1/18/17 Lecture #2 How Xrays are produced Flashcards
What seven items make up the tube head of an X-Ray machine?
- Metal housing
- Power supply (transformer)
- X-ray tube
- Oil
- Collimator (lead lined)
- Aluminum filters
- Aiming cylinder (lead lined)
What is Bremsstrahlung X-rays?
-“Braking Radiation”
If the incident electron runs directly into the nucleus what type of energy is produced?
-Maximum energy
If the incident electron goes past the nucleus at a distance and does a slight turn what type of energy is produced?
-Low energy
If the incident electron goes closely past the nucleus and does a sharp turn what type of energy is produced?
-Moderate energy
If you have a 70kVP tube head what is the maximum energy x-ray that you can produce?
70 keV
What is characteristic radiation?
-When a high speed electron collides with an orbiting inner ring electron ejecting it from the orbit.
What happens when an electron is bumped out from an inner ring?
-An electron from the next energy ring jumps to fill the void
When an electron jumps from an energy ring down to the next one what occurs?
-A photon of energy equal to the difference in binding energy between the rings is released
Why is it called characteristic radiation?
-It is the energy given up that is “characteristic” for that target atom
What kV number corresponds with characteristic radiation from Tungsten?
59 keV
An incident electron directly striking a tungsten nucleus produces what?
-Maximum energy Bremsstrahlung radiation
Cathode or Anode: A tungsten filament and molybdenum focusing cup provide the source and direction of electrons of the cathode ray.
Cathode
Cathode or Anode: The tungsten target and copper stem convert the kinetic energy of the electron stream of photons to x-rays and heat
Anode
Is the anode or cathode (+) charged in an X-ray rube?
-Anode
Why do you use a vacuum tube in an x-ray tube?
- So there are no gas molecules for electrons to collide with
- No oxidation or “burnout” of filament
Why is the molybdenum focusing place in a cup shape?
-It electrostatically focuses the electrons into a narrow beam
What are four important characteristics of the target in an x-ray tube?
- High atomic number
- high melting point
- High thermal conductivity
- Low vapor pressure
What is the point on the target where the stream of x-ray photons originate in an x-ray tube?
-Focal spot
Why do you place the target at an angle?
-Provides a smaller effective focal spot for sharp images while maintaining a larger actual focal spot for heat dissipation
A molybdenum focusing cup in an x-ray tube:
A) narrows the beam of electrons
B) Is located in the cathode
C) Directs electrons toward the tungsten target
D) all of the above
D) All of the above
What power supply heats the tube filament and generates a stream of electrons between the cathode and anode?
-Low voltage tube current
What power supply is between the cathode and anode and accelerates the electron stream to a level that x-rays can be produced?
-High tube voltage
How many Volts are needed to heal of the filament?
3-5 Volts
What type of transformer reduced the incoming 110V to the approximate 3-5 V needed to heat the filament?
-Step-down filament transformer
What type of transformer provides large electrical force needed to propel electrons from cathode to anode and increases the voltage from 110 V to 60-100 kV?
-Step up transformer
What type of transformer is controlled by the kVp dial on the control panel, this transformer varies the voltage to the step-up transformer to achieve the desired high energy kVp to the tube?
-Auto transformer
What is the most frequent source of malfunction in the X-ray tube?
-Filament burnout
Should the X-ray machine be left On all day or turned on and off after each use?
-Left on to prevent burnout
If you wish to increase the energy of the emitted x-ray beam, you would:
A) increase power to the high voltage step up transformer
B) Increase power to the low voltage tube current
C) increase the size of the focal spot
D) all of the above
A) increase power to the high voltage step up transformer
What does changing the 3-5 V (milli amps) on the step down transformer do?
-Change the number of x-rays emitted in other words the quantity (changes the electron cloud)
What are two ways to change the quantity of radiation?
- The step down transformer
- Exposure time
What is the quantity of radiation produced directly proportional to?
-The tube current
As the tube current (mA) is increased _______ are released to collide with the target?
- More electrons
T/F
More collisions = more radiation
True
T/F
Increase in kVp increases the x-ray penetration of matter
True
What does filtration do?
-Absorbs low energy photons that would not reach the image receptor
What types of filters are used to absorb the low energy photons?
-Aluminum filters
What is a collimator?
-A metal barrier used to reduce the size of the x-ray beam and reduce some of the scatter radiation
T/F
The intensity of the x-ray beam is inversely proportional to the square of the distance between the source and the target.
True
T/F
The distance between the x-ray tube and the patient has no effect on exposure.
False
-The distance has a marked effect on exposure
You expose an x-ray and it comes out underexposed. How do you try to correct the exposure on a retake?
A) move tubehead back to increase focal distance
B) increase kVp on the control panel
C) increase exposure area by using a round collimator
D) increase exposure time
B and D
If you go from 1 foot to 2 feet away from the object being exposed what happens to your exposure?
-It goes from 1 at 1 foot to 1/4 at 2 feet
If you have an underexposed x-ray is it dark or light?
-Light
What does turning up the kVp do?
-Increases the energy
What is photoelectric absorption?
-Incident photons are absorbed by an atom by interacting with inner shell electrons
What does photoelectric absorption depend on?
-Density and thickness of the object
If you have something that is high density what happens to the x-rays and how does that appear on a film?
-The x-rays are absorbed giving a light appearance on the radiograph
What occurs when low energy photons are deflected by outer ring electrons and cause the photon to bend?
Coherent Scattering
What type of scattering occurs when a photon interacts by colliding with an outer orbital electron?
-Compton scattering
Which type of scattering is responsible for more interactions in dental radiography?
-Compton scattering
What do scattered photons do?
-Darken and degrade the image while giving no useful information
What is Beam attenuation?
-Lower energy photons are more readily absorbed as they travel through matter
T/F As the energy of the photon beam increases, so does the transmission of the beam through the absorber.
True
Which of the following photon interactions make it possible to produce radiographic images? A) coherent scattering B) photoelectric absorption C) compton scattering D) all the above
B) photoelectric absorption
