x-ray generator Flashcards
when were x-rays discovered
1895
who discovered x- rays
Roentgen
two different mechanisms by which x-rays are produced
bremsstrahlung x-rays and characteristic x-rays
process of braking radiation as a result of a radioactive collision between a high speed electron and a nucleus
bremsstrahlung x-rays
how does bremsstrahlung mechanism produce x-rays
as electron passes in vicinity of a nucleus, it suffers sudden deflection, the loss in energy is radiated as electromagnetic radiation.
energies are continuous
get photon as a result
result of bremsstrahlung interactions in terms of energy
complete loss of energy or partial loss of energy
the resulting photon may have up to, but not exceeding the energy of the incident electron
electron interacts by ejecting an orbital electron such as K, L, or M electron
characteristic x-rays
result of characteristic x-rays in terms of energy
energies are discrete or specific
original electron continues with an energy equals to its initial energy minus energy given to the orbital electron
Kilovoltage units
Grenz- ray therapy Contact therapy Superficial therapy Orthovoltage therapy Supervoltage therapy
negative electrode
cathode
consists of a wire filament, a circuit for current, and a negatively charged focusing cup
cathode
when heated, emits electron by thermionic emission
tungsten filament
tungsten filament’s purpose
heats up to emit electrons by thermionic emission
cathode cup
directs electron from cathode to anode so that they strike the target in a well defined area, the focal spot
positive electrode
the anode
consists of thick copper rod at the end
the anode
the tungsten target consists of
thick copper rod
why is the tungsten used as the target material
because of high Z and high melting point
what is the anode consist of
thick copper
what is the cathode consist of
wire filament, a circuit, and a negatively charged focusing cup (to direct electron from cathode to anode)
anode hood
prevents stray electrons from striking non target tube components
Beryllium window and thin glass window
absorbs some of photon/x-rays
also called inherent filter
the focal spot
small for diagnostic x-rays and larger for therapy x-ray tube
removal of heat
achieved by circulating oil, water, air, rotating the anode in diagnostic x-ray
X-ray tube efficiency
ratio of output energy emitted as X-ray to input energy deposited by electrons
efficiency equation
9x10^-10 ZV
what happens to the rest of the energy
converts to heat
efficiency depends on what quantities
output energy x- rays and input energy electrons
what are the specifications of quality of x-ray beam
quality of the beam in the range of kvp is specified in terms of energy and half value layer
what is half value layer
the thickness of an absorber of specific composition required to attenuate the intensity of radiation to half of its original value under the condition of good geometry
average energy
1/3 of the maximum energy of kVp
x-ray energy spectrum for bremsstrahlung (unfiltered)
a continuous distribution of energies for photons
what is the purpose of added filtration
enriches the beam with higher energy photons by absorbing the lower energy spectrum components
x-ray energy distribution for characteristic x-rays
individual discrete energies
where is added filtration placed
externally to the tube, further modifies the spectrum
relationship between added filtration and beams
as filtration increases, x-ray beams hardens, which leads to achievement of higher average energy, therefore leads to greater penetrating power
inherent filtration
absorption of low energy photons in the target, glass walls of tube, or thin beryllium window
what is a LINAC
a device that uses high frequency electromagnetic waves to accelerate charged particles such as electron to high energies through a linear tube
electron gun (LINAC)
filament when heated by an electrical current, releases electrons into the space around it
accelerator guide (LINAC)
energized by microwave power
mounted horizontally for high energies
mounted vertically for low energies
circulator (LINAC)
prevents any microwaves from returning to the Klystron- acts like valves in veins
Water Cooling System
allows components in the drive stand and gantry to operate at a constant temp by circulating water
Microwave system magnetron
magnetron is a device that produces microwaves
Klystron
is a microwave amplifier, not generator- driven by a low power microwave oscillator
pros of klystron
higher power microwave applications due to:
- excellent stability
- load isolation
- high gain
using E/M waves to accelerate electron
an electron will traverse a distance = 1/2 lambda in the time =1/2 period, accelerates continuously by the standing wave
what is the function of a bending magnet
treatment units with a nominal90 degree or 270 degree beam bending magnet (mounted horizontally in the gantry) to redirect beam and select energy spectrum
effects of bending magnet
spread of energy of the actual beam
spread of focal point at the x-ray target
3 pole 270 bending magnet
limit energy spectrum exiting the magnet
refocuses electrons of correct energy
LINAC treatment head
- primary collimator
- beam flattening system (flattening filter, scattering foil)
- Dosimetry monitoring system
- Moveable collimators
beam collimation in x-ray mode
treatment field size is defined by the secondary collimator
beam collimation in electron mode
must be achieved close to the skin surface of the patient due to their readily scatter in air
beam flattening
produce a beam of uniform intensity
flattening filter is for
x-rays
scanning system or scattering foil is for
electrons
what happens if flattening filter is removed
- non uniform beam profile (highest intensity at center)
- increase in dose rate, decrease in treatment time
- softening of x-ray beam (increases surface, skin dose, shift dmax toward surface)
- reduction in head scattered radiation
- less neutron production due to higher dose per MU
Flattening Filter Free (FFF) applications
small fields (SRS) IMRT
purpose of FFF
- higher dose rate
- short term treatment as an advantage in motion management, patient comfort, treatment accuracy, reduce patient and target motion
- beneficial for small fields and IMRT
Scattering system
- most common for flattening method
- 1 or 2 metal foils are placed in beam to scatter electrons, results in spreading and flattening beam
- produces x-ray contamination of the electron beam
- initial alignment involves more mechanical adjustments
- components are small and cheap
Scanning system
- a set of electromagnetic coils are placed around the beam
- current to the coils vary in time, beam is bent such that it sweeps the field
- relies on precise control of electromagnets
- more expensive
- if scanning fails and beam remains pencil beam for a long time, a high dose may be delivered to a very small volume
Dosimetry Monitor
monitor dose rate, integrated dose, field flatness and symmetry
Secondary blocking
after passing through ion chambers, beam is further collimated by continuously moveable X-ray collimator (produce rectangular field sizes)
Teletherapy
external beam treatment in which source of radiation (gamma emitter) is at a large distance from patient
Trimmers
metal bars extend downward from collimators to trim penumbra near the treatment surface by attenuating the edge of the beam, place no closer than 15 cm
Transmission penumbra happens if the edges of the block are parallel with the central axis of the beam
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