Xray Physics Flashcards
cathode consists of?
large filament
small filament
focusing cup
filament serves as?
the source of electrons in the production of xrays and are made of tungsten
when does thermonic emission occur?
when a current is applied to a filament
coil of the wire becomes very hot, boiling off electrons
the temp of the filament controls?
quantity of electrons (mA) emitted from it
the number of electrons determines the number of xrays created
the temp of the filament controls?
quantity of electrons (mA) emitted from it
the number of electrons determines the number of xrays created
focusing cup/filament cup
part of the cathode
encases the two filaments
this is where thermionic emmision occurs
each filament sits in its own cup, the cup consolidates the electron cloud
anode
positive electrode in the xray tube
most xray tubes consist of a rotating anode
rotating target receives electrons as they move from cathode to anode
rotating anode target
dissipates the heat generated
creates the focal spot
focal spot
an area of tungsten target of the anode
will be determined by the size of the filament that is chosen
why is a smaller filament better?
better detail, less penumbra (unclear borders)
line focus/beam hardening principle
has the effect of making the actual focal spot size appear smaller when viewed from the position of the film
the smaller the target angle, the smaller the effective spot
stream of electrons narrows down to an effective beam
mA
milliamperage
heats the filament causing a boiling off of electrons
mAs
more current=more electrons produced= greater radiograph density (darker)
density
overall blackening of the film
radiolucent
structures that produce more blackening on the film
radiopaque
structures that produce more whitening on the film
the relationship between mAs and density
directly proportional
what does changing mAs do to the film?
controls the blackening of the film
more mAs= more film exposure
how much change in mAs does it change to perceive a change in the film?
30%
if mAs is doubled..
density is doubled
kVp
force applied to accelerate the electrons from the cathode to the anode at the time of exposure
greater force= greater number of high energy photons will be produced
wavelength and frequency are..
inversely related
low kVp=
low energy= weak penetration
high kVp=
high energy= greater penetration
higher kVp produces
shorter wavelengths with a greater ability to penegrate the body tissue
contrast
difference in density between 2 structures
contrast makes detail visible
what controls contrast?
kVp, they are inversely related
low kvp looks like..
high contrast, short scale
low kVp produces less
scatter, however, the amount of radiation absorbed by the patient is increased
to lower your contrast you/longer scale of contrast you..
have more shades of grey
you must increase kVp by 15% and decrease mA by 50%
to raise your contrast/shorter scale of contrast you…
more black and white
decrease kVp and increase mAs by 100%
an increase in ___ by 15% will____the density of the film
kVp
double
a decrease in ___by 15% will____the density of the film
kVp double
describe what the different physiologic parts of the body will show up on film
gas-black
fat- black
muscle, water, soft tissue- grey
bone, metal- white
anode heel effect
when theintensity and desnity decrease on the anode side of the film, place the thinner portion of what is being xrayed toward this end
give an example of the anode heel effect
the toes are thinner than the ankle
if you ray the foot, you aim the tube so the anode is toward the toes and the cathode is at the ankle
anode side
less xray
smaller effective focal spot
more detail
use thinner body part
cathode side
more xray
larger effective focal spot
less detail
use on thicker body part
FFD/SID
focal film distance
source image distance
represents the length of space from the focal spot to the recording medium
as you increase FFD..
the image gets smaller and clearer
decreases magnification distortion
inverse square law
2/1 inversed= (1/2)^2 AKA 1/4
example of inverse square law for xray
if a 40 ubcg FFD is doubled to an 80 inch FFD, the intensity of the radiation at 80 inches is 1/4 the intensity at 40 inches if no factors are changed
intensifying screens
consists of fluorescent crystals
when hit by the xray, fluorescent salt changes xray photons into visible light photons
can expose the film and decrease patient exposure
large crystals in an intensifying screen..
work quickly
less detail
faster screens=higher contrast= short scale (less pt exposure)
small crystals in an intensifying screen..
work slowly, but offer more detail
slow screens=lower contrast= longer scale (more patient exposure)
definition
describes the clarity and sharpness of an image
umbra
picture made by useful beam
penumbra
blurry halo parts of the image (unsharpness)
caused by having tube too close to the film (short FFD), having patient too far from the film (long OFD), having too fib filament on cathode, making focal spot too big
OFD
distance between patient and film
as you increase OFD, image becomes bigger and blurrier
as you decrease OFD, image becomes smaller and more clear
screen film contact and definition
screen film contact is the pressure eerted by the film holder as it encloses the xray film
pressure should be evenly distributed across the surface of film
wire mesh test is best for screen film contact
poor screen film contact reduces the image sharpness of the image
scatter radiation
when xrays strike an object one of 3 things can occur: scatter, absorb, or penetrated
scatter can be reduced by?
grids
air gap technique
collimators
filters
grid
device of alternating strips of lead and spacer material
placed between patient and film
sole purpose of grid
improve quality of radiograph by abdorbing scatter radiation
what will be required if you use a grid?
increase in exposure factors to maintain density (increases patient dose of radiation)
grid ratio
height of the lead strips to the distance between the strips
12:1 ratio in chiro office
air-gap technique
6-10” between patient and film
lat. cervical, flexion, extension
distance between body part and film causes?
scatter radiation to diverge away from the film
reduces patient dose, can distort/magnify image
collimation
limits the size of the beam to the size of the body part you need to capture on film
collimation is the best way to?
reduce scatter radiation to patient and film
filtration
preferential removal of low energy photons
usually made of aluminum
what does the filter do?
stops the weak photons from entering the aptient’s body thus protecting them from scatter radiation
radiation absorbed dose (RAD)
used to identify irradiation in patients
measures the radiation energy absorbed
1 RAD= 1 REM
radiation equivalent man (REM)
typically applied to occupationally exposed persons
identifies the biological effectiveness of the radiation abdorbed
5 REMS is the safe limit per year
maximum permissible dose (MPD)
rumber of REMS a person can get in a lifetime without getting radiation poisoning
MPD= 5(age-18)
bremsstrahlung radiation
AKA braking radiation
majority of useful beam
interacts with target nucleusq
characteristic radiation
high intensity electron hits an inner shell electron, which is knocked out of its position in orbit
outer shell electron then fills its spot
types of scatter radiation
compton scatter
classical scatter
photoelectric effect
quantum mottle
compton scatter
primary form of scatter
moderate energy xray knocks out an outer shell electron, which causes a loss of energy
classical scatter
AKA thompson scatter
AKA coherent scatter
low energy xray that schanges direction with no energy loss
contributes to film god
photoelectric effect
low energy photon is absorbed by the subject creating a latent image on the film
quantum mottle
AKA radiographic noise
fluctuation of the number of photons absorbed by the intensifying screens to form the image on the film
faster screens porduce more quantum mottle
what decreases quantum mottle?
high mAs
low kVp
