Production of X Rays, MMED Physics, Lesson 1

MMED Physics Notes, Week1, Production of X Rays
A:F2.1

• Tube voltage 20keV to 150keV
• Heat tungsten filament with sufficient energy to overcome the tungsten binding energy and release electrons towards the anode, therefore would need 70keV or greater to remove -69 keV k shell electron binding energy. If less than 69 keV no characteristic x-ray only Brehemstrauling. greater than 69keV gives characteristic xrays
•  Vacuum needed because:
• Oxygen at 3000 degrees Celsius would incinerate the filament
• electrons mass is 2000 x smaller than the mass of a proton, therefore if there was no vacuum  there would be:
• more collusions with other atoms
•  inefficient generation of x-rays
• Focussing cup:
• A device that surrounds the filament on the cathode side of the x-ray tube to prevent electrons from being emitted in all directions. Without the focusing cup, the electrons would repel each other and spread in different directions.
• Tungsten would eventually evaporate and cote the glass surface with a red layer (sun burn)
• Tube Current: CXR requires alot of current and electrons
• Angle of the anode - 6 to 20 degrees:
• sin angle - smaller angle gives a smaller focal spot
• the smaller the focal spot size the better the resolution
•  Mamography - molebdinum and Rhodium target

F2.2 - EM waves, Particle Waves
Particle characteristics 

• 
• 10 keV = longer wave length
• 100 keV = Shorter wave length
• f proportionate to the inverse of wave length as the speed of EM waves is consistent 

Ionising vs non-ionising radiation

• Ionizing: high frequency radiation with enough energy to remove bound electrons leaving ionized atom (UV, XRay and Gamma Ray)
• Non-ionizing:  EM radiation with energy below UV region
• Binding energy: energy required to move an electron completely from an atom, determined by the atomic number of the atom
• more negative for electrons closer to the nucleus (K shell)

Diagram of an Xray tube

Coolidge side-window tube (scheme)

• C: filament/cathode (-)
• A: anode (+)
• W_in and W_out: water inlet and outlet of the cooling device

1. Draw diagram
2. Explain anode and cathode
3. function of components of XRay Tube (briefly, no long stories)
4. Characteristics
1. eg filament made of tungsten
5. Line focus principle
6. Anode (5 points)
7. Rotating anode vs Stationary Anode - Why use rotating?
1. 
   
8. Heel effect, 
1. F2.8
2. Define Anode Heel effect
• X Ray is attenuated as it travels through material
• The anode is angled, the intensity of the x-ray beam along the longitudinal axis of the tube varies. The variation in intensity results from absorption of some photons by the target itself. The photons that make up the anode side of the x-ray beam stand a greater chance of being absorbed because they have to travel through more material than those which make up the cathode side of the beam. Notice the difference from the point where the photons are given off to the the edge of the target. Consequently, the intensity of the x-ray beam is greater on the cathode side than on the anode side. This nonuniformity is known as the anode heel effect.
• Examples of application of the anode heel effect:
• Focus-film distance and the area of the x-ray beam to be used
• heel effect causes problems at short focus film distances and is improved at longer focus film distances
• heel effect is greater at larger film area than focused film area 
9. Heat unit defined in bold
• Heat units - like BTU of Air conditioners. BTU = VI
• SI Units: P = VI/ = watts
• BTU will obviously be greater than SI units as it not /
10. X Ray rating Chart
11. Cooling of Anode of Xray tube - discuss
1. The only way energy is released is by radiation - there is no conduction in the newer ceramic x-ray tubes, even the motor is an induction motor that does not conduct heat 
12. Electron orbits and energy levels
13. Process of Xray generation
1. Characteristic Xray
2. Bramstrauling
14. Define Intensity
1. Intensity = photon energy x energy of each photon (number)
2. Tube rating Charts = mA vs log of time
1. Increasing voltage - increases 
2. Decreasing voltage - decreases

College of Medicine CD of exam questions

Notes

Production of X Rays

v  Diagnostic X-Ray Tubes

Energy conversion

Ø  Glass enclosure

Ø  Cathode

§  Cathode, is also referred to as a filament. The filament is the source of electrons

§  The x-ray tube current, measured in milliamperes (1 mA = 0.001 A), refers to the number of electrons flowing per second from the filament to the target.

