X-ray Tube, Housing, & Collimater Flashcards
Cathode rays
Electrons emitted by a cathode and propelled across a vacuum tube from cathode to anode
The early X-ray tubes
Had no cathode filament, incomplete vacuum chamber
Cathode tube
Hot filament utilizes the process of thermionic emission to provide a source of free electrons.
The Coolidge tube was produced by
GE in 1936 and sold for a price of $40
3 things for X-ray production
Source of free electrons
Acceleration of free electrons
Abrupt halting of high speed electrons
Dense anode target material
Made of tungsten
Primary components of X-ray tube
Cathode
Anode
Glass envelope
All above encased in a protective housing
Why a glass envelope?
To maintain a vacuum within
Why a vacuum
So that electrons accelerated from cathode to anode don’t “bump” into anything and be deviated or lose kinetic energy, also to prevent the filament from burning as it heats up.
Mammographic tubes
Can also be composed of metal or ceramic, however these are rare.
The cathode
The negative side of the X-ray tube whose function is to emit electrons.
The cathode consists of
Filament(s)
Focusing cup
Associated wiring
Filament
Most X-ray tubes are dual focus (two filaments - large and small).
Only one filament is used at a given time.
Filament size corresponds to
Focal spot size
Filaments undergo
Thermionic emission
- electron emission from a heated source
Tungsten is the primary component of the filament, why?
Heat resistance, over 6,000 degrees.
Thermionic emission
Electron emission from a heated source
Thermionic emission source
Filament of cathode
- tungsten wire with very narrow diameter
- as diameter decreases resistance increases
Electrical resistance
= heat
Thermionic emission produces free electrons for
X-ray production
As mA increases, the rate of thermionic emission
Increases
Electrons hover off of the
Filament
The collection of electrons is referred to as the
Electron cloud or space charge
mA =
The number of electrons thermionic ally emitted per second
mA Stations
Selected by technologist at the control console.
Controls amperage to cathode filament.
- this controls rate of thermionic emission
mA
The number of electrons thermionically emitted per second.
Routine radiography is done between
100 to 400 mA
MAS
mA and S combined
The number of X-rays in beam - has primary control of image density
Density
Overall image blackness
Focusing cup
Metallic cup that encases the filaments, composed of nickel or molybdenum; given a negative charge.
Focusing cup keeps emitted electrons in a tight fist until
Accelerated to anode.
Electrons impact the anode
In the smallest area possible - minimizes focal spot
Mutual repulsion
Negative repels negative
- this is how focusing cup works
Anode
Positive side of the X-ray tube whose function is to receive stream of john speed electrons
Anode consists of
Anode
Stator
Rotor
Associated wiring
Types of anodes
Stationary anodes
Rotating anodes
Stationary anodes
Anode stationary as electrons impact to produce X-ray
Tungsten button in a copper core
Dental X-ray, limited medical machine
Rotating anode
Anode rotates as electrons impact to produce X-ray.
Capable of higher technical factors
Rotating anode spreads heat
Over a larger area, heat due to impact of high speed electrons
Composition of anode
Tungsten - able to withstand great amounts of heat
High melting point - 6192*f
High atomic number - 74
Good conductor of heat/electricity
Line focus principle
The effective focal spot will always be smaller than the true/actual focal spot.
Anode face is angled
7-15 degrees is typical - 12 degrees is standard
True/actual focal spot
Area of electron impact
Effective focal spot
True focal spot projected towards the patient.
Focal spot sizes are measured by
The effective focal spot
Anode heel effect
Construction of the anode results in a disparity of X-ray intensity from cathode to anode.
There will be more X-rays on the cathode side of the tube than the anode side.
Stator
Induction motor that turns the anode disk located on the outside of the glass envelope and tube housing. No physical contact between stator and anode
Rotor
Rotates, located inside the glass envelope connected to disk by shaft of molybdenum.
Can be noisy and can continue to spin long after exposure
Rhenium
Blended in Tungsten for additional strength
Other metals anode/cathode
Graphite - substrate of anode disk (weight)
Molybdenum - stem/neck of anode (light weight and strong)
Copper -rotor portion of anode (good heat/electric conductor)
Iron - rotor portion of anode (responds to magnetic field of stator)
Composition of X-ray tube housing
Metallic housing, usually lead lined
Functions of X-ray tube housing
Protects X-rays tube within
Absorb unusable X-ray
Protect from unnecessary radiation
Protect from electrical risk (high voltage/amperage)
Oil found within the X-ray tube housing
Sometimes called dielectric oil, located between glass envelope and metal housing.
Functions of the oil
Absorb unusable X-ray
Electrically insulate
Conduct heat away from X-ray tube
X-ray proceeds isotropically from
The point of origin, originates at anode focal spot
Most of the X-ray created is
Unusable, only tiny portion of X-ray that happens to travel toward patient are useful in creating am image
Leakage radiation
Radiation that leaves the tube housing at a point other than the tube window.
Must be less than 100mR/hr at 1 meter
X-ray tech controls
When X-rays are created (exposure switch)
Strength of X-rays created (KVP adjustment)
How many X-rays created (mAs adjustment)
How long X-rays created (exposure timer)
How X-rays are created
X-ray machine is a device that converts one form of energy that we have in abundance into the form of energy we want.
Electrical energy into electromagnetic energy as X-ray
Electrical energy Thermal energy Potential energy Kinetic energy Electromagnetic energy
X-ray production begins and ends with
Electrical energy
Properties of electricity
Voltage (the force/strength of electron propulsion)
Amperage (the number of electrons in motion)
The properties of X-ray are similar to
The properties of electricity (kilovoltage and miliamperage seconds)
X-ray photons are produced on demand when
Electrons are made available by thermionic emission of the cathode filament (these electrons possess potential energy)
OR
Electrons boiled off the filament are accelerated toward the anode disk up to half the speed of light and collide with the positive anode.
Incident electrons
Possess massive kinetic energy
When incident electrons strike the anode target
Electrons come to an abrupt halt, kinetic energy is lost
Lost kinetic energy must take new energy forms:
Heat (thermal energy)
Electromagnetic (in the form of X-ray)
X-ray production is a very inefficient process
99% heat
1% X-ray
X-ray exposure switches are two stage
Pushing the prep button
- thermionic emission occurs (boiled electrons float in vacuum as a space charge)
- anode rotates in preparation for electrons (anode rotates to spread heat over larger area)
Pushing the exposure button
Kilovoltage is applied
- boiled off electrons are accelerated from cathode to anode
- these electrons possess massive kinetic energy
Primary beam/useful beam
Polyenergetic/heterogeneous in nature - exists in a variety of strengths - from 1kv to KVP
Primary beam exits tube housing
At tube window, proceeds toward patient, passes through colimator
Collimater function
Control size and shape of primary beam, minimize the amount of tissue irradiated and amount of scatter produced.
Centering light
With “crosshairs” for central ray, visible light bounced off mirror, mirror acts as X-ray filter
Colimator contains
2 sets of lead shutters within
Scaled colimator dial
3 things for X-ray production
All come together in the X-ray tube... High amperage for thermionic emission - source of free electrons - delivered by filament/step down transformer High voltage/kiliovoltage -acceleration of free electrons -delivery of high voltage/step up transformer Dense anode target material -abrupt halting of high speed electrons -Tungsten
