Study Notes on Chapter 6: X-ray Production
Chapter 6: X-ray Production
Overview of X-ray Production
Focuses on micro-level interactions at the anode target that lead to the creation of x-ray photons.
Discusses properties, characteristics, and influencing factors of the x-ray beam.
Highlights the complexity of processes that contribute to radiographic imaging.
Tube Housing
Function of Tube Housing:
Eliminates air to prevent interference with x-ray production.
The tube/envelope is constructed of glass and encases the cathode and anode assembly.
Sources of Free Electrons
Requirement for Free Electrons:
Essential for x-ray production; must be readily available.
Thermionic Emission:
Defined as the ejection of electrons from the wire surface due to increased heat.
Produces an electron cloud known as the space charge cloud.
Heat (termed thermionic) causes ionic separation, resulting in released electrons.
Acceleration of Electrons
Process:
Free electrons are accelerated from cathode to anode within the x-ray tube.
KVP Influence:
The kilovolt peak (kVp) applied forces and energizes the electrons; higher kVp equates to higher electron energy.
Deceleration of Electrons
Mechanism:
To convert kinetic energy of electrons into x-ray energy, the electrons must be decelerated upon striking the anode.
X-rays vs. Gamma Rays
X-ray Characteristics:
Man-made process originating from the electron cloud of atoms.
Gamma Ray Characteristics:
Naturally occurring, derived from atomic nucleus through radioactive decay.
Photon Interactions with Target
Initial Conditions for Photon Production:
Selection of exposure factors and the flow of electricity to the anode, cathode, and filament.
Filament electrons emitted and traversing to the anode.
Interaction Details:
Majority of filament electrons interact with tungsten atom outer shell electrons.
Insufficient energy transfer to cause ionization; instead, energy raises electrons to higher excitation levels.
Excited electrons release excess energy as infrared radiation (heat) when returning to normal states.
X-ray Production Mechanisms
Two Main Types of Interactions:
Characteristic Interactions
Bremsstrahlung Interactions
Characteristic Interactions
Sequence of Events:
Filament electron interacts with an orbital electron within a target atom.
The filament electron must possess energy greater than the binding energy of the orbital electron to remove it from orbit.
Resulting Interactions:
Outer-shell electrons fall into inner-shell vacancies, resulting in x-ray photon emissions dependent on energy differences between shells.
This energy release constitutes the characteristic cascade.
Energy Characteristics:
Photons emitted are named based on the shell position (e.g., K characteristic).
Binding energy values for tungsten electrons include:
K shell: 69.5 keV
L shell: 12.1 keV
M shell: 2.82 keV
N shell: 0.6 keV
O shell: 0.08 keV
P shell: 0.008 keV
Photon Energy Calculation:
Energy of a characteristic photon = binding energy of closer shell - binding energy of farther shell.
Example:
If an electron filling a K shell comes from an L shell, the energy of the K characteristic photon equals 69.5 keV (binding energy of K) - 12.1 keV (binding energy of L).
Bremsstrahlung Interactions
Definition:
Bremsstrahlung, meaning “braking radiation,” occurs when filament electrons approach atomic nuclei without interacting with orbital electrons.
Energy Losses:
Electrons slow down and change trajectory, emitting energy as a brems photon.
Energy Calculation:
The energy of a brems photon = initial energy of filament electron - final energy after interaction.
Example: An electron enters at 100 keV, leaves at 30 keV, thus producing a brems photon of 70 keV (100 keV - 30 keV).
Photon Production Dominance:
With tungsten targets, the majority of x-ray photons are brems due to the high likelihood of filament electrons missing orbital electrons and only targeting the nucleus.
X-ray Beam Properties
Key Properties for Radiographers:
Beam quantity and quality characterized by production methods and interactions with matter.
Filtration defined as the application of materials (typically aluminum) to absorb non-contributory x-ray photons.
Filtration Types
Inherent Filtration:
Naturally occurring within the tube assembly, especially at the target window.
Added Filtration:
Comprises additional aluminum (2.0 mm Al) placed between the target window and collimator.
Total Filtration:
The sum of inherent and added filtration creating total attenuation of x-ray photons.
Beam Quantity
Definition:
Total number of x-ray photons emitted in a beam.
Influenced by factors such as:
mAs (milliampere-seconds)
kVp (kilovolt peak)
Distance from the radiation source
Filtration types.
Dose Relationship:
Radiation dose correlates with quantity; thus, adjustments to mAs directly affect exposure.
Intensity Changes:
Beam quantity increases with kVp due to greater kinetic energy imparted to filament electrons.
Mathematical Relationships:
Beam quantity varies as the square of kVp change:
If kVp is doubled, intensity increases by a factor of four.
A 15% increase in kVp equates to doubling mAs.
Inverse Square Law:
Beam quantity is inversely related to the square of distance:
$I1/I2 = d2^2/d1^2$
Factors Affecting Beam Quantity
Factor | Effect on Quantity |
|---|---|
mAs | Increases |
kVp | Increases |
Distance | Decreases |
Filtration | Decreases |
Filtration Effects:
Reduces x-ray quantity; effectiveness depends on thickness and material type.
Beam Quality
Definition:
Refers to the penetrating quality of the x-ray beam.
Penetration Explanation:
Photons that penetrate the body contribute to image formation, manifesting as darker shades in the resultant image.
Absence of photons results in lighter or clearer image areas.
Influencing Factors:
KVP: Higher kVp enhances penetration abilities.
Filtration: Removes lower-energy photons, elevating the average energy of the x-ray beam.
Quality Relationships:
Factor
Effect on Quality
kVp
Increases
Filtration
Increases
Half Value Layer (HVL)
Definition:
The thickness of absorbent material required to halve the beam energy.
Normal diagnostic beams exhibit HVL between 3 to 5 mm Al.
Measurement Procedure:
Assess beam intensity using a detector and calculate necessary filtering thickness to achieve a reduced intensity of one-half the original value.
Primary and Remnant Beams
Primary Beam:
The x-ray beam exiting the collimator before patient exposure.
Remnant Beam:
The x-ray beam that exits the patient after interactions and before reaching the image receptor.
Emission Spectrum
Definition:
A graphical representation of the x-ray beam.
Characteristic vs. Brems Photons:
Characteristic photons: Discrete emission spectrum.
Brems photons: Continuous emission spectrum.
Discrete Emission Spectrum
Illustration of Photon Production:
Reflects characteristic x-ray production with limited energy values.
Characteristic photons are associated with specific energy levels determined by shell filling.
Bar Height Significance:
Represents the number of emitted photons at particular energy levels.
Continuous Emission Spectrum
Representation of Brems Radiation:
Displays a bell-shaped continuum indicating a range of photon energies.
Combined Emission Spectrum
Graphical Composition:
Merges discrete and continuous spectrums, with the continuous spectrum dominating due to brems contributions.
Influencing Factors on Emission Spectrum
Factors That Modify Appearance:
Changes in mA, kVp, tube filtration, generator type, and target material.
Effect of Changes on Quantity and Quality:
| Factor | Effect on Quantity | Effect on Quality |
|------------------|---------------------|---------------------|
| mA | Increases | No effect |
| kVp | Increases | Increases |
| Tube filtration | Decreases | Increases |
| Generator type | Increases | Increases |
| Target material | Increases | Increases |