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X-Ray Circuit & Tube Questions With Accurate Answers
How many steps are there to pressing the X-ray button? - Answer-Two
What does the first step of pressing the button do? - Answer-Puts the x-ray into "prep" mode
What happens in prep mode? - Answer-The filament heats
A cloud of electrons are emitted around the filament
What is the cloud of electrons around the filament called? - Answer-The space charge
What is the space charge? - Answer-Electrons leave the filament as a cloud.
This creates a net +ve (positive) around the very negative cathode
What happens when the exposure button is pressed? - Answer-High electrical kilovoltage is applied.
Electrons accelerate across the tube
What energy do electrons have when they accelerate across the tube? - Answer-Kinetic energy
How can the electrons travels freely across the tube? - Answer-They travel in a vacuum
Is the cathode positive or negative? - Answer-negative
Is the anode positive or negative? - Answer-positive
What is the anode target made from? - Answer-Tungsten
What is the atomic number of tungsten? - Answer-
What force does the tungsten target exert on the electrons? - Answer-A strong positive force
What three things control thermionic emission? - Answer-Temperature
Material type
Surface area of coil
What three things does an x-ray tube provide? - Answer-A supply of electrons
The ability to move them at high speed
A way to change their direction
What is the mains for an x-ray circuit? - Answer-Mains AC 240v
How many circuits are there in an x-ray circuit? - Answer-Two.
An xray circuit
A filament circuit
Where does the voltage go first when the button is pressed? - Answer-It goes through an auto transformer
What are the role of the auto transformer? - Answer-It ensures that the voltage remains set at a constant level to compensate for any fluctuations
It allows different kV values to be selected
What does the filament transformer do? - Answer-It provides a means to reduce the supplied voltage by means of a step down transformer
How does the filament transformer reduce voltage? - Answer-It receives AC current in in primary winding
It has a secondary winding that has LESS turns
This ensures a turns ratio that allows it to step DOWN the voltage to the required level
Where does the AC current go in the main circuit once it has passed through the auto transformer and HT transformer? - Answer-The AC current goes into the rectifiers
What is the role of the rectifiers? - Answer-To change the current from AC to DC
This ensures that the cathode is negative and the anode is positive
This ensures that electrons always travel from cathode to anode to PRODUCE XRAYS!!!!!!!
What are the two components of the xray tube? - Answer-The insert and the shield
What is the tube made from? - Answer-Borosillicate glass
What are the functions of the borosillicate glass tube? - Answer-Strong to support a vacuum
Allows thermal expansion to stop it cracking using seals made from different glass
Good electrical insulator
Rounded to prevent static
Radiolucent to transmit the beam(some is filtered)
Whats are the functions of the tube shield(housing) - Answer-Shock resistant
Radiation is sealed (lead lined 3mm)
Insulated
Well cooled
Built in filtration to remove low energy beams
What is the minimum total filtration that must be built into the xray tube shield? - Answer-2.5mm aluminium equivalent
What does the cathode operate at? - Answer-High negative potential
What does the cathode contain? - Answer-Filament
Focussing cup
wires and support
What is the focussing cup in the cathode made from? - Answer-Nickel or stainless steel
How is the focussing cup able to reduce the focal spot? - Answer-By using a negative bias
What are the features of the filament? - Answer-Made from tungsten
Low work function(4.5 ev) allows easy emission of electrons
High melting point
Low vapour pressure - doesnt evaporate
The stem contacts oil which uses convection to heat the oil
The oil expands and can cause expansion bellows to trip a microswitch preventing further xray exposures until cool down.
Heat from the oil passes through the metal casing via conduction
How can heat loss be maximised in the anode? - Answer-High conductivity material used
Large anode block
Anode length as short as possible
Anode stem in oil
Why is lead used on the outside of the casing? - Answer-It has a high atomic number (82) and provides good radiation protection
How is electrical safety achieved in the xray tube housing? - Answer-Live components are insulated using oil.
