x ray production Flashcards
two methods of x ray production
bremsstrahlung (breaking radiation) or characteristic (fluorescence)
features of the general x ray energy spectrum (3)
low energy part (limited by self absorption), characteristic lines, intensity proportional on KV^2
high energy electron interaction with bound electron
characteristic x-rays
high energy electron interaction with the nucleus
highest energy bremsstrahlung
high energy electron interaction near the nucleus
high energy bremsstrahlung
high energy electron interaction further away from the nucleus
low energy bremsstrahlung
what do clinical x-ray beams (excl. mammo) mostly consist of and what percentage
~ bremsstrahlung (90%), characteristic (10%)
x ray generation: components needed (3)
cathode: electron source
applied kV: to accelerate electrons
anode: made of a material where electrons interact and their energy converted into x rays which rotates to cool it down (99% of electron energy is converted into heat)
what encloses the vacuum of the xray tube
glass (pyrex) or metal/ceramic envelope
what is contained in the cathode (2)
multiple tungsten filaments
focusing cups
how does rotating anode cool it down
spreads heat over large area by rotating up to 3500 rpm
purpose of tungsten filaments in the cathode
source of electrons
what kind of tungsten are the filaments
thoriated (alloyed with thorium - greater thermionic emission)
thermionic emission process to produce electrons
heated ___
___ escape
forming ___ ___
Electron emission from heated surface.
cathode
Electrons
space charge
purpose of focusing cups
Focus Electron Beam which strikes the focal spot
Diagnostic kV (accelerating potential) Range
Typically spans 40 kV to 150 kV.
purpose of stator windings
coils of wire around the ___
produce ___ ___ which ___ the anode rapidly by ___ ___
tube
magnetic field
spins
em induction
rotating anode:
made of ___ because of it’s high __ and __ ___.
___ disk to help ____ ___
___ mount for the ___ and ___
tungsten
Z - 74
melting point = 3400 deg C
pmolybdenum
dissipate heat
graphite
mo disk and W target, helps dissipate heat)
x ray tube housing made of ___ with ___ lining and containing ___ ___
steel
lead
insulating oil
what is blurring at the penumbra
Blurred or indistinct edge in a radiographic image, reducing image sharpness and detail/clarity, specifically at the edges of the object
. This is most easily seen at the penumbra which is the blurring specifically at the edges of the object image. When the spot is small, the radiation effectively originates from a point source, thus the boundary of the image is very sharp, and hence the penumbra does not appear. In contrast, when the spot is large, the radiation path depends on the location in the spot from which it originates, causing the edges to be less well-defined, creating a large penumbra as seen in Figure 1. Typically the size of the spot, and hence of the penumbra, increases with the power of the electron beam.
Causes: e.g. X-ray beam divergence, and scatter.
Mitigation: proper positioning, collimation, and exposure techniques.
Significance: Clear visualization of anatomical structures is essential for accurate diagnosis in medical imaging.
what causes penumbra blurring
x-ray beam divergence: Focusing the electron beam results in an X-ray spot instead of a point source so when the X-rays interact with the object, and are then recorded by the detector, blurring in the image can be observed
equation that links effective focal spot size to penumbra size, how to minimise penumbra
X1/X2=Y1/Y2, where X1 is the effective focal spot, X2 is the penumbra at the image plane, Y1 is the distance between the target and object, Y2 is the distance between the object and image plane.
penumbra minimised for smaller X1 (focal spot) and Y2 (distance to image plane)
a cathode is _______ charged and the anode is _________ charged.
negatively, positively
the source of electrons for x-ray production comes from the ______ ________
cathode filament
What purpose does the focusing cup serve being negatively charged?
it helps to “herd” negatively charged electrons into a space cloud
what is the boiling off of electrons from the filament called?
thermionic emission
the cathode focusing cup is made of?
molybdenum (or nickel)
What part of the cathode holds the filament?
focusing cup
What is the target material made of and why is it used?
tungsten, it has a high melting point
the induction motor is made up of what 2 parts?
the rotor and stator
Why must the target be angled?
in order for the x-ray photons to exit the window
what is the negative aspect of angling the target called?
Heel effect
what causes Heel effect
self absorption within the heel of the angled target depending on the angle of the target
consequences of Heel effect
% of central x ray energy varies with the target angle, may limit minimum target angle in certain circumstances e.g. radiotherapy
is the beam intensity higher on the cathode or anode side
cathode side - greater self absorption at lower angles on anode side
Greater than ____% of heat is produced in x-ray production and less than ____% of x-rays are produced.
