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X-ray Production 10047/10046 Cheat Sheet by

Cheat sheet for: X-ray production and X-ray interactions with matter

Types of Radiation

Part­icu­late
Elec­tro­mag­netic
Alpha
X-ray
Beta
Gamma
Neutrons

Partic­ulate Radiation

All radiation particles have two features in common: they all have a mass and they are all subatomic particles. Since particles have a mass (and sometimes a charge) = HIGH intera­ction between radiation and matter. This is one of the reasons partic­ulate radiation is not used for medical imaging; low pentating ability (we need radiation to pass through the patient and still interact with the patient, but partic­ulate radiation have very high Linerar Energy Transfer

Alpha Paticles
> released by nuclei of unstable atoms (urani­um-238, pluton­ium­-236).
> consist of 2 protons and 2 neutrons (net positive charge: +2 --> identical to helium atom).
> high L.E.T; because of their heavy weight and high charger.
> can be stopped by a piece of paper

Beta Partic­les
> two types: electrons and positrons. Both orginate from an unstsbale atom and have high energy and speed
> Beta+ decay: when excess protons --> proton converts into. Position and neutrino produced.
> Beta- decay: when excess neutrons --> neturon converts into proton. Electron and antine­utrino produced (rearely used in medical imaging)
> Positrion has low atomic mass, (+1 charge). Low mass and charge = lower LET, higher penetr­ability compared to alpha particles.
> stopped by a few milimetres of aluminium

Neut­rons
> bypoduct of nuclear fission or fusion.
> similar mass to proton, but no charge

Radiation Penetr­ating Power

Partic­ulate vs Electr­oma­gnetic Radiation

Generator type and Emission Spectrum

Change in Genera­tor: note that as the efficiency of the generator increases, so does the x-ray quantity given the same amounf of electr­icity used. This goes back to X-ray circuitry: reduced ripple effect, consistent levels of kVp with high-f­ree­quency generator.
 

Electr­oma­gnetic Radiaiton

They are chargless and mass-less; "­packets of energy­"
* they can travel in straig­htlines through empty space/­vacuum.
* they are transm­itted by electric and magentic fields oscilating at right angles to each other
* travel at the speed of light (in a vacuum).
* they are unafected by external magnet­ic/­ele­ctric fields
* wave-p­article duality. for medical imaging, we view X-rays and Gamma rays more as a wave.
* low wavenl­engths, high freque­ncies; higher freque­ncies = higher energy

X-ray Produc­tion
* Created from the intera­rctions of high-speed (high KE) electrons with target (e.g. tungsten).
two types: chara­cte­rsitic and bremss­thr­alung radiation

Charac­ter­sitic Radiation

Charac­ter­istic radiation involves the fillament electrons intera­cting with orbital electron of target atom. Created when orbital electrons are removed from their shell and outer-­shell elelctrons fill inner-­shell vacancies (usually K-Shell electrons that are ejected). To fill vacancy: potential energy is releaserd as a charac­ter­istic photon. AKA. since the binding energies differ between orbiting shells: outer-­shell electrons (low BE) fills inner-­shell vacancy (high BE). Energy released is the difference between the inner-­shell BE and outer-­shell BE.

For charac­ter­istic radiation to even occur, the incident electron (fillament electron) MUST have a HIGHER than the relevant BE.
resu­ltant charac­ter­istic x-rays are specific to certian shell-­shell transi­tions and USUALLY do not provide sufficient energy to even leave the target atom, never mind the patient

Binding Energies for Tungsten

K Shell
69.5 keV
L Shell
12.1 keV
M Shell
2.82 keV
N Shell
0.6 keV
O Shell
0.08 keV
P Shell
0.008 keV

Charac­ter­sitic Radiation

Filtration and Emission Spectrum

Added filtra­tion: increases in tube filtration causes a decrease in X-ray beam quantity and an increase in quality, but the energy of charac­ter­istic x-rays are unaffe­cted.

Target Material and Emission Spectrum

Change in Target Materi­al: note that as the atomic number of the material increases, so does the average energy and qunatity of the x-rays and the position of the discrete line (chara­cte­ristic x-rays) changes. Greater atomic numbers represent 'bigger' targets for the fillament electrons to interact with. This increases the likelihood of intera­ctions and the number of photons produced. The charac­ter­istic x-rays are different as they are atom specific.
 

Bremss­thr­ahlung Radiation

Brems photons are produced when filament electrons miss all of the orbital electrons of the target atoms and interact with the nucleus. The attraction of the fillament electron to the nucleus causes it slow down and change direction. the resultant loss of energy is given off as a brems photon.

Unlike charac­ter­istic x-rays where very specific energies are produced, brems photons have a much larger range of energy levels. The amount of direct­ional change imposed on the incident electron dictates the amount of energy released

Important conclu­sions about X-ray produc­tion:

1) Knowing that the average energy of brems is 1/3 of the kVp selected and that most of the beam is made up of brems: we can predict average energy of an x-ray beam to be 1/3 of the kVp selected
2) A number of X-rays are at very low energies (& have no diagnostic value). This highlights need for filtra­tion. Inherent filtration from X-ray tube housing (glass envelope, oil) removes ~50% of X-rays generated at the anode. The added filitr­ation of aluminium removes 80% of THE REMAINDER. this means that there is leakage radiation.
3) X-ray production is not efficient: Most intera­ctions (99%) do not result in X-rays, but produce only heat. only 1% of intera­ctions result in X-ray production either by charac­ter­istic or brems intera­ctions. Basically, when incident electrons hit the target, 99% only result in excitation of the target atom's electrons and 1% results in ionisa­tion.

Brems vs. Charac­ter­istic

kVp and Emission Spectrum

Change in kVp: purple curve (increased kVp), increases the quantity and quality of brems x-rays. It does not change the position of the charac­ter­sitic radiation line = does not change the energy of charac­ter­istic x-rays, just the quantity of them.

mAs and Emission Spectrum

Change in mAs/ma: increasing mAs or mA will increase the quantity of radiation (because increased current supply to the fillament = more incident electrons hitting the target anode). Increasing mAs/mA has no affect on the quality (average energy) of X-rays and the energy of the charac­ter­istic x-rays

Factors Affecting Emission Spectrum

Increase in
Effect on Quantity
Effect on Quality
mA/mAs
increases
no effect
kVp
increases
increases
Tube Filtration
decreases
increases
Generator type
increases
increases
Target Material (atomic number)
increases
increases
 

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