Chapter 5 Flashcards
Within a vacuum tube, high energy electrons are converted to
X-ray photons
Energy changes in form - kinetic to
X-ray
4 Conditions Necessary for the Production of X-rays
• A source of electrons that will be converted to x-ray photons.
• Focusing of the electrons to the anode.
• Production of High Speed Electrons. By using the laws of electricity, electrons are put into motion.
• Sudden stopping of the Electrons
Cathode Assembly has what 3 things
o Filament
• Focusing cup
• Associated wiring
Cathode Function
• Produces a thermionic cloud
• Conducts high voltage to the gap between the cathode and anode
• Focuses the electron stream as it heads towards the anode
Source of ElectronsCathode • FILAMENT
current flows through the wire, heating it and causing electrons to “boils off”
Source of Electrons
• FILAMENT
• THERMIONIC EMISSION
current flows through the wire, heating it and causing electrons to “boils off”.
FILAMENT
Filament is made with
Tungsten and Rhenium and molybdenum
Within a vacuum tube, high energy electrons are converted to
X-ray photons
created in a vacuum tube
X-rays
Energy changes in form - kinetic to
X-ray
4 Conditions Necessary for the Production of X-rays
• A source of electrons that will be converted to x-ray photons.
• Focusing of the electrons to the anode.
• Production of High Speed Electrons. By using the laws of electricity, electrons are put into motion.
• Sudden stopping of the Electrons
Cathode Assembly
o Filament
• Focusing cup
• Associated wiring
Cathode Function
• Produces a
thermionic cloud
Cathode Function
• Conducts high voltage to the gap between the
cathode and anode
Cathode Function
• Focuses the __ as it heads towards the anode
electron stream
Tungsten selected due to:
High melting point
• Difficult to vaporize
*Filament length and width impact
recorded detail
Focusing Cup is
Composed of
Compresses
And
Composed of nickel
• Compresses thermionic cloud
• Grid-biased focusing cup
• Space charge effect
• Saturation current
directs the electrons towards the anode.
FOCUSING CUP -
• Regulates the flow of electrons, producing x-ray photons
Grid-biased (more negative charge)
• Precise control of thermionic cloud
• Permits very short, rapid sequencing of exposures
Grid-biased (more negative charge)
Grid biased is used when
• Angiography
• Pulsed fluoroscopy
The electrons will hang around the filament in a __ until they are put in motion or until the filament cools when they will settle back on the wire.
“space cloud” or “space charge”
As kVp increases, a greater percentage of the __ are driven toward the anode.
thermionically emitted electrons
Because electrons are electrically negative and the cathode becomes strongly negative, the electrons speed away from it. The strong positive attraction of the anode put them in __
motion at the speed of light
ANODE - The high speed electrons slam into the anode at the
target track.
It is at this point that x-rays are created. When the electrons interact with the__ material a transfer of energy takes place and the electron __ is converted into __. Electrons are converted into X-ray photons.
target
kinetic energy
x-ray energy
• Serves as the target surface for high voltage electrons from the filament.
Anode Functions
Conducts the high voltage from the cathode back into the x-ray generator circuitry.
Anode
Serves as the primary thermal conductor.
Anode
Anode
assembly consists
a specific area where the electrons hit The angle (and the size of the filament) will
• assembly consists of anode, stator and rotor
• Anodes can be rotating or stationary.
• Regardless of the type, each have a target track - a specific area where the electrons hit
• They may have different angles.
• The angle (and the size of the filament) will control the detail that is created on the x-ray.
Material used for Rotating Anode
Molybdenum with tungsten target tract.
For a Rotating Anode the material used is Molybdenum with tungsten target tract. These two materials are used because:
High atomic number
•High melting point
• Greater conductivity of heat
Stationary Anode
• Static target area
• Smaller target area
• Low power, used mainly in dental units
Anode Heat Loading
• Rotating anode
Revolutions per minute (rpm)
• Diameter of disk
ANODE ANGLES
• Most common 12 degrees
• Diagnostic 7 - 17 degrees
Anode Angles give a FS size of 0.1mm to 3.0mm
• The smaller the FS size, the greater the
image detail
The smaller the tube angle….
the smaller the FS size
• the more detailed the resultant image
The Larger the FS size
• the greater the heat capacity.
• the greater thermal conductivity
•which lessens the chance of tube damage
• the lesser the capability of demonstrating image detail.
used to reduce the effective area
of the actual focal spot because of the anode angle.
Line Focus Principle-
Effective focal spot (projected area of x-ray beam) Controlled by:
- Actual focal spot (physical portion of focal track being hit by electrons)
-Target angle
Anode Heel Effect
• Radiation intensity is greater on the __ side of the x-ray beam, due to the geometry of the anode.
cathode
Due to the anode heel effect, the __ portion of the body should be placed at the cathode
thicker
X-RAYS are emitted (direction they go)
isotropically
ANODE HEEL EFFECT
• Less radiation intensity on the __ side of the x-ray beam.
anode
PREVENT TUBE DAMAGE
Warm-up Anode
• Test exposures to heat anode
• Refer to Rating Charts and Cooling Curves
• Protective Housing
Warm-Up Procedure is to prevent
Gradually warms anode
• Prevents cracking
• Helps maintain vacuum
• Stress relieved anode-feature that allows efficient heat dissipation and doesn’t require an elaborate warmup procedure
- produce magnetic field
Stator
Stator- produce magnetic field
Located
Bank of
Failure Results in
• Located outside envelope
• Bank of electromagnets
• Stator failure
• Results in suboptimum anode rotation speed
Anode cooling is
rapid at first then gradually slows
Anode cooling Most integrated into
generator software controls
Tube rating charts
• Designed to
protect
X-ray tube and lengthen tube life
Tube rating charts Any combo of kVp and mAs __ are safe for the x-ray tube
at or under the red curve
Collimator
• Lead strips used to
restrict the primary beam (exit)
Off-Focus Radiation is Undesirable part of the beam; because
photons not produced at the focal spot
Contributes up to 25 percent of total primary beam
Off focus radiation
Produces “ghosting” of image and reduces image quality
Off focus radiation
Can change histogram during post processing in digital radiography
Off focus radiation
Protective Housing
• Controls
leakage and scatter radiation
• Additional lead at cathode end because of
direction of photons
photons that escape from the protective housing except at the port.
Leakage radiation-
Provides means to cool tube
-Contains dielectric oil which insulates and absorbs heat
-Small air fan
maintains the vacuum between the cathode and anode
Glass Envelope-
The glass envelope is made of
Made of (pyrex) glass or metal (eliminates the occurrence of tungsten vaporization)
The glass envelopes contains
Contains the cathode and anode assemblies except the stator
Copper cylinder connected to anode disk by
molybdenum stem
Rotator Turns when
stator is energized
• Bearing failure due to
excessive heat, lubricant break down & friction