Components are earthed using copper wire braiding
Access to live components is restricted
Name the two types of filtration that protects against radiation within the xray tube? - Answer-Inherent filtration: glass and oil
Added filtration: aluminium
How is the xray tube designed to as small a radiation source as possible? - Answer-Using electron beam focussing
Target angulation
What faults can occur with the xray tube? - Answer-The filament will eventually break due to vaparisation
Oil seal and shield could leak
Glass may break and stress fracture
Heat can reduce xray efficiency over time
Glass insulating properties can reduce over time
How does an x-ray tube provide the means for x-ray production? - Answer-It provides :
A supply of electrons
A way to get the electrons to travel at high speed
A force that causes them to change directions
What energy does an a x-ray tube change? - Answer-Electrical energy into x-ray energy
What happens when electrons collide with atoms? - Answer-99% are lost to heat, 1% produce x-rays
What is electrons moving at high speed called? - Answer-Thermionic emission
How is an exposure made? - Answer-Turn on machine
Set kVp
Set mA
Set time
Press switch
Voltage is supplied across the tube
Electrons are accelerated across the tube
X-rays are produced
Why is tungsten used as the target - Answer-1) Low Vapour pressure(5000 kPa) - otherwise will become a conductor
High melting point -3370c - Gets very hot when bombarded with electrons at high speed
High atomic number - 74 - This increases the number of positive protons creating a greater positive attraction. Efficient converter of xrays
Same thermal conductivity as Copper - Expansion rate is similar so doesnt get distorted when heat is applied
5 )Good Thermal conductor- Transfers heat to the Copper
Temperature increase of the target material in a stationary anode X-ray tube must be reduced to save the tube life. This can be achieved by: - Answer-reducing the mA
Copper is used to make the anode block because of its - Answer-High thermal conductivity
The glass envelope of an X-ray tube - Answer-insulates the anode from the cathode
The anode angle of a diagnostic X-ray tube - Answer-influences the size of the effective focal spot
The focal spot size of an x-ray tube influences - Answer-the unsharpness of the image
The mA is - Answer-the voltage between the filament and the anode of the x-ray tube
Tungsten is used as the target material in an X-ray tube because it has - Answer-a high atomic number
The oil in the X-ray tube housing - Answer-provides a method of heat dissipation
Lead is incorporated into the housing of an x-ray tube because - Answer-it has a high atomic number
An x-ray tube converts electrical energy directly into x-rays and - Answer-heat
In an X-ray circuit the high tension transformer - Answer-is used to raise the value of the mains supply voltage
Tungsten wire is used for the filament of the X-ray tube because it - Answer-Low vapour pressure so doesnt evaporate
Low work work function (4.5eV)
Easily emits electrons
Very strong & can be drawn into a thin spiral of wire to increase surface area - DUCTILE
Copper is used in a stationary anode x-ray tube because - Answer-it is a good conductor of heat
The tungsten target of an X-ray tube - Answer-is bombarded by electrons during the exposure
How can heat be reduced in a tube - Answer-Reducing kVp, mA & time
What is the inherent filtration - Answer-Glass envelope
Oil
What is the added filtration - Answer-Aluminum
What reduces spot size - Answer-Light beam diaphragm
Three principles of electrical safety - Answer-Live components insulation
Component housing earthing
Restricted access to live components
What is the area bombarded by electrons - Answer-True focal area
The area below the true focal area - Answer-Effective spot size
What does the rotating anode achieve - Answer-Spreads heat
What faults can occur with the x-ray tube - Answer-Tungsten deposits reducing vacuum
Glass may be broken/fractured
Filament may break
Oil seal leakage
Heat causes crazing on anode causing unsharpness
Tube shield construction requirements - Answer-No danger of shock
Radiation must not escape
Support for tube & cables
Insulation between insert & shield
Cooling
Filtration
Why use a focussing cup - Answer-Plume of electrons will increase in size as they travel
