10. Exposure technique factors: Primary and Secondary technique factors Flashcards
what are 2 factors that contribute to compton scatter
how
kVp = affects beam penetrability
Volume of irradiated material = field size and patient thickness
increasing the kVp has what effect on transmission
increases transmission
increasing the kVp has what effect on photoelectric scatter
decreases photoelectric scatter
increasing the kVp has what effect on compton scatter
increases compton scatter
increasing the kVp has what effect on patient dose
how
decreases dose
increase penetrability of beam so less is attenuated in tissues
increasing the kVp has what effect on image quality
lower amounts of contrast
decreasing the kV means what needs to be done for the mAs
Decrease in kv is more absorbtion but need to increase mAs to compensate (15% rule to maintain image quality)
what does increasing mAs result in for the beam
increases number of photons in the beam
what are the 3 factors of the volume of irradiated material
field size
patient thickness
average tissues density
increasing the field size does what to the volume of tissue radiated
what effect does this have on the scatter
increases it
increases scatter
increasing FS has what effect on the beam quantity
what are the 3 factors causing this
increases it
increases scatter and amount of remnant radiation hitting the receptor
can increase image noise without mAs compensation
what is the field size
how big the image is
what is the tradeoff when you decrease the field size
what needs to be done to compensate for this trade off
fewer photons reach the IR so exposure decreased
increase in mAs must accompany significant reduction in the field size to maintain image exposure
what 2 factors of the nature of body parts produce more scatter
why
thicker and denser parts = more scatter
both increase number of interactions x-ray beams undergo as it passes through the body
why do denser body parts produce more scatter
higher e- density present in thicker/denser tissues
increased likelihood of interactions occurring especially photoelectric
compression devices are used to improve spatial resolution and contrast how
in terms of thickness and dose and OID
decrease patient thickness
makes thickness more uniform
lowers patient dose and brings tissue closer to receptor by decreasing the OID
what are the 3 primary exposure factors
mAs
kV
exposure time
what does increasing mAs do to the beam
more photons in beam
what does increasing the kVp do to the beam
increase quantity and quality
mAs increase has what effect on the quantity of radiation reaching the IR
what is the relationship
direct proportional
increases quantity
mA and exposure time have an ___ ___ relationship when maintaining the mAs
inverse proportional
how does kV affect the x-ray beam penetration and absorption in anatomic tissues
increase kV increases penetration and decreases absorption
how does kV affect the subject contrast
increasing kVp decreases subject contrast
what does lower kV do to the variation in the x-ray intensities exiting the patient
greater variation in the x-ray intensities exiting the patient
what is the 15% rule in terms of the IR exposure
what are the equations for increasing and decreasing the exposure and maintaining the exposure
to increase exposure to the IR multiply the kVp by 1.15
1.15 x (original kV + 15%)
to decrease the exposure to the IR multiply by 0.85
0.85 x (original kV - 15%)
what is the 15% rule in terms of the IR exposure
what are the equations for maintaining the exposure when kVp is increased or decreased
to maintain the exposure to IR, when increasing the kVp by 15% (kVp x 1.15), divide the original mAs by 2
when decreasing the kVp by 15% (kVp x 0.85), multiply the mAs by 2
is image brightness dependent on mAs and kVp
no
is image contrast dependent on kVp
no
what does it mean when people say that digital IRs have a wide dynamic range
can use a wide range of exposure factors and still get a good quality range
what happens to the exposure and image brightness/contrast if there is too much mAs and/or kV used
patient overexposed
computer processing adjust image brightness and contrast
what happens to the quantum noise if there is too little mAs and/or kV used
it will be visible
the screen film comes in different speeds what does it mean when its faster/higher speed
in terms of exposure factors
Faster/higher speed = more exposure characters, syst takes the few pixel it has and gives you the info you want
Fast = lower exposure factors
what are 10 secondary factors
focal spot size
SID
OID
central ray-anatomic part image receptor alignment
Grids
beam restriction
generator output
tube filtration
compensating filters
patient factors
what do secondary factors affect
don’t affect production of beam directly
affects the radiation but not directly during its production
how does the focal spot size affect unsharpness and spatial resolution
increasing focal spot size increases unsharpness and decreases spatial resolution
what is the penumbra
unsharpness/blurring at the edge of the beam
what is the size of a small focal spot
0.5-0.6mm
what is the size of a large focal spot
1.0-1.2mm
how does SID affect the radiation intensity
inverse square law
increasing SID decreases the radiation intensity reaching the IR
how does SID affect the size distortion (magnification) and recorded detail/spatial resolution
increasing the SID will decrease magnification and increase resolution
what is the SID
distance between x-ray tube and IR
what is the SID and mAs equation
mAs1/mAs2 = (SID1)^2/(SID2)^2
what is the equation for the radiation intensity and distance
I2/I1 = (d1)^2/(d2)^2
what is the OID
distance between the anatomic part and the IR
which 4 things does the OID affect
radiation intensity reaching the IR
amount of scatter radiation reaching the IR
magnification
spatial resolution
a smaller OID has what effect on magnification
less magnification
in terms of OID an air gap will have what effect on the radiation intensity reaching the IR and scatter
increase OID so decreases intensity and increases scatter
increasing the OID ahs what effect on the IR exposure, contrast and magnification and spatial resolution
decrease IR exposure
increase contrast and magnification
decrease spatial resolution
what is subject contrast
the anatomy and the contrast due to difference of structures
eg. anatomy in chest has got many differences of structures (bone, ribs, lungs all diff thicknesses in chest)
what is the magnification factor
what is the equation for the MF and SOD and object size
MF = SID/SOD
what is the equation for the SOD
SOD = SID - OID
what is the equation for the object size
object size = image size/MF
as OID increases the image size ___
decreases
what is the equation for MF, Lim and Lobj
MF = Lim/Lobj
Lim = width of image Lobj = width of object
what are the 3 factors of the CR misalignment
tube
part
image receptor
what is the purpose of the grid
limit the amount of scatter radiation that reaches the IR and improves the quality of the radiograph
what effect of less scatter or unwanted exposure on the image is to
increase the radiograph contrast
what does beam restriction do
what is the tradeoff
increased collimation = smaller field size = reducees the amount of tissue irradiated and reduces both the scatter radiation produced and the amount of radiation reaching the IR
higher contrast but needs increase in mAs
generators with more efficient output require what exposure techniques to produce an image comparable to those of a single phase unit
lower exposure techniques
how might tube filtration affect the quality of images
how significantly
small variations wont have any effect on the quality
what are 3 patient factors
body habitus
part thickness effects
pathology
what 3 things does the part thickness affects
beam attenuation
exposure reaching the IR
scatter production and image contrast
what does pathology affect
increase or decrease kV (penetration)
