Generation of X-Rays

Robert Metzger, Ph.D.

Outline Production of X-rays Production of X-rays X-ray Tubes X-ray Tubes X-ray Tube Insert, Housing, Filtration and Collimation X-ray Tube Insert, Housing, Filtration and Collimation X-ray Generator Function and Components X-ray Generator Function and Components X-ray Generator Circuit Designs X-ray Generator Circuit Designs Making Correct X-ray Exposures in Radiography Making Correct X-ray Exposures in Radiography Factors Affecting X-ray Emission Factors Affecting X-ray Emission Power ratings and Heat Loading Power ratings and Heat Loading X-ray Exposure Rating ChartsX-ray Exposure Rating Charts

Production of X-Rays X-rays are produced by the conversion of the kinetic energy X-rays are produced by the conversion of the kinetic energy

(KE) of electrons into electromagnetic (EM) radiation.(KE) of electrons into electromagnetic (EM) radiation.

Bremsstrahlung A large potential difference is applied across the two electrodes A large potential difference is applied across the two electrodes

in an evacuated (usually glass) envelope.in an evacuated (usually glass) envelope. Negatively charged cathode is the source of electrons (eNegatively charged cathode is the source of electrons (e - - ).). Positively charged anode is the target of electrons.Positively charged anode is the target of electrons.

Electrons released from the cathode are accelerated towards Electrons released from the cathode are accelerated towards the anode by the electrical potential difference and attain kinetic the anode by the electrical potential difference and attain kinetic energy.energy.

Bremsstrahlung

About 99% of the KE is converted to heat via collision-like About 99% of the KE is converted to heat via collision-like interactions.interactions.

About 0.5%-1% of the KE is converted into x-rays via strong About 0.5%-1% of the KE is converted into x-rays via strong Coulomb interactions (Bremsstrahlung).Coulomb interactions (Bremsstrahlung).

Occasionally (0.5% of the time), an eOccasionally (0.5% of the time), an e -- comes within the comes within the proximity of a positively charged nucleus in the target proximity of a positively charged nucleus in the target electrode.electrode.

Coulombic forces attract and decelerate the eCoulombic forces attract and decelerate the e --, causing a , causing a significant loss of kinetic energy and a change in the electron’s significant loss of kinetic energy and a change in the electron’s trajectory.trajectory.

An x-ray photon with energy equal to the kinetic energy lost by An x-ray photon with energy equal to the kinetic energy lost by the electron is produced (conservation of energy).the electron is produced (conservation of energy).

Bremsstrahlung

This radiation is termed This radiation is termed bremsstrahlungbremsstrahlung, a German word , a German word meaning “braking radiation”.meaning “braking radiation”.

The impact parameter distance, the closest approach to the The impact parameter distance, the closest approach to the nucleus by the enucleus by the e- - determines the amount of KE loss.determines the amount of KE loss.

The Coulomb force of attraction varies strongly with distance The Coulomb force of attraction varies strongly with distance (( 1/r 1/r22); as the distance ↓, deceleration and KE loss ↑.); as the distance ↓, deceleration and KE loss ↑.

A direct impact of an electron with the target nucleus (the A direct impact of an electron with the target nucleus (the rarest event) results in loss of all of the electron’s kinetic rarest event) results in loss of all of the electron’s kinetic energy and produces the highest energy x-ray.energy and produces the highest energy x-ray.

BremsstrahlungCreates a

polychromatic spectrum

Bremsstrahlung

The probability of an electron’s directly impacting a nucleus is The probability of an electron’s directly impacting a nucleus is extremely low; the atom is mainly empty space and nuclear extremely low; the atom is mainly empty space and nuclear cross-section is small.cross-section is small.

X-rays of low energies are generated in greater abundance.X-rays of low energies are generated in greater abundance. Fewer x-rays are generated with higher energies. The number of Fewer x-rays are generated with higher energies. The number of

higher-energy x-rays decreases approximately linearly with higher-energy x-rays decreases approximately linearly with energy. energy.

The maximum x-ray energy is the maximum energy of the The maximum x-ray energy is the maximum energy of the incident electrons (at kVp).incident electrons (at kVp).

Bremsstrahlung

Eavg ≈ ⅓ - ½ kVp

A graph of the bremsstrahlung spectrum shows the distribution A graph of the bremsstrahlung spectrum shows the distribution of x-ray photons as a function of energy.of x-ray photons as a function of energy. The unflitered bremsstrahlung spectrum shows a ramp-shaped spectrum shows a ramp-shaped relationship between the number and the energy of the x-rays relationship between the number and the energy of the x-rays produced, with the highest x-ray energy determined by the peak produced, with the highest x-ray energy determined by the peak voltage (kVp) applied across the x-ray tubevoltage (kVp) applied across the x-ray tube.

