Transcript Diagnostic Radiology III

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
VP N P

VS N S
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:
VP I P  VS I S
• 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:
Vmax  Vmin
% voltage ripple 
100
Vmax
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