Podcast
Questions and Answers
The cathode in X-ray production is made of a material other than tungsten.
The cathode in X-ray production is made of a material other than tungsten.
False
Electrons are released from the tungsten filament only when the temperature exceeds 2000 degrees Celsius.
Electrons are released from the tungsten filament only when the temperature exceeds 2000 degrees Celsius.
True
Bremsstrahlung accounts for the highest percentage of X-ray production during electron interaction with the anode.
Bremsstrahlung accounts for the highest percentage of X-ray production during electron interaction with the anode.
False
A potential difference between the cathode and anode is essential for accelerating electrons in X-ray production.
A potential difference between the cathode and anode is essential for accelerating electrons in X-ray production.
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Thermionic emission occurs when electrons from the cathode move into the vacuum chamber without any energy input.
Thermionic emission occurs when electrons from the cathode move into the vacuum chamber without any energy input.
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Electrons can be accelerated towards the positively charged anode without the need for a vacuum in the glass enclosure.
Electrons can be accelerated towards the positively charged anode without the need for a vacuum in the glass enclosure.
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Bremsstrahlung, excitation, and ionization are the three interactions that occur when electrons strike the anode.
Bremsstrahlung, excitation, and ionization are the three interactions that occur when electrons strike the anode.
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The glass enclosure in an X-ray tube serves to increase the pressure within the tube, allowing for better control of electron speed.
The glass enclosure in an X-ray tube serves to increase the pressure within the tube, allowing for better control of electron speed.
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Tungsten must be heated to a temperature of at least 2500C for effective X-ray production.
Tungsten must be heated to a temperature of at least 2500C for effective X-ray production.
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The legal limit of leakage radiation from an X-ray machine is 1 mGy/hr at a distance of 1 meter from the anode.
The legal limit of leakage radiation from an X-ray machine is 1 mGy/hr at a distance of 1 meter from the anode.
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Study Notes
X-Ray Production Overview
- X-ray production requires specific components:
- Tungsten filament (cathode)
- Tungsten target (anode)
- Vacuum chamber (glass)
- Focusing cup
Thermionic Emission
- Thermionic emission is the emission of electrons from a heated metal, such as tungsten (cathode).
- As temperature increases, surface electrons gain energy, overcoming binding forces, allowing them to escape from the surface.
X-Ray Production Process
- Thermionic emission releases electrons from the heated filament.
- High voltage accelerates these electrons towards the anode.
- When high-energy electrons strike the tungsten target, they lose energy via excitation, ionization, and radiative losses (Bremsstrahlung).
- Some of this energy loss results in the production of X-rays.
Components of an X-Ray Tube
-
Glass Enclosure/Envelope:
- Purpose: maintains a vacuum to control the movement and energy of electrons.
- This is achieved through this enclosure/envelope.
-
Filament:
- Material: Tungsten (sometimes tungsten & rhenium)
- Reason for choice of material: high melting point (3422°C) and high atomic number → good thermionic emitter.
-
Tungsten Anode:
- Ideal characteristics: high melting point, high atomic number ( Z ), high conductivity, low vapor pressure, mechanically stable.
- Stationary anode applications: used in mobile fluoroscopy or dental (low current/low mAs) studies up to 8mA.
- Rotating anode applications: used for high tube current studies such as general X-ray, cardiology, and computed tomography (CT). This is better for high current studies.
Cathode
- Material: Tungsten, sometimes with tungsten-rhenium additive (10%)
- Temperature for x-ray production : 2200°C
- Space charge effects: Filament= thin coiled wire of tungsten, current through filament heats it up, stimulating thermionic emission of electrons. Imbalances create uneven numbers of negative charges (electrons) and positive charges (protons in the tungsten nucleus). This creates a net positive charge that pulls on the emitted electrons. The electron cloud around the filament reaches an equilibrium. Electrons emitted into this cloud leave for the anode. This limits electron emission.
- mA : unit of measurement that describes electron current / movement of electrons from cathode to anode. Measures electron quantity.
Anode
- Types: Stationary and Rotating
- Characteristics: High melting point, High atomic number Z (intensity is proportional to Z), high conductivity, low vapor pressure, mechanically stable.
Generator
- Generates high voltage across the x-ray tube, creating a negative-to-positive gradient.
- Accelerates electrons towards the anode.
Focal Spot
- Actual Focal Spot: Spot on anode where electrons strike.
- Effective Focal Spot: Spot on the image receptor (patient). Affects blur (smaller sizes are better).
Anode Angle
- Angles the anode to 6-20°
- Increases surface area for heat dispersion.
- Decreases focal spot size → increased spatial resolution and image quality.
Factors Affecting Heel Effect
- Anode Angle: Steeper angles lead to increase in heel Effect (variation of beam intensity across the x-ray field), sometimes called heel cut-off. Beam intensity is reduced on the anode side of the beam compared to the cathode side because the beam travels a farther distance on the anode side before emerging from the material.
- Focal Film Distance (FFD): Higher FFDs result in less pronounced heel effect. This is because more rays from the central region of the focal spot are collected on the image receptor.
- Image Receptor Size: Smaller image receptors reduce the heel effect as more central rays are collected.
Filtration
- Removes low-energy x-rays, increasing the average energy of the beam (hardening).
- Materials used in filters: Aluminium (2–3 mm thick)
- Removal of low-energy x-rays decreases patient dose.
X-Ray Interactions at the Anode
- Excitation: Causes electron movements to lower energy levels. Primarily results in heat.
- Bremsstrahlung: X-ray production through electron deceleration.
- Ionization (Characteristic radiation) : Inner-shell electron removal.
XR Spectrum Graphs
- Y-axis represents the number of X-rays produced.
- X-axis represents the photon/x-ray energy in keV.
- Displays continuous spectrum of Bremsstrahlung radiation, overlaid with discrete characteristic x-rays peaks.
Characteristic Radiation
- Electrons interact with inner-shell electrons in the target atom.
- Removal of the inner-shell electron creates a vacancy.
- Filled by an outer-shell electron, with energy released as characteristic radiation.
- discrete energy lines representing photons with specific energies.
X-Ray Generators
- Produce high voltages, crucial for x-ray production.
- Types: single-phase, three-phase, high-frequency.
- High-frequency inverters: lower ripple and overall improved performance/efficiency.
Beam Quantity/Quality
- Quantity: Total number of X-rays
- Quality: Average energy of X-rays
- kVp affects Quantity and Quality: Higher kVp ⇒ higher average energy / quality, and greater quantity of x-rays.
- mA/Exposure time(mAs) affects Quantity : Higher mA/s ⇒ greater quantity of x-rays.
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Description
This quiz covers essential concepts in X-ray production, including the key components like the tungsten filament and target, as well as the thermionic emission process. Understand how high voltage accelerates electrons and leads to X-ray generation through various energy loss mechanisms. Ideal for students studying radiology or medical imaging.
