X-ray Generators and Transformers Rad T 290 X-ray Generators and Transformers

Transformers are principal components of generators Principal function of x-ray generator is to provide current at high voltage to the x-ray tube Electrical power available to hospital/clinic provides up to about 480 V; need 20,000 to 150,000 V Transformers are principal components of generators Convert low voltage into high voltage through process of electromagnetic induction

Transformers “Transform” alternating input voltage into alternating output voltage using principles of electromagnetic induction

Transformer Law The ratio of the number of coil turns in the primary winding to the number of coil turns in the secondary winding is equal to the ratio of the primary voltage to the secondary voltage

Transformers (cont.) Transformer can increase or decrease voltage Depends on ratio of the number of turns in the two coils NS > NP: “step-up” transformer, increases secondary voltage NS < NP: “step-down” transformer, decreases secondary voltage

Power Power is the rate of energy production or expenditure per unit time (1 Watt = 1 J/s) For electrical devices, P = IV For an ideal transformer: Decrease in current must accompany increase in voltage, and vice versa

Autotransformers Consists of a single coil of wire wrapped around an iron core Law of Transformers still applies Operates on principle of self-induction rather than mutual induction Smaller increases or decreases in secondary voltage than normal transformers Does not electrically isolate primary from secondary circuit

Diodes - Rectifier Electrical devices with two terminals that allow current flow in one direction only Example of a diode is the x-ray tube itself Solid-state diode contains a crystal of a semiconductor material Crystal “doped” with trace amounts of impurity elements Conductivity increased when voltage applied in one direction but reduced to very low level when voltage applied in opposite polarity

Operator Console Operator selects the kVp, the mA (proportional to the number of x-rays in the beam at a given kVp), the exposure time, and the focal spot size Peak kilovoltage (kVp) determines x-ray beam quality (penetrability), which plays a role in subject contrast Tube current (mA) determines the x-ray flux Selection of focal spot size usually determined by mA setting Some generators support preprogrammed techniques

Voltage ripple Voltage ripple of a DC waveform is defined as the difference between the peak voltage and the minimum voltage, divided by the peak voltage and multiplied by 100:

Timing the x-ray exposure Digital timers have largely replaced electronic timers (high reproducibility and microsecond accuracy) Mechanical switches only used in single-phase, low-power generators High-voltage triode switches used in 3-phase and constant potential circuits High-frequency inverter uses electronic switching on the primary side of the high-voltage transformer

Phototimers Measure the actual amount of radiation incident on the image receptor Terminate the x-ray production when the proper amount is obtained Provides a consistent exposure to the image receptor by compensating for thickness and other variations in attenuation in a particular patient and from patient to patient

Falling-Load Generator Works in concert with the phototiming (AEC) subsystem Delivers the maximum possible mA for the selected kVp by considering the instantaneous heat load characteristics of the x-ray tube Continuously reduces the power as the exposure continues Delivers the desired amount of radiation to the image receptor in the shortest possible exposure time