LECTURE 5: X-RAY PRODUCTION Flashcards
What is kVp?
It is the potential difference between the cathode and the anode
Where does the external energy source to accelerate the e- come from?
generator! high-voltage generator supplies the energy to the cathode to fire e- at anode
Where does the KE of e- come from?
As the e- are accelerated to the anode, they gain kinetic energy
How are e- accelerated to the anode? Where does its energy come from?
high-voltage generator
What is KE of e- called?
Kilo-electron-volts - keV
T or F: kVp is proportional to keV
TRUE - the kinetic energy of an e- is proportional to kVp (potential difference between cathode and anode)
If the atomic number of the target is higher, does this increase or decrease x-ray production?
Increase because more probability of hitting an e- since theres more of them
Formula for KE?
KE = 1/2m v^2 JOULES
What is the units for KE?
Joules
Do all e- have the same mass?
YES!!! Always
Kinetic energy: if you double V, what happens to KE?
It will quadruple! x4 because it is ^2
i.e. 5^2 = 25 but 10^2 is 100
Kinetic energy: if you double m (mass), what happens to KE?
It will double
If you decrease mass by 1/4, but increase speed by double, what happens to KE?
Stays the same!
Do all e- have the same energy if you supply a certain kVp?
No - because it depends on a lot of factors like e- e- interaction, space defect…
T or F? e- travel at half the speed of light
True - more than half
Where does heat come from when e- interact with target?
Collision with atom but KE incident e- is less than BE of orbital e-; only get excitation
What is coulomb interaction?
INDIRECT INTERACTION of electrically charged particles - i.e. electrons with nucleus - Bremsstrahlung x-rays
What is the main principle of bremsstrahlung x-rays, what about characteristic?
Brem: coulomb interactions
Characteristic: IONISATION - incident e- collision with orbital e-
Describe what interaction makes characteristic x-rays
IONISATION incident e- comes and hits orbital e-; with KE larger than BE, it will eject the orbital e- from the atom
All atoms have same binding E. T or F?
FALSE - “Characteristic” of each atom - they have diff properties!
Why is it called “characteristic” x-rays?
First, we have incident e- ejecting an orbital e-; (produces heat) then, as each e- from outer shells jump down to fill the space, energy is given off in the form of CHARAC X RAYS
What does “ISOPROPIC” x-ray production mean?
Means that e- has no set path when it is ejected; goes wherever they want to
What makes outer shell e- want to jump closer to the nucleus?
the proton positive charges
Can you get more than one characteristicc x ray during just one interaction?
Of course. because all the e- keep jumping down the spaces!
As e- transitions to a lower E state, what happens?
The difference in BE of the e- from outer and inner shell is released as characteristic x-ray photons
Is BE unique to given elements/atoms? (Z)
YES- characteristic x-rays!
What is the concept of brems that releases energy?
High energetic e- misses outer shell e- > goes close to nucleus > attraction to nucleus causes KE to slow down > e- must release some energy > energy in form of brem x-rays > continuous
What are white radiations?
Brem x-rays
For brems; a small deflection means ____ wavelength and ____ freq?
Large wavelength and low freq
Brems: when e- loses KE, where does it go?
Energy is in the form of a photon
T or F? Coulomb force varies strongly with distance
VERY TRUE - this is the basis of brems; closer to nucleus, more force
Brems: closer e- comes to nucleus, higher energy emitted
True - decrease distance - increase amount of E lost by incident E - increase photon E (conservation law)
Can an e- hit the nucleus?
YES - but very rare. in this occasion, the MAX x-ray energy is released (Emax)
Is the brems process poly or monoenergetic?
Poly - there are many diff energies being released
X-ray production efficiency is influenced by what 2 main factors?
- kVp (max energy)
- Atomic number
think about it - we need the target and we need the energy to hit it
How is efficiency measured?
Ratio between the radiative energy loss : collisional energy loss
Why are there different energies on the x ray spectrum?
- Random events - K shell or L shell- could be from any
2. Interactions along the way changes KE of each e-
If you increase kVp, low energy x-rays will increase. T or F? Explain
True!!! More e- fired, more interaction, but more heat produced; more interaction between e- (?) - space charge?
Explain why the x-ray spectrum shows there are small amounts of low energy x-rays
Low energy filtered out by window + surroundings
how do you calculate the peak value in an x-ray spectrum
The peak - will be halfway between one third and half of kVp
Characteristic x-rays other than those generated through K-shell transitions are not important in diagnostic imaging
TRUE - we only want K shell!
As the E of the incident e- increases above the threshold E for characteristic x-ray production, the % of characteristic x-rays ↑
TRUE
How is the peak voltage and lowest energy x ray determined? (Brems)
Peak E of x-ray is determined by the kVp - Emax
Lowest E is not easiy determined due to filter and other interactions
Theoretically, how should an x-ray spectrum look?
negative linear graph with characteristic spikes in them
Brems vs. Charac: what is the “loss” called?
Radiation vs Collision loss
Are collision or radiation loss more common?
Collision - characteristic
Brems efficiency is much higher at higher kVp than it is at low
TRUE
How to measure efficiency of x-ray production?
Amount of energy emerging as x-rays vs the amount we put into the e- to hit the target
