Ch. 5: The X-Ray Tube

4 Conditions for production of X-rays

1) Source of electrons

2) Target

3) High-voltage

4) Vacuum

Thermionic Emisson

• Electrical process of liberating electrons from wire filament

• The filament is heated to a very high temperature due to creating resistance with small diameter

• “Boiling off” electrons create electron cloud around filament

• Principle of incandescents light bulb

• Very important x-ray design

Grid-Based Tubes

• Precise control of the thermionic cloud

• Brief change of focusing cup from negative to positive

• Permits very short, rapid sequencing of exposure

• Commonly used in:

  • Angiography

  • Pulsed fluoroscopy

Tube Failure (cathode)

• Tube arching (like short circuit)

• Filament breakage

Tube arcing

Vaporized tungstem collection envelope

Filament breakage

• ‘Boost’ and hold vs. repeated tube boosting

• Filaments become increasingly thin due to vaporization over time (too much heat)

Tube failure (Anode)

• Ball bearings worn (overuse) at high temp.

• Melting of anode disks (rotor failure)

  • Stator fails and rotor ceases to move

Tube failure (anode)(causes)

• Overload (high temp)

• Poor power conditions

• Mechanical failure

Target Area

• Portion of anode that electrons stream contacts

• Referred to by several names

• Point of source of X-ray photons

Target Area (several names)

Target, focus point, focal point, focal track

Production fo X-rays (4 basic things)

• Source of electrons by thermionic emissions

• Means of accelerating electrons (kVp)

• Means of decelerating electrons (target)

• A vacuum (controlled environment)

2 focal points

• Actual focal spot

• Effective focal spot

Actual focal spot

• Physical area where electrons strike the target

• Controlled by: length of filament + target angle

• Small actual focal spot mA limited

Effective Focal spot

• The area projected out of the tube toward the patient.

• Controlled by actual focal spot

Spatial Resolution (Actual vs Effective focal spot)

Smaller focal spot = Increased (better) resolution

Lines Focus Principle

• Relationship between actual focal point and effective focal point

• Size of effective focal point depend on angle of anode

The most common angle of the target

12 degrees

Anode Heel Effect

• Results in variation of x-ray beam intensity along longitudinal axis of x-ray beam (can vary 45%)

• More intense under cathode side

• Intenional positioning of body parts can take advantage of heel effect

Warm-up procedure

• Only sometimes

• Gradually warms up anode

  • Prevents craking glass envelope in older machines

  • Maintains vacuums

Envelope (functions)

• Supports anode/cathode assemblies

• Maintains a vacuum to avoid debris and disturbances

Envelope (Modern tubes now metal)

• 10” long

• 6” central diameter

• 2” peripheral diameter

Envelope (Tube window)

Area constructed for the X-ray beam to exit

Protective housing

• Metallic lead-lined cast steal

• Supports X-ray tube

• Controls leakage and scatter radiation (lead-lined)

• Isolates high voltage

Protective Housing (mechanisms to cool tube)

• Cooling oil surroundings tube

• Cooling fans

• Water colling in high end tube designs

Off-Focus Radiation

• Undesirable part of beam produced away from the target and therefore considered off-focus

• Contributes up to 25% of the total primary beam

• Produces ghosting of image and reduces image quality

• Can be corrected with post-processing in digital radiography

Extending X-ray tube life

• Follow tube warm-up procedures

• Avoid frequency and successive boosting of tube

• Use low mAs settings

• Avoid rough handling of x-ray tube head

• Listen for unusual sounds

Cathode Assembly (includes)

• Filaments

• Focusing cup

• Associated wiring

Filament

• Dual focus

• Thermionic emissions

• Coil of thoriated tungsten

  • Greater current (is radiative)

  • Provides resistance (to make heat)

Focusing Cup

• Composed of nickel

• Low negative potential applied

• Compresses/narrow thermionic cloud (space charge)

• Two working systems

Focusing cups (2 working systems)

• Space charge effect

• Saturation Current

Space Charge Effect

• Congested electrons limiting current flow

• Limits exposure miliamperes (mA) to 1000-1200

Saturation current

• Typically controlled by X-ray circuitry (kVp)

• Higher kVp gives a bigger push of electrons toward the anode

• Eliminates the space charge effect

Anode Assembly (Functions)

• Target surface for X-ray production

• Conducts high voltage (closed circuit pathway)(recyles energy)

• Serves as primary thermal conductor (allows heat to be safely conducted)

Anode assembly (components)

Anode (with target), stator, rotor

Rotating Anode (types)

• Stationary anode (only used for low-energy machines) (e.g, dental units)

• Rotating anode (modern X-ray machines

Rotating anode (Tungsten-rhenium alloy)

• High atomic #

• High melting point

• Heat-conducting ability

the rotating anode is made of tungsten-rhenium

Rotor

• Ferromagnetic bars arranged in cylindrcal pattern

• Inside stator + envelope

• Copper cylinder connected to anode disk by molybdenum stem

• Turns when stator energized (3000-12000)

• Balls bearings (inside rotor)

  • Silver plated

  • Reduces surface contact/friction

Stator

• Wraps around rotor outside envelope

• Electromegnets consisting of copper windings

Stator failure

• Suboptimal anode rotation speed (rotor stops turning)

• Immediate melting on target results from too much heat (bullet heat)