6/7: Radiation Physics - Mahoney Flashcards
ionizing
removes an electron from an atom
roentgen
quantity of exposure
Rad
quantity of absorbed dose
Rem or Sv sievert
quantity of effective dose equivalent received by radiation workers
curie or becquerel
quantity of radioactivity
amount of ionization that is produced when radiation passes through matter
exposure
amount of energy absorbed by matter when radiation passes through it
absorbed dose
measure of biological damage caused by radiation
dose equivalent
product of absorbed dose multiplied by quality factor that accounts different types of radiation causing different amounts of damage
measure of biological damage caused by radiation to a specific organ
effective dose equivalent
product of dose equivalent multiplied by measure of risk from exposure to that organ compared to risk from whole body exposure to same dose
1 Gy = _____ rad
100
1 Sv = _____ rem
100
1 rad = _____ rem
1
1 Gy = _______ Sv
1
smallest quantity of any type of EM radiation
photon
has high energy and short wavelength
results in dif. shades of gray on film
attenuation
matter that absorbs x rays = white
matter that is penetrated = black
inverse square law
light intensity from a source is inversely proportional to the square of the distance b/w object and source
x ray intensity is 1 rad at 2 ft and .25 rad at 4 ft
energy of x-ray beam
kVp kilovoltage peak
manipulates primarily the energy or quality of x-rays and to a lesser extent the quantity of x-rays
kVp
manipulates the quantity of x-rays
mA and Timer
mA range
10-30 mA
timer range
1/60th -1/100th of a second
tube current and controls the number of photons produced
mA milliamperage
negative electrode of x-ray tube
cathode
positive electrode of x-ray tube
anode
inherent v. added filtration
tube housing absorbs off-focus radiation that does not contribute to the useful x-ray beam
filters teh useful x-ray beam and absorbs low energy rays that cannot reach image receptor
collimation
shape dimensions of useful x-ray beam so that only the area of interest is x-rayed
x-ray production summary
electrons generated at the cathode end at the filament
focusing cup directs electrons toward the anode target
electrons interact with the target where x-rays are produced
characteristic v. bremsstrahlung radiation
x-rays produced as outer-shell electrons fill the inner shells
+ charged nucleus changes course of electron which causes it to lose energy in the form of x-ray photons
penetrability of the x-ray or how much energy it has
quality
range of x-ray energies present in a quantity of x-rays
x-ray emission spectrum
increasing mAs increases ___ but not _____
quantity
quality
increases kVp increases _____ and _______
quality
quantity
decreases quantity and increases quality
filtration
the quality is increased with an _______ in kVp or an _______ in filtration
increase for both
why do you position the thicker portion of the part to be examined closer to the cathode?
heel effect - useful x-ray beam has greater intensity on the cathode side than the anode side which is a result of the line-focus principle
x-rays produced by this process
bremsstralung radiation
coherent scattering
as x-ray photons excite the target atom, a secondary photon of equal energy is released in dif direction which is absorbed by the pt but does not contribute to diagnostic image
occurs primarily, but not the most useful: comptom scattering
occurs when x-ray photon interacting w/ matter ejects the outer-shell electron causing ionization
original x-ray continues in dif direction w/ decreased energy
creates FOG that impairs image quality by reducing its contrast
produces the image: photoelectric effect
x-ray photon is totally absorbed and an inner-shell electron is ejected during ionization
process causes beam attenuation and image formation
also contributes to pt absorbed dose
diagnostic image is formed by photoelectric effect and
remnant radiation (xprays that pass through pt and strike image receptor)
most x-rays interact w/ matter by comptom scattering which causes fog
favors photoelectric effect but increases dose of radiation
low kVp
high kVp favors compton scattering (more radiation will make its way to x-ray film)
differential absorption is manifested by ________ which is the ___________ as it is absorbed and scattered by matter
attenuation
reduction in x-ray beam intensity
ability to separate and distinguish b/q 2 separate objects
resolution
able to distinguish b/w two objects that are aligned closely together
high spatial resolution
ability to distinguish b/w differences in density or intensity
contrast
amount of darkening in radiograph
optical density
primary controlling factor of optical density
mAs
as mAs increases, both xpray quantity and radiographic density increase proportionally
law of reciprocity
as mAs increases, the image becomes darker
need 30% change in mAs for human eyes to see visisble change
if you need to make an adjustement in density either increase the mAs by a factor of ____ to increase darkness or decrease by a factor of __ to increase lightness
2 - darker
1/2- lighter
change either the mA or the s but not both
low kVp produces ____ contrast; high kVp produces ___ contrast
high
low
kVp governs the penetrating power of electrons and alters both quality and quantity of photons produced
15% Rule
a 15% increase in kVp will cause the same change in radiographic density (darker) as doubling the mAs
a 15% decrease in kVp will cause image to look lighter to the same extent as halving the mAs
source to image distance will alter…
number of photons striking film
inverse square law
if tube is too far from film, too few photons strike film producing a lighter image
if too close to film, more photons produce a darker image
the ____ the speed of the film/screen, the ____ radiation is necessary to produce an image, but the ____ image detail is produced
higher
less
less
why do you need a compensation filter?
