x ray physics content

what are the internal parts of the cathode

• Cathode is negatively charged and contains a filament which is typically a tungsten alloy wire , which when heated via a tube current, produces electrons via thermionic eversion

• the cathode has negatively charged focusing cups that focus the beam into a small tight area on the anode. Since the focusing cups are negatively charged this means that the electrons produced will be repelled from the focusing cups towards a small spot on the anode.

• smaller target area on the anode reduced penumbral blur (sharper image)

Protective Housing

• X-rays are produced at the anode (target) in the X ray tube and emitted isotropically (at equal intensity in all directions)

• Most X-rays produced are not useful as they are absorbed by the anode (target) or tube housing (outer protective casing around X-ray tube)

• The useful beam is a small fraction of X-rays that exits through the tube window

• There can be some leakage of radiation through the tube housing, which is not part of the final useful beam thus, patient and radiographer must be protected from leakage radiation

• The housing may also contain oil that acts as a thermal cushion (against thermal shock) and an insulator (against electrical shock)

• The housing also protects the tube from mechanical shock damage

• The metal protective housing reduces leakage radiation to safe levels (less than 1 mGy/hr at 1 m)

Internal Components - Anode what is the anode

• The anode is the positive side of the X-ray tube and attracts electrons from the cathode.

• When electrons hit the anode, ~99% of their energy becomes heat, and ~1% becomes X-ray radiation.

• The anode is designed to conduct heat away efficiently to prevent damage.

what are the materials usually used in the anode

• the anode is typically made of Copper (Cu), Molybemum (Mo) or Tungsten (W)

• the anode target material is typically W or Mo ( esp Tungsten as it does not melt quickly and has a high melting point)

• the target material is the surface that the energy electrons strike or hit

• copper is typically wrapped alrunf the outside of the anode , as it acts as a good conductor allowing heat to be carried away, preventing overheating

rotating vs stationary anode

• a stationary anode is limited to short exposures and currents

• a stationary anode is prone to overheating compared to rotating anodes which spread and distribute heat around a large surface area, preventing the target from melting

anode focal spot

the anode focal spot is the area on the anode where the electrons hit the anode and x rays are produced

the goal is to make the focal spot as small as possible for a sharper image and preventing penumbral blur

however a focal spot too small can lead to electrons overheating the anode

effective focal spot: how large the x ray source appears from the patient or detectors view

actual focal spot: physical area where the electrons hit the anode to make x rays

distance from source penumbral blurring

→ increased distance from the source means lower magnification, lower skin enternece dose ( due to diverging x ray beams less x rays hit the patient) and higher sharpness and less penumbral blurring

→ close distance can lead to higher magnification, high skin entrance dose, lower sharpness and higher penumbral blurring, however this can be overcome by smaller focal spot

size of focal spot and penumbral blurring

→ a smaller focal spot leads to less penumbral blurring and higher brightness. however, this can lead to lower brightness as there is an increased likelihood that the target may melt or a given number of projectile electrons per second

→ a larger focal spot size can lead to more penumbral blurring

autotransformer

an autotransformer has a single coil of wire which provides precise voltage to the filament and high voltage to the x ray tube.

the autotransformer supplies AC current which is then amplified by a step up transformer to produce high voltage AC currents which is rectified by a diode bridge which coverts the Ac current into DC current allowing uni directional flow from the cathode to the anode.

focal spot heel effect

→ the effective focal spot effect means that the x ray beam is more intense on the cathode side than the anode side where some of the x rays are absorbed by the anode target material such as tungsten

→ the heel effect can be overcome by increasing the angle of the anode

→ since focal spot intensity is not uniform for this reason it can be used to radiographers advantage as one examinations would want one area to be more sharper than the other

reasons for x ray tube failure and how to prevent

thermal shock: prevent by using the correct starting procedures so the anode does not heat up too quickly

prolonged ON times: prevent by reducing thermal load

filament damage: evaporated tungsten by collect on the filament and cause internal electric discharge

