Unit 1
Creating the Beam
Chapter 6
The X-Ray Tube
Production of X-Rays
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Source of electrons
Target
High-voltage
Vacuum
Tube Components
The Cathode Assembly
• Filament
• Focusing cup
• Associated wiring
The Filament
• Coil of thoriated tungsten
– 0.1: 0.2 mm thick
– 1: 2 mm wide
– 7: 15 mm long
• Filament length and width impact recorded detail
Filament Material
• Tungsten selected due to:
– High melting point
– Difficult to vaporize
• Rhenium and molybdenum
– Also good choices
Dual Focus Arrangements
Thermionic Emission
• Filament is heated
• Causes electrons to be released from filament
Tube Failure
• Tube arcing
– Vaporized tungsten collection on the envelope
• Filament breakage
The Focusing Cup
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Composed of nickel
Low negative potential applied
Compresses the thermionic cloud
Biased focusing cup
Space charge effect
Saturation current
The Focusing Cup
Grid-Biased Tubes
• Precise control of thermionic cloud
• Instead of having the same potential of the filament, Gridbiased tubes can increase their negative charge therefore
making the electron stream more narrow.
• Used predominantly in mammo.
Space charge effect
• As more and more electrons build up in the area around
the filament, their negative charges begin to oppose the
emission of additional electrons.
• There is just not enough room
• Limits x-ray tubes to a maximum of 1,000 to 1,200 mA
range.
Saturation current
• At this point an increase in kVp will not increase the tube
mA.
• It is another filament phenomenon that affects the
efficiency of the x-ray tube.
• The filament amperage curve flattens out when there are
no further thermionic electrons to be driven toward the
anode.
The Anode Assembly
• Three functions
– Target surface
– Conducts high voltage
– Primary thermal conductor
The Anode Assembly
• Components
– Anode
– Stator
– Rotor
Stationary vs. Rotating Anode
Rotating Anode
• Tungsten-rhenium alloy
– High atomic number
– High melting point
– Heat-conducting ability
Anode Layering
• Assists with heat loading
• Backed with molybdenum and/or graphite
Mammographic Equipment
• Molybdenum target material
– Creates needed lower energy photons
Normal Anode Wear
Warm-Up Procedure
• Gradually warms the anode
– Prevents cracking
• Helps maintain the vacuum
• Stress relieved anode
The Target Area
• Portion of anode that electron stream contacts
– Target
– Focus
– Focal point
– Focal spot
– Focal track
• Point source of x-ray photons (SID)
Anode Heat Loading
• Rotating anode
– RPM
– Diameter of disk
• Target material
• Actual vs. effective focal spot
Line Focus Principle
• Used to reduce the effective area of the focal spot. This
permits the best resolution of detail
• Effective focal spot
– Controlled by:
• Actual focal spot (length of filament)
• Target angle
Line Focus Principle
• Angle
– When the target angle is less than 45 degrees, the effective focal
spot is smaller than the actual focal spot.
– The most common diagnostic radiography target angle is 12
degrees.
– Geometry of angle can limit the size of the beam. An 14 x 17 can
use no less than 12 degrees.
