Generation of X-Rays

Robert Metzger, Ph.D.

 Outline
 Production of X-rays
 X-ray Tubes
 X-ray Tube Insert, Housing, Filtration and Collimation
 X-ray Generator Function and Components
 X-ray Generator Circuit Designs
 Making Correct X-ray Exposures in Radiography
 Factors Affecting X-ray Emission
 Power ratings and Heat Loading
 X-ray Exposure Rating Charts
 Production of X-Rays
 X-rays are produced by the conversion of the kinetic energy
(KE) of electrons into electromagnetic (EM) radiation.
 Bremsstrahlung
 A large potential difference is applied across the two electrodes
in an evacuated (usually glass) envelope.
-
 Negatively charged cathode is the source of electrons (e ).

 Positively charged anode is the target of electrons.

 Electrons released from the cathode are accelerated towards

the anode by the electrical potential difference and attain kinetic
energy.
 Bremsstrahlung
 About 99% of the KE is converted to heat via collision-like
interactions.
 About 0.5%-1% of the KE is converted into x-rays via strong
Coulomb interactions (Bremsstrahlung).
-
 Occasionally (0.5% of the time), an e comes within the
proximity of a positively charged nucleus in the target
electrode.
-
 Coulombic forces attract and decelerate the e , causing a
significant loss of kinetic energy and a change in the
electron’s trajectory.
 An x-ray photon with energy equal to the kinetic energy lost
by the electron is produced (conservation of energy).
 Bremsstrahlung
 This radiation is termed bremsstrahlung, a German word
meaning “braking radiation”.
 The impact parameter distance, the closest approach to the
-
nucleus by the e determines the amount of KE loss.
 The Coulomb force of attraction varies strongly with distance
2
( 1/r ); as the distance ↓, deceleration and KE loss ↑.
 A direct impact of an electron with the target nucleus (the
rarest event) results in loss of all of the electron’s kinetic
energy and produces the highest energy x-ray.
Bremsstrahlung
Creates a
polychromatic
spectrum
 Bremsstrahlung

 The probability of an electron’s directly impacting a nucleus is

extremely low; the atom is mainly empty space and nuclear
cross-section is small.
 X-rays of low energies are generated in greater abundance.
 Fewer x-rays are generated with higher energies. The number of
higher-energy x-rays decreases approximately linearly with
energy.
 The maximum x-ray energy is the maximum energy of the
incident electrons (at kVp).
 Bremsstrahlung

Eavg ≈ ⅓ - ½ kVp

 A graph of the bremsstrahlung spectrum shows the distribution

of x-ray photons as a function of energy.
 The unflitered bremsstrahlung spectrum shows a ramp-shaped
relationship between the number and the energy of the x-rays
produced, with the highest x-ray energy determined by the peak
voltage (kVp) applied across the x-ray tube.
 Bremsstrahlung
 Filtration refers to the removal of x-rays as the beam passes
through a layer of material.
 A typical filtered bremsstrahlung spectrum shows that the
lower-energy x-rays are preferentially absorbed, and the
average x-ray energy is typically about one third to one half of
the highest x-ray energy in the spectrum.
 X-ray production efficiency (intensity) is influenced by the
target atomic number and kinetic energy of the incident
electrons (which is determined by the accelerating potential
difference).
 Characteristic Spectrum
 Each electron in the target
atom has a binding energy
(BE) that depends on the shell
in which it resides
 K shell – highest BE, L shell
next highest BE and so on
 When the energy of an
electron incident on the target
exceeds the binding energy of
an electron of a target atom, it
is energetically possible for a
collisional interaction to eject Characteristic x-ray:
the electron and ionize the from L → K e- transition
atom
 Characteristic Spectrum
 The unfilled shell is
energetically unstable, and
an outer shell electron with
less binding energy will fill
the vacancy.
 As this electron transitions to
a lower energy state, the
excess energy can be
released as a characteristic
x-ray photon with an energy
equal to the difference
between the binding Characteristic x-ray:
energies of the electron from L → K e- transition

shells.
 Characteristic Spectrum

 Binding energies are unique to a given element. The emitted x-

rays have discrete energies that are characteristic of that
element.
 The target materials used in x-ray tubes for diagnostic medical
imaging include W (Z=74), Mo (Z=42) and Rh (Z=45): BE  Z2.
 As the E of the incident e- increases above the threshold E for
characteristic x-ray production, the % of char. x-rays increases
(5% at 80 kVp versus 10% at 100 kVp).
 A variety of energy transitions occur from adjacent (α)and non-
adjacent (β) e- orbitals (shells) in the atom giving rise to discrete
energy peaks superimposed on the continuous bremsstrahlung
spectrum.
 Characteristic Spectrum