Lack of use: the vacuums can fail

tube life is typically expressed in the number of exposures

what is the kinetic energy of en electron . when the KVP is 70. what is the electron speed at the target

KEe= charge x voltage

1.6 × 10^-19 × 70 × 1000 =1.12 x 10^-14 J

at the target the electrons speed is about ½ the speed of light.

anode heat

• Electrons which hit the target atoms’ outer shells produce anode heat

• When outer shell electrons are hit by projectile electrons, they become excited to higher energy levels

• When they fall back to original energy level, they release energy as infrared radiation (heat)

• Doubling the electron current (mA) → doubles the heat

BREMSSTRAHLUNG RADIATION:

→ where most x rays come from

→ occurs when projectile electrons are attracted to the positively charged nucleus, it slows down or decelerates, which leads these electrons to loose kinetic energy , this energy is seen as an x ray

→ the closer the projectile electron slows down near the nucleus , the larger energy loss meaning a higher energy x ray is produced

→ the further away the projectile electrons slow down near the nucleus , the smaller energy loss meaning a low energy x ray is produced

→ this model produces a range of x ray energies as these projectile electrons don’t all loose the same amount of energy.

→ electrons that avoid atomic electrons can interact with the electric field of the nucleus, as they slow down the energy loss can be seen as an x ray.

→ low energy x rays do not contribute to the final useful beam and is often absorbed by the anode target or the housing

→ very few projectile electrons slow down very close to the nucleus thats why there aren’t many that are commonly produced

how can u determine the KVP applied by looking at a graph

the max energy an x ray photon can have is equal to the max KVP that is set for the electron., where the number of x rays equal zero on the right side

characteristic radiation

→ the projectile electrons hits an orbital electron, this leads it to be knocked out of its shell. when a higher shell electron replaces it and falls into the gap, the difference in energy between the shells is called an x ray

→ the energy difference between shells K (inner shell) and L is higher than L and M , meaning that a higher energy X ray will be produced . but very few projectile electrons interact with inner shell electrons closer to the nucleus

→ the shell that the electron is knocked out of the atom is Called eg: k shell bc knocked out of k shell ect , k x-ray

→ the x rays of these energies will always be the same

charateritsic x rays 2

the electron that helps to fill the vacancy in the K shell can be classified as Kbeta, gamma or alpha depending on which shell the electron that replaces it comes from. higher energy drop higher energy x ray produced.

characteristic x rays have discrete energy levels explain

→ x-rays produced from specific electron transitions, will have energies corresponding to e difference in binding energy between shells

→ with tungsten anode targets only K x rays will be emitted, and L or N ect x rays will be absorbed by the anode target or tube housing as they only contribute to patient dose.

factors inflcuencing the x ray emission spectrum

→ the total number of x rays is defined as the area under the spectrum plot

→ the larger the area under the graph, the higher quantity of x rays

→ higher KVp can also increase not only quantity but also quality of the x rays

effect of Ma and MAS

→ doubling the current only doubles the amount of x rays produced by does not increase the x ray energies, essentially it produced more x rays of the same energies

EFFCT OF KVP

→ KVP influences both quality and quantity of X rays , area under the graph increases and the energies of the x rays increase as KVP increases

→ the characteristic part of the x ray stays the same

→ around a 15% increase in KVP doubles the x ray input intensity

x ray filteration

→ x rays produced at the anode is often filtered by a filter places immediately after the tube window

→ filters contribute to average increase in x rays energies or quality but a decrease in x ray quality, as low energy x rays only contribute to patent dose and does not improve image quality.

the effect of target material

This effects both quality

and quantity of the beam,

• Bremsstrahlung radiation

increases with atomic

number, as the nucleus has a stronger positive charge , increasing the strength of the nuclear electric field, causing greater deceleration of electrons

• Characteristic radiation

energy also changes with

material

• Mammography uses tubes

with Mo or Rh target

material