Test 2 Flashcards
why can’t x-rays be focused by a lens or reflected by a mirror?
high frequency = penetration
why do x-rays always carry the risk of molecular damage?
xrays ionizing
why are x-rays highly penetrating?
high energy and high frequency
why are x-rays not affect by either electric or magnetic fields?
neutrally charged
why do x-rays travel in a straight line
they are a photon –> like light
neutrally charged
what characteristic or property of x-rays allows us to produce and view them on an image receptor?
ability to effect change on phosphors = image
who discovered electromagnetic induction
michael faraday
who created vacuum tube/cathode ray
william crookes
who produced the first film
richard leach maddox
how are x-rays produced
accelerating electrons
what does a vacuum tube produce
cathode rays –> invisible x-rays
who discovered x-rays
Wilhelm Roentgen
x-rays have
_____ wavelengths and _____ frequency
this makes them ____
short
high
highly penetrating
what is an x-ray considered as?
a wave but has particle like characteristics
what is the difference between a wave and a particle
a wave is neutral
a particle has a charge
do x-rays have consistent energies or wavelengths?
no –> energies and wavelength cannot be controlled
what kind of frequency is reflective
low frequency
what are the components to an x-ray circuit
operating console
high-voltage generator
x-ray tube
what does an x-ray imaging system do
control intensity of electron flow to make x-ray beam
converts potential energy –> kinetic energy –> electromagnetic energy
why does electron intensity be high?
to make sure electrons are going fast and are strong enough
what is the potential energy in an x-ray system
voltage
what is kinetic energy in an x-ray system
tube current –> mAs
what occurs during kinetic energy conversion to electromagnetic energy
electrons are flowing from cathode to anode
what does an operating console do
controls tube current and voltage
controls line compensation, kVp, mA, and exposure time
why is it important to control tube current and voltage
proper radiation quantity (mAs) and quality (kVp)
what does radiation quantity control atomically
current of electrons flowing from cathode to anode
what is line compensation
measures voltage going to the system and adjusts it to 120-220V + or - 5%
mA and exposure time are ____ proportional
inversely
is it better to have higher mA or exposure time
mA –> cardinal rules –> goal: reduce time exposed
what components are in an operating console
line compensator
autotransformer
major and minor kVp selector
kVp meter
timing circuit and selector
What does a autotransformer do
- receives energy from AC source and adjusts to what is needed –> voltage varies greatly
- powers rest of unit
- directly determines voltage
what is another name for autotransformer
adjustable transformer
what affects autotransformers
kVp selection –> voltage adjusted to kVp needed
how is voltage and kVp related
voltage = potential energy
kVp = kinetic energy
x-ray needs higher energy –> gets it through more voltage (PE) TRANSFORMING to kVp (KE)
what does AC do in a line compensator
????? self induction?
what does an autotransformer consist of?
solenoid –> coil of insulated wire around iron core
purpose of major and minor kVp selector?
minor changes in voltage
purpose of kVp meter?
reads voltage before exposure
aka pre-reading voltmeter
purpose of timing circuit
- regulate x-ray exposure by turning off x-ray tube automatically
- reduce heat and radiation dose
types of timing circuits
synchronous
electronic
mAs
automatic exposure control
synchronous timer
- uses motor that operates at frequency of 60hz @ 60 rotations per second
- timer is in 1/60s increments
cons of synchronous timers
cannot be used in series of exposures –> has to be redialed in after each exposure
purpose of automatic exposure control (AEC)
- measures mAs reaching IR by using ionization chamber
- controls time/exposure
purpose of ionization chambers
control exposure
how to AEC relate to ionization chambers
enough voltage hits chamber selected the operational amplifier terminates exposure
cons of using ionization chambers
- large room for error
- patient position correctly or risk overdose
- metal absorbs = longer radiation exposure
purpose of AEC back up timer
- used if AEC malfunction
- protects from heat and overexposure
cons of AEC back up timer
150% over suggested time –> can still overexpose
if thick area and AEC back up kicks on it can underexpose
purpose of step up transformer
uses AC to get high voltage to supply tube with energy
purpose of step down filament transformer
supplies low current to heat up filament for thermionic emission about 12V
what controls step down transformer
mA adjustments
purpose of mA selector
controls amperage in filament circuit
purpose of rectifiers
- changes AC to DC before releasing to xray tube
- ensures electrons flow from cathode to anode only
cons of step up transformer
cannot be adjusted or controlled –> increases voltage by fixed amount
purpose of high voltage generator
increases output voltage from autotransformer to kVp needed
parts of a high voltage generator
high voltage step up transformer
step down filament transformer
rectifiers
mA meter
monitors xray tube current in secondary circuit
what is a semiconductor diode?
