Diagnostic xray production Flashcards
Components of an xray tube
Main component of the circuit is the x-ray tube
This component actually produces the x-rays
Two major components:
Cathode
Anode
When exposure switch is closed electrons are drawn across from cathode to anode by electromagnetic force
Density of metal of anode causes:
Electrons to lose speed
Forward motion energy converted to heat and x-rays (99% heat, 1 % x-rays)
X-ray tube is
Consists of glass enclosure housing cathode and anode
Glass enclosure is made from special heat-resistant glass
X-ray tube is shielded with a metal covering
Restricts any radiation from exiting the tube other than at the tube port
Cathode purpose and consists of
Purpose:
Provide source of electrons
Direct electrons towards anode
Negative side of the x-ray tube
Consists of:
Coiled filaments
Focusing cup
Filaments of the cathode
Cathode contains 2 filaments: a large and a small
Large filament: used for denser body parts
Small filament: used for detailed work such as extremities or pocket pets
Older or portable machine usually only have one filament
Which filament is used is dependent on mAs setting on unit
Filaments are made of tungsten
Able to withstand high temperatures without melting
High potential electron availability
Filaments are held in a focusing cup aimed directly at anode
Focusing cup of the cathode
Cup shaped device positioned opposite outer rim of the anode
Has slightly negative charge to help focus boiled off electrons towards anode
Beveled edges designed to focus on very small areas on anode
Focal spot of anode that x-rays are produced
Ionization of the cathode
When the circuit is activated, the exposure switch is closed
Filament transformer sends electricity to the cathode
This increases temperature of cathode and the electrons are “boiled off” the filament
Called thermionic emission
Cloud of electrons is produced
Called space charge
The whole process is called the space charge effect
Cathode failure, areas, result, outcome, prevention
Filament evaporation
Due to inappropriate technique and use of the pre-exposure setting
Result
No electron cloud can be formed
Outcome
Film will be blank
No movement of mA meter on control panel
Prevention
Correct use of pre-exposure switch and appropriate technique settings
Anode is and purpose
Purpose:
Attract electron stream thus producing x-rays
Positive side of x-ray unit
Consists of bevelled target on cylindrical base
Target composed of Tungsten
High atomic number allows it to absorb electrons and heat
This target is where the electrons (now called photons) are focused
Focal spot is the small area where electrons are focused and x-rays are produced and sent downward toward patient
Two types of anodes:
Rotating
Stationary
The rotating anode is
Entire structure rotates
On beveled front edge of rotating anode is the target of the x-ray tube
Anode stem serves as thermal dissipator by directing heat emitted along stem
Rotates so electrons (photons) are not always focused on the same spot
Outer edge of disk beveled at specific angle to direct x-rays down toward patient
Angle is fixed, cannot be adjusted
Usually between 11 and 15 degrees
The stationary anode is
Found in portable units, dental units, and Fluoroscopy C-arms
Needs efficient method to dissipate heat
Composed primarily of copper and tungsten
Targeted end of anode angled to direct beam towards patient
Important to follow warning and read lights on unit so heat can properly dissipate between exposures
Ares of tube failure in the anode result, outcome, and prevention
Damage to Glass Envelope
Due to:
Arcing
Gassy tube
Result
Deposits act as secondary anode
Air molecules interact with electron stream
Outcome
Inconsistent radiographic pictures with identical exposures
Prevention
Arcing – Appropriate technique settings
Gassy tube – None available
The line focus principle
Describes how electrons interact with the anode and change direction so the x-rays are directed toward the patient being radiographed
Angle of the bevel of anode and resulting change of direction is called the line focus principle
Narrow x-ray beam (smaller focal area) results in higher resolution image
Wider x-ray beam (larger focal area) results in lower resolution image
Focal spot is and difference between small and large
Area on anode target where electrons (photons) contact
Smaller focal spot
Sharper image
More heat produced
Larger focal spot
Picture lacks sharpness – Penumbra (halo) effect
Less heat produced
Actual focal spot
Area on target where electrons contact
Effective focal spot
Area projected onto patient and film
The Effective focal spot size is stamped onto the tube housing not the actual focal spot size
Anode heel affect
Intensity of radiation is greater on cathode side than on anode side
Due to target angulation
Important when patient is thicker on one side of anatomy than other
Thicker end of animal should be placed on cathode end of take advantage of greater amount of radiation
Ways to turn off the xray unit
All x-ray units have an on/off switch as well as a wall switch and a line voltage compensator
The on/off switch is located on the control panel (or x-ray console)
The wall switch is a legal requirement that is located at eye level within reach of the x-ray generator
Exposure switch
Two-stage activation:
1st stage – activates rotating and boosts filament circuit
2nd stage – activates exposure and x-rays are produced
Waiting for “ready signal” very important to avoid overheating tube
Foot pedal switches used to allow restraint of animal during exposure
Timmer switch
Controls time which voltage is applied across the x-ray tube therefore determining the duration of x ray production
To prevent motion from appearing on the radiograph exposure times of 1/30 sec or less are required
Collimator is
Prevents unnecessary irradiation of patient and technologist
Decreases amount of scatter radiation
Wide variety with adjustable collimators being most common
Often contain a light bulb directed onto a mirror which reflects down onto the tabletop
Must be tested to ensure tabletop illumination is same as radiographic field
Cross hairs painted on Plexiglas window to identify center of radiographic field
Knobs enable to adjust size of radiographic field
The smaller the radiographic field the less scatter radiation produced
Control panal contains
On/off switch – closes electrical current
Line voltage compensator (may be built-in so not seen on control panel)
Kilovoltage selector – selects penetrating power of x-rays
Milliamperage selector – selects quantity of the x-ray beam
Timer – selects time of exposure
Warning light – shows exposure being made and x-rays being emitted
Large animal portable units
Need to be able to travel
Less powerful and less mA than installed units
Adequate for legs and feet
Generator, transformers and x-ray tube are all miniaturized and compressed into very small space