Bremsstrahlung Filtration refers to the removal of x-rays as the beam passes Filtration refers to the removal of x-rays as the beam passes

through a layer of material.through a layer of material. A typical filtered bremsstrahlung spectrum shows that the A typical filtered bremsstrahlung spectrum shows that the

lower-energy x-rays are preferentially absorbed, and the lower-energy x-rays are preferentially absorbed, and the average x-ray energy is typically about one third to one half of average x-ray energy is typically about one third to one half of the highest x-ray energy in the spectrum.the highest x-ray energy in the spectrum.

X-ray production efficiency (intensity) is influenced by the X-ray production efficiency (intensity) is influenced by the target atomic number and kinetic energy of the incident target atomic number and kinetic energy of the incident electrons (which is determined by the accelerating potential electrons (which is determined by the accelerating potential difference).difference).

Characteristic Spectrum Each electron in the target Each electron in the target

atom has a binding energy atom has a binding energy (BE) that depends on the shell (BE) that depends on the shell in which it resides in which it resides

K shell – highest BE, L shell K shell – highest BE, L shell next highest BE and so on next highest BE and so on

When the energy of an When the energy of an electron incident on the target electron incident on the target exceeds the binding energy of exceeds the binding energy of an electron of a target atom, it an electron of a target atom, it is energetically possible for a is energetically possible for a collisional interaction to eject collisional interaction to eject the electron and ionize the the electron and ionize the atomatom

Characteristic x-ray: from L → K e- transition

Characteristic Spectrum The unfilled shell is The unfilled shell is

energetically unstable, and energetically unstable, and an outer shell electron with an outer shell electron with less binding energy will fill less binding energy will fill the vacancy.the vacancy.

As this electron transitions to As this electron transitions to a lower energy state, the a lower energy state, the excess energy can be excess energy can be released as a characteristic released as a characteristic x-ray photon with an energy x-ray photon with an energy equal to the difference equal to the difference between the binding between the binding energies of the electron energies of the electron shells.shells.

Characteristic x-ray: from L → K e- transition

Characteristic Spectrum

Binding energies are unique to a given element. The emitted x-Binding energies are unique to a given element. The emitted x-rays have discrete energies that are characteristic of that rays have discrete energies that are characteristic of that element.element.

The target materials used in x-ray tubes for diagnostic medical The target materials used in x-ray tubes for diagnostic medical imaging include W (Z=74), Mo (Z=42) and Rh (Z=45): BE imaging include W (Z=74), Mo (Z=42) and Rh (Z=45): BE Z Z22..

As the E of the incident eAs the E of the incident e-- increases above the threshold E for increases above the threshold E for characteristic x-ray production, the % of char. x-rays increases characteristic x-ray production, the % of char. x-rays increases (5% at 80 kVp versus 10% at 100 kVp). (5% at 80 kVp versus 10% at 100 kVp).

A variety of energy transitions occur from adjacent (A variety of energy transitions occur from adjacent (αα)and non-)and non-adjacent (adjacent (ββ) e) e-- orbitals (shells) in the atom giving rise to discrete orbitals (shells) in the atom giving rise to discrete energy peaks superimposed on the continuous bremsstrahlung energy peaks superimposed on the continuous bremsstrahlung spectrum.spectrum.

Within each shell (other than the K shell), there are discrete Within each shell (other than the K shell), there are discrete energy subshells, which result in the fine energy splitting of the energy subshells, which result in the fine energy splitting of the characteristic x-rays characteristic x-rays Characteristic x-rays other than those generated by K-shell Characteristic x-rays other than those generated by K-shell transitions are unimportant in diagnostic imaging because they are transitions are unimportant in diagnostic imaging because they are almost entirely attenuated by the x-ray tube window or added almost entirely attenuated by the x-ray tube window or added filtrationfiltration

Characteristic Spectrum

X-ray Tubes

+75 kV-75 kV

X-ray Tube Cathode Source of electrons is Source of electrons is

cathode, which is a helical cathode, which is a helical filament of tungsten wire filament of tungsten wire surrounded by a focusing surrounded by a focusing cup.cup.

Filament circuit - (10V, 7A).Filament circuit - (10V, 7A). Electrical resistance heats Electrical resistance heats

the filament and releases the filament and releases electrons via electrons via thermionic thermionic emission.emission.

Adjustment of the filament Adjustment of the filament current controls the tube current controls the tube current (rate of ecurrent (rate of e-- flow from flow from cathode to anode).cathode to anode).