2 electrodes –> n-type and p-type
ex. rectifiers
half wave rectification
- voltage wave is unable to dip below zero
- no electric current
cons of half wave rectification
wastes half of power and 2x exposure time
half wave has ____
full wave has _____
zero, one or two diodes
4 diodes
pros of full wave rectification
uses less mAs
what occurs during full wave rectification
negative half-cycle is reversed so anode is positive
cons of single phase power
low energy
low penetrability
little diagnostic value
100% ripple
single vs three phase
pulsating
multiple waves and constant high voltage
pros of three phase power
voltage never drops to zero during exposure = constant energy
less ripples
what is a ripple
fluctuations of energy from zero to max voltage
what is good for ripple
low ripples –> output is similar to input = less fluctuations
purpose of high frequency generator
helps waveform, voltage, and reduces ripples
how does a high frequency generator work
increases the voltage frequency through AC and DC power converters
what are the external components of x-ray tube
support structure/mechanism
protective housing
glass or metal enclosure
how many diodes are in an x-ray tube
2 diodes –> + and - sides
what is the primary parts of xray tube
cathode and anode
purpose of glass or metal enclosure
creates vacuum tube to pack electrons tightly together = more efficiency and longer tube life
why use glass?
high melting point to withstand heat
why metal?
maintains constant electric potential
why is it good to maintain a constant electrical potential
to accelerate emitted electrons to the anode
purpose of x-ray tube window
thin area that xray beams are emitted from
can beam particles leak out from the window
yes because xrays cannot be controlled as radiation leaks out but environment can be modified
purpose of protective housing
covers tube with lead to reduce leakage
prevents electric shock
mechanical support
protect tube damage
what is considered as a useful beam
xrays beams coming from window
oil
purpose?
located?
insulator for electricity
thermal cushion for heat
protective housing
cathode + or - ?
negative
components of cathodes
filament and focusing cup
where does thermionic emission occur
in filament aka step down transformer
what is thermionic emission
the filament heating up to create free electrons by boiling electrons off of the alloy
purpose of focusing cup
negatively charged to repulse and accelerate electrons within a confined space –> focuses electrons to hit focal spot
location of focusing cup
cathode
location of focal spot
anode
is anode + or - ?
positive side
purpose of anode
electrical conductor
mechanical support for target
thermal dissipater
how is heat produced
%?
when electrons hit the anode energy is released as 99% heat and 1% xray
types of anode?
stationary and rotating
pro of rotating anode
- spins target = more surface area for electrons to hit high intensity xray
- beams in short time
con of rotating anode and solution
no vacuum or mechanical connection to turn anode
solution:
electromagnetic induction motor
stator?
purpose?
series of electromagnets
spins stem of anode = higher efficiency
rotor
shaft made of bars of copper and soft iron
what is the target
area of where electrons hit the anode from cathode
ex. tungsten
purpose of high frequency generator
improve waveform and voltage
reduces ripple
how does high frequency generator improve voltage
uses AC and DC converters to alter 60hz to 25000hz
what are the primary parts to the xray tube
anode and cathode
what are the cathode and anode considered as
a type of diode –> + and -
what the are the external components of the xray tube
support structure
protective housing
glass or metal enclosure
why do we use a glass enclosure
pyrex glass can withstand high heat
why do we use a vacuum tube
tightly packs electrons to be more efficient xray production = longer tube life
why is the glass enclosure mixed with metal?
