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Questions and Answers
What are the two types of x-rays that can be identified?
Characteristic x-rays and Bremsstrahlung x-rays
How does the mA affect x-ray production?
Characteristic radiation results from outer shell electrons filling vacancies left by inner shell electron ejections.
True
What is the main purpose of added filtration in an x-ray beam?
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The formula for kinetic energy of electrons in a kVP tube is KEelectron = ______.
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Which target material is most commonly used for x-ray production?
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What does increasing kVP do to the x-ray emission spectrum?
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What is Bremsstrahlung radiation?
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What is the effect of increasing kVP on the x-ray emission spectrum?
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Which two types of x-rays are produced during the interaction of electrons with the anode?
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How does the added filtration affect the x-ray emission spectrum?
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What is the role of target material in x-ray production?
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What formula represents the kinetic energy of an electron in a 70 kVP tube?
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What happens to the kinetic energy (KE) of projectile electrons when they collide with the target atoms?
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When a projectile electron interacts with inner shell electrons, what occurs?
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What is the effect of doubling the electron current in terms of heat energy output?
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Characteristic radiation occurs when which of the following happens?
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What primarily happens to the projectiles' electrons as they travel through the target material?
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Study Notes
X-ray Production
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X-ray production involves accelerating electrons to high kinetic energy and then depositing this energy into the target material (Anode) of the x-ray tube.
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In a 70 kVP tube, each electron gains a kinetic energy of 1.12 x 10-14 J.
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At a current of 100 mA, approximately 6 x 1017 electrons strike the target every second.
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The projectile electrons, upon colliding with the target, transfer their kinetic energy to the target atoms, converting it mainly into heat (approx. 99%) and a small amount of x-radiation energy (approx. 1%).
Electron-Target Interactions
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Projectile electrons interact with outer shell atomic electrons, causing them to jump to higher energy levels. When these electrons fall back to their original level, they release heat energy as infrared radiation and x-rays.
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When projectile electrons interact with inner shell electrons, they can eject these electrons, ionizing the atom. An outer shell electron then fills the resulting vacancy, releasing energy in the form of an x-ray.
Characteristic Radiation
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X-rays produced from specific transitions into the inner shells are called characteristic x-rays. They have energies specific to the electron energy levels of the target atom.
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While the energy depends on the specific electron transition, the x-ray energy is characteristic of the target atom.
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For a tungsten target, only K x-rays are emitted as the L, M, and N x-rays are absorbed by the target material.
Bremsstrahlung Radiation
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Bremsstrahlung radiation is electromagnetic radiation produced by the deceleration of a charged particle (electron) after passing through the electric and magnetic fields of an atom's nucleus.
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This radiation occurs when electrons 'avoid' atomic electrons and interact with the electric field of the nucleus, causing them to slow down and lose energy.
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The Bremsstrahlung radiation results in a continuous spectrum of x-rays.
X-ray Emission Spectrum
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The x-ray emission spectrum represents the probability distribution of all the interactions that produce x-rays as projectile electrons lose their kinetic energy.
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The spectrum consists of two components: the Bremsstrahlung component (continuous spectrum due to electron deceleration) and the characteristic component (discrete spectrum due to electron transitions).
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The characteristic x-ray spectrum is a result of discrete energy level differences of the stable electron states in the atom, represented by binding energy levels.
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For tungsten, the L x-rays are absorbed by the target material, only the K x-rays are emitted.
Factors Affecting the X-ray Emission Spectrum
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The total number of x-rays produced is proportional to the area under the spectrum curve.
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mA (beam current): Doubling the current doubles the number of x-rays but doesn’t change the shape of the spectrum.
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mAs (current times time): Increasing exposure time increases the number of x-rays but doesn't alter the shape of the spectrum.
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kVP (tube voltage): Increasing kVP expands the spectrum towards higher energy levels, improving the quality and increasing the quantity of x-rays. However, the characteristic part of the spectrum remains unchanged.
