Dental Radiology
Properties of x-rays
Appearance - x-rays are invisible
mass - x-rays have no mass or weight
Charge - no charge
Speed - travels at speed of light
Wavelength - x-rays travel in waves and have short wavelengths with a high frequency
Long wavelengths have LOW energy, & low frequency - thus poor penetrating ability
Short wavelengths have HIGH energy, & high frequency - thus high penetrating ability
Extension Arm - suspends the x ray tubehead, allows for movement and positioning of the tubehead
Inside tubehead-
A tightly sealed, heavy metal housing
- Contains the x-ray tube that produces dental x-rays
Metal housing
• Surrounds the x-ray tube and transformers, protects tube and grounds high voltage components
Insulating oil
• Surrounds x-ray tube and transformers, prevents overheating
Tubehead seal **
• Permits exit of x- rays from tubehead, seals the oil, filters x- ray beam
• X-ray tube
• Heart of generating system
• Transformer
• Alters voltage of incoming electricity
Aluminum disk
Filter out nonpenetrating, longer wavelength x-rays
1.5mm - 2.5 mm
Aluminum filtration
– Preferentially removes long wavelength photons not useful for dental imaging
Lead collimator
Restricts the size of x-ray beam
Collimation restricts the beam of radiation to a size appropriate for the size of film used in dentistry
2 ¾ Inches at patient’s skin
Position-Indicating Device PID
Aims and shapes the x-ray beam
Dental X-ray Tube
The heart of the x-ray generating system and is critical to the production of x-rays.
Glass vacuum tube: all air removed
The x-ray tube used in dentistry measures several inches long by 1 inch in diameter
Component includes: leaded-glass housing, negative cathode, and positive anode
Leaded Glass Housing
• A leaded glass vacuum tube
• Prevents x-rays from escaping in all directions**
• One area has a “window” that permits the x-ray beam to exit the tube and directs the beam toward the aluminum disks, lead collimator, and PID
Cathode (-)The negative electrode **
• Consists of a tungsten wire filament in a cup-shaped holder made of molybdenum
• Supplies the electrons necessary to generate x-rays
• The tungsten filament produces electrons when heated = thermionic emission.
• Tungsten filament has a high melting point
• The molybdenum cup focuses electrons into a narrow beam and directs the beams toward the tungsten target (anode)
Tungsten Filament supplies the electrons to generate x-rays (- charge)
Source of electrons = As the filament heats up, electrons boil off, creating light and a cloud of electrons around the filament
Anode (+) The positive electrode **
• Consists of a wafer-thin tungsten plate (tungsten target) embedded in a solid copper rod
• Converts electrons into x-ray photons
- The tungsten target serves as a focal spot and converts electrons into photons
- The copper stem functions to dissipate heat away from the tungsten target.
99% of the x-rays are absorbed by the copper stem, only 1% is actually directed out toward the patient
Focal spot- exact area where electrons strike target
• Focal spot is important in how sharp the radiographic image will be
• The smaller the focal spot the sharper the image
• Electrons move at high speeds (kVp) when exposure button is pushed
• Impact creates a huge amount of heat 99% and a small amount of x- rays (1%)
Line Focus Principle (Benson Principle)-
• Places the target at a 15-20 degrees to the cathode
• Distributes the intense heat
• Projects an “effective focal” spot- smaller than actual focal spot on the target
• The small focal spot the sharper the image
What happens to the rest of the x-rays?
• Lead glass housing- stops x-rays
• Insulating oil-helps absorb high amounts of heat
• Aperture- opening for x-ray to leave tube head
• Lead collimator-restricts beam size
• Federal standards require collimation to 2.75”
• Aluminum filter-1.5-2.5mm of aluminum, removes “soft x- rays”, long wavelength, weak ray
What are the 4 conditions for x-ray generation?
1. Generation of electrons
2. Focusing of those electrons
A.) Thermionic emission- the release of electrons from the tungsten filament when the electrical current passes through it and heats the filament. This happens at the cathode**
B.) When the exposure button is activated, the electrons are accelerated from the cathode to the anode
C.) The electrons strike the tungsten target, and their kinetic energy (energy an object possesses due to its motion) is converted to x -rays and heat.
3. Production of high-speed electrons
4. Stopping of high-speed electrons with target
Circuits used in the production of x-rays.
A Circuit is the path of the electrical current.
• Filament circuit: Low voltage (3-5 V)
• Controlled by the mA setting in the control panel
• Regulates flow of current to the filament (quantity on electrons)
High-voltage circuit: Uses 65,000- 100,000 V
• Controlled by the kVp setting in the control panel
• Accelerates electrons from cathode to anode to generate the x-ray in the x-ray tube (quality- energy or penetrating power of the x-ray beam)
kVp controls speed of the electrons
Transformers- device that is used to either increase or decrease the voltage in an electrical circuit
• Step-down transformer- used to decrease the voltage from the incoming 110 or 220 line to 3-5 V.
• Step-up transformer – used to increase the voltage from the incoming 110 or 220 line to 65000-100,000 V.
• Autotransformer- serves as a voltage compensator that corrects for minor fluctuations in current.
Know periapical, bitewing, occlusal - know two uses of each.
Know difference between paralleling and bisecting
Know terms like radiopaque
Know electromagnetic spectrum, parts of atom,
Know four conditions
Parrelleing - sensor and tooth are parallel