Unit 1 - Quia
Unit 1
Creating the Beam
Chapter 6
The X-Ray Tube
Production of X-Rays
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
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, Grid-biased 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
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
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
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 (λ)
Angstrom
Frequency (υ)
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 = υ x λ
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 = hυ
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
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
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
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
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
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