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Added Filtration: Using material to filter the x-ray beam increases the average energy (quality) of the beam while reducing the quantity. Filters selectively remove low-energy x-rays, which contribute to patient dose without significantly improving image quality.
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Target Material: Target material affects both quality and quantity of the beam. The atomic number of the target material determines the intensity of the Bremsstrahlung radiation, and the characteristic radiation energy also changes. Most images are taken with tungsten targets, while mammography often uses molybdenum or rhodium targets.
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Voltage Variation: Different tube voltage schemes affect the ripple amplitude and frequency of the waveform. Three-phase tube power supplies result in a more constant, more efficient output compared to single-phase supplies. The maximum x-ray output is associated with the peaks of the voltage waveform.
X-ray Production
- The generation of x-rays through a tube involves accelerating electrons to high kinetic energy and depositing this energy into the target material (anode)
- Each electron in a 70 kVp tube gains a kinetic energy of 1.12 x 10-14 J.
- At 100 mA, 6 x 1017 electrons strike the target every second traveling at approximately half the speed of light.
- Upon colliding with the target, projectile electrons transfer their kinetic energy to the target atoms, releasing heat energy (99%) and x-radiation energy (1%).
Electron Target Interactions
- Projectile electrons primarily interact with the outer shell atomic electrons, causing them to transition to higher energy states and release heat energy as infrared radiation.
- When projectile electrons interact with inner shell electrons, an atomic electron is ejected, causing ionization and creating a vacancy in the shell.
- An outer shell electron filling this vacancy releases energy in the form of an x-ray, specific to the energy levels of the target atom.
Characteristic Radiation
- X-rays produced from specific transitions into inner shells are called characteristic x-rays, as they are unique to the emitting atom’s element.
- With a tungsten target, only K x-rays are emitted, while the L, M, N, or lower-energy x-rays are absorbed by the target material.
Bremsstrahlung Radiation
- Electromagnetic radiation is produced when a charged particle (electron) decelerates due to its interaction with the electric and magnetic fields of a nucleus.
- Electrons that deviate from atomic electrons interact with the nucleus’s electric field, losing energy in smaller increments, creating a continuous spectrum called Bremsstrahlung radiation (braking radiation).
X-ray Emission Spectrum
- The x-ray emission spectrum is a probability distribution function reflecting the interactions between projectile electrons and target atoms.
- It consists of a continuous Bremsstrahlung component and a discrete characteristic component.
- The low-energy x-rays are absorbed by the target and window materials.
Factors Affecting X-ray Emission Spectrum
- The area under the spectrum curve represents total x-ray quantity.
- Increasing kVP enhances beam quality (penetration ability), while mAs affects the total number of x-rays (quantity).
- Doubling mA or exposure time doubles the x-ray quantity but doesn't alter the spectral shape.
- Increasing kVP extends the spectrum to higher energies, but does not affect the characteristic component.
Effect of Added Filtration
- Added filters placed after the tube window increase the average energy (beam quality) and reduce intensity (quantity).
- This ‘hardens’ the beam by removing low-energy x-rays, reducing patient dose while preserving image quality.
Effect of Target Material
- Bremsstrahlung radiation increases with the target’s atomic number.
- Characteristic radiation energy becomes dependent on the target material.
- Tungsten targets are typically used for general imaging, while mammography often utilizes molybdenum or rhodium targets.
Effect of Voltage Variation
- Different tube voltage schemes result in varying ripple amplitudes and frequencies.
- Three-phase power supplies produce a more constant, efficient output compared to single-phase supplies.
- Maximum x-ray output occurs at the peak of the voltage waveform.
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Description
Explore the principles of x-ray production and the interactions between electrons and target atoms in x-ray tubes. This quiz covers the energy transformations and the mechanisms involved in generating x-rays and heat. Test your understanding of kinetic energy transfer and ionization processes.