§  Thermoionic emission

·         Tungsten is chosen – not because it is it is efficient in emitting electrons as other materials, but is chosen because:

¨        it can be drawn into  a thin wire that is quite strong, 

¨       Has a high melting point (3370 degrees), and

¨       Has little tendency to vaporise – thus the filament has a reasonably long life expectancy

§  Space charge and Space charge effect

§  Electron current in an x-ray tube is in one direction only

§  Focussing cup, which surrounds the filament.

·         Maintained at the same the same negative potential as the filament. The nickel focussing cup, is designed so that its electrical force cause the electron stream to converge onto the target anode in the required shape and size.

§  Vaporization of the filament when it is heated – shortens the life of the x-ray tube because the filament will break off if it becomes too thin. Standby mode.

§  Tungsten that is vaporised from the filament is deposited as an extremely thin cote on the inner surface of the glass wall of the x-ray tube. It produces a colour that becomes deeper as the tube ages. The bronzed coloured “sunburn”

·         Changes the quality of the image

·         Arcing between the glass and the electrodes at high kVp – which may result in puncture of the tube. This is why they have developed metal tube enclosures to replace the glass enclosures.

Ø  Line Focus Principle

§  Most of the energy of the electrons is converted into heat

§  Conflict in the need for a large tungsten focal spot, with a high melting point, to allow greater heat loading, and the conflicting need for a small focal area to produce good radiographic detail.

§  The apparent focal spot is smaller than the actual focal spot – directly related to the sine of the angle of the anode. As the sine of the angle becomes smaller , the apparent focal spot becomes smaller. An anode angle of 16 degrees will produce a smaller focal spot size than an angle of 20 degrees.

§  New 0.3mm focal spot tubes use an angle of 6 degrees – these small angles use larger areas for electron bombardment and heat dissipation, yet achieve a smaller apparent focal spot area.  The angle used is limited by the heel effect.

Ø  Anode

§  Stationary or rotating

§  Stationary anode.

·         Small rectangular plate of tungsten embedded at 15 to 20 degrees in a large mass of copper .

·         Tungsten:

¨        has a high atomic number (74) which makes it efficient for the production of x-rays

¨       High melting point – 3370 degrees. Good for the rapid absorption of heat and rapid dissipation of heat away from the target area.

¨       Copper facilitates the heat dissipation and enhances rate of cooling.

¨       Tungsten and Copper have different expansion coefficients when heated – and if not bonded correctly the tungsten would peel off from the copper.

§  Rotating Anode. X-ray circuits are limited by the x-ray tubes themselves – specifically the heat generated at the anode. Anodes are assumed to rotate at about 3000 rpm.

·         Tungsten dist has a bevelled edge – 6 to 20 degrees – taking advantage of the line focus principle

·         The purpose of the rotating anode is to spread the heat produced during the exposure over a large area of the anode.

·         Increases the total loading area by about 125 times for a 40 mm disc, with the apparent focal spot area remaining the same.

·         Stator coils use magnetic induction motor to rotate the anode assembly in a vacuum.

·         Magnetic lubricants (silver) are used in a high vacuum – (better than the oils that vaporize at high temperatures)

·         Rotating anode tubes – absorption of heat by the anode tube is undesirable because heat would be absorbed by the bearings – would case them to expand and bind. The stem is made of molybdenum – a poor conductor of heat with a high melting point – provides a barrier between the tungsten disc and the bearing of the anode assembly.

·         Short stem reduces the load on the bearings of the anode assembly.

·         Inertia – short delay of 0.5 to 1s  - between the application of force to the anode assembly and the time the motor reaches full angular velocity.  

·         Life of the anode is limited by : roughening and pitting of the anode exposed to the electronic beam – diminish x-ray output of the tube, increase scattering and increase absorption of the target itself. 

·         90% tungsten and 10% rhenium (a heavy metal with good thermal capacity) produces an anode that is more resistant to surface roughening and has a higher thermal capacity than an anode of pure tungsten.

·         Increase the anode rotation to 10000 rpm

¨       Short anode stem, 2 sets of bearings, and decreasing the weight of the anode itself – using molybdenum or graphite discs

§  Grid-Controlled X-Ray tubes

·         The third electrode is the focussing cup that surrounds the filament. Focussing cup is Even more electrically negative relative to the filament. The voltage across the filament grid produces an electron beam that pushes the electrons even closer together. (may even pinch the current off if too high)

§  Saturation voltage -   

·         Residual space charge –

¨        limits the number of electrons available, and limits the current flowing in the x-ray tube. As the kilovoltage increases the  produces a significant increase in tube current even though the filament heating remains the same.