99, 1
When using techniques of under _____ kvp you will have 100% ______ production.
70, brems
Characteristic radiation only occurs when using techniques over _kVp.
70
What is the binding energy of a tungsten atom?
69.5 (which is why we need at least 70 kVp to knock an inner shell electron from orbit
purpose of light beam diaphragm
enables the radiographer to visualise and control the field size and pathway of the x ray beam. the “gatekeeper” for the X-ray beam, ensures that only the necessary portion of the beam reaches the patient, optimizing safety and diagnostic accuracy
what determines the max spectral energy
the generating voltage (keV=kV)
bigger patients need more/less kV
more (penetration)
_____ _____ _____ and _____ _____ go up with kV
maximum spectral energy, output intensity
why do lower energies need to be filtered
they increase skin dose
what material is used for filtration
aluminium
why is aluminium used for filtration
low atomic number (atomic number 13).
absorbs most low-energy photons.
allows transmission of most high-energy photons
What is the equation for total filtration in an X-ray machine
Total filtration = Inherent Filtration + Added Filtration
what are the three key radiographic settings
what do they determine
kV (determines penetration) beam current mAs (beam current and exposure time ->no, of xrays produced/dose)
relationship between kV and mA
inverse
Give the properties of the glass envelope
what is it made of and why
what is the window
contributes to what
made of pyrex = withstands a lot of heat
maintains a vacuum
window:
- segment of glass thinner than rest of glass envelope
- contributes to inherent filtration (.5mm Al equivalent)
what does the rotating anode contain
what is it made of
- contains rotating disks of 5-13cm in diameter
- made of Molybdenum
what are two advantages of using a rotating anode ?
greater target area
greater heat dissipation
Describe the focal spot
portion of the – where – – – – will impact
size compromise
smaller =?
larger =?
the portion of the anode where the high voltage electron stream will impact
must be small enough to produce sharp images but large enough to absorb heat without melting
What is the effective focal spot?
the projection perpendicular to the central x-ray which is its apparent area
the focal spot produced as a result of angulating to target material
what is the target rating chart used for
by manufacturers to programme x ray sets
what information does the target rating chart give
max mA for any kV and exposure time
what could go wrong (5)
- anode damage from heating
- failure of filaments, high voltage wiring etc
- failure of heat sensors, oil pumps
- oil leaks
- loss of insulation on x ray housing
developments (2)
- larger/faster anode discs -> greater thermal capacity
- metal-ceramic x-ray tubes -> greater resilience
Define the use of the collimator
used to restrict the size and shape of the x-ray beam to reduce patient exposure (scatter radiation)
why is tungsten thoriated
increase thermionic emission
KV selection based on desired __ and __ ___.
contrast, tissue penetration
diagnostic KV for soft tissue imaging, like chest X-rays.
Lower kV (40-70 kV)
diagnostic KV for denser structures, such as bones.
Higher kV (80-150 kV)
what does rotating anode allow
continuous X ray generation by spreading heat
What does the x-ray tube insert provide
the electron source, vacuum environment, target electrode
what does the external power source provide
high voltage potential difference to accelerate the electrons
what does the x-ray tube housing that the insert is mounted within include
metal enclosure
protective radiation shielding
X-ray beam filters
collimators
what is the purpose of x-ray beam filters
to shape the x-ray spectrum
what is the purpose of collimators
to define the size and shape of the x-ray field
what does the x-ray generator supply
the cheaper to ensure to accelerate the electrons
a filament circuit to control tube current
and and exposure time
definition of exposure
dosimetry quantity describing the amount of charging coulombs release in a known mass of air in coulombs per kilogram
definition of exposure rate
the increment of exposure in a time interval in the units of coulombs per kilogram per second
definition of electron volt
the energy obtained by an electron after it is accelerated across the potential difference of 1 V. One electron volt is equal to 1.6 X 10 ^ -19 Jules
and applied x-ray tube potential of 50 kV accelerates the electrons to a kinetic energy of __
50 K EV
what happens during most interactions
99% of interactions are collision or with other electrons produce nothing but heat
what is bremsstrahlung radiation
radiative emission of an x-ray photon Caused by
electrical forces attracting and decelerating an electron changing its direction and velocity
What factor determines the X-ray energies of the resulting bremsstrahlung radiation
The Coulombic attraction
Which is proportional to the inverse square of the distance
What happens at closer interaction distance
increase coulombic force causes greater electron deceleration and conversion to hire x-ray energies
what happens when there is a direct impact of an electron of the target nucleus
