Emission and HT generation Flashcards
What is the aim of the x ray generator ?
Switch on and off the exposure every few milliseconds. Fluoroscopy is on for longer periods but has a lower dose.
What qualities are necessary for an x ray tube ?
thick glass x ray tube to withstand differential pressure of the vacuum inside and the atmospheric pressure outside. (roughly 1-2mm)
Why does the x ray tube contain a vacuum ?
to prevent an electrical explosion.
What is aluminium used for in the x ray tube ?
used to filter and improved beam quality . Must be measured in its thickness and calibrated properly before fitting. Must be done individually for each tube.
What width of aluminium filtration must all standard x rays be bought up to ?
must be higher than 2.5mm of Aluminium overall.
Why is a centre tap used in the secondary of the transformer ?
To be able to measure the tube current and reduce the thickness of insulation required. Therefore the anode and cathode of the tube are above and below earth respectively.
Why is a medium/ high frequency used to generate a beam ?
to improve the beam quality compared to lower frequency sources.
How is power supplied to the tube and what are the minimum kV limits of the tubing?
Supplied through 2 identical cables either side of the centre tap. X ray uses voltages between 40- 150 kV so each cable must be able to withstand half ( 75 kV at minimum).
What are the qualities of a High Tension cable ?
Most x ray tubes have 2 filaments which requires 3 conductors for the cathode with each having its own thin insulation.
this is wrapped in a conductive plastic tubing layer which has a much thicker overall insulation layer.
it also has a copper braid layer which is grounded to earth to prevent shocks in a fault condition.
it finally has another conductive plastic layer and an outer coating.
How is tube current measured ?
the current flowing in the circuit is the same at all points, therefore an mA meter is placed in the centre point and measures the high tension (secondary current).
What is the function of a rectifier and what are the types of rectification x ray tubing supply had ?
A rectifier converts A.C voltage to D.C. The 3 unit types are self rectifying, 2 pulse unit and 3 phase unit.
How does the current waveform and output waveform compare for self rectifying units ?
Produces a lot of soft radiation at the start of a positive cycle. therefore little radiation escapes the tube which causes the radiation to become harder. Only uses the positive cycles
How does a 2 phase unit work and what does its waveform look like ?
Usually contains a bridge rectifier in the supply to the tube to prevent the anode from going into the negative. Waveform is sinusoidal and symmetrical about the x axis.
Not very crisp and poor image quality in 2 phase units.
How does a 3 phase, 6 pulse unit HT generator work
Steps up 3 phase A.C into the secondary of the transformer. Primary in Delta (Triangular ) formation and secondary in Star (3 phases going outward in different directions from one point). Each arm of the star has full wave rectifier connected and the centre point is earthed. 3 phases produce 6 pulses.
How does a 3 phase unit waveform compare to other units ?
3 phase units produce higher quality waveforms than 2 phase or self rectifying units. Due to there being 6 pulses, there a smaller gaps between waveforms and the ripple is lower in 3 phase units. This means that less soft (lowly penetrating) radiation is produced by 3 phase systems.
