12 Questions
What is the primary function of the focusing cup in an x-ray tube?
To focus the electron beam onto the anode, preventing electrostatic repulsion and allowing for a targeted spot on the anode.
What is the purpose of the anode disc in an x-ray tube?
To convert the kinetic energy of the electron beam into x-rays, while also providing thermal dissipation and mechanical support for the target.
What determines the voltage in an x-ray tube?
The kVp, which decides the voltage, and the electrical supply (mA) to the filament.
What is the purpose of the rotor in an x-ray tube?
To rotate the anode at high speeds (3000-10000 rev/minute) to prevent repetitive targeting of electrons on the anode.
What are the three main functions of the anode in an x-ray tube?
Electrical conduction, thermal dissipation, and mechanical support for the target.
What determines the focal spot size in an x-ray tube?
The size, shape, and position of the filaments in the focusing cup.
Describe the process of thermionic emission in the production of X-rays.
Electrons from the cathode filament are accelerated to high speeds by a potential difference between the cathode and anode, resulting in the emission of electrons.
What is the primary difference between Bremsstrahlung and characteristic X-rays?
Bremsstrahlung X-rays are produced when an electron beam travels close to the nucleus, resulting in the loss of energy and re-emission as X-rays, whereas characteristic X-rays are produced when an electron beam hits the tungsten atom and causes an inner electron to be ejected.
What is the relationship between the energy and wavelength of X-rays?
The energy and wavelength of X-rays are inversely proportional, with higher energy corresponding to shorter wavelengths.
How do X-rays interact with matter in medical imaging?
X-rays transfer energy to matter through electrical, electromagnetic, and chemical means, resulting in the creation of a latent image on the imaging receptor.
What is the purpose of the focusing cup in the X-ray production process?
The focusing cup targets the electrons emitted from the cathode filament towards the anode, ensuring that they are all accelerated to high speeds.
What is the typical energy range of diagnostic X-rays?
The typical energy range of diagnostic X-rays is 30-150 kVp.
Study Notes
X-ray Tube Components
- 2 primary internal components: Cathode and Anode
- Secondary external components: Glass enclosure, Cathode assembly, and Anode assembly
Cathode Assembly
- Focusing cup:
- Focuses the beam to the anode
- Prevents electron beam from repelling itself
- Targets a spot on the anode
- Effectiveness depends on size, shape, and position of filaments in the cup
- Filament:
- Receives electrical supply (mA)
- kVp determines the voltage
Anode Assembly
- Anode:
- Tungsten-outer layer (focal track)
- Internal layers: generally graphite, for dissipating heat and lower mass density
- Bevelled edge forms anode angle at the target surface (focal track)
- Angled at 5-20 degrees for x-ray photons to exit the tube
- Anode stem:
- Connects rotor and high tension cables
- Rotor:
- Rotates at 3000-10000 rev/minute in high capacity x-ray tube
- Anode disc:
- Tungsten focal track
- Prevents repetitive targeting of electrons on anode
Focal Spot Size
- Fine:
- Used for smaller body parts
- Better spatial resolution
- Used for fine details
- Broad:
- Used for large body parts
- More heat is generated
Anode Functions
- Electrical conductor
- Thermal dissipater
- Mechanical support for the target
X-ray Formation
- Select voltage, current (mAs to filament), and focus
- mA sends current to filament, starting the process of sending electrons to anode
- Apply high voltage, creating potential difference between anode and cathode, pulling electrons to anode
- Focusing cup targets electrons at the anode
X-ray Spectrum
- Beam typically consists of a continuous spectrum of x-ray photon energies
- Bremsstrahlung:
- Diagnostic energy range: 30-150kVp
- Characteristic (peak):
- 69kVp (for tungsten target)
Factors Affecting X-ray Spectrum
- Bremsstrahlung:
- Electron beam travels close to nucleus
- Proton causes electron to change direction and lose speed and energy
- Loss of energy reappears as bremsstrahlung x-rays
- Characteristic x-rays:
- Electron beam hits the tungsten atom, causing an inner electron to be ejected (ionisation)
- An outer electron fills the gap, releasing energy as x-ray photons
Properties of X-rays
- Short wavelength and high frequency
- Higher the energy, shorter the wavelength
- Wavelength and energy are heterogeneous
- Electromagnetic radiation
- Travels through air in straight lines
- Divergent beam once emitted from x-ray tube
- Energy transfer:
- Electrical: in x-ray machine (control at console)
- Electromagnetic: when x-rays are produced at the anode
- Chemical: conversion to electrical signal at image receptor/detector
Medical Imaging
- X-rays pass through patient and are captured on the imaging receptor
- Creates a latent image, which is then processed through chemical energy to produce a 2D x-ray image
This quiz covers the primary internal components and secondary external components of an X-ray tube, including the cathode, anode, and cathode assembly. Learn about the functions of the focusing cup and filament in X-ray production.
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