X-ray Tube Cathode Focusing cup (cathode Focusing cup (cathode

block) block) Shapes the electron Shapes the electron

distribution when it is at distribution when it is at the same voltage as the same voltage as the filament (unbiased) the filament (unbiased)

Width of the focusing Width of the focusing cup slot determines cup slot determines the focal spot width the focal spot width

Filament length Filament length determines the focal determines the focal spot length spot length

Small and large focal Small and large focal spot filamentsspot filaments

X-ray Tube Cathode Focusing cup (cathode Focusing cup (cathode

block) block) Shapes the electron Shapes the electron

distribution when it is distribution when it is at the same voltage at the same voltage as the filament as the filament (unbiased) (unbiased)

Isolation of the Isolation of the focusing cup from the focusing cup from the filament and filament and application of a application of a negative bias voltage negative bias voltage reduced the electron reduced the electron distribution further distribution further (biased).(biased).

Width of the focusing Width of the focusing cup slot determines cup slot determines the focal spot width.the focal spot width.

Space Charge Cloud The filament current The filament current

determines the filament determines the filament temperature and thus the temperature and thus the rate of thermionic rate of thermionic emission emission

When no voltage is When no voltage is applied between the applied between the cathode and anode, an cathode and anode, an electron cloud, also electron cloud, also called a space charge called a space charge cloud, builds around the cloud, builds around the filament filament

Space Charge Cloud

This space charge cloud shields the electric field for tube This space charge cloud shields the electric field for tube voltages of 40 kVp and lower, only some electrons are voltages of 40 kVp and lower, only some electrons are accelerated towards the anode (space charge limited) accelerated towards the anode (space charge limited)

Above 40 kVp, the space charge cloud effect is overcome by Above 40 kVp, the space charge cloud effect is overcome by the voltage applied and tube current is limited only by the the voltage applied and tube current is limited only by the emission of electrons from the filament (emission-limited emission of electrons from the filament (emission-limited operation) operation)

Tube current is 5 to 10 times less than the filament current in Tube current is 5 to 10 times less than the filament current in the emission-limited range the emission-limited range

Anode Configuration Tungsten anode disk Tungsten anode disk

Mo and Rh for Mo and Rh for mammography mammography

Stator and rotor make up the Stator and rotor make up the induction motor induction motor

Rotation speeds Rotation speeds Low: 3,000 – 3,600 rpm Low: 3,000 – 3,600 rpm High: 9,000 – 10,000 rpm High: 9,000 – 10,000 rpm

Molybdenum stem is a poor Molybdenum stem is a poor heat conductor and connects heat conductor and connects the rotor to the anode to the rotor to the anode to reduce heat transfer to the reduce heat transfer to the rotor bearings rotor bearings

Anode cooled through Anode cooled through radiative transmission radiative transmission

Focal track area (spreads heat Focal track area (spreads heat out over larger area than out over larger area than stationary anode configurationstationary anode configuration

Anode Angle/Focal Spot Size The anode angle is defined The anode angle is defined

as the angle of the target as the angle of the target surface with respect to the surface with respect to the central ray in the x-ray field central ray in the x-ray field

Anode angle range: 7° - 20° Anode angle range: 7° - 20° Line focus principle Line focus principle

(foreshortening of the focal (foreshortening of the focal spot length) spot length) The effective focal spot The effective focal spot

size is the length and size is the length and width of the focal spot width of the focal spot projected down the projected down the central ray in the x-ray central ray in the x-ray field field

Effective focal length = Effective focal length = actual focal length ∙ sin(actual focal length ∙ sin() )

Anode Angle/Focal Spot Size

Three major tradeoffs to consider for the choice of anode angle Three major tradeoffs to consider for the choice of anode angle Field coverage and effective focal spot length vary with the Field coverage and effective focal spot length vary with the

anode angle anode angle A smaller anode angle provides a smaller effective focal spot A smaller anode angle provides a smaller effective focal spot

for the same actual focal area for the same actual focal area However, a small anode angle limits the size of the usable x-However, a small anode angle limits the size of the usable x-

ray field owing to cutoff of the beam ray field owing to cutoff of the beam Field coverage is less for short focus-to-detector distancesField coverage is less for short focus-to-detector distances

Heel Effect Reduction of x-ray beam intensity Reduction of x-ray beam intensity

towards the anode side of the x-ray towards the anode side of the x-ray field field

Although x-rays generated Although x-rays generated isotropically isotropically Self-filtration by the anode Self-filtration by the anode More attenuation and More attenuation and

diminished intensity on the diminished intensity on the anode side of the x-ray field anode side of the x-ray field

Can use to advantage, e.g., Can use to advantage, e.g., Cathode over thicker parts Cathode over thicker parts Anode over thinner parts Anode over thinner parts

Less pronounced as source-to-Less pronounced as source-to-image distance (SID) increases, image distance (SID) increases, because the image receptor because the image receptor subtends a smaller beam angle.subtends a smaller beam angle.

