X-Ray Tubes (2) Flashcards
How are x-rays developed?
Generated when an electron is accelerated and decelerated rapidly - interacts with dense atomic particles (tungsten target)
Prior to 1913 most x-ray tubes were referred to as___.
Gas tubes due to the fact that they generally contained a high amount of residual gases.
prior 1913 x-ray tubes were
- unreliable (fluctuations in gas temp and pressure)
- max energy 130kV
- taget material = platinum
What are the different types of x-ray tubes?
- bipolar
- unipolar
- rotating anode tube
Bipolar x-ray tubes
- most common
- most portable
- built-in high voltage transformer
- 150-450kV max
different housing can emit 4000kV max
Unipolar x-ray tubes
only cathode has a potential difference, anode is at ground potential
Rotating Anode Tubes
Rotating target self-cools by constantly rotating
exposures can be performed in “fractions” of seconds
list the X-Ray tube controls
- Milliamperage (mA)
- Kilovoltage (kV)
- timer
What does the mA control do?
Governs the amount of electricity going to the negative cathode.
1amp = 6.3x10¹⁸ electrons through a conductor per second
most tubes are limited to 10mA
What does the kV control do?
Governs the energy potential between the negative cathode and positive anode.
voltage that moves electrons (current) through the circuit, higher voltage = faster moving electrons.
amount of voltage required to move 1amp of current through the circuit per second.
What does the Timer control do?
Governs the duration for the x-ray shot.
longer the tube is energized the more x-rays will penetrate the part and more photons will reach the film.
Two types of X-ray Generation
AC (alternating current)
Constant Potential Systems
What are AC x-rays?
Alternating current is the power supply for the x-ray tube, but the current will only produce x-rays during 1/2 of the 1/60th second AC cycle
- valiance in energy levels
- needs filters
What are Constant Potential System x-rays?
tube rectifiers convert AC supply into a rectified AC that resemble a DC current
- uniform stream of mono-energetic radiation.
What are fashion tubes?
X-Ray Tubes that make use of AC for the production of x-rays in short bursts or pulses dependant on the frequency of AC applied.
What are Rectifiers?
Used to convert the AC power supply to emulate uniformity or DC.
What are Transformers?
X-ray systems generally require 20-450kV +. most voltage supplies do not support this, transformers are used to “step up” voltage to something that the tube can use.
Amperage is normally to high so “step down” transformers are used
Types of Transformers and what they do
Step up = convert to higher
Step down = convert to lower
auto = automatic adjustment for what is needed
What are Rheostats?
Used to control the filament heating current for an x-ray tube
Three different ways that Rectification can be done
- valves and diodes
- special rectification circuits
- semi-conductor rectifiers
What is a Solid-state rectification system?
commonly used rectification system
- simplified circuits allow for a high voltage filtering and capacitor system
- use of thyristor-based circuits
- high-frequency square-waves (10-16kHz)
Describe the Self-rectification x-ray circuit
The tube acts as a valve, only half the sine wave is used, unused portion dissipates as heat.
- anode may overheat forming electrons
Describe the Half-wave rectification x-ray circuit
Adding a single valve (diode) into the circuit eliminates the reverse half of the sine wave, creating a pulsating x-ray.
Describe a Full wave rectification x-ray circuit
Both positive and negative sine waves are rectified, leaving only the positive portion with a frequency that is 2x the half wave rectified current at the anode.
three parts to producing an x-ray
- source of electrons (heated tungsten filament)
- way to accelerate electron movement
- high-density target to be impacted by electrons
How much electrical energy actually produces the x-ray?
2% remaining 98% is the heat
Name the two interactions that occur to produce an x-ray
- bremmstrauhlung
- characteristic
Bremmstrauhlung
Braking
Electrons slow as they pass through the atoms composing the anode target.
x-ray energy released is dependent on original velocity - new slowed velocity.
Characteristic
Electron knocks one of the anode target atoms electrons out, orbital electrons domino to fill inner rings.
x-ray energy released is = change in energy level of transferred shell electrons.
What is an X-ray tube focal spot?
the area of the target that is impacted by the electrons (actual focal spot).
What is so important about the x-ray tube focal spot?
the size of the area affects the amount of power that can be applied to the x-ray tube and the quality of the resultant image on the film.
must be large enough to transmit and dissipate heat, yet small enough to maintain image sharpness.
actual focal spot vs. effective focal spot?
actual = area of anode target impacted with electrons
effective = portion of x-ray beam projected from the tube window (smaller than actual focal spot)
The focal spot in an x-ray tube is supposed to be as ____ as possible to create a _____ image.
small as possible to create a sharper image.
What is the anode hood (window)?
hood design over the anode that:
- filters radiation outside of the “cone” area
- provides an electrical shield with the glass envelope
The anode hood and window act as a filter that creates a point that is referred to as_______
inherent filtration; the filtration of energy due to absorption by the tube envelope, insulators, and mechanical components
Parts of the x-ray tube
- Glass envelope vacuum
- anode hood
]- anode + - cathode -
- target
- window
What is the x-ray window made of? and what is its purpose?
Beryllium
reduces the longer wavelength radiation while not affecting the short/hard radiation
What role does the Glass envelope play on an x-ray tube?
traditionally pyrex glass, now metal-ceramic tubes made from a steel cylinder fused to alumina-ceramic insulating caps.
- hermetically sealed under an extreme internal vacuum.
- waves pass through the glass in areas that are thinner or have a beryllium window.
How can x-ray tubes be cooled?
- suitable oil that is circulated through the anode on the back surface of the target
- coolant around target + anode
- constant water or air flow
- utilizing “duty cycle”
What is an x-ray tube duty cycle?
tube is idle for a duration of time relative to the time it was activated for between exposures to avoid overheating.
50% duty cycle = 1min exposure, 1min cool down