¨       Above 40 kVp (the saturation voltage) – increases in kilovoltage produces very little change in tube current

·         Heel effect. The intensity of the x-ray beam that leaves the x-ray tube is not uniform throughout all portions of the beam.

¨       First, the intensity of film exposure on the anode side of the x-ray is significantly less than that on the cathode side of the tube.

Ø  This can be used to obtained balanced densities in radiographs of body parts of different thicknesses – thicker parts placed near the cathode

Ø  CXR – the anode side should be oriented so that the anode end is over the upper thorax and the cathode end is over the lower thoracic spine.

¨       Second, the heel effect is less noticeable when larger focus-film distances are used.

¨       Third, for equal targeted film distances, the heel effect will be less for smaller films. Intensity of the x-ray beam is more uniform nearest the central ray than towards the periphery of the beam.

·         Tube Shielding and High Voltage Cables.

¨       The effectiveness of tube housing in limiting leakage radiation must meet the specifications of the National Council on Radiation Protection and Measures Report No 49

Ø  “the leakage radiation measured at a distance of 1m from the source shall not exceed 100 mR in an hour when the tube is operated at its maximum continuous rated current for the maximum rated tube potential.”

Ø  Tube housing provides shielding for high voltages required to produce x rays.

v  Tube Rating Charts

§  The Heat Unit (HU) is defined as the product of  current (mA) and kVp and time (sec) for single-phased power supplies

·         In a single phase generator the peak voltage (kVp) is not the average voltage. The kVp – peak voltage is 1.4 times the average voltage. Average voltage is referred to as the root mean square voltage.

¨       Single phase

Ø  70 kVp x  100mA x 0.1 s = 700 HU

¨       Constant potential (3 phase power supplies – where the average voltage and the peak kV are the same)

Ø  70 kVp x 1.4 x 100 mA x 0.1s = 980 HU

§  A watt (W) is a unit of power

·         1W = 1 V x 1A, also 1W = 1 KV x 1 mA – where the volt is the average volt

·         Joule = watt-second = watt x second or kVp x mA x Second

·         In a single phase system conversion is required because the kVp and the average voltage are not the same. One converts kVp to average voltage by dividing by 1.4 or root of 2 (or multiplying by 0.7)

¨       Single Phase

Ø  70/1.4 kVp x 100 mA x 0.1 sec = 500 J

¨       Constant potential:

Ø  70 kVp x 100 mA x 0.1 sec = 700 J

§  Kilowatt rating – of an x-ray tube is

·         used to express the ability of the tube to make a single exposure of reasonable duration.

·         0.1 s.

·         Kilowatt ratings are always used with a Constant potential generator and high-speed rotation.

¨       70 kVp x ? mA = 30 kW

¨       ? mA = 30 000 W / 70 kVp = 429 mA

§  Heat loss by radiation is proportionate to the fourth power of the temperature

Ø  Metallic / ceramic X-Ray Tubes

§  High performance x-ray tubes –

§  Reduced stress on the shaft

§  Ceramic insulators allows for a more compact design

§  Advantages of using a metal tube enclosure

·         Less off-focus radiation

·         Longer tube life with high tube currents

·         Higher tube loading

§  Off-Focus Radiation

·         Off focus radiation is produced by an x-ray tube when high-speed electrons interact with metal surfaces other than the focal track of the anode

¨       Sources – electron back scatter from the anode

¨       Controlled by:

Ø   placing a collimator or lead diaphragm as close to the x-ray tube as possible

Ø  Metal enclosure decreases off-focus radiation by attracting off focus electrons to the grounded metal wall of the x-ray tube

§  Longer Tube Life

·         Sunburn effect of tungsten and arcing effects at high currents in glass tubes

·         Metal enclosure grounded would have a longer life – especially for angiography

§  High Tube Loading

·         Massive anode allows greater tube currents because of the larger heat storage capacity of the anode

¨       Better cooling,

¨       more efficient transfer of heat to the oil by metal enclosure

v  Interaction of Electron Beam with X-Ray Tube Target

Ø  Atomic Structure

§  Nucleus

·         Nucleons – protons and neutrons

·         Atomic number – Z

·         Mass Number  - A

§  Electron orbits and Energy Levels

·         Bound particles always have negative energy

·         Binding energy

·         Energy shells

·         Forbidden transition

§  Process of X-Ray Generation

·         Energy conversion

¨       Kinetic energy of the electron in passing along a voltage is increased by

Ø  E =  eV, where e is the electron charge (e = 1.6 x 10^-19 C and is constant). Therefore, increasing the voltage across the x-ray tube will increase the kinetic energy of the electron.

