Grad class notes IV Flashcards
what is matter
physical manifestation posessing mass
-can also possess charge
chemically inert atoms
have no vacancy in outer shell
cross section of radiation vs elastic scattering
-cross sections for radiation are 1/137 that of elastic scattering
radiative and collisional stopping power dependence
Irad ~ Z^2 T/A
I coll ~ Z/AT (falls to min at 1 MeV then increases slightly with T)
Irad/Icol = TZ/n, n is 700-800
Irad/Itotal = TZ/(TZ+n)
brems. yield for thick vs thin targets
thick- total energy is less than thin target but xray number is higher
-total brems energy = area under curve of radiant energy vs hv
brems angular distribution
-for T< 0.5 MeV - photon goes in any direction
-for T> 5 MeV- photon and electron go in direction of incident electron
-at lower energies, max intensity is at 90 degrees to incident electron
fluorescene yield, L and K
Yl < Yk/20
yield of auger electrons
1- fluorescene yield
auger
-transition to lower shell
coster kronig
transition between subshells in lower shell
super coster kronig
transitions between same shell
fluorescene photon angular distribution
isotropic
characteristic vs brems in beams
mammo and diagnostic- 30% is characteristic
therapy: few percent is characteristic
impact of scattered photons on image
-reduces image contrast
-resolution is the same
mean free path
1/mu
-after travelling this distance, 63% if incident photons have interacted with the material
homogeneity coefficient
HC= HVL1/HVL2
HC = 1 if monoenergetic beam
2 ways to create and accelerate electrons
-gas discharge
-heated cathode
therapy xray tube
-stationary anode
-copper shield stops secondary electrons
-W shield attenuates photons in non=patient directions
-oil surrounds tube for cooling and insulation
-Pb casing for further xray attenuation
rotating anode
-used for diagnostic: superior heat loading
-Mo surrounds tungsten and helps dissipate heat
-graphite mounts target and dissipates heat
recoil electrons in xray tube
-add dose, don’t contribute to image
capture of recoil electrons:
-decreases anode heat load
-improves image contrast
-reduce patient dose
what is tube current a function of?
-filament current
-operating kV
space charge cloud
-tubes below 40 kVp are space charge limited
effective focal spot
actual focal spot size * sin(theta)
larger filament = higher power loading, but larger effective focal size
smaller angle = smaller effective focal spot = better spatial resolution
smaller focal spot = smaller coverage
focal spot size distribution
focal spot length is shortest on anode side and longest on cathode side
-focal spot width is constant and depends on cathose width and focusing cup
heel effect
photons emitted towards anode side travel a larger distance therough anode and are therefore attenuated more
-thicker part of patient should be placed under cathode side
-beam at anode side is therefor harder
transformer
ferromagnetic core with windings
-AC through primary winding produces changing magnetic field, which produces AC on secondary winding
ideal transformer
-electrical power input into primary winding = electrical power output of secondary winding
types of transformer
-step-up
-isolation
-step-down
i2/i1= N1/N2 = V1/V2
N is number of windings
rectification
-current only flows when cathode is negative wrt anode
semiconductor diodes
-current flows if positive potential on p side and negative on n-side
-or else, current can’t flow
-current flows after depleted zone narrows to 0
bridge rectifier
- a single diode uses only 1 pulse per AC cycle
-bridge rectifier uses both AC lobes for xray
disadvantage of sine waves for xray tubes
-during portion of AC cycle when voltage is low, electrons are not accelerated sufficiently to produce xrays of high enough energies
-low energy xrays contribute heat, dose, and no info to exam
-mean intensity of radiation decreases since no useful radiation is generated for large part of AC cycle
3 phase current
-phase difference between any 2 of 3 lines is always 1/3 of variation of the voltage on a single line
DC voltage ripple
measure of how closely the xray tube’s potential mimics a DC potential
-for 3 phase, 12 pulse circuit, DC ripple is only 5 %
parts of xray power supply
AC- transformer - rectifier - smoothing- regulator
what affects voltage ripple?
decreases with large capacitor
increases load current
decreases with frequency
output voltage changes with input voltage
voltage regulator
keeps output voltage at constant value
-independent of load current and input voltage
% ripple
(Vmax- Vmin)/Vmax
-single phase, 1 pulse, ripple = 100%
3 phase, 6 pulse, 13-25%
3 phase, 12 pulse, 3-10%
constant potential, < 2 %
medium-high frequency, 4-15%
effect of kV ripple on xray spectrum
-average energy decreases as ripple increases
6 factors that affect xray efficiency, quantity, quality, and exposure
-target material
-tube voltage
-tube current
-exposure time
-beam filtration
-generator waveform
effect of kV on xray output
-exposure is proportional to kV^2
-exit exposure through a patient varies as 4th to 5th power of kV
effect of tube current on xray output
-exposure is porportional to tube current
effect of exposure time on xray output
of photons is product of current and exposure times
what increases power rating?
