Rad-103 Ch 3: The X-Ray Beam

at what speed do electrons moving away from cathode travel at

half the speed of light

what type of interactions are seen when electrons reach anode and interact with tungsten focal tract

bremsstrahlung interaction

characteristic interactions

what percent of x-ray beams will be bremsstrahlung at what kVp

85% at >70 kVp

100% < 70 kVp with tungsten target

when does braking radiation occur

when projectile incident electron completely avoids orbital electron of tungsten atom

how does bremsstrahlung electron travel in regard to nucleus and why

very close due to electrostatic force that causes the electron to slow down and abruptly change direction

how does bremsstrahlung energy loss reappear as

an x-ray photon

relationship between attraction and photon energy

the stronger the attraction the higher the photon

what is x-ray energy measured in

kiloelectron volts (keV)

how can the energy of a bremsstrahlung x-ray photon found

subtracting the energy that projectile electron exits the atom with from the energy it had upon entering

diagnostic energy range of bremsstrahlung x-ray photon

30 - 150 keV

most type of interaction are from

braking radiation

what yields 100% braking radiation

kVp below 70 + tungsten target

what is the projectile electron energy of characteristic interactions

69.5 keV

what kind of shell atoms are ionized

inner shell atoms

how are characteristics interactions produced

when incident electron interacts with inner k shell electron of tungsten atom

what is needed for incident electron to eject k shell electron

equal or greater than energy

what is the binding energy of k shell electrons

69.5 keV

what happens once k shell electron is ejected

other shell electron drops down to fill void causing the release of energy as x-ray photon

what electron orbits produce energy

k-l-m-n-o-p

what is the energy range of k shell characteristics x-ray

57-69 keV

what percent of interactions above 70 kVp are from characteristic interactions

15%

x-ray beam is

polyenergetic

what is space charge effect

thermionic cloud formed around filament during thermionic emission by the liberated electron

what does space charge effect refer to

tendency of space charge to prevent more electrons to be boiled off of the filament

how does focusing cup affect space charge

has a negative charge that forces electrons in space charge to remain together

what does deadman switch mean

exposure is terminated as soon as pressure removed

what does the activating the rotor do

sends current through filament in cathode and starts rotor

what range does filament current operate

3-5 amps at 10 volts

what does amount of current flow depend on

mA set by technologist on control panel

what leads to thermionic emission

current heating tungsten filament

what happens once exposure button is pressed

kVp accelerates the electrons across positively charged anode

what is tube current

flow of electrons from cathode to anode

what is tube current measured in

milliamperes

what does quantity of x-ray beam indicate

number of x-ray photons in primary beam

what does quality of x-ray beam indicate

penetrating power

how is quality of beam controlled

through kVp settings

quality of x-ray beam also refers to

energy level of radiation production

what does kVp determine

speed at which electrons in tube current move

higher beam quality allows photons the greater ability to

penetrate or move through tissue

kVp and penetrability relation

direct

kVp and efficiency of production relation

direct

why is generator required to convert low voltage into high voltage

to provide the energy necessary to produce x-rays

types of x-ray generators

single phase

three phase

high-frequency

what do x-ray generator waveforms reflect

consistency of voltage supplied to x-ray tube during exposure

voltage-ripple describes waveforms in terms of

how much voltage varies during x-ray production

what is voltage ripple for single-phase generator and why

100% due to variations in voltage waveform going from peak volts to zero volts

what is voltage-ripple for three-phase generator and why

13% in 6 pulse mode

4% in 12 pulse mode

due to voltage waveform never going to zero

what is voltage ripple for high frequency generator

less than 1%

what does more consistent voltage applied to x-ray tube throughout exposure cause

greater quantity and quality of x-ray beam

what is tube current

number of electrons flowing per unit time between cathode and anode

what does mA setting allow for

specific amount of tube current to be applied to filament allowing a specific amount of thermionic emission

what cascade effect does more mA cause

more thermionic emission → more space charge → more electrons

what does mA determine

number of electrons flowing in the tube and quantity of x-rays produced

mA is directly proportional to

number of electron in tube current

changing mA results in

proportional change in quantity of x-rays produced

what does exposure time determine

length of time tube current is allowed to flow from cathode to anode

exposure time is directly proportional to

quantity of x-rays

tube current

what is actual focal spot

area on anode target exposed to tube current electrons

what is effective focal spot

focal spot size as measured directly under anode target

what does a smaller anode angle cause

smaller effective focal spot size

what focal spot size can handle heat from large exposure

large

what focal spot size produces good image quality

small

what is line focus principle

relationship between size of focal spot and effective focal spot

what side of the x-ray tube has greater intensity

cathode

how to employ anode heel effect

position thicker anatomical areas towards the cathode side

where are x-rays from anode heel effect absorbed and not absorbed

absorbed from anode side

not absorbed from cathode side

how much can the different in intensity be during anode heel effect

45%

why is added filtration used

filter out the low energy photons from x-ray beam that cannot penetrate anatomy and are only absorbed in the body

what is the preferred material for filtration and why

aluminum because of it’s ability to absorb low energy photons

what contributes to inherent filtration

tube envelope

oil surrounding the tube

window in tube housing

what object in the collimator adds extra filtration

mirror

what is total filtration

sum of added and inherent filtrations

what is current guideline for tube filtration

x-ray tubes operating over 70 kVp must have minimum total filtration of 2.5mm of aluminum or equivalent

how does increasing tube filtration increase x-ray beam quality

there’s a greater percentage of x-rays with higher energy

what does increasing tube filtration decrease

quantity of x-rays

what reduces beam intensity to half of its original value

amount of filtration added to the beam

one hvl reduces beam intensity by what

50%

what is hvl an effective method for

tracking x-ray system output over time

what is purpose of heat units

measure heat produced by an exposure

how to calculate heat units (HU)

mA x time x kVp x generator factor

generator factors for generator types

single phase - 1.00

three phase - 1.35

high frequency - 1.40

how to extend x-ray tube life

warm up tube

avoid excess heat generation

don’t hold down rotor without making exposure

use lower mA and long exposure time

don’t move tube while energized

recognize and report unusual noises