The target nucleus stops the electron and convert all its kinetic energy into an equivalent x-ray photon
the highest bremsstrahlung x-ray energy
what is a bremsstrahlung spectrum
the probability distribution of x-ray photon fluence produced as a function of energy expressed in K EV
What is filtration
the removal of x-rays by attenuation
what Material causes inherent filtration
materials that are inherent in the x-ray tube such as the glass or metal window of the cheap insert
What Material is used for added filtration
Materials intentionally placed in the beam
thin aluminium and copper sheets which preferentially attenuate lower energy x-rays
what are the two main factors that affect x-ray production efficiency
the kinetic energy of the incident electrons
The atomic number of the target material
The blank is directly related to the tube potential
the kinetic energy of the incident electrons
what is characteristic radiation
discrete x-ray energy peaks
what do continuous x-ray energies depend on (2)
the elemental composition of the anode
the applied x-ray tube voltage
What is the order of shells from inner to outer
KLMN
which Shell has the high select from binding energy
the innermost shell K
how is a vacancy caused in a target atom
the kinetic energy of an incident electron exceeds the binding energy of the electron shell so the interaction can inject an electron
why can an outer shell electron feel the vacancy by immediately transitioning
because it has less binding energy
what is the energy of the characteristic x-ray equal to
the difference in the electron binding energies of the two shells
when are characteristic K x-rays produced
only when the impinging electrons on the target exceed the binding energy of a K shell electron
Major components of the x-ray tube
cathode
anode
Rotor/stator
glass or metal envelope
Tube port
cable sockets
cheap housing
what is the x-ray tube voltage for mammography
25 to 49 kV
what is the x-ray tube current
the number of electrons per second flowing from the cathode to the anode
What does the cathode comprise of
An electron emitter and focusing cup
how are filaments positioned in most x-ray tubes for diagnostic imaging
Two filaments of different lengths positioned in a slot Which is machined into the focusing cup
What is Thermionic emission
electrical resistance heating the filament to a temperature determined by the amplitude of the current resulting in a release of electrons from the filament surface
when is a space charge( static electron cloud) formed around the filament
when the repulsive force of the negative charge of emitted electrons equalling the thermionic emission force
What is the function of the focusing cup
to surround the filament emitter with a slotted half cylindrical structure to reduce the spread of electrons during exposure
what determines the focal spot dimensions (2)
the length of the filament in one direction
the width of the focusing cup in the perpendicular direction
What anode materials are used for mammography x-ray tubes And why
molybdenum and radium because they provide useful characteristic x-rays for breast imaging
what are the two anode configurations
Stationary and rotating
what are the advantages of rotating anodes
high x-ray output
what spins the anode in a rotating anode
the stator/rotor induction motor which is energised
Why is there a short delay before x-ray exposure when the button is pushed for General radiography x-ray systems
the x-ray tube generator is designed so the x-ray tube voltage is applied only when the anode is at full speed
Why are Rotor bearings often the cause of x-ray tube failure
they are heat sensitive
how to achieve thermal insulation from the hot anode
Using a connector stem to the rotor made of molybdenum which is a poor heat conductor
what is the heel effect
reduction in the x-ray beam fluence on the anode side of the x-ray field due to the steep anode angle causing x-rays directed towards the aThisnode side of the beam attenuated by greater path lengths through the anode material
purpose of x-ray tube housing
mechanically support ,
Electrically and thermally insulate ,
and protect the x-ray tube insert
Purpose of the lead shielding inside the x-ray tube housing
attenuate nearly all x-rays not directed to the tube port
The purpose of automatic exposure control
to measure the actual amount of radiation incident on the image receptor during the acquisition and terminate x-ray production when the optimal radiation levels are obtained
What does automatic exposure control compensate for
patient thickness and other variations in attenuation
what is the advantage of automatic exposure control over manual exposure time settings
more consistent exposures
how does the anti-scatter good work
place between the detector and patient
allows all primary radiation incident upon it to pass while absorbing all of the scattered radiation
Obliquely angled photons from scattered x-rays emanating from the interior of the patient have a higher probability of striking the highly absorbing grid septa
what is the design of the scatter grid
open into space regions and alternating x-ray absorbing septa are aligned with the x-ray tube focal spot
