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Principles of Imaging Science I (RAD119)

X-ray Production & Emission

X-ray Production

• X-rays are produced inside the x-ray tube when high energy projectile electrons from the filament interact with the atoms of the anode

• Conditions necessary: – Source of electrons

– Target (anode)

– High potential difference

– Sudden deceleration of projectile electrons

Target Interactions • All occur within 0.25 to 0.5 mm of target

surface

– Heat production

– Bremsstrahlung interactions

• Braking or slowed-down

– Characteristic interaction

• Target material

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Heat Production

• 99.8 percent of incident electrons’ kinetic energy converted to heat

• Incident electrons transfer kinetic energy to outer shell electrons of target atoms

– Causes them to emit infrared radiation

• Heat (heat units = joules)

X-ray Production

Most of the kinetic energy of projectile electrons is

converted to heat by interactions with outer-shell electrons

of target atoms. These interactions are primarily excitations

rather than ionizations

Bremsstrahlung Radiation

• Projectile electron enters an atom in the metal of the anode and does not strike any of the electrons

• It may continue toward the center of the atom and come near the nucleus due to the electrostatic attraction. This attraction slows the electron down as it passes the nucleus and alters the direction of the projectile electron.

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Bremsstrahlung

• As incident electrons get closer to nucleus, the following occurs:

– Photon energy increases

• Photon energy dependent on how close electron comes to nucleus

• Due to larger deflection of incident electron

Bremsstrahlung

• Direct interaction between nucleus and incident electron

– Possible, but not probable

– Maximum energy photon

Bremsstrahlung Radiation

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Video

Characteristic Radiation • Characteristic of the target element

• Projectile electron strikes an atom and knocks a K shell electron out of its orbit. This leaves the atom in an unstable state

– Tube potential must be at least 70 kVp to eject a tungsten K shell electron

• K shell binding energy is 69 keV

– Only electron that drops into K-shell will contribute to beam

Characteristic Radiation

• An electron from a higher orbit moves down to the “hole”

– Called a K characteristic x-ray

• X-ray photons are produced when the electron changes orbital shells.

– Cascade effect

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Characteristic Radiation

• Characteristic x-rays are produced after the ionization of a K-shell electron. When an outer-shell electron fills the vacancy in the K shell, an x-ray is emitted.

The energy is calculated by the difference between the electron orbits

Video

Characteristic Radiation – Calculate the the

energy of a K characteristic photon with the transition from the L shell. M shell?

– Calculate the energy of an L characteristic photon with the transition from the M shell. N shell?

Graphed as discrete

spectrum (measurable)

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X-ray Production (Mostly Brems) • First, only a high-energy projectile

electron has enough energy to knock a K-shell electron out of its orbit to produce a characteristic x-ray. – At kVp settings lower than 70, all brems. .

• Second, the projectile electron is more likely to miss the K-shell electron of the target atom than it is to hit it due to atom’s open space

Off-Focus Radiation Effect on Image

Emission Spectrum

• General form of an x-ray emission spectrum.

– Characteristic radiation

– Bremsstrahlung radiation

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Emission Spectrum • Graphical representation of characteristic (discrete) and bremsstrahlung

radiation (continuous)

– Y axis = x-ray quantity

• Height of the curve or bar graph

• Change in amplitude = change in quantity

– X axis = x-ray quality (keV)

• Shown on horizontal axis

• Change in position horizontal axis = change in quality

Emission Spectrum: Brems Radiation

– Tube potential based on manufacturer specs

• kVp range <70 - 120+

– Graphed as continuous spectrum (wide range of energies) • Selected kVp will determine maximum keV possible

for any photon

• Minimum kev could be just above zero

Factors Affecting the Emission Spectrum • Milliamperage (mA)

– Quantity, number of photons

– Amplitude of continuous and discrete spectra are affected

– No change in position

• Milliamperage-seconds (mAs)

– mA X time

Change in mA results in

a proportionate change

in the amplitude of the x-

ray emission spectrum at

all energies.

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Factors Affecting the Emission Spectrum

• Kilovoltage (kVp)

– Quality, penetrability

– Amplitude and position of continuous spectrum are affected

– Amplitude of discrete spectrum is affected

Change in kVp results in an increase in the

amplitude of the emission spectrum at all

energies, but a greater increase at high

energies than at low energies. Therefore,

the spectrum is shifted to the right or high-

energy side.

Emission Spectrum

Factors Affecting the Emission Spectrum

• Anode atomic number

– Slight change in Amplitude of continuous spectrum

– Amplitude and position of discrete spectrum is affected Discrete emission spectrum shifts

to the right with an increase in the

atomic number of the target

material. The continuous spectrum

increases slightly in amplitude,

particularly to the high-energy side,

with an increase in target atomic

number.

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Factors Affecting the Emission Spectrum

• Voltage Waveform

– Amplitude and position of continuous spectrum is affected

– Amplitude of discrete spectrum is affected

Three-phase and high-frequency

operation are considerably more efficient

than single-phase operation. Both the x-

ray intensity (area under the curve) and

the effective energy (relative shift to the

right) are increased. Shown are

representative spectra for 92-kVp

operations.

Filtration

• Process of eliminating undesirable low-energy x-ray photons by insertion of absorbing materials into primary beam

• Allows radiographer to shape emission spectrum

Factors Affecting the Emission Spectrum

• Filtration – Inherent

• Window of x-ray tube

• O.5 mm Al equivalent

– Added

• Aluminum added between tube housing and collimator

• 1.0 mm Al equivalent

– Total Filtration = Inherent + Added

• 2.5 mm Al equivalent

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Filtration

Filtration

• “Hardening” of beam

– Removes low energy “soft” photons

– Increases average beam energy

• Soft tissue penetration requires approximately 30-40 kiloelectronvolt (keV) photons

Filtration

• Low energy photons cannot penetrate the part

– Only contribute to patient dose

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Factors Affecting the Emission Spectrum

• Purpose of added filtration is to remove low energy, long wavelength photons

– Amplitude and position of continuous spectrum is affected

– Amplitude of discrete spectrum is affected

Adding filtration to an x-ray

tube results in reduced x-ray

intensity but increased

effective energy. The emission

spectra represented here

resulted from operation at the

same mA and kVp but with

different filtration.

Measurement

• Aluminum

– Standard filtering material

– Filtration expressed as Al/Eq

• Half-value layer (HVL)

– Filtration needed to reduce beam to one half of its original intensity

Types of Filtration

• Inherent filtration

• Added filtration

• Compound filtration

• Compensating filtration

• Total filtration

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Filtration Types • Inherent

– 0.5 mm Al equivalent • X-ray tube design

• Added – 1.0 mm Al equivalent

– Any filtration outside x-ray tube and housing • Silver on collimator mirror

• Thin layers of aluminum or copper permanently added between the collimator and protective housing

• Filters may be changed

Inherent Filtration

• Glass or metal envelope

• Dielectric oil bath

• Glass window of housing

Inherent Filtration

• Tube aging increases inherent filtration

– Vaporized tungsten coats tube window

– HVL testing important

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Total Filtration = Inherent + Added Filtration

Does not take into account any compound or compensating filtration

Effect on Tube Output

• Ideally, filtration would only remove low-energy photons

• Some high energy photons are removed

• Results in decrease in radiographic density that must be compensated for with increase in technique

Compound Filtration

• K-edge filters

– Two or more materials

– Each layer absorbs characteristic photons created in previous layer

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Compensation Filtration

• Evens radiographic density with parts that have uneven tissue thickness or densities

– E.g., wedge for foot or T-spine, trough for CXR

Compensating Filters

Compensation Filtration Applications