to maintain a constant electric potential = longer tube life
what is within the glass enclosure
xray tube window
purpose of xray tube window
thin area that allows useful beams to be emitted
what does an xray tube contains
2 electrodes
purpose of protective housing
reduces leakage of secondary radiation
protects against electric shock
mechanical support
protects tube
direction of xray movement
everywhere –> radiates
how does the protective housing protect against electrical shock
oil acts as insulator and thermal cushion for heat
cathode is + or -?
negative side of xray tube
components of cathode
filament and focusing cup
purpose of focusing cup
confines electron beam to small area of anode
how does the focusing cup work
it is negatively charged which repulses electrons to help accelerate and concentrate them to hit the focal spot on the anode
what is thermionic emission
when the filament heats up creating free electrons by boiling off electrons from the alloy
purpose of filament
coil of wire used for thermionic emission
is anode + or -?
positive side of xray tube
purpose of anode
electrical conductor
mechanical support for target
thermal dissipater
what is considered the target
tungsten
how is heat created
when electrons hit the anode
types of anodes
rotating
stationary
rotating vs stationary
rotating: high intensity xray beams in short time –> 500x more area
stationary: doesn’t produce enough energy to penetrate thick tissues
pros of rotating
500x more area = 500x more xrays
con of rotating
solution?
no mechanical connection to outside to spin anode
solution: electromagnetic induction motor
components of anode
stator
rotor
purpose of stator
series of electromagnets to spin anode = higher efficiency
rotor
shaft made of bars of copper and soft iron
what is a target
area of the anode that gets hit by electrons flowing from the cathode
what is the target made out of and why?
tungsten in the copper anode
why? high melting point = more electrons can hit
pros of tungsten
high atomic number = high efficiency for xray
high thermal conductivity
efficient heat dissipator
withstand high tube current without pitting or bubbling
what does pitting affect
decreases in imaging quality
where does pitting occur
focal spot
what is a focal spot
area of target –> electrons hit
what produces fine image details
small focal spot = better spatial resolution
good for thin tissue
cons of small focal spot
high heat concentration –> less anode surface area used
pros and cons of large focal spots
Pros:
good heat dissipator
thick body parts
short exposure time
con:
large effective spot
what is effective focal spot
area projected onto patient
what is actual focal spot
physical area on the anode
goal of line focus principle
create small effective focal spot to increase image sharpness
how do we create a small effective focal spot
angle the target (anode) 5-20 degrees to change size of effective focal spot
how does angle affect effective focal spot
angle decreases the width of the effective focal spot without altering the actual focal spot
line focus principle is good for what?
to get larger heating area and a smaller effective focal spot by adding an angle
cons of small anode angle
small effective focal spot
limits size of useable field
anode heel effect
reasons for xray tube failures
heat
having anode at high heat for long time periods
filament
why does the filament need low current
high mAs can vaporize the filament
what % does a xray tube produce
1% and 99% heat
methods to dissipate heat
radiation
conduction
convection
radiation
transfer heat by emission of infrared radiation
conduction
transfer energy from one object to another
ex. oil, glass or other materials
convection
transfer heat by movement
ex. spinning of anode
anode heel effect
the intensity on the cathode side is higher than anode side
why do we need a low current for a step down transformer
so we can get a low voltage since voltage controls the damage to the tube
why is the intensity higher on the cathode side
due to the angled anode the beams close towards direction of the cathode has less material to pass through to exit the anode
what effect does a larger anode angle have on anode heel effect
less effect
what affect does a smaller anode angle have on anode heel effect
greater effect
radiographic rating chart
lists safe and unsafe techniques for xray tube operation
anode cooling chart
thermal capacity for anode and time it takes for cool down
housing cooling chart
thermal capacity for housing and time it takes for cool down
primary function of xray tube
accelerate electrons from cathode to anode
how do we accelerate electrons
increase kVp
what occurs by increasing the kVp
intensity of mAs and energy of xray beam is increased
AKA quality and quantity
projectile electrons?
