Ch11: Primary Factors Flashcards
Define mA
of electrons flowing from cathode to anode
Milliamperage
of the current to the filaments
determines thermionic emission
Tube Current
What two components make up mAs?
milliamperage and time (in seconds)
What are primary exposure factors?
mA
seconds (time)
KvP (kilovoltage peak)
Which factor(s) affect quality of the xray beam?
kvp
Which factor(s) affect quantity of the xray beam?
ma
time
kvp
How does mA increasing affect tube current/quantity?
increases
How does mA increasing affect # electrons at the filament?
increases
How does mA increasing affect pt dose?
increases
How does mA decreasing affect tube current/quantity?
decreses
How does mA decreasing affect # of electrons at the filament?
decreases
How does mA decreasing affect pt dose?
decreases
What does time of exposure mean?
amount of time the electrons are flowing in the tube and xrays are being created
What measurement is time recorded?
seconds
What benefits are there to a shorter exposure time?
decreases motion or blur
What q’s are related to time?
quantity only
How does increased time affect # of electrons at filament?
increases
How does increased time affect # of xrays and patient dose?
increases
How does decreased time affect # of electrons at filament?
decreases
How does decreased time affect # of xrays and patient dose?
decreases
What does mAs control?
amount of thermionic emission at the filament and the number of photons created at the anode
Define thermionic emission
boiling off of electrons from a filament (within the cathode) by a flow of electrical current
What is quantity?
intensity/amount of radiation striking/reaching the IR
Is ma and time a inverse or direct relationship?
inverse
as mA increases, time needs to decrease in order to maintain exposure
is mAs indirect/directly proportional to quantity?
direct
need the right combo to penetrate at the correct EI
What does mAs affect?
exposure to IR
Patient dose
quantity/intensity of the beam, x-ray photons
What does mAs NOT affect?
penetration ability (beam quality)
spatial resolution/recorded detail
brightness of an image
How do you calculate mAs?
ma x time
T/F Different combos of ma and time will create the same resulted mAs.
True
Why would you change the combination of mA and time?
if you want to manipulate the time of exposure
Shorter time: peds, decrease motion
Longer time: blur anatomy (ribs during breathing)
T/F Changing the combination of ma and time will result in a change in patient dose.
False
As long as the resulting mAs is constant, the patient dose will also be the same
To maintain exposure/pt dose - as time decreases, ma must….
increase
To maintain exposure/pt dose - as ma increases, time must….
decrease
Why is it important to check EI after each exposure?
digital processing will adjust an image even if the exposure it too high or low
ensure the EI is in range and adjust technique if pt dose is too high or low
T/F increasing mAs will increase the brightness of an image
False
mAs has no direct affect on image brightness
What determines brightness?
computer processing
T/F Digital imaging will alert you if the EI is out of range. The image will appear noisy.
False
digital images will appear consistent despite improper exposure
Automatic rescaling “edits” the image
Can fix overexposure, not always underexposure
T/F In days of film, an image that is too bright means there was a high EI.
False
brightness on film indicates a low EI
T/F On film, an image that is dark and has a short grayscale means there was a high EI
True
Over exposure means that more will reach the IR, so the image will appear dark
T/F kilovoltage peak affects just quality
False
kvp affects both quantity and quality
Define kVp
kilovoltage peak
energy of electrons and # of photons created
beam penetrability
define kilo
1,000 or 1.0 x 10^3
What is the purpose of kvp within the tube?
accelerates the electrons
w/o it, the electrons would be created but wouldn’t move into the anode
How does kvp affect quality?
affects the beam’s energy and how much penetrability it has
How does kvp affect quantity?
affects the conversion of electron energy into xrays (photons)
Increasing kvp will ______ voltage potential
increase
Increasing kvp will _____ electron energy
increase
decreasing kvp will ______ xray beam energy
decrease
decreasing kvp will ______ penetrability of the beam
decrease
increasing kvp will ______ pt dose
increase
Decreasing kvp will ______ receptor exposure
decrease
Which has the lowest EI?
70 kvp @ 2 mas
50 kvp @ 2 mas
93 kvp @ 2 mas
50 kvp @ 2 mas
Which has the highest EI?
70 kvp @ 200 mA, 0.01 sec
50 kvp @ 75 mA, 0.027 sec
93 kvp @ 60 mA, 0.03 sec
93 kvp @ 60 mA, 0.03 sec
T/F Before you adjust mAs, you need an adequate kvp or the technique adjustment won’t improve your image.
True
Adequate kvp is needed in order for the beam to penetrate the patient and reach the IR
T/F It is better to adjust your kvp in your technique.
False
kvp also affects contrast, so your image quality will decrease if kvp is decreased too much
kvp changes absorption and transmission properties b/c it changes the amount of penetration
What is the 15% rule?
15% increase in kvp will double # of photons
15% decrease in kvp will half # of photons
In order to keep the exposure/quality, need to adjust the mas accordingly.
increase in kvp = double mas
decrease in kvp = half mas
What does “more k’s, more grays” mean?
higher kvp means that there is a lower contrast, longer gray-scale/more shades of gray
T/F A higher kvp will equal less transmission, lower subject contrast, and more scatter
False
High Kvp = more transmission
lower contras (long grayscale)
increases scatter while decreasing photoelectric interactions
T/F A lower kvp will equal less transmission, higher subject contrast, shorter grayscale, and more absorption
True
T/F Level of subject contrast depends on the body part being examined
True
some body parts may not show all anatomy/detail b/c of differential absorption, but there is enough contrast for the doctors to see what they need
What is kvp range for extremities?
in the 60s
hands may be 50s
T/F For a particularly large or small patient, you can adjust the kvp so you have proper penetration.
True
the desktop has an option for “large” or “small” or “peds” which will adjust the technique for you to avoid over or under exposure
What is kvp range for abdomens?
75-80
85 for large patient
What is kvp range for chests?
90-110
T/F the computer can fix images as long as the kvp is at an acceptable range for penetration
True
to an extent, computer processing can fix the radiographic contrast (cannot fix contrast issues due to scatter)
T/F Exposure techniques using higher kvp and lower mAs are recommended.
True
If a patient has a cast on their leg, how would you change your technique?
increase
What special conditions would require you to adjust your technique?
Casts
implants or replacements
Both require an increase