X-ray Filtration Filtration is the removal of x-Filtration is the removal of x-

rays as the beam passes rays as the beam passes through a layer of material through a layer of material

Inherent (glass or metal Inherent (glass or metal insert at x-ray tube port) and insert at x-ray tube port) and added filtration (sheets of added filtration (sheets of metal intentionally placed in metal intentionally placed in the beam) the beam)

Added filters absorb low-Added filters absorb low-energy x-rays and reduce energy x-rays and reduce patient dose patient dose

HVL – half value layer (mm HVL – half value layer (mm Al)Al)

X-ray Collimators

Collimators adjust size and Collimators adjust size and shape of x-ray beam shape of x-ray beam

Parallel-opposed lead Parallel-opposed lead shutters shutters

Light field mimics x-ray Light field mimics x-ray field field

Reduces dose to patient Reduces dose to patient and scatter radiation to and scatter radiation to image receptor.image receptor.

Positive beam limitation Positive beam limitation (PBL) – automatic beam (PBL) – automatic beam sizing.sizing.

X-ray Generator Function and Components

The principal function of the The principal function of the x-ray generator is to provide x-ray generator is to provide current at a high voltage to current at a high voltage to the x-ray tube the x-ray tube

Transformers are the Transformers are the principal components of the principal components of the x-ray generators; they x-ray generators; they convert low voltage into high convert low voltage into high voltage through a process voltage through a process called electromagnetic called electromagnetic inductioninduction

X-ray Generator Function and Components

The principal function of the x-The principal function of the x-ray generator is to provide ray generator is to provide current at a high voltage to the current at a high voltage to the x-ray tube x-ray tube

Transformers are the principal Transformers are the principal components of the x-ray components of the x-ray generators; they convert low generators; they convert low voltage into high voltage voltage into high voltage through a process called through a process called electromagnetic inductionelectromagnetic induction

Transformer Relationships

Mutual induction Mutual induction Law of Transformers: Law of Transformers:

VVpp/V/Vss = N = Npp/N/Nss

Step-up transformer: Step-up transformer: NNss > N > Npp

Isolation transformer: Isolation transformer: NNss = N = Npp

Step-down transformer: Step-down transformer: NNss < N < Npp

Power output (IxV) = Power output (IxV) = Power input (IxV) Power input (IxV) VVppIIpp = V = VssIIs s

Autotransformer Autotransformer Autotransformer

It is an iron core It is an iron core wrapped with a single wrapped with a single wire wire

Self induction Self induction Conducting taps allow Conducting taps allow

the input to output turns the input to output turns to vary, resulting in to vary, resulting in small incremental small incremental change between input change between input and output voltages and output voltages

A switching A switching autotransformer allows autotransformer allows a greater range of input a greater range of input to output valuesto output values

X-ray Generator Components

Diodes – either vacuum Diodes – either vacuum tube or solid-state device: tube or solid-state device: ee-- flow in only a single flow in only a single direction (cathode to anode direction (cathode to anode only) only)

High-Voltage power circuit High-Voltage power circuit Low input voltage Low input voltage High output voltage High output voltage Autotransformer allows Autotransformer allows

kVp selection kVp selection Filament circuit Filament circuit

Tube current (mA)Tube current (mA) Timer sets the exposure Timer sets the exposure

duration (S or mS)duration (S or mS) manual exposure or manual exposure or

phototimedphototimed

Operator Console The operator selects the tube potential [the peak kilovoltage The operator selects the tube potential [the peak kilovoltage

(kVp)], the tube current (mA), the exposure time (S) and the (kVp)], the tube current (mA), the exposure time (S) and the focal spot size.focal spot size.

The kVp determines the x-ray beam quality (penetrability), The kVp determines the x-ray beam quality (penetrability), which plays a role in subject contrast.which plays a role in subject contrast.

The x-ray tube current (mA) determines the x-ray flux rate The x-ray tube current (mA) determines the x-ray flux rate (photons per square cm per second) emitted by the x-ray tube (photons per square cm per second) emitted by the x-ray tube at a given kVp.at a given kVp.

mAs = mA x sec (exposure time).mAs = mA x sec (exposure time). Low mA selections allow small focal spot size to be used, and Low mA selections allow small focal spot size to be used, and

higher mA settings require the use of large focal spot size due higher mA settings require the use of large focal spot size due to anode heating concerns.to anode heating concerns.