Ø  Electron volt as the energy a single electron obtains when crossing one volt

¨       E = 1.6 x 10^-19 C x 1 V = 1.6 x 10^-19 J

·         We must clearly distinguish between kVp and keV

¨       100 kVp means the maximum voltage across the tube causing acceleration of the electrons in 100,000 V.

¨       keV denotes the energy of any individual electron in the beam (is 100keV)

¨       At 100 kVp few electrons will acquire a kinetic energy of 100 keV – but the applied voltage pulsates between some lower value and 100 kVp selected.

¨       Voltage providing the potential to accelerate the electrons is pulsating, so the energy (eV) of electrons that encounter the target covers a broad range.

·         High speed electrons striking the target do not have the same energy

·         X-Rays are generated by two different processes

¨       General radiation or bremsstrahlung ­ - involves the reaction of the electrons with the nucleus of the tungsten atoms       

·         Characteristic radiation -  involves the collusion between high speed electrons and the electrons in the shell of the target tungsten atoms.

·         General radiation or bremsstrahlung – Most of the radiation has low energy and 99% of the energy is lost as heat. The energy of a photon of radiation is inversely related to its wavelength. Energy of the electron is related to the potential difference (kVp) across the x-ray tube.

¨       Minimum wavelength (in angstroms) of an electron that collides with x-ray photon can be calculated:

Ø  Min wavelength = 12,4 / kVp

Ø  (this is the maximum energy as the electron collides with the nucleus and releases the maximum possible breaking radiation – therefore the minimum wavelength.)

Ø  Using 100-kVp x-ray tube potential, the maximum energy (eV) that an electron can acquire is 100 keV. An electron with this energy can produce an x-ray photon with minimum wavelength of 0.124 Angstroms:

§  Minimum wavelength = 12.4 / 100 kVp = 0.124 Angstroms

Ø  This is the shortest wavelength and highest energy – most x-rays produced will have greater than 0.124 Angstrom wavelengths – and 99% will be so long as to only produce heat.               

¨       The wavelength of x rays in the continuous spectrum varies. The variation is produced by different energies with which the electrons reach the target, and by the fact that most electrons give up their energies in stages

Ø  The maximum wavelength will be determined by the filtering action of the enclosure of the x-ray tube and on any added filtration.

·          Characteristic radiation. Characteristic radiation results when electrons bombarding the target eject electrons from inner orbits of the target atoms.

¨       The atom may get rig of excess energy in one of two ways:

Ø  An additional electron (Auger electron) may be expelled by the atom and carry off the excess energy. This does not produce X-rays!

Ø  X-Rays produced by filling the inner-shell vacancy, is constant and characteristic for each atom. Again, The x-rays are characteristic of the atom that has been ionised.

Ø  The outer-shells from M outward, release lower energy – not sufficient to form x rays, and is mostly released as heat. The low energy x-rays released are absorbed by the walls of the x-ray tube.

Ø   Characteristic x-rays make up 10% (80 kVp) and 28% (150 kVp) of the useful x-ray beam. Above 150 kVp the contribution of characteristic x-rays decrease and above 300 kVp it is negligible.

§  Intensity of X-Ray beams - # photons in the beam x the energy of each photon – R/min or in SI units: C/kg. Varies with:

·         Target material. The atomic number of the target material determines the quantity (number) of Bremsstrahlung produced and determines the quality (energy) of the characteristic radiation.

¨       The higher the atomic number of the target atoms, the greater the will be the efficiency of the production of x-rays.

Ø  Tungsten (z=74) produces much more bremsstrahlung than tin (z=50), if the tube potential (kVp) and current (mA) are consistant.

¨       For the continuous spectrum, the atomic number of target material partly determines the quantity of x rays produced.

Ø  Melting point is also important

¨       The atomic number of target material determines the energy, or quality, of characteristic x rays.

¨       Molybdenum Target

Ø  Efficiency of bremsstrahlung diminishes with lower atomic numbers and lower tube voltages. Characteristic radiation take more importance.

Ø  Used in mammography

·         Voltage (kVp) Applied.

Ø  The kVp determines the maximum energy (quality) of x rays produced.

Ø  Intensity (amount of radiation produced) is proportional to the (kVp)²

·         Wavelength is not changed by the kVp 

·         X-Ray Tube Current, and

¨       The number of x-rays produced is determined by the number of electrons that strike the target of the x-ray tube – depends on the tube current (mA)

·         Filtration