principles of imaging: production of xrays

xrays are a form of

electromagnetic radiation

electromagnetic waves low frequency vs high frequency

long wavelength vs short wave length

conditions required for xray production

source of electrons (filament)

accelerate electrons (high voltage)

decelerate electrons (target)

all occurs in a vacuum

production of xrays occurs when

fast moving electrons are suddenly decelerated by interaction with target material

production of xrays occurs within _________ of target surface

.25 to .5 mm

energy=

amount=

quality

quantity

incident electrons

what is coming from cathode and hitting out anode

"the incident"

xray photons

after they hit the harget

anode is made up of

tungsen

kinetic energy of incident electrons

99% is converted to heat

1% is used for the production of xray

xray energy is directly proportional to the...

kinestic energy of the incident electrons

heat production (the 99%)

-electrons interact with outer shell electron of target atoms

-outer shell electrons raised to a higher energy level

-electrons drop back to relaxed state

-transition accompanied by infrared radiation

two types of target interactions produce xray photons

diagnositc radiation production (1%)

bremsstrahlung

characteristic

diagnositc radiation production (1%)

-electrons give up energy to target atoms

-type of interaction depends on kinetic energy of incoming electron and binding energy of electron shells

bremsstrahlung is the german word for

braking or slowing

bremsstrahlung interactions make up

80-90% of the primary beam

in bremsstrahlung interactions high speed electron passes near the _______

nucleus

in bremsstrahlung interactions the ________ charge of the nucleus ______ the electron from the ______________

positive charge

deflects

original path of travel

electrostatic force

power of force field

brems radiation

-incident electron slowed by force field of nucleus

-incident electron continues in a different direction with reduced energy (can cause numerous interactions)

brems reaction is the

loss of kinetic energy given off as an xray photon

energy of brems radiation depends on

-the energy of incident electron

-the charge of the nucleus

-the proximity of the incident electron to the nucleus

interactions further from the nucleus equals

lower energy brems

interaction closer to the nucleus equals

higher energy brems

averag beam energy equals

30-40% of kVp selected

incident electron may lose _________ of its kinetic energy in an interaction with the nucleus

any amount

when 70 kVp is set on the control panel

-produce kinectic energy up to 70 kVp

-electrons may use some, none or all kinetic energy

-brems xrays produced my have energies up to 70 kVp

kVp selected vs kEv

kVp the peak energy selected

kEv energy can only go up to kVp

in brems radiation the energies are

unpredictable

-may range from very low levels to peak kV

-very low energies are absorbed within the tube

in brems radiation incident electrons may have ______ interactions before it _____ all its energy

multiple

loses

in brems radiation a ________________ beam contains a variety of energies

heterogenous/ polyenergetic

characteristic radiation

-10-20% of primary beam

-incoming electron ejects inner shell (k shell)

-leaves vacancy in inner orbital shell

-characteristic xrays are emitted when an outer shell electron fills an inner shell void

characteristic radiation/ cascade effect

xrays are emitted here

the energies are predictable (we know the energy of K shell)

tungsten atom

atomic number 74

110 neutrons

74 electrons

electron shell of tungsten

K= 2 electrons

L= 8 electrons

M= 18 electrons

N= 32 electrons

O= 12 electrons

P= 2 electrons

tungsten binding energies

-emitted radiation is characteristic of the target element

-binding energies of specific element have distinct values

K shell

number of electrons 2

keV 69.5

tungstem electron transition

-xray energy equals difference in binding energies of the orbital electrons involved

-electrons dropping into K shell may be from any shell (not always subsequent)

in tungstem electron transition, the ______ out the electron comes out to "drop" into vacancy, the _______ the characteristic radiation produced

further

greater

in characteristic radiation only ______ interactions contribute to the _______ beam

K shell

useful

lower energies ______ the tube

exit

characteristic radiation SUMMARY

-incident electron must have enough energy to knock an inner shell electron from orbit

-interaction will only occur at 69.5 keV (70 kVp) and above

-the ejection of the innershell orbital electron creates a temporary hole

-an outer shell electron moves in to fill the void

-outer shell electron moves into fill the void

-outser shell electron drops to inner shell and xray is emitted

-energy= to the difference in binding energy of the shells involved

emission spectrum graphically represents xray beam (brems and characteristic)

energies emitted in the beam at a particular kVp setting

brems emission spectrum

-plot of the number of xrays emitted per energy level

-heterogenous beam contains broad spectrum of energies

-kEv is calculated from peak (30-40% of peak)

characteristic emission spectrum

-characteristic energies are predictable

-energies emitted through k shell interactions (57, 66, 68, 69)

-L shell interactions will be absorbed (will not contribute to the image)

-anything below 70 wont contribute

xray production emission spectrum: characteristic

xrays emitted at 70 kVp and above (69.5)

produces specific discrete xray energies

represented by spike

xray production emission spectrum: brems

xrays emitted over entire spectrum of beam

area under curve

emission spectrum summary:

most photons produced by ________ target interactions within the diagnotic range

bremsstrahlung

above 70 kVp approximately ______ of the beam consists of characteristic xrays

10-20%

average energy beam (keV)=

30-40% of set kVp

factors affecting the spectrum

mAs

kVp

filtration

generator type- voltage waveform

generator type- voltage waveform

single

3 phase 6 pulse

3 phase 12 pulse

high frequency

mAs determines the:

number of electrons that boil off during thermonic emission, number of xray photons in beam, and beam quanity

mAs and amplitude are:

directly proportional

mAs and exposure are:

directly proportional

mAs is doubled, exposed doubled

kVp determines the:

quality (energy) of beam

kVp: affects penetrability/ quality

affects the quantity of photons, but NOT in a directly proportional manner

-kVp effects quantity but not proportionally

change in kVp affects:

both the amplitude and postion of the spectrum

-curve shifts to the right

-amplitude increases

filteration

xray beams through materials as exiting xray tube and housing

higher filtration = decreased quantity= increased average energy

increased generator efficiency

-increased average photon energy

-increased number of photons in beam and/ or amplitude of spectrum