Lecture 2 - Radiation Flashcards
Why is the tube head filled with oil?
Heat transfer from the photon
An incident electron directly striking a tungsten nucleus produces what?
Maximum energy Bremsstrahlung radiation
When the incident electron passes near the atom, but doesn’t hit it, what occurs?
Low energy Bremsstrahlung radiation
When an electron passes close to the nucleus and bend around it, when is produced?
Moderate energy Bremsstrahlung radiation (x-ray photon)
*The electron decelerates and bends, when this occurs, energy is given off
What element is the target for electrons?
Tungsten
Bremsstrahlung Radiation:
High energy
Moderate energy
Low energy
High - Some electrons directly collide with the tungsten nucleus - produces a high energy photon equal in energy to the incident electron
Moderate - Electrons pass close to nucleus - Decelerates and bends electron - Moderate energy photon given off
Low - Electrons pass at a distance from nucleus - deflected and slowed - Low energy photon given off
A 70 kVp tube head will produce x-rays with a max energy of how much?
70 keV of energy
What is characteristic radiation?
Incident electron hits another electron, and kicks it out of its orbital. This creates a void in the orbital, and a lot of energy is released in the form of photons
T/F - Characteristic radiation accounts for only a small fraction of x-ray photons generated.
TRUE
The tungsten characteristic X-ray is found at what photon energy?
59 keV
The cathode is __________
The anode is ___________
Negative
Positive
*This is opposite of common sense
What is the cathode made of?
How does it work?
Tungsten filament and molybdenum focusing cup.
Low voltage heats the filament, and this produces a cloud of electrons. The molybdenum cup focuses the electrons into a narrow beam
What is the anode made of?
How does it work?
Tungsten target and copper stem
Electron stream turned into x-rays and heat
Why is the X-ray tube in a vacuum?
It helps prevent burnout of the filament, also no oxidation
What is the element used as the typical target on the anode?
Tungsten
What is the focal spot?
The point on the target where the stream of X-ray photons originate.
What does the low voltage tube current do?
Heats the tube filament - this generates a stream of electrons b/t the cathode and anode
What does the high tube voltage current do?
It accelerates the electron stream from the cathode to the anode to a level that X-rays can be produced (80,000 volts)
What does the step-down filament transformer do?
Reduces incoming voltage (110-120V) to approximately 3-5 volts needed to heat the filament
What does the step-up transformer do?
Provides large electrical force needed to propel electrons from cathode to anode (from 110-120 V to 60,000 to 100,000 V)
What is the auto transformer and what does it do?
It is controlled by the kVP dial on control panel. It varies the voltage to the step-up transformer to achieve the desired kVp to the tube
What is the most frequent source of malfunction in the X-ray tube?
Filament burnout
*Keep the machine on during the day. More strain occurs when it is repeatedly warmed and cooled by being turned on and off repeatedly
How do you increase the energy of the emitted X-ray beam?
Increase power to the high voltage step up transformer. This increases the acceleration of the electrons from cathode to anode, resulting in an energy increase
Doubling the exposure time _________ the exposure.
Doubles
If tube current increases, what happens to the quantity of X-rays?
Higher quantity
What does the body absorb better (what is more dangerous), high kV or low kV?
LOW kV.
*Counter-intuitive, but the higher the kV, the more the rays pass thru the tissue instead of being absorbed
What is kVp?
Tube voltage
If the potential b/t the anode and cathode increases, then what happens to the energy of each electron when it strikes the target?
Increases
*THIS INCREASES EFFICIENCY OF THE CONVERSION OF ELECTRON ENERGY INTO X-RAY PHOTONS
**INCREASE IN kVp INCREASES X-RAY PENETRATION OF MATTER
What is filtration and what is used?
It absorbs low energy photons that would not reach the image receptor
Aluminum filters absorb these low energy photons
What is a collimator?
Metal barrier used to reduce the size of X-ray beam
*Lined with lead to absorb stray photons
**Reduces scatter radiation and the patient surface exposed
The intensity of the X-ray beam is __________ proportional to the square of the distance b/t the source and the target.
Inversely
*If I’m 1 foot away, my exposure is 1. If I’m 2 feet away, my exposure is 1/4. If I’m 3 feet away, my exposure is 1/9.
A radiograph come out underexposed. How do you try to correct the exposure on a retake? (2 ways)
Increase kVp on control panel
Increase exposure time
What is photoelectric absorption?
Incident photons are absorbed by an atom by interacting with inner shell electrons.
This produces the lighter areas on the film, making radio graphic imaging possible
Black on a radiograph means space because all the rays go direct to the film
Photoelectric absorption depends on the _______ and _______ thickness of the object, which creates the contrast b/t different tissues.
Density
Thickness
What is coherent scattering?
Low energy photons are deflected by OUTER ring electrons
*Only about 7% of interaction for dental X-rays, and contributes very little to film fog
What is Compton scattering?
Photon interacts with OUTER orbital electron
*Accounts for about half of interactions in dental X-rays
**These darken and degrade the image while carrying no useful information
What is beam attenuation?
Lower energy photons are more readily absorbed, while an increase in energy results in an increase of transmission of the beam thru the absorbing tissue
