Rad T 290

X-ray Generators and Transformers

• 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

S

P

S

P

N

N

V

V

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

SSPP IVIV

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:

100ripple voltage%max

minmax

V

VV

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