Dental x-rays

The production of

Dental x-rays

Machine, Tube, Generation, ProductionChapter 2 Continued

X-Ray Machine

3 Main Components:

1. Control Panel
1. on/off switch
2. indicator light
3. control devices
1. exposure time, kilovoltage, milliamperage
2. Extension Arm
3. Tubehead
1. Tightly sealed, heavy metal housing that contains the X-ray tube capable of producing dental X-rays
2. The components of the tubehead will be the same in an handheld unit

Parts of the Tubehead

1. Metal Housing
2. Tubehead Seal
3. Transformer
1. high and low
4. Aluminum discs
5. Lead collimator
6. Position or Beam Indicating Device (P/BID)
7. X-ray tube

Parts of the X-ray Tube (x-ray maker)

1. Leaded-glass housing (glass envelope)
1. Vacuum-sealed tube that prevents x-rays from escaping in all directions. One central window allows the X-ray beam to exit the tube toward the PID. (Do you remember from Ch. 1 who invented that?)
2. Cathode (negative electrode)
1. Supplies the electrons necessary to generate X-rays
2. Tungsten Filament - coiled tungsten wire that can produce electrons when heated (check out tungsten and molybdenum on the periodic table)
3. Molybdenum cup (electron focusing cup) - focuses and directs the electrons across the tube to the anode
3. Anode (positive electrode)
1. Converts the electrons into X-ray photons
2. Tungsten target - the thin plate that serves as the focal spot on the anode and converts the electrons into X-ray photons
4. Copper Stem -
1. moves heat away from the tungsten target

Electricity! The tubehead doesn't work without it :)

Electrical Current - Flow of electrons through a conductor

• Direct Current (DC)-flows in one direction, produces smooth, and consistent X-rays
• Alternating current (AC)-flows in two, opposite directions
• from the wall to the unit

Rectification - conversion of an AC current to a DC current. X-ray tube is a self-rectifier, converting AC to DC.

• This makes sure the electrons are always flowing in one direction - cathode to anode

More on electricity

Production of Dental X-rays

Thermionic Emission

• When the electricity (step-down transformer) heats the tungsten filament at the (-)cathode (mA), outer shell electrons of the tungsten atom move away from the filament and form an electron cloud.

• The electrons in the cloud stay at the (-) cathode until the step-up transformer is activated by the exposure button (kVp) moving the electrons to the tungsten target at the (+) anode, creating x-ray photons

• KVP means kilovolt peak and is the same as kV
• see pages 14 and 15 in your radiology textbook
• The photons are directed out of the tubehead, through the PID to the receptor

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Circuit: Path of electrical current

• In the production of X-rays 2 circuits are used:
• Filament circuit: 3-5V = regulates the flow of electrical current to the tungsten filament in the cathode (- electrode)
• controlled by the milliampere (mA) setting of the X-ray machine
• Milliamperage (mA) = measurement of the number (quantity) of electrons
• High-voltage circuit: 65,000-100,000V = voltage required to accelerate the electrons and to generate x-rays in the x-ray tube
• controlled by the kilovolt (kV) setting of the X-ray machine
• Kilovolt (kV) = measurement of the electrical force (quality) that causes electrons to move

Transformer: Device used to increase or decrease the the voltage in an electrical current.

• Step-down: Decreases the voltage from 110V (from the outlet) to the 3-5V used to heat up the tungsten filament at the (-) cathode (mA)
• Step-up: Increases the 110V (from the outlet) to the 65,000-100,000V used to accelerate the electrons to the (+) anode (kV)
• Autotransformer: corrects minor fluctuations in the current

Tubehead Seal

• Aluminum or leaded glass covering
• Permits the X-rays from exiting the tubehead
• Seals the oil in the tubehead
• Acts as a filter to the X-ray beam

Types of X-rays Produced

Not all the x-rays produced in the x-ray tube are the same.

• General Radiation (braking/bremsstrahlung):
• When an electron hits the nucleus of a tungsten atom or when an electron passes very close to the nucleus of the tungsten atom.
• Consists of X-rays with many different energies and wavelengths.
• 70% of the x-ray energy produced at the anode
• Characteristic Radiation:
• When an electron dislodges an inner-shell electron from the tungsten atom and causes ionization of that atom, producing an x-ray photon
• Accounts for a very small amount of x-rays produced in the dental x-ray machine.
• Must have 70kV or above because the binding energy of the k-shell (innermost shell) is 70keV.