Anode Heel Effect
The Stator
• Located outside the envelope
• Bank of electromagnets
• Stator failure
The Rotor
• Copper cylinder connected to anode disk by molybdenum
stem
• Turns when stator is energized
• Ball bearings
– Bearing failure
The Envelope
• Pyrex glass or metal
– 10” long
– 6” central diameter
– 2” peripheral diameter
• The window
• The vacuum
Protective Housing
• Controls leakage and scatter radiation
• Isolates high voltages
• Provides means to cool the tube
Control of Leakage Radiation and Scatter
Radiation
• Housing made of lead-lined cast steel
• Leakage radiation limit
– 100 mR/hr at 1 meter
High-Voltage Isolation and Tube Cooling
• Dielectric oil
– Insulates
– Promotes cooling
– Sometimes circulated through heat exchanger
• Air fan
Off-Focus Radiation
Rating Charts and Cooling Curves
• Tube Rating Charts
• Anode Cooling Curves
• Housing Cooling Curves
Anode Cooling Curves
Calculation of Heat Units
• kVp x mA x time x rectification constant
X-Ray Production
Conditions
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X-rays vs. gamma rays
Gap between filament and target
Velocity of accelerated electrons
Incoming electrons = incident electrons
(Solid arrow)
• Departing photons
(Wave arrow)
Target Interactions
• All occur within 0.25 to 0.5 mm of target surface
– Heat production
– Bremsstrahlung interactions
– Characteristic interactions
Heat Production
• 99+% of the incident electrons’ kinetic energy is converted
to heat
• Incident electrons transfer kinetic energy to outer shell
electrons of the target atoms
– Causes them to emit infrared radiation (heat)
Target Materials
• Tungsten and rhenium
– High Z#’s
– High melting points
– Similar electron binding energies
• Mammography
– Molybdenum
Bremsstrahlung “Brems” Interactions
• German word for braking
• Incident electron interacts with electrostatic force field of
the nucleus
– Mutual attraction - slows electron
– Strong nuclear force - keeps them apart and deflects incident
electron
Brems Interactions
• Result is x-ray photon production
– Accounts for 85-100% of the beam
– Photon energy dependent on how close electron comes to
nucleus
Brems Interactions
• As incident electrons get closer to the nucleus the
following occurs:
– Photon energy increases
– Larger deflection of the incident electron
Brems Interactions
• Direct interaction between nucleus and incident electron
– Possible, but not probable
– Maximum energy photon
Characteristic Interactions
• Incident electron interacts with K-shell electron
– Incident electron continues in slightly different direction
• Kinetic energy must overcome binding energy
– Occurs in techniques using 70 kVp or higher
Characteristic Interactions
• Characteristic cascade
– Hole in inner shell and must be filled by an electron from outer
shell
– Electron energy difference
– Secondary photons produced
• Only electron that drops into K-shell will contribute to the
beam
Emission Spectrum
• Brems and characteristic emissions combined
• Selected kVp will determine the maximum keV possible for
any photon
Emission Spectrum
• Average keV is approximately 30-40% of the selected kVp
• Characteristic peaks at 69 and 59 keV
– Increased output due to tube potential change to 69 or 70
Summary
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Conversion to x-ray photon energy in the x-ray tube
Bremsstrahlung target interaction
Characteristic target interaction
Characteristic K-shell photon production
X-ray photon emission spectrum curve
The x-ray beam
Electromagnetic (EM) Energy
• Combination of electric and magnetic fields traveling through space
Electromagnetic Energy
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Results from acceleration of a charge
EM Radiation can travel through a medium or vacuum
Wave/particle duality
Excitation/ionization
Characteristics of EM Radiation
• Wavelength
• Energy
• Frequency
Wave Theory
• Waves are disturbances in a medium
– Ocean, sound, etc.
• Wavelength ()
– Angstrom
• Frequency ()
– Cycles per second (Hz)
Wave Equation
• Frequency and wavelength are inversely related
• Velocity = frequency x wavelength
• Velocity of all EM radiation is c
– c = 3 x 108 m/sec
• c =  x 
Particle Theory
• High frequency, high energy EM radiation
– Interacts like a particle when contacting matter
• Photon energy and frequency are directly related
• If frequency is doubled, energy doubles
• E = h
X-Ray Properties
• Penetrating and invisible form of EM radiation
• Electrically neutral
• Can be produced over a wide variety of energies and
wavelengths.