-large focal spot
-high anode rotation speed
-massive anode
-large anode angle
-large anode diameter
energy of generator
root mean square voltage X tube current X exposure time
Vrms = Vpeak/root2
heat unit
-peak voltage X tube curent X exposure time X generator factor
-generator factor = 1 for single phase, 1.35 for 3 phase, 1.4 for constant potential
falling load generator control circuit
-minimize exposure time while not exceeding tube’s capacity to absorb heat (decreases possibility of patient motion artifacts)
-works with AEC to deliver max possible mA for selected kV in shortest time
AEC
-measures radiation and terminates exposure after treshold
-can prodive consistent exposure for differen patient size and attenuation
-backup timer ensures that if AEC fails, the patient doesn’t get large dose
what is xray image map of?
photon survival probabilities
5 factors that determine xray spectrum
-accelerating potential
-waveform
-target material
-tube and housing materials
-added filters
electron density of bone vs muscle vs fat
electron density of bone is higher than muscle, and muscle higher than fat
are narrow beam attenuation curves straight lines?
no, because of beam hardening
f-factor
0.876 cGy/R * [uen/p) tissue to air
skin entrance dose/BSF in diagnostic
skin entrance dose- largest dose
backscatter from inside patient needs to be accounted for
magnification
SID/SOD
size of off-axis radiation penumbra
F(M-1)
F= focal spot spize
M is magnification
size of object motion unsharpness
delta m * magnification
m is motion
size of focal spot motion unsharpness
delta m * (M-1)
m is motion
M is magnification
size of blur at 3D contrast boundary
height * M/diameter
M is magnification
3 zones of an image
shadow zone: object fully intercepts focal spot
light zone: none of focal spot is shadowed by object
penumbra zone: object intercepts a portion of focal spot
focal spot vs magnification
-For M=1, focal-spot blur is insignificant, blur is due to detector
-For large M (every object detail is projected onto large area of detector), detector- induced unsharpness is negligible, focal spot blur dominates
quantum detection efficiency
probability a photon incident on the receptor will be detected
intensification
conversion of high energy xray photon (> 10 keV) into many light photons (2-3 eV)
QDE of direct film (no screen)
< 0.25%
-with screen, QDE is > 50%
latent image center
-introduce impurities into crystal structure to create sensitivity specks
-light photons or xrays interact with electrons in crystal
-electrons contact speck, Ag+ becomes Ag
-As more Ag atoms are formed, they combine as Ag2
nucleation phase of image formation
-form Ag2
growth phase of image formation
nucleation center acts as site for attracting other free electrons and mobile Ag+ ions, forming more Ag atoms
film developer
-developer reduces Ag+ in latent image center to Ag
4 stages of film development
-developing
-fixing
-washing
-drying
what kind of recorder is film?
negative recorder
-region receiving most light appears darkest
OD = log(Io/I)
high contrast film is what kind of latitude?
small latitude
sensitometry
-chemical processing of film can result in underdevlopment or overdevelopment
-sensitometers ensure that development is working optimally for a given film type
-sensitometer exposes film to step series of different light intensities
-densitometer measures OD of film
-OD values compared against standard
quenching
-non-radiative transition
-competes with luminescence
quantum detection efficiency
-fraction of xray energy actually absorbed by screen to energy incident on screen (30-80%
light conversion efficiency
-fraction of energy emitted as light photons to energy absorbed from incident photons (5-15%)
light escape fraction
fraction of light photons created that escape the phosphor (50%)
overall screen efficiency
product of QDE, energy absorption efficiency, light conversion efficiency, and light escape fraction
intensification factor
-ratio of xray exposure w/o screen to that with screen for a given OD
-overall figure of merit
impact factors on screen efficiency
-QDE increases as screen thickness increases
-QDE decreases as kVp increases (except for k edge)
-intensification factor increases as kVp increases
-energy absorption factor decreases as kVp increases
how to increase QDE of screen
-increase phosphor thickness
-double emultion film between 2 screens
how to increase light escape efficiency
-coat side of phosphor layer opposite of film with reflective coating to reflect light photons can toward film
what happens to system’s contrast and resolution as object becomes smaller?
rose model
SNR > 5 for detection
-SNR is proportional to contrast * diameter * square root of photon fluence
minimum contrast required to see an object
10%
why do scattered photons make the image worse?
-lost geometric correlation with focal spot
-energy discrimination not possible
-difficult to tell between scattered and primary photons
for what energy is compton most probable interaction?
> 10 keV to 1 MeV