high speed electrons moving from cathode to anode
xray tube current
electrons moving from cathode to anode
mA –> amp
when an electron’s velocity is at zero what occurs
electrons are boiling off the filament
when a projectile electron interacts with the target what is produced
thermal energy
electromagnetic energy (infrared radiation)
xray
location of projectile electrons?
focusing cup
what energy is converted during an interaction from a projectile electron
kinetic energy
excitation
charged particle transfers all or some of energy to orbital electron temporarily raising to higher energy shell
what is produced during excitation
heat
when does ionization occur
enough energy transferred to overcome binding energy of orbital electrons to eject electron from atom
what determines ionization
speed of electron = strength
anode heat is mainly what?
excitation
what law is applied when heat is generated
law of energy conservation –> the kinetic energy from electron has to go somewhere
what controls heat
tube current (mA) and kVp
directly proportional
how do we increase the efficiency of xrays
increase kVp
why not always high kVp for better images
HEAT BAD FOR TUBE
bremsstrahulung radiation
an incident electron slows down through interaction from force field of nucleus
no physical interactions
what happens to the electron’s energy?
kinetic energy turns into xrays
what dictates how much energy the xray photon will have
the greater deviation of the electron = more xray photon energy
bremsstrahulung energy reaction ratio
xray photon exactly the difference between entering and exiting kinetic energy
where is a bremsstrahulung xray produced
at any cathode
how many interactions can be made from a single bremsstrahulung electron
multiple until loss of energy
what happens once a bremsstrahulung electron loses energy
it becomes part of the current flow
what determines the characteristic radiation energy level needed
the target material
what is characteristic radiation
when an incident electron interacts with an inner shell (k) electron with enough energy to knock electron out of orbit
what is the typically energy needed for radiation to be characteristics
69kVp +
what would raise the energy level needed for characteristic radiation
if binding energy of electron is higher
electron closer together = higher binding energy
what happens to the projectile electron from the characteristic interaction
it goes to a slightly different direction from incident electron
electron cascade
outer shell electron falls in to fill hole left by projectile electron and continues on until no more outer shell electrons or pulls in electron from surroundings
what is produced from electron cascade
xray photon from lower energy state (energy difference) between the 2 shells
characteristic energy reaction ratio
exact difference between binding energy between inner and outer shells from where electron dropped
why with k shell first?
outer shell electron’s energy is too low for diagnostic purposes
what increases effective energy of characteristic xray
increasing atomic number of target element
as cascade occurs what are the energy levels
inner shell highest
outer shell lowest
what is heterogeneous
xray beams with various energies
why are xray beams heterogeneous
energy levels are random from nucleus deviation (bend)
also varying frequencies and wavelengths
xray emission spectrum
the collective of all different energies of xrays
aka the different potential energy in single beam
what makes up the primary beam spectrum
both brems and characteristic
kvp range for brems and percentage of occurrence
80-100kVp
80-90%
relation of kVp and keV
keV = 30-40% of kVp
more kvp = more kev but not equal
what is discrete xray spectrum
a graph-able spectrum using characteristic xrays
shows different electron binding energies for differing elements
why do we use characteristic xrays for the discrete xray spectrum graph
specific energies used for target = predictable
characteristic peak
peak = k shell emissions
varies if xray output is above kshell peak
continuous xray spectrum
uses brems radiation
measures any produced cathode electron energy
from peak to 0
what affects the spectrum
mA = amplitude
kVp = amplitude/energy of photon
generator = intensity of beam, phasing efficiency, and average photon energy
atomic number = kvp and ma –> characteristic made?
voltage ripple = decreases ma and kvp
filtration = decrease in ma and kvp
half value layer
the thickness of absorbing material that attenuation half of xray
measures quality of xray and sets radiation safety
attenuation
reduction in xray intensity occurring from absorption and scattering
what is quality of xrays measured by
half value layer
typical half value layer?