• Release heat when passing through matter
X-Ray Properties
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Travel in straight lines
Travel at the speed of light
Can ionize matter
Cause fluorescence in certain crystals
Cannot be focused by a lens
X-Ray Properties
• Affect photographic film
• Produce chemical and biological changes in matter
through ionization and excitation
• Produce secondary and scatter radiation
Prime Exposure Factors
• -Kilovoltage peak (kVp)
• -Milliamperage second-(mAs)
– mA
– Exposure time
• -Distance (d)
– SID
Quantity and Quality
• Quantity of the beam
– Intensity of the beam
– How many photons are within the beam
– Measured in Roentgen (R)
• Quality refers to beam penetrability
– How many of the photons will penetrate the anatomy
– Numerically represented by HVL
mA
• Determines beam quantity or intensity
• Change the mA station on equipment
– Change current delivered to filament
• Change current to filament
– Change how many electrons are released through thermionic
emission
mAs
• mA x seconds = mAs
– Controls
-Quantity
-Radiographic film density
-Patient dose
kVp
• Controls beam quality
• Energy and penetrability
– Influences Scatter
• Dramatic effect on radiographic contrast
• Influences beam quantity
– Increased target interactions with increased kVp
• Directly squared relationship to change in kVp selected
Density Relationship
• How will changing kVp affect beam quality and quantity?
– Increasing kVp
• Increases beam penetrability
• In addition it increases beam quantity
– Decreasing kVp
• Vice versa
15% Rule
• Because kVp affects both quality and quantity, a change of
only 15% will demonstrate a doubling of film density
• In order to obtain on overall image quality, when kVp is
increased or decreased by 15% mAs must either be
halved or doubled.
Inverse Square Law
• Intensity of radiation at a given distance from point source is
inversely related to the square of the distance between the object
and the source
Exposure (Film Density) Maintenance Formula
• As SID increases, beam intensity decreases
– And vice versa
• Provides technique correction for change in SID
– Maintains the same film density
Chapter 12
X-Ray Interactions
Attenuation
• Definition
– Reduction in the number of x-ray photons in the beam
Attenuation
• Definition
– Result of x-ray photons interacting with matter, and therefore giving up their
energy to the matter they interact with
Interaction Basics
• X-rays can:
– Be transmitted without interaction
– Or interact with:
• Entire atom
• Orbital electron
• Nucleus of an atom
Photon Energy Dependent Interactions
• Low energy photons interact with whole atom
• Moderate energy photons interact with orbital electrons
• High energy photons interact with nucleus
Atomic Structure
• Nucleus
• Orbital electrons
– Electrons close to nucleus are “bound”
– Electrons further away are “loose” or “free”
Five Basic Interactions Between
X-rays and Matter
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Coherent scattering
Photoelectric (PE) absorption
Compton scattering
Pair production
Photodisintegration
Photon energy range
– Low
– Moderate
– High
Photoelectric Absorption
• Incident photon energy is completely absorbed by an inner
shell electron
– Most likely to occur when x-ray photon has just slightly more
energy than Eb of a K or L-shell electron
Photoelectric Absorption
• Ion pair is formed when:
– An electron is ejected from the atom
– It becomes known as the photoelectron
– Remaining atom has a vacancy in its inner electron shell
The Photoelectron
• Photoelectron characteristics:
– Kinetic energy (Eke)
– Mass
– Reabsorbs quickly
• Within 1-2mm of tissue
Ionized Atom
• Inner shell electron vacancy makes atom electrically
unstable
• Characteristic cascade
– Vacancy filled by an outer shell electron
– Electron undergoes change in energy level
– Emits characteristic photon
Secondary Radiation
• Radiation that originates from irradiated material outside of
x-ray tube
• Production similar to characteristic x-rays production within
target
• Characteristic photons emitted from atoms of patient after
PE absorption interaction has occurred
Secondary Radiation Energy
• Low Z# in tissue
– Low energy secondary radiation
• Higher Z# with contrast agents
– Higher energy secondary radiation
Photoelectric Absorption
Condition #1
• Incident photon energy (Ei) must be greater than or equal
