Final

Question 1

Properties of X-Rays

Answer 1

1. electromagnetic radiation
2. ionizing radiation
3. travel at constant velocity
4. energy is proportional to their frequency and inversely proportional to their wavelength
5. travel in straight lines, direction may be changed
6. no electric charge, thus unaffected by magnetic or electric fields
7. interact with matter (absorbed or scattered)
8. interaction with certain substances causes fluorescence
9. react with photographic film and produce a visible image
10. penetrate tissue although some of the energy is absorbed
11. can cause (damaging) biological changes following interaction with living tissue

Question 2

X-Ray Generation

Answer 2

• x-rays are produced in the x-ray tube
• Cathode (tungsten filament) is negatively
  charged
• Anode (rotating tungsten disk) is positively
  charged
• Electrons are produced at the cathode (filament) by running an electric current through it -> heating -> thermoionic emission
• Electrons (e- ; negatively charged) leave the
  cathode(-) and travel through the vacuum inside the x-ray tube to the anode (+)
• the amount of current (and thus electrons
  produced at the cathode) is controlled by the mA
• electrons (negatively charged) are accelerated due to a voltage differential between the filament (cathode, negatively charged) and the target (anode, positively charged)
  -> this voltage is controlled by the kVp

Question 3

Radiative interaction

Answer 3

• production of Bremsstrahlung
• > braking radiation
  1. Electron from filament bends around the nucleus and “brakes” as a result of the large difference in electric charge
  2. As the electron slows, it releases energy as an x-ray that forms part of the useful x-ray beam
  3. This electron may produce more useful x-rays through additional interactions

Question 4

Collisional interaction

Answer 4

• Production of characteristic radiation
  1. Electron from filament deposits energy and ejects the electron from the inner shell
• the electron from the filament continues at a lower energy
  2. The Outer shell electron fills the inner shell void and energy deposited by the electron from the filament is released as a characteristic x-ray that is part of the useful x-ray beam
  3. These electrons may participate in other interactions and produce x-rays, but the x-rays are of low energy and will not escape the target

Question 5

X-ray generation IN the x-ray tube

Answer 5

• The quantity of radiation produced is determined
  by the tube current: mA
• The energy of the x-rays produced at the target/
  anode is a function of the energy of the electrons
  striking it and thus a function of the kVp
  -> the potential difference between cathode and
  anode is adjusted with the kilovoltage peak kVp
  -> increasing the kVp increases the potential difference between the filament and target and electrons are accelerated to higher velocities and
  have more energy when striking the target
• only 1-5 % of the kinetic energy of the electrons is
  transformed into x-rays, the rest is converted into
  heat
• x-rays are produced in the x-ray tube

Question 6

Components of the X-ray tube

Answer 6

1. Cathode
   - tungsten filament in a focussing cup made
   from molybdenum
   - Cathode using a filament circuit-tungsten wire
2. Anode
   - tungsten target (focal spot) on
   -> copper rod : stationary anode
   -> molybdenum rod : rotating anode
   - Anode which may be rotating or stationary
3. Glass Tubing with a vacuum
   - evacuated glass envelope
4. Surrounded by oil and a lead lines metal housing
   - Surrounded by a lead shielded metal casing

Question 7

Anode Types

Answer 7

1. Stationary

2. Rotating

Question 8

Cathode/Filament in the X-ray Tube

Answer 8

• The cathode is a coiled wire filament
• A low current (mA) is applied to it and electrons boil off it as it is heated (to 2000’C), so a cloud of electrons hang around the cathode
• The higher the temperature of the filament, the more electrons are produced
• The temperature of the filament is controlled by the mA setting on the X-ray machine

Question 9

Focal Spot

Answer 9

• Location on the target (anode) where x-rays are produced
• Up to 99% of the energy created by electrons hitting the
  target is transformed into heat not x-rays
  -> target must
  have high melting point
• Made of Tungsten: Melting point: 6192°F / 3422°C
• High atomic number: 74
  -> higher positively charged
  nucleus (74 nuclear protons)
  -> more effective braking
  -> more Bremsstrahlung (i.e. x-rays) is produced
• the target rotates, to prevent it from melting (heat dissipation) in x-ray tubes with rotating anodes

