CT

1
CT

• X-Ray Production

George David Associate Professor
2
The Atomic Nucleus

• Protons
• Charges
• protons atomic (Z)
• Neutrons
• No charge
• Mass about the same as proton
• Atomic Weight(mass) protons neutrons

3
Orbital Electrons

• Electrons
• - charges
• very small mass compared with protons / neutrons
• Electrons reside only at certain energy levels or
  Shells
• Designations start at K shell
• K shell closest to nucleus
• L shell next closest
• Shells proceed up from K, L, M, N, etc.
• Except for K shell, all shells contain sub-shells

L
K
-






-
-
4
Binding Energy

• energy required to remove orbital electron from
  atom
• Negative electrons attracted to positive nucleus
• more binding energy for shells closer to nucleus
• K shell has highest binding force
• higher atomic materials (higher Z) result in
  more binding energy
• more positive charge in nucleus

5
Electron Shells (cont.)

• Electrons can only reside in a shell
• electron has exactly the energy associated with
  its shell
• electrons attempt to reside in lowest available
  energy shell

L
K
-






-
-
-
6
The Shell Game

• Electrons can move from shell to shell
• to move to higher energy shell requires energy
  input equal to difference between shells

L
Requiresenergyinput!
K
-


-




-
-
7
The Shell Game (cont.)

• to move to a lower energy shell requires the
  release of energy equal to the difference
  between shells
• characteristic x-rays

-
-
8
Output X-Ray Beam
9
Producing X-Rays

• Electrons emitted by filament
• Electrons slam into target
• Reminder
• Electrons carry their energy as kinetic energy
• Energy of motion

10
Requirements to Produce X-Rays

• Filament Voltage
• High Voltage

filament
anode
filament voltage source

high voltage source
11
X-Ray Production(cont.)

• X-Rays are produced in the x-ray tube by two
  distinct processes
• Characteristic radiation
• Bremsstrahlung

12
Output Beam Spectrum

• Output photon beam made up of
• Characteristic Radiation
• characteristic of target material
• several discrete energies
• Bremsstrahlung
• continuous range of energies
• 0 - kVp setting
• most photons have low energy
• Spectrum
• depicts fraction of beam at each energy value
• combination of Bremsstrahlung and characteristic
  radiation

Energy
13
Characteristic Radiation

• Interaction of high speed incident electron with
  orbital electron of target
• 1 orbital electron removed from atom
• 2 electrons from higher energy shells
  cascadedown to fill vacancies
• 3 characteristic x-ray emitted

L
K
-






1
-
-
-
2
-
3
14
Characteristic Radiation

• Consists only of discrete x-ray energies
  corresponding to energy difference between
  electron shells of target
• Specific energies are characteristic of target
  material
• for tungsten 59 keV corresponds to the difference
  in energy between K and L shells

15
Bremsstrahlung

• interaction of moving electron with nucleus of
  target atoms
• Positive nucleus causes moving electron to change
  speed / direction
• Kinetic energy lost
• Emitted in form of Bremsstrahlung x-ray

-
16
Bremsstrahlung (cont.)

• Bremsstrahlung means braking radiation
• Moving electrons have many Bremsstrahlung
  reactions
• small amount of energy lost with each

17
Bremsstrahlung (cont.)

• Energy lost by moving electron is random
  depends on
• distance from nucleus
• charge (Z) of nucleus
• Bremsstrahlung Energy Spectrum
• 0 - peak kilovoltage (kVp) applied to x-ray tube
• most x-ray photons low energy
• lowest energy photons dont escape tube
• easily filtered by tube enclosures or added
  filtration

Energy
18
Beam Intensity

• Product of
• photons in beam
• energy per photon
• Units
• Roentgens (R) per unit time
• Measure of ionization rate of air
• Depends on
• kVp
• mA
• target material
• filtration

19
Intensity Technique

• beam intensity proportional to mA
• beam Intensity proportional to kVp2

filament voltage source

high voltage source
20
keV kilo-electron volt

• energy of an electron
• Kinetic energy
• Higher energy electron moves faster
• Electrons can be manipulated by electric fields
• Accelerated
• Steered

21
kVp kilovolts peak

• peak kilovoltage applied across x-ray tube

kVp
22
kVp

• kVp corresponds to maximum photon energy in beam
  spectrum
• kVp affects quality (spectrum) quantity of
  x-rays produced
• energy spectrum changed
• subject contrast changes
• higher kVp reduces subject contrast

-------- Higher kVp
Energy
23
mA

• mA affects only quantity of x-rays
• does not affect
• quality
• spectrum

-------- Higher mA
Energy
24
X-Ray Technique

• Kilovoltage peak (kV or kVp)
• maximum high voltage applied between cathode
  anode
• Exposure time
• Length of time high voltage is applied
• Milliamps (mA)
• see following slides

time
mA
kVp
77
200
.040
Joe Blow X-Ray Company
25
Tube Current (mA)

• rate of electron flow from filament to target
• Measured in milliamperes (mA)
• mA controlled primarily by filament voltage
• increasing filament voltage / current results in
  increased
• filament temperature
• emission of electrons

26
Tube Rating Chart

• Indicates load limit for tube
• Maximum time for given kVp mA
• Maximum time depends upon
• Rate at which heat generated
• kVp
• mA
• Speed of rotating anode
• Anode characteristics
• See next slide

27
Anode Characteristics

• Tube heat ratings depend upon surface area of
  tungsten bombarded by electrons
• focal spot size (apparent)
• target angle
• anode diameter

Actual FS
Apparent FS
28
Typical Single-Exposure Tube Rating Chart

• shows maximum exposure time for single exposure
  at given kV mA

29
Example

• What is the maximum exposure time at 90 kVp 300
  mA?

30
Example

• What is the maximum exposure time at 120 kVp
  400 mA?

Cant do 120 kVp at 400 mA for any exposure time.
?
31
Tube Anode Damage

• Single exposure heat capacity exceeded
• melted spots on anode

32
Anode Thermal Characteristics Chart

• 2 charts in one
• cooling curve in absence of heating
• anode heating
• for continuous heat input (Spiral CT)

33
Cooling

• Start on cooling curve with current heat units
• 100,000 heat units in this example
• Cool for2 minutes

x
x
2 minutes
34
The Many Ways Tubes Die
Warning
35
Tube Anode Damage

• thermal shock (high mA on cold anode)
• can cause in cracks in anode (tube death)
• Tube warm-up
• eliminates thermal shock from high mA exposures
  on cold anode
• warm-up needed whenever tube cold
• once in the morning not sufficient if tube not
  used for several hours

36
High Voltage Arcs

• electrons move from filament to tube housing
  instead of to anode
• can be caused by filament evaporation
• deposition of filament on glass envelope as
  result of high filament currents or filament
  boost time
• very short exposure with instantaneously very
  high mA
• Generator often drops off line

arcing
37
Tube Insert Damage

• Bearing Damage
• prevents proper rotation of anode
• Anode can
• run too slow
• stop
• results in thermal damage to anode (melted spots)
  
• Anode not running at design speed
• Filament breaks
• renders one focal spot completely inoperative

38
Oil Leaks

• May be accompanied by air bubble in housing
• Eventually causes high voltage arcing
• Requires immediate service attention