to balance unequal radiographic density b/w forefoot and rearfoot
subject v. radiographic contrast
subject - result of attenuation dif as xrays pass through body
radiographic - result of dif in densities tha allows viewer to discern b/w two adj densities w/i same image
scale of contrast used in pod
short
distinguishes bone margins from adjacent soft tissues
*** primary factor controlling contrast
kVp
higher kVps produce _____ contrast
lower
to produce shortened contast ….
to produce lengthened contrast…
increase mAs by 2 and decrease kVp by 15%
decrease mAs by 1/2 and increase kVp by 15%
the greater the SID distance, the _____ the image
sharper
but requires increased mAs to maintain the same radiiographic density
slower speed produces __ image
sharper
but require more radiation
speed dependent on silver halide crystal size larger - faster- less exposure
** to limit motion of a part (pediatrics/parkinsons) …
decrease exposure time by 1/2 and icnrease kVp by 15% to produce same radiographic density
plaster cast modifications ***
increase mAs 2X normal or +10kVp
fiberglass cast modifications ***
increase mAs 1.5X normal
soft tissue masses, foreign bodies modifications ***
decrease kVp 15% and double mAs
fixed kVp is typically 60, the mAs are then..
if fixed mAs, the kVp is…
varied per part thickness
typically lower than 60kVp and varies on thickness
how do traditional xrays produce an image
- emerging pattern of photons of differing energies affect the film emulsion to produce latent image
- developing the film allows this image to become visible
- film emulsion consists of gelatin containing finely dispersed grains of silver bromide or halide
- absorption of xprays by silver bromide grains renders them developable
- silver bromide grains which have been exposed to xrays will be turned black by developer
- unexposed grains are dissolved and removed and leave a white area on film
intensify screens
present w/i cassette on either side of film
contain materials which fluoresce when bombarded by xrays
using both visible light and xrays allows reduction of exposure times (90% of exposure comes from intensifying screen light)
not used when very fine detail required
traditonal xray housed in cassett, digital screen housed in …
imaging plate
how does digital xray work?
- PSP electrons energized when exposed to xrays and form latent image
- IP placed into PSP reader, phosphor screen removed and scanned by laser beam
- laser stimulates excited electrons to their ground state and PSPs emit light
- light collected and directed to photodetector via optical system
- photodetector measures light signal, amplifies it and sends electrical signal to analog-to-digital converter
- phosphor plate exposed to bright light which erases latent image, so IP can be used again
- time to process image takes 1 min
smallest unit in an image is a
pixel
a pixel is 2D representation that corresponds to 3D volume of tissue known as voxel
a _________ signal-to-noise ratio correlates to higher contrast resolution
higher
noise - anything that interferes with image formation
signal- xrays that exit pt and correspond to anatomical attenuation
used to determine sites of abnormal glucose metabolism
PET positron emission tomography
inflammatory cells increase uptake of FDG
produces a SD image
SPECT single photon emission computed tomography
uses xprays to produce real-time moving images
fluoroscopy
utilizes xray image intensifier to reduce radiation
is exposure with fluoroscopy greater to thin or thick pt
thick
image intensifier automatically adjusts brightness by increasing kVp (penetration) and mA (intensity)
Fluoroscopy Recommendation
- image intensifier input should be positioned as close to pt as practical
- use exposure pedal as sparingly as possible
- use last image hold and pulsed fluoro whenever possible
- use smallest field of view allowed
- high dose or detail modes should be used only sparingly
- magnification should be used only when necessary
- for carm type fluoroscopy units, the pt should be positioned as FAR from the xray tube as practical to minimize pt entrance dose (xray tube should also be beneath pt)