Single-phase (Half-wave & Full-wave) Rectifier Circuit

Single-Phase Rectifier Circuit

Different Types of Generators

Single-phase Single-phase Uses single-phase input line voltage source (e.g., 220 Uses single-phase input line voltage source (e.g., 220

V at 50 A) V at 50 A) Three-phase Three-phase

Uses three voltage sources, (0, 120 and 240 deg) Uses three voltage sources, (0, 120 and 240 deg) Constant-Potential Constant-Potential

Provides nearly constant voltage to the x-ray tube Provides nearly constant voltage to the x-ray tube High-Frequency Inverter High-Frequency Inverter

State-of-the-art choice State-of-the-art choice High-frequency alternating waveform is used for High-frequency alternating waveform is used for

efficient transformation of low to high voltageefficient transformation of low to high voltage

Voltage Ripple and Root Mean Square Voltage

% voltage ripple = % voltage ripple = (V(Vmax max - V- Vminmin)/ V)/ Vmaxmax ∙ ∙

100% 100% Root-mean-square voltage: Root-mean-square voltage:

(V(Vrmsrms) ) The constant voltage The constant voltage

that would deliver the that would deliver the same power as the time-same power as the time-varying voltage varying voltage waveform waveform

As %VR ↓, the VAs %VR ↓, the Vrmsrms ↑ ↑

Phototimers Although the x-ray exposure technique (mA and exposure time Although the x-ray exposure technique (mA and exposure time

or the mAs) can be manually set, phototimers help provide a or the mAs) can be manually set, phototimers help provide a consistent exposure to the image receptor.consistent exposure to the image receptor.

Ionization chambers produce a current that induces a voltage Ionization chambers produce a current that induces a voltage difference in an electronic circuit.difference in an electronic circuit.

Tech chooses kVp; the x-ray tube current terminated when Tech chooses kVp; the x-ray tube current terminated when this voltage equals a reference voltage.this voltage equals a reference voltage.

Phototimers are set for only a limited number of anatomical Phototimers are set for only a limited number of anatomical views, thus +/- settings.views, thus +/- settings.

Phototimers

Factors Affecting X-ray Emission

Quantity = number of x-rays in Quantity = number of x-rays in beam beam ZZtargettarget ∙ (kVp) ∙ (kVp)22 ∙ mAs ∙ mAs

Quality = penetrability of x-ray Quality = penetrability of x-ray beam and depends on: beam and depends on: kVp kVp generator waveform generator waveform tube filtration tube filtration

Exposure depends on both Exposure depends on both quantity and quality quantity and quality Equal transmitted exposure: Equal transmitted exposure: (kVp(kVp11))

5 5 ∙ mAs∙ mAs11 = (kVp = (kVp22))55 ∙ mAs ∙ mAs22

Generator Power Ratings and X-ray Tube Focal Spots

Power (kW) = 100 kVp ∙ Power (kW) = 100 kVp ∙ AAmaxmax (for a 0.1 second (for a 0.1 second

exposure) exposure) AAmaxmax limited by the focal limited by the focal

spot: ↑ focal spot → spot: ↑ focal spot → ↑ power rating ↑ power rating

Generally range between Generally range between 10 kW to 150 kW 10 kW to 150 kW

Typical focal spots Typical focal spots Radiography: 0.6 and Radiography: 0.6 and

1.2 mm 1.2 mm Mammography: 0.1-0.3 Mammography: 0.1-0.3

mmmm

X-ray Tube Heat Loading Heat Unit (HU) Heat Unit (HU)

HU = kVp ∙ mA ∙ sec ∙ factor.HU = kVp ∙ mA ∙ sec ∙ factor. HU = kVp ∙ mAs ∙ factor.HU = kVp ∙ mAs ∙ factor. factor = 1.00 for single-phase generator.factor = 1.00 for single-phase generator. factor = 1.35 for three-phase and high-frequency factor = 1.35 for three-phase and high-frequency

generatorsgenerators.. factor = 1.40 for constant potential generators.factor = 1.40 for constant potential generators.

Energy (J) = VEnergy (J) = Vrmsrms ∙ mA ∙ sec ∙ mA ∙ sec VVrmsrms = 0.71 ∙ kVp (1 phase), 0.95-0.99 ∙ kVp (3 phase & = 0.71 ∙ kVp (1 phase), 0.95-0.99 ∙ kVp (3 phase &

HF) and 1.0 ∙ kVp (CP).HF) and 1.0 ∙ kVp (CP). Heat input (HU) ≈ 1.4 Heat input (J)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