HVL of 3-5 mm Al
how does kvp relate to HVL
more kvp = more intensity = more HVL
filtration
removes low energy brems from beam –> not useful diagnostic by using absorbing materials in primary beam
effects of filtering
decreases intensity of beam
increases average photon energy
lowers radiation dose
control photon energy
density –> mA / gray scales
name layers of xray filters
1st glass envelope
2nd compensating filter (Al)
hardening of beam
filters beam of soft photons for highest quality/intensity
aluminium equivalency
many materials like lead shielding is not 100% lead made out of other materials
why do we use wedges and trough filters
visualizes fine details by lowering xray beams to certain parts of body –> decreases darkness (air)
inherent filter
glass or metal enclosure of xray beam
added filter
thin sheet of Al between protective xray tube housing (window) and beam collimator
compensating filter
additional filter added to compensate differences in subject radiopacity
between collimator and patient
total filteration
total sum of inherent and added filtration
at least 2.5mm Al equivalent
how to compensate for decrease in xray intensity after filtering
increase kvp
what occurs during ionization
addition or removal of an electron
what are the effects of ionizing radiation
ability to ionize atoms
changes the charges and force fields of atom
can any form of energy ionize?
yes
interactions of xrays
attenuation
some energy transfers and scatter
direct transmission
direct transmission
xray can pass through unaffected –> pushes straight through to IR
when does a photon interact with biological matter
random occurrence –> cannot predict single photon interaction reaction
is the kVp ratio interact with cells equal?
no –> random interaction
how can we predict the probability of photon interaction with matter
radiation decay –> large numbers of photons rather than single photon
when does an electromagnetic radiation interact with matter?
matches size of wavelength of the radiation
what does low energy xray interact with?
type of interaction?
whole atoms
excitation
what does moderate energy xray interact with?
electrons
what does high energy xray interact with?
nuclei
what is the wavelength of higher energy xray
shorter wavelength
what are interactions dependent on
kvp
body mass
SID
energy of photons
atomic number
types of interactions
classical (coherent scattering)
photoelectric interaction
compton interaction
pair production
photodisintegration
does the photons always interact with atoms?
no –> atoms = large space –> passes through without interactions
Coherent scattering energy levels
below 10 keV
coherent scattering alternate name
thompson or classical
what occurs during coherent scattering
excites target atom –> movement = energy emitted –> scattered xray produced
output of coherent scattering
scattered xray wavelength = wavelength of incident xray
direction changes without energy change
image noise
does energy transfer for coherent?
ionizing?
no energy transfer
no ionization
who discovered photoelectric effect
albert einstein
when does photoelectric occur
energy of incident photon is slightly higher than binding energy of k or L shell
what happens to xray in photoelectric
entire xray is absorbed when incident xray removes an electron
how does energy transfer in photoelectric
the inner shell electron absorbed all energy –> ejecting a photoelectron
traits of photoelectron
negative charge
can ionize atoms
low energy
energy ratio of photoelectron
kinetic energy = difference between energy of incident xray and binding energy of electron
after effects of photoelectric effect
characteristic xrays –> low energy and locally absorbed
probability of photoelectric
directly dependent on atomic number of irradiated tissue
indirectly with photon energy
how to decrease photoelectric effect
pro?
