X-RAY GENERATOR

Ariño Celjhon B.BS. Rad.Tech. II

Generator

Heart and Brain of X-ray System

Tube requirements

Needs DC (is in fact a vacuum tube diode) Can only conduct in one direction:

cathode negative with respect to anode Because of thermionic emission Therefore need rectifiers; convert AC to DC

Need high voltage for X-ray production ½ mv2 into h 110 Volts vs 110 thousand volts Therefore need transformers (changes voltage)

Transformer

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Single Phase

3 Phase

Voltage Ripple

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One vs. Three Phases

Other generators

Battery storageCapacitor dischargeConstant potential gradient

(CPG) Tetrodes (high voltage vacuum

tubes) control kV and exposure time directly on high voltage side

Flat waveform but expensive High freq nearly as good

Transformers

Two separate coils of wire wrapped around closed core

Many configurations Electrical supply

connected to 1 Output device to 2 Step up or step

down

Laws of Transformers

1) Voltage in two circuits proportional to number of turns in the two coils

2) Power (Energy) is

conserved: As Power (watts) is voltage x

current: Therefore as voltage

increases by turns ratio, current decreases

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Bushberg

Autotransformer

Unique single winding design Self inductive

1 & 2 defined by number of turns enclosed by taps

Variable number of turns from taps allows voltage control at relatively low potential

Feeds primary of high voltage transformer and filament transformer

Can be both step up and down

Filament circuit

Step down transformer drops voltage 10 V @ 3-5 A

Filament current (A) indirectly controls tube current (and output X-ray intensity)

High Voltage Circuit

Step up transformer > 500 fold voltage increase Immersed in dielectric Secondary side of autotransformer Fixed # of transformer windings Grounded at center (mA meter)

So for 100 kVp, potential on one side is +50,000 V & other is –50,000 V

Less of an insulation problem

Rectification

Converts AC (needed by transformer) to DC (needed by tube)

Conduct current in one direction only

Vacuum tubes (old style) large, bulky, and burnout

Solid state semiconductor diodes Made of N-P semiconductors

Conduct only on forward bias

Diode Bridge (Wheatstone bridge)

Four diode arrangement to allow current to flow in one direction through tube regardless of polarity of secondary side of high tension transformer

Full wave rectified generator

2x as efficient as self (half) wave rectified

But inefficient compared to high freq & CPG generators

Generator Efficiency

Single Phase – 100% ripple w/ half or full wave rectified

High voltage varies between 0 and max

For single phase, average voltage is R.M.S.

peakpeak

SMR 707.02

...

Three phase generators

Recall AC power avail. in 3 3 voltage peaks per 1/60 sec 3, 6 pulse

High volt transform & rectify 13.5% ripple

3, 12 pulse 2 different winding config on 2°▪ Delta and wye

Another 30° