Ch. 5 Xray Rube Rad 154

X-Ray Tube

Essential for radiographic imaging, operates on the principle of electron flow through a vacuum.

Production of X-Rays

Involves a source of electrons (cathode filament), a tungsten anode target, under high voltage and within a vacuum.

Source of Electrons

Cathode filament.

Target

Tungsten anode.

Vacuum

Necessary for the operation of the x-ray tube.

Stator

Contains electromagnets and bearings.

Envelope

Encloses the internal components of the tube.

Rotating Portion

Includes filament and electronic circuits.

Filament

Made from thoriated tungsten, impacts radiation detail.

Electron Beam

Generated from the filament and directed towards the target.

Bearing

Ensures stability and alignment of electron beam.

Tungsten Anode

High melting point allows energy absorption and minimizes heat damage.

Armature

Provides structural support for the anode and proper positioning of X-ray tube components.

Molybdenum Neck and Base

Ensures efficient thermal conduction and stability during operation.

The Cathode Assembly

Includes components like filament, focusing cup, and wiring.

Filament (Cathode Assembly)

Produces electrons when heated, negative side of x-ray tube, coiled tungsten wire in focus cup.

Focusing Cup

Directs electrons towards the anode.

Wiring

Connects the components for operation.

Filament Specifications

Coil of thoriated tungsten, measuring 0.1-0.2 mm thick, 1-2 mm wide, and 7-15 mm long.

Filament Material

Tungsten is preferred due to high melting point; alternatives include rhenium and molybdenum.

Thermionic Emission

Heating of filament releases electrons into cathode space, starting from low temperature until exposure.

Tube Failure

Common issues include tube arcing and filament breakage.

Tube Arcing

Caused by tungsten vapor deposit on the envelope due to short-circuits.

Anode Heel Effect

Causes tube arcing due to uneven distribution of electrons.

Filament Breakage

Results in loss of electron generation.

The Focusing Cup Features

Composed of nickel, applies a low negative potential for thermionic cloud compression.

Key Effects of Focusing Cup

Enhances focus and compresses the electron cloud.

Biased Focusing Cup

Adjusts charge to enhance focus by alternating negative and positive charges.

Space Charge Effect

Limits the number of electrons that emit from the cathode affecting the density for x-ray production.

Grid-Biased Tubes

Improve control over thermionic cloud by regulating electron flow.

The Anode Assembly

Serves as the target surface, conducts high voltage, and acts as a thermal conductor.

Stationary vs. Rotating Anode

Stationary anodes consist of copper and tungsten; rotating anodes allow for heat dissipation and higher exposure rates.

Rotating Anode Material

Made of tungsten-rhenium alloy.

Warm-Up Procedure

Evenly heats the anode to prevent cracks and relieve stress.

The Target Area

Where the electron stream hits the anode, critical for x-ray production.

Anode Heat Loading Factors

Includes specifications like RPM, disk diameter, and target material.

Effective Focal Spot

Controlled by actual focal spot and target angle.

Stator location

Located outside the envelope, consists of electromagnet banks.

The Rotor Design

Copper component connected to anode disk, helps in heat dissipation.

The Envelope Specifications

Made of Pyrex glass or metal, 10" long with varying diameters.

Vacuum in X-Ray Tube

Near perfect vacuum is critical to efficient x-ray production.

Tube Window

Area for x-ray beam exit to receptor.

Protective Housing Purpose

Controls leakage and stray radiation, isolates high voltages, and provides cooling.

Control of Leakage and Scatter Radiation Construction

Lead-lined cast steel housing, limits leakage to 100 mR/hr at 1 meter.

High-Voltage Isolation and Tube Cooling

Dielectric oil provides insulation and cooling, sometimes circulated through a heat exchanger.

Cathode

The negative side of the x-ray tube responsible for producing a thermionic cloud, conducting high voltage to the cathode-anode gap, and focusing the electron stream towards the anode.

Saturation Current

The condition in an X-ray tube where, as the kVp increases, more thermionically emitted electrons accelerate towards the anode until no further electrons are driven towards it, affecting the efficiency of the tube.

Thermionic Emission

The process of heating a filament which releases electrons into the cathode space; not all emitted electrons reach the anode, causing scatter and additional X-rays.

Tungsten

A metal with a high melting point, difficult to vaporize, and does not produce thermionic emission below 2,200 celsius.

Tube Arcing

A failure in an X-ray tube caused by tungsten vapor deposit on the inner surface of the glass envelope, leading to destruction of the tube and decreased efficiency due to increased filtration of the primary beam.

Biased Focusing Cup

Enhances the focus by adjusting charge, maintaining a more negative voltage than the filament.

Space Charge Effect

Occurs when accumulated negative charges from electrons oppose the emission of additional electrons, limiting the x-ray tube output.

Tungsten

The metal choice for the source of x-ray photons.

3 Primary Reasons Tungsten is chosen

High atomic number, Heat conduction, & High melting point

Focal Track Temperature Range

The focal track can reach temperatures between 1000-2000 degrees Celsius, and higher if tube load increases.

Rhenium

A material that provides greater elasticity when the focal track expands rapidly due to intense heat.

Anode Heat Management

Procedures specified by tube manufacturers designed to manage and ensure proper heating of the anode.

Actual Focal Spot

Describes the physical area of the focal track that is impacted.

Effective Focal Spot

Describes the area of the focal spot that is projected out of the tube toward the object being radiographed.

Effective Focal Spot Size Control

Controlled by the size of the actual focal spot (controlled by the length of filament) & the anode target angle.

Target Angle Effect

When the target angle is less than 45 degrees, the effective focal spot is smaller than the actual focal spot.

Common Diagnostic Radiography Target Angle

The most common target angle is 12 degrees.

X-ray Emission

When electrons hit the target, x-rays are produced and primarily emitted at angles of 45-90 degrees towards the direction of the electrons.

Stator

A component of the x-ray tube made up of induction-motor electromagnets that rotate the anode and is the only part of the cathode or anode assemblies located outside the vacuum.

Stator Failure

If the stator fails, the rotor can't turn the anode, causing the anode to overheat and potentially melt in one spot.

Rotor

Located inside the envelope of the X-ray tube, the rotor is connected to the anode disk by a molybdenum shaft and is turned by the electromagnetic field produced by the stator, aiding in the functioning of the X-ray tube.

X-ray Tube Contains

The envelope of an X-ray tube that contains the cathode and anode assemblies, typically made of heat-resistant Pyrex glass or metal.

Glass envelope

The glass envelope of an X-ray tube made from heat-resistant Pyrex glass, shaped into a tube connecting the cathode assembly at one end and the anode assembly at the other.

X-Ray Tube Envelope diameters

The X-ray tube envelope is made of heat-resistant Pyrex glass or metal, measuring 10 inches long with a 6-inch central diameter and a 2-inch peripheral diameter.