 Within each shell (other than the K shell), there are discrete
energy subshells, which result in the fine energy splitting of the
characteristic x-rays
 Characteristic x-rays other than those generated by K-shell
transitions are unimportant in diagnostic imaging because they are
almost entirely attenuated by the x-ray tube window or added
filtration
 X-ray Tubes
-75 kV +75 kV
 X-ray Tube Cathode
 Source of electrons is
cathode, which is a helical
filament of tungsten wire
surrounded by a focusing
cup.
 Filament circuit - (10V, 7A).
 Electrical resistance heats
the filament and releases
electrons via thermionic
emission.
 Adjustment of the filament
current controls the tube
-
current (rate of e flow from
cathode to anode).
 X-ray Tube Cathode
 Focusing cup (cathode
block)
 Shapes the electron
distribution when it is at
the same voltage as
the filament (unbiased)
 Width of the focusing
cup slot determines
the focal spot width
 Filament length
determines the focal
spot length
 Small and large focal
spot filaments
 X-ray Tube Cathode
 Focusing cup (cathode
block)
 Shapes the electron
distribution when it is
at the same voltage
as the filament
(unbiased)
 Isolation of the
focusing cup from the
filament and
application of a
negative bias voltage
reduced the electron
distribution further
(biased).
 Width of the focusing
cup slot determines
the focal spot width.
 Space Charge Cloud
 The filament current
determines the filament
temperature and thus the
rate of thermionic
emission
 When no voltage is
applied between the
cathode and anode, an
electron cloud, also
called a space charge
cloud, builds around the
filament
 Space Charge Cloud

 This space charge cloud shields the electric field for tube
voltages of 40 kVp and lower, only some electrons are
accelerated towards the anode (space charge limited)
 Above 40 kVp, the space charge cloud effect is overcome by
the voltage applied and tube current is limited only by the
emission of electrons from the filament (emission-limited
operation)
 Tube current is 5 to 10 times less than the filament current in
the emission-limited range
 Anode Configuration
 Tungsten anode disk
 Mo and Rh for
mammography
 Stator and rotor make up the
induction motor
 Rotation speeds
 Low: 3,000 – 3,600 rpm

 High: 9,000 – 10,000 rpm

 Molybdenum stem is a poor

heat conductor and connects
the rotor to the anode to
reduce heat transfer to the
rotor bearings
 Anode cooled through
radiative transmission
 Focal track area (spreads heat
out over larger area than
stationary anode configuration
Anode Angle/Focal Spot Size
 The anode angle is defined
as the angle of the target
surface with respect to the
central ray in the x-ray field
 Anode angle range: 7° - 20°
 Line focus principle
(foreshortening of the focal
spot length)
 The effective focal spot
size is the length and
width of the focal spot
projected down the
central ray in the x-ray
field
 Effective focal length =
actual focal length ∙ sin(q)
Anode Angle/Focal Spot Size

 Three major tradeoffs to consider for the choice of anode angle

 Field coverage and effective focal spot length vary with the
anode angle
 A smaller anode angle provides a smaller effective focal spot
for the same actual focal area
 However, a small anode angle limits the size of the usable x-
ray field owing to cutoff of the beam
 Field coverage is less for short focus-to-detector distances
 Heel Effect
 Reduction of x-ray beam intensity
towards the anode side of the x-ray
field
 Although x-rays generated
isotropically
 Self-filtration by the anode

 More attenuation and

diminished intensity on the
anode side of the x-ray field
 Can use to advantage, e.g.,
 Cathode over thicker parts

 Anode over thinner parts

 Less pronounced as source-to-

image distance (SID) increases,
because the image receptor
subtends a smaller beam angle.
 X-ray Filtration
 Filtration is the removal of
x-rays as the beam passes
through a layer of material
 Inherent (glass or metal
insert at x-ray tube port)
and added filtration
(sheets of metal
intentionally placed in the
beam)
 Added filters absorb low-
energy x-rays and reduce
patient dose
 HVL – half value layer
(mm Al)
 X-ray Collimators

 Collimators adjust size and

shape of x-ray beam
 Parallel-opposed lead
shutters
 Light field mimics x-ray
field
 Reduces dose to patient
and scatter radiation to
image receptor.
 Positive beam limitation
(PBL) – automatic beam
sizing.
 X-ray Generator Function and
Components
 The principal function of the
x-ray generator is to provide
current at a high voltage to
the x-ray tube
 Transformers are the
principal components of the
x-ray generators; they
convert low voltage into high
voltage through a process
called electromagnetic
induction
 X-ray Generator Function and
Components
 The principal function of the x-
ray generator is to provide
current at a high voltage to the
x-ray tube
 Transformers are the principal
components of the x-ray
generators; they convert low
voltage into high voltage
through a process called
electromagnetic induction
 Transformer Relationships
 Mutual induction
 Law of Transformers:
 Vp/Vs = Np/Ns

