Chapter 5 Flashcards by Shaylee Meier

Within a vacuum tube, high energy electrons are converted to

X-ray photons

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Energy changes in form - kinetic to

X-ray

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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

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Cathode Assembly has what 3 things

o Filament
• Focusing cup
• Associated wiring

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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

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Source of ElectronsCathode • FILAMENT

current flows through the wire, heating it and causing electrons to “boils off”

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Source of Electrons

• FILAMENT
• THERMIONIC EMISSION

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current flows through the wire, heating it and causing electrons to “boils off”.

FILAMENT

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Filament is made with

Tungsten and Rhenium and molybdenum

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Within a vacuum tube, high energy electrons are converted to

X-ray photons

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created in a vacuum tube

X-rays

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Energy changes in form - kinetic to

X-ray

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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

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Cathode Assembly

o Filament
• Focusing cup
• Associated wiring

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Cathode Function
• Produces a

thermionic cloud

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Cathode Function
• Conducts high voltage to the gap between the

cathode and anode

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Cathode Function
• Focuses the __ as it heads towards the anode

electron stream

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Tungsten selected due to:

High melting point
• Difficult to vaporize

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*Filament length and width impact

recorded detail

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Focusing Cup is
Composed of
Compresses
And

Composed of nickel
• Compresses thermionic cloud
• Grid-biased focusing cup
• Space charge effect
• Saturation current

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directs the electrons towards the anode.

FOCUSING CUP -

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• Regulates the flow of electrons, producing x-ray photons

Grid-biased (more negative charge)

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• Precise control of thermionic cloud
• Permits very short, rapid sequencing of exposures

Grid-biased (more negative charge)

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Grid biased is used when

• Angiography
• Pulsed fluoroscopy

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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