Radiography: x-ray machine circuit
Podcast: Radiography: x-ray machine circuit
- X-ray 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 220V to 10V.
- 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 are not 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:
- Pre-reading 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 pre-determined 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 240V 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 > 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:
- If a circuit includes four rectifiers it can be constructed so that current flows from cathode to anode unimpeded and retrograde current is reversed to ensure current flows continously.
- The voltage across the tube head varies with time .
- This may be important if a large number of exposures are made rapidly, eg angiography.
- A constant voltage may be achieved by placing condensors across the output from the rectifier circuit.
- The condensor 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:
- This circuit consists of 3 separate transformers linked to the mains supply with each out of phase by 120° .
- A special mains supply is required.
- As the voltage from one transformer starts to drop the current is boosted by the other two.
- This results in an almost constant voltage across the tube head .
- The kV selector provides a variable but pre-determined voltage to the primary turnings of the step-up transformer.
- X-ray machines haveelectronictimers which are accurate down to <0.01 sec - which is important for measuring the length of exposure.
- Electronic timers are based on a thyrotron tube (a gas filled triode).
- Older machines may use mechanical timers based onspring loadedtimers orsynchronoustimers which use an electric motor to turn a cam.
- These are less accurate than electronic timers and have to be reset between exposures so that rapid serial exposures cannot be taken.
- Phototimerslimit the exposure after a predetermined amount of radiation has reached the film, or a set mA has passed through the tube.