Question 10

Effective Focal Spot

Answer 10

• size depends on the filament size and focal spot angle:
  -> Angling the anode/ target allows to make the original (actual) focal spot smaller
  -> Overheating can be reduced and the image made sharper
• Focal spot angle at the same filament size (FS) determines
  effective focal spot size
  -> larger angle = larger effective focal spot on the anode
  -> smaller angle = smaller effective focal spot

Question 11

Focal Spot Size Affect on Image Resolution

Answer 11

• Focal spot size affects image resolution (the sharpness
  of the image )
• small focal spot means sharper/more detailed image
• large focal spot means less sharp image/penumbra (Edge
  blurring)

Question 12

Electrical Supply of the X-ray Tube

Answer 12

• heating the tungsten filament to produce the electrons
  using electric current (mA)
• accelerating the electrons from the cathode to the anode
  applying a voltage (kVp)

Question 13

Components of the Electrical Supply of the X-ray Tube

Answer 13

1. a power supply to the tungsten filament
2. A high voltage transformer
3. An auto-transformer
4. A rectifier circuit

Question 14

There are a few problems: The mains electrical supply is not appropriate for the x-ray machine:

Answer 14

1. (mains) mA current for the filament is too high:
   - The Step down transformer
   reduces the incoming current Ampères (A) to milliAmpères (mA)
2. (mains) kV voltage is too low:
   - The Step up transformer increases voltage from volts to Kilovolts (kV)
3. Level of the incoming current fluctuates
   - The Autotransformer enables mains electricity voltage compensation:
   -> provides constant voltage to the filament circuit and the primary voltage to the step
   up transformer
4. Alternating current (ACDC)
   Rectification
5. Amount of heat that is generated
   - Rotate the anode/disc shaped anode
   - Use a large actual focal spot and angle the anode to create a small effective focal spot

Question 15

Rectification of alternating current

Answer 15

• A generator is needed to increase the voltage
  and rectify the current
  -> make it flow only in one direction
• normal current is alternating (AC) and depending on the location at a voltage of 110
  or 220V
• radiography needs direct current (DC) at higher voltage (ca. 440V)
• rectifying is necessary to keep the target positive and determine the electron flow
  from – to + ( with alternating current, the flow direction of electrons would change in
  50% of every cycle,
  -> electrons would hit
  the cathode and damage the filament)
• Step-up transformer increases the voltage

Question 16

Rotating the disk- shaped Anode angle

Answer 16

- Rotating the anode
helps to dissipate the
heat and thus conserve
the anode 
- a beveled surface of the ring-like target on the rotating anode disk allows transforming a large area that is hit by the incoming
electrons to create a small
effective focal spot (-> sharper
image, less penumbra) an
minimize effect of the heat
created in the target

Question 17

X-ray machine controls

Answer 17

1. kV selection
2. mAs selection
3. Exposure button
   - 2 stages (‘prep’ and ‘take’) of advancing the button
4. Timer
   - Electronic
5. Light Beam Diaphragm
6. Support for an x-ray machine

Question 18

kV an mAs

Answer 18

• kVp = Maximum energy of X- rays
• mAs = Number of X-rays
• Machine specific differences:
  1. kV/mA linked
  2. mA and time linked (mAs)
  3. kV, mA and time all separate

Question 19

mAs

Answer 19

• mAs = mA x time (s)
• This is the number or quantity of X-rays
• A certain mAs, 5mAs, can be made up of various combinations of mA and seconds
  -> consider that time may be critical when radiographing
  structures like the thorax -> movement-> motion unsharpness/ blur
• to minimize motion/blur, keep the time low

Question 20

X-Ray exposure determined by kVp and mAs

Answer 20

• These factors are operator controlled
• combinations of high / low kVp and mAs determine image quality (image contrast)
• These factors should be determined by a Technique
  Chart which is specific to the x-ray machine and imaging system used and thus varies somewhat between different set- ups / clinics

Question 21

The light beam diaphragm / Collimator

Answer 21

• an x-ray beam restriction device

- Serves to reduce the area on the patient (and around it) to be irradiated/ exposed to the area of interest

Question 22

Collimation

Answer 22

• Lead shutters that restrict the x-ray beam and reduce the volume of tissue exposed, so less scattered radiation is created in the patient
  -> improved image quality
  -> improved radiation safety
• light beam diaphragm = collimator
  -> permits accurate collimation, or delineation of the area
  of the patient that is irradiated
• works best with a dimmed main light