Download X-Ray Production

CT
X-Ray
Production
George David
Associate Professor
The Atomic Nucleus
 Protons
+ Charges
# protons = atomic # (Z)
+
+
+
 Neutrons
~
 No charge
~
~
 Mass about the same as proton
Atomic Weight(mass)= # protons + # neutrons
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
-
~ +
~ +
+ ~
-
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
L
K
~ +
~ +
+ ~
-
-
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
~ +
~ +
+ ~
-
-
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
K
~ +
~ +
+ ~
-
-
-
Requires
energy
input!
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
L
K
~ +
~ +
+ ~
Energy
released -
-
Output X-Ray Beam
Producing X-Rays
 Electrons emitted by filament
 Electrons slam into target
 Reminder
 Electrons carry their energy as kinetic energy
 Energy of motion
+
Requirements to Produce X-Rays
 Filament Voltage
 High Voltage
anode
+
high
voltage
source
filament
filament
voltage
source
X-Ray Production(cont.)
 X-Rays are produced in
the x-ray tube by two
distinct processes
 Characteristic
radiation
 Bremsstrahlung
Output Beam Spectrum
 Output photon beam made up of
 Characteristic Radiation
 characteristic of target material
 several discrete energies

#
Energy
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
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 cascade
down to fill vacancies
L
K
-
 #3: characteristic x-ray emitted
+
~
+
~
#1
+
-
#2
~
-
#3
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
#
Energy
L
K
~ +
~ +
+ ~
-
-
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
L
K
~ +
~ +
+ ~
-
-
Bremsstrahlung (cont.)
 Bremsstrahlung means braking radiation
 Moving electrons have many Bremsstrahlung
reactions

small amount of energy lost with each
L
K
~ +
~ +
+ ~
-
-
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 don’t escape tube

easily filtered by tube enclosures or added filtration
#
Energy
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
Intensity & Technique
 beam intensity proportional to mA
 beam Intensity ~ proportional to kVp2
+
high
voltage
source
filament
voltage
source
keV = kilo-electron volt
 energy of an electron
 Kinetic energy
 Higher energy electron moves
faster
 Electrons can be manipulated by
electric fields
 Accelerated
 Steered
+
*
kVp = kilovolts peak
 peak kilovoltage applied across x-
ray tube
kVp
Time
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
mA
 mA affects only quantity of x-rays
 does not affect


quality
spectrum
#
-------- Higher mA
Energy
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
kVp
77
mA
200
time
.040
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
+
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
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
Typical Single-Exposure Tube
Rating Chart
• shows maximum exposure time for
single exposure at given kV & mA
Example
• What is the maximum exposure time at
90 kVp & 300 mA?
Example
• What is the maximum exposure time at
120 kVp & 400 mA?
Can’t do 120 kVp at 400 mA
for any exposure time.
?
Tube Anode Damage
 Single exposure heat capacity exceeded
 melted spots on anode
Anode Thermal Characteristics Chart
 2 charts in one
 cooling curve in
absence of
heating
 anode heating

for continuous
heat input
(Spiral CT)
Cooling
 Start on cooling
curve with current
heat units
 100,000 heat units in this
example
 Cool for
2 minutes
x
x
2 minutes
The Many Ways Tubes Die
Warning
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
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
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
Oil Leaks
 May be accompanied by air bubble in housing
 Eventually causes high voltage arcing
 Requires immediate service attention