lecture 6/7 review

Internal Components of the X-Ray Tube

• Cathode:

  • Negatively charged component.

  • Contains dual filaments: small and large focal spots.

  • Focusing cup houses the filaments.

• Anode:

  • Positively charged.

  • Focal spots are located on the anode where electrons strike.

• Electron Flow:

  • Electrons flow from the cathode (negative) to the anode (positive).

  • Electrons have a negative charge, hence they are attracted to the positive anode.

Filament Characteristics

• Filament is a coil, similar to a toaster wire.

• Made of thoriated tungsten to withstand high temperatures (high melting point).

• Thermionic Emission:

  • Process where electrons boil off from the filament due to high temperature.

• Space Charge Effect:

  • Electrons form a cloud around the filament, limiting further emissions due to their like charges repelling each other.

Rectification Process

• Rectification Definition:

  • Changing alternating current (AC) to direct current (DC) which is necessary for X-ray tube operation.

• AC and DC Needs:

  • X-ray tubes need DC for proper function, transformers utilize AC for circuit operation.

  • Rectification ensures a one-direction flow from cathode to anode.

• Halfway vs Full Wave Rectification:

  • Halfway Rectification:

    • Single-phase waveform; only uses the positive half of the wave, resulting in a % ripple and energy loss.

  • Full Wave Rectification:

    • Also a single-phase, flips negative to positive, still has a % ripple but retains more usable energy.

• Three-Phase Systems:

  • Three-Phase Six Pulse:

    • More efficient with 14% ripple, uses three overlapping waveforms.

  • Three-Phase Twelve Pulse:

    • Most efficient with only 4% ripple from fully rectified waveforms.

Importance of Voltage in X-ray Production

• High voltage ensures efficient X-ray production, while low voltage increases patient dose and reduces image quality.

• Sufficient voltage is necessary to produce high energy photons.

Focal Spot Considerations

• The focal spot on the anode is where X-rays are produced; smaller focal spots improve spatial resolution.

• Spot Selector:

  • Controls filament circuit; small/large filaments used depending on the body part size and required technique.

Components of Rectifiers

• Rectifiers are semiconductors, also known as diodes made from P and N type materials.

• A minimum of four rectifiers are needed for full-wave rectification.

X-ray Timer Control

• Automatic Exposure Control (AEC):

  • Terminates exposure when sufficient radiation reaches the image receptor.

Anode Heel Effect

• Thickest part of the body should be positioned on the cathode side where the beam intensity is higher.

Anode and Cathode Components

• Types of Anodes:

  • Stationary: Fixed in place, typically used for simpler machines like dental X-rays.

  • Rotating: Spins to distribute heat and increase lifespan.

• The anode dissipates heat and serves as a target for electrons.

X-ray Production Process

• Electrons move from the cathode to the target (anode) and interact with tungsten atoms, producing X-rays.

• Kinetic energy of electrons determines interaction type and energy level of X-ray photons produced.

Transformers in X-ray Circuits

• Step-up transformers increase voltage (low to high); step-down reduces voltage (high to low).

• Transformers work on the principle of mutual induction between primary and secondary coils.

Reasons for Using Magnets in Radiography

• Essential for the rotation of the anode and for the function of transformers used in the circuit.

Filament and Anode Maintenance

• High heat during operation can lead to tungsten vaporization, causing arcing and efficiency loss; the lifespan of components is affected.

Composite Structure of X-ray Systems

• Understanding key components such as the control panel, high voltage section, and X-ray tube is crucial for operation and troubleshooting.