Species: Feline | Classification: Miscellaneous
- Xray machine circuits comprise three main components:
- A circuit for heating the filament.
- A circuit for applying a large potential difference ( high voltage) between cathode and anode to accelerate electrons.
- A timing device to control the length of exposure.
The filament circuit
- The tungsten filament at the cathode is the source of electrons used to produce x-rays.
- The number of electrons produced at the tungsten filament is dependent upon the temperature of the filament.
- A tungsten filament needs to be heated to at least 2200°C to emit useful numbers of electrons.
- Electrons are produced by thermionic emission.
- When a metal is heated its atoms absorb energy which allows some electrons to move a small distance from the surface of the metal.
- The filament circuit consists of a step down transformer to reduce the voltage from 220 V to 10 V.
- A variable resistor to control filament current and therefore filament temperature.
- An ammeter to give an indication of filament current which is directly related to mA.
- The tube current is directly proportional to the filament current so the ammeter is usually placed on the secondary circuit of the high voltage circuit as transformers aren't 100% efficient.
High voltage circuit
- This circuit produces a large potential difference between cathode and anode to accelerate electrons produced at the filament to high velocities.
- High tension transformers convert high Amp and low kV to mA and high kV.
- It consists of 2 circuits (an autotransformer and step up transformer) .
- The autotransformer:
- Prereading voltmeter and line voltage compensation.
- Measures incoming voltage and allows adjustments on the autotransformer so that the incoming line voltage remains constant.
- This compensates for fluctuations in national grid.
- Usually automatic on modern x-ray machines.
- KV selector provides predetermined voltage to primary turnings of the transformer.
- Step up transformer:
- These alter the incoming voltage to kV.
- The kV meter is placed across the primary circuit and measures incoming voltage but calibrated to read across the x-ray tube voltage (kV).
- Mains electricity is 240 V and has to be modified to produce a high voltage across the x-ray tube head and low voltage to heat the filament.
- Transformers comprise two coils of wire wound around an iron core .
- When current flows through one coil (primary) a magnetic field is generated which induces a current to flow in the secondary wire coil.
- The ratio of the incoming voltage to outgoing kilovolts is proportional to the number of turns on both the primary and secondary side (Vp/Vs = Np/Ns).
- If the number of turns in the secondary coil is > than the number in the primary the voltage is increased.
- Step-up transformer has many more turns on the secondary coil than the primary coil.
- The production of x-rays is dependent on the current flowing in one direction across the x-ray tube.
- The step-up transformer on the high voltage circuit requires alternating current (AC) to function.
- The current to the tube changes from AC to direct current (DC) by the process of rectification.
- Ideally voltage across the tube head is kept constant so that x-ray production is consistent.
- Half wave rectification:
- Full wave rectification:
- This may be important if a large number of exposures are made rapidly, eg angiography.
- A constant voltage may be achieved by placing condensers across the output from the rectifier circuit.
- The condenser supplies voltage when the rectifier output is less than maximal.
- Capacitor discharge unit:
- A capacitor is charged from the mains to a preset kV.
- During exposure kV decreases with time.
- High frequency machines use a high frequency alternating current copper coil - this reduces the ripple effect and allows a higher kV compared with traditional machines.
- Three phase rectification:
- The kV selector provides a variable but predetermined voltage to the primary turnings of the step-up transformer.