to binding energy (Eb) of inner-shell electron
Photoelectric Absorption
• PE absorption interaction is more likely to occur if:
– Incident photon energy (Ei) and inner-shell electron binding
energy (Eb) are close to each other
Photoelectric Absorption
• As photon energy increases, chance of PE interaction
decreases dramatically
Photoelectric Absorption
• PE absorption interaction is more likely to occur in
elements with a higher Z#, and therefore higher binding
energy (Eb) of inner-shell electrons
Photoelectric Absorption
• Increased Z# has a dramatic impact on the amount of
PE absorption
– Direct cubed relationship
• Double Z#
– Increase chance of PE absorption interaction by a factor of 8
Photoelectric Absorption
• Low Z# atoms experience PE absorption interaction with
the K-shell
• Higher Z# atoms experience PE absorption interaction in
the K, L, or M-shell
• Example
– Bone vs. soft tissue
Coherent Scatter
• Involves low energy photons (below 10keV)
• Two types with same result
– Thompson (single outer-shell electron)
– Rayleigh (all electrons of the atom)
Coherent Scatter
• Electrons are excited and vibrate at photon frequency
• No electrons are ejected
• No ionization takes place
Coherent Scatter
• Atom stabilizes itself by releasing a photon equal in energy to
incident photon (Ei), but in a different direction
Compton Scatter
• Incident photon (Ei) interacts with outer-shell, loosely bound
electron and ejects it
• Ion pair is formed
Compton Scatter
• Photon transfers some of its kinetic energy to the recoil (Compton)
electron and continues on in a different direction
Compton Scatter
• Energy transferred to recoil electron (Eke) affects angle and
energy of scattered photon (Es)
– And therefore, the frequency and wavelength of the scattered
photon
Compton Scatter
• Recoil electron travels until it fills a vacancy in another
atom
• Scattered photon continue to interact until absorbed
photoelectrically
Compton Scatter
• Source of occupational exposure and radiation fog
• Most scatter travels in forward direction
• Backscatter
Pair Production
• Incident photon energy must be 1.02 MeV
or higher
• Photon energy absorbed by nucleus
Pair Production
• The nucleus becomes unstable
• Nucleus releases a positron and a negatron to stabilize itself
Pair Production
• Both have mass equal to an electron but with opposite
charges
– Negatron - negative
– Positron - positive
• Negatron acts like a free electron and will combine with a
nearby atom
Pair Production
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Positron is unstable antimatter
Combines with nearest electron
Annihilation reaction occurs
Matter of particles is converted to energy
– Results in two photons of .511 MeV traveling at 180o to each
other
Pair Production
• Does not occur in diagnostic range of energies
• More significant in radiation therapy
• Not a significant interaction until energies of 10 MeV are
being used
Photodisintegration
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Extremely high energy photon (10 MeV or greater)
Absorption of photon by nucleus
Excited nucleus releases alpha particle
Not significant in diagnostic imaging range
Effect on Technical
Factor Selection
• Most of the x-ray beam is attenuated while some of the
beam is transmitted
Effect on Technical
Factor Selection
• As kVp increases the number of photons transmitted
without interaction increases
– Decreased probability of PE absorption and Compton
interactions
– Vice versa is true, too
Effect on Technical
Factor Selection
• Within the attenuated beam…
– As kVp increases
• PE absorption decreases
• Compton effect increases
– Increases percentage of scatter and decreases percentage of
absorption
Effect on Technical
Factor Selection
• Compton scatter typically predominates within diagnostic
x-ray energy range
Effect on Technical
Factor Selection
• PE absorption interactions predominate in two
circumstances:
– Lower energy ranges (25-45 keV produced by 40-70 kVp
techniques)
– In elements with higher Z#’s
– Introduction of contrast agents results in increase PE absorption
Effect on Technical
Factor Selection
• When PE absorption predominates
– Resulting image will have short scale contrast
– Low kVp, high mAs
Effect on Technical
Factor Selection
• When Compton interactions predominate
– Resulting image will have long scale contrast
– High kVp, low mAs