lighter film image
overall blackness
Density in film
-how dark or light digital image appears on monitor
-does not correspond to IR exposure
Brightness
-primarily controls beam quality
-energy and penetrability
-influences: scatter/secondary radiation production
-dramatic effect on radiographic contrast if using film technology
kVp
Increases beam penetrability and quantity
increasing kvp
decreases beam penetrability and quantity
decreasing kvp
Because kvp affects both quality and quantity, change of only 15% will cause doubling of exposure
15%
what should not be used to control IR exposure
kvp should not be used to control IR exposure
15%
if maintenance of exposure desired:
-decrease kvp by 15% and double mAs
-increase kvp by 15% and reduce mAs by 50%
-xrays produced from a “point source” in the xray tube
-inverse square law applies
as it increases, beam intensity decreases
Distance (SID)
Intensity of radiation at given distance from point source is inversely related to the square of distance between object and source
Inverse Square Law
Doubling the distance, reduces the beam intensity by a factor of 4
Inverse Square Law
-provides technique correction for change in SID
-maintains same image density/IR exposure
Exposure Maintenance formula
Be sure to look at the separate handouts that have been given to us throughout the semester (math, relationship charts, packets)
what does longer SID do
-reduced entrance skin exposure
-improves image quality
tabletop radiography
40 inches =
100 cm
chest radiography and other selected exams
72 inches=
180cm
Exposure indicator values used to assess image quality and ALARA compliance
-kvp selected to maximize receptor exposure
-distance selected based on desired beam geometry
-mAs adjusted to meet exposure needs
ALARA
As low as reasonably achievable
for Digital image receptor systems what is contrast controlled through:
controlled through computer post processing
-image noise values impacted
-digital data drop with excessive exposures
extremely high or low exposures do not produce acceptable image quality
what is the primary controlling factor for image receptor exposure
mAs
what does window width associate with:
contrast window width left and right
associates with contrast and kvp
Low contrast
-Long Scale (many grays)
-increased kvp (ex.chest xray)
-wide window width
-wide latitude
-elephant herd
High Contrast
-short (fewer grays)
-decreased kvp (ex.hand xray)
-narrow window width
-narrow latitude
-zebra
increased kvp increases transmission, increases energy, increases Compton which increases what dose?
occupational
when you increase your kvp what happens to contrast
decreases
when you decrease your kvp what happens to your contrast
increases
when you increase your time what happens to your time
decreases and vise versa
Practice:
-indirect square long
-reciprocity
-direct square law
-15% rule
The radiograph must exhibit differences in the brightness levels or densities in order to differentiate among the anatomic tissues
image contrast
a result of the absorption characteristics of the anatomic tissue radiographed and the quality of the xray beam
subject contrast
the ability to distinguish among types of tissues is determined by the differences in brightness levels or densities in the image or contrast
subject contrast
number of different shades of gray that can be stored and displayed in a digital image
Gray scale
the range of densities visible on film
Scale of contrast
