X-ray circuits Flashcards
What does kV (tube voltage) affect?
-type of photons
-number of photos
(desired tube voltage is at least 50kV and constant)
Why is the alternating mains supply voltage not suitable for X-rays?
The energy is inconsistent, you would only get X-rays half of the time.
The average energy would be lower than the peak energy, producing X-rays with lower energy
Higher exposure times would be needed and resolution would be worse
Low energy electrons are more likely to be absorbed which results in a higher radiation dose and chance of ionising cells
What does mA (current) affect?
number of photons in an exposure
What is the solution if the mains voltage is low and alternating?
High Tension Transformer provides high voltage
Rectification and Smoothing determine the constant tube voltage
Faraday’s Laws
A changing magnetic field linked to a magnetic field induces an Electromotive Force (electromagnetic induction)
How do we produce Filament electrons?
Filament of X-ray tube needs low voltage so voltage needs to be decreased = filament transformer (Low Tension Transformer)
Increase temperature and mAs
More thermionic emission
Anode rotating
Giving electrons energy equivalent to kVp set
Explain the features of a transformer
Soft iron core- ferromagnetic
Primary coil- alternating current induced electrons that move back and forth (alternating magnetic field)
Secondary coil- Faraday’s Law. has EMF induced, more turns= higher voltage
Explain Rectification
Only allowing current to flow in one direction
PN junctions allow flow of electricity or block flow of electricity (redistribute the flow of electricity so the electrons flow in one direction, from filament to anode)
However the value of energy of X-rays is still inconsistent
Explain Smoothing
Capacitors charged to peak voltage and discharge as the supply voltage falls, reducing voltage ripple
Thermionic emission
The temperature of filament atoms must be raised sufficiently to give electrons enough energy to overcome binding energy
When emitted, the electrons collect in a cloud or space charge around the filament
They remain there until the exposure switch is closed and a high voltage applied to the tube
What are the key components of a High Tension Circuit?
kV selector- determines operating voltage of tube
HTT- produces operating voltage
Rectification and Smoothing- determine how constant and high the voltage will be
Why is having a consistent supply from filament to anode good?
Constant potential, always have X-rays produced during exposure time
Less skin dose
Shorter exposure
What does the inverter do?
Increases supply frequency
Improves HTT efficiency
increases voltage for X-ray tube for rectification and smoothing