high kVp –> by hardening beam
lower radiation dose
compton scattering energy level
30-40keV
when does compton scattering occur
incident xray interacts with outer shell electron and ejects electron= ionizing atom
energy ratio of electron in compton
ejected electron = binding energy and kinetic energy that leaves the atom
what happens to ejected electron in compton
ejected electron = scattered xray –> goes to interact with another electron –> gets absorbed photoelectrically
energy of scattered xray in compton
2/3 of original energy
what makes the most molecular damage
scattering
what is the greatest interaction for diagnostic imaging
compton scattering
compton scattering ______ as xray energy increases
____ with lower energy xrays
increases with kVp increase
decreases with kVp decrease
compton scattering ____ proportional to kVp
photoelectric effect ____ proportional to kVp
directly
indirectly
cons of compton scattering
reduces image contrast = useless
energy level pair production
probability of occurrence
1.02MeV
higher probability at 10 MeV +
if you have alot of energy ex. MeV….. what do you also have?
high energy photons
what occurs in pair production
incident xray photon strongly interacts with nucleus of atom of irradiated tissue and disappears
what happens to energy in pair production
energy of photon transforms to 2 new particles
negatron (reg electron)
positron (positive charged electron)
what does negatron do
loses kinetic energy through excitation and ionizing atoms in path
what does positron do
antimatter –> ANNIHILATES nearby free electron = 2 photons created at 0.511MeV each
why is annihilation good?
used in positron emission tomography PET
what is used for radiation therapy
pair production
photodisintegration
photodisintegration energy level
more than 10MeV –> very high energy
how does energy transfer in photodisintegration
high energy photon collides with nucleus of atom and directly absorbs all the energy of photon
then absorption by patient = excitation –> no ionization
what occurs in photodisintegration
energy transfer too high = unstable atom –> nucleus blows up scattering nucleus pieces
nucleus pieces are radioactive
how is radiation intensity reduced
absorption through tissue and scattering
what does attenuation measure
how much is absorbed
% of xray beam left
xray attenuation relationship
exponentially –> can absorb forever
how can xrays be stopped
lead ABSORBS the xray by fractional amounts
differential absorption
different materials absorb at different rates
dependents of differential absorption
atomic number
mass density
xray energy
___ atomic number = ____ attenuation
why
increase
increase
higher atomic number = more atoms to interact with
___ mass density = ____ attenuation
why
decrease
decrease
less atoms to interact with
___ xray energy = ____ attenuation
why
increase
decrease
more push power = less interactions
2 important interactions for xray image and why
photoelectric –> affects density
compton scattering –> creates noise
kVp affect on xray image
high kVp = less grays
types of cell dysfunctions
repaired
altered
dead
what is the most dangerous cellular dysfunction
why?
altered
given long exposures –> less immunity = less likely to repair –> risk of cancer
what can occur with dead cellular dysfunction
risk of organ failure because of apoptosis
what does exposure measure
radiation intensity in the air
what is exposure measured in
roentgen
what does dose measure
radiation absorbed from radiation exposure
what is dose measured in
rad
what is exposure used for
calculates entrance skin exposure
what is dose used for
estimates irradiation of patients
what does entrance skin exposure measure
skin dose of radiation exposure as beam enters
looks at CR area
where is ESE calculated at
minimum SOD
areas of high ESE
L-spine
pelvis
hip
abdomen
high thickness = high intensity
priority for patient dose
avoid repeats > all other methods to reduce patient dose
for radiosensitive tissues what is a good position
ex. breast –> PA
place sensitive tissue furthest away from entrance exposure
why are immobilizations good
eliminate motion
decrease tissue density –> compression
how to reduce patient dose with kVp
increase kVp and decrease mAs
too low mAs = decrease sharpeness
too high kVp = high dose
BALANCE
SID/SOD increase = ESE ____
why
decreases
more distance = less dose
why not have max SOD/SID?
too far = decrease in xray beam intensity
radiosensitive organs (3)
lens of eye
breast
reproductive organs
SOD
source object distance
anode target to patient
SSD
source skin distance
ex. CR