 Step-up transformer:
 Ns > Np

 Isolation transformer:
 Ns = Np

 Step-down transformer:
 Ns < Np

 Power output (IxV) =

Power input (IxV)
 VpIp = VsIs
 Autotransformer
 Autotransformer
 It is an iron core
wrapped with a single
wire
 Self induction

 Conducting taps allow

the input to output turns
to vary, resulting in
small incremental
change between input
and output voltages
 A switching
autotransformer allows
a greater range of input
to output values
 X-ray Generator
Components
 Diodes – either vacuum
tube or solid-state device:
e- flow in only a single
direction (cathode to anode
only)
 High-Voltage power circuit
 Low input voltage

 High output voltage

 Autotransformer allows
kVp selection
 Filament circuit
 Tube current (mA)

 Timer sets the exposure

duration (S or mS)
 manual exposure or
phototimed
 Operator Console
 The operator selects the tube potential [the peak kilovoltage
(kVp)], the tube current (mA), the exposure time (S) and the
focal spot size.
 The kVp determines the x-ray beam quality (penetrability),
which plays a role in subject contrast.
 The x-ray tube current (mA) determines the x-ray flux rate
(photons per square cm per second) emitted by the x-ray tube
at a given kVp.
 mAs = mA x sec (exposure time).
 Low mA selections allow small focal spot size to be used, and
higher mA settings require the use of large focal spot size due
to anode heating concerns.
Single-phase (Half-wave &
Full-wave) Rectifier Circuit
Single-Phase Rectifier Circuit
 Different Types of
Generators
 Single-phase
 Uses single-phase input line voltage source (e.g., 220
V at 50 A)
 Three-phase
 Uses three voltage sources, (0, 120 and 240 deg)

 Constant-Potential
 Provides nearly constant voltage to the x-ray tube

 High-Frequency Inverter
 State-of-the-art choice

 High-frequency alternating waveform is used for

efficient transformation of low to high voltage
 Voltage Ripple and Root
Mean Square Voltage
 % voltage ripple =
(Vmax - Vmin)/ Vmax ∙
100%
 Root-mean-square voltage:
(Vrms)
 The constant voltage
that would deliver the
same power as the time-
varying voltage
waveform
 As %VR ↓, the Vrms ↑
 Phototimers
 Although the x-ray exposure technique (mA and exposure time
or the mAs) can be manually set, phototimers help provide a
consistent exposure to the image receptor.
 Ionization chambers produce a current that induces a voltage
difference in an electronic circuit.
 Tech chooses kVp; the x-ray tube current terminated when this
voltage equals a reference voltage.
 Phototimers are set for only a limited number of anatomical
views, thus +/- settings.
Phototimers
 Factors Affecting X-ray
Emission
 Quantity = number of x-rays in
beam
  Ztarget ∙ (kVp)2 ∙ mAs

 Quality = penetrability of x-ray

beam and depends on:
 kVp

 generator waveform

 tube filtration

 Exposure depends on both

quantity and quality
 Equal transmitted exposure:

 (kVp1) ∙ mAs1 = (kVp2) ∙ mAs2

5 5
 Generator Power Ratings and X-ray
Tube Focal Spots
 Power (kW) = 100 kVp ∙
Amax (for a 0.1 second
exposure)
 Amax limited by the focal
spot: ↑ focal spot →
↑ power rating
 Generally range between
10 kW to 150 kW
 Typical focal spots
 Radiography: 0.6 and
1.2 mm
 Mammography: 0.1-
0.3 mm
 X-ray Tube Heat Loading
 Heat Unit (HU)
 HU = kVp ∙ mA ∙ sec ∙ factor.
 HU = kVp ∙ mAs ∙ factor.
 factor = 1.00 for single-phase generator.
 factor = 1.35 for three-phase and high-frequency
generators.
 factor = 1.40 for constant potential generators.
 Energy (J) = Vrms ∙ mA ∙ sec
 Vrms = 0.71 ∙ kVp (1 phase), 0.95-0.99 ∙ kVp (3 phase &
HF) and 1.0 ∙ kVp (CP).
 Heat input (HU) ≈ 1.4 Heat input (J)
Single-exposure Rating Chart
Single-exposure Rating Chart
Anode Heat Input and Cooling
Chart
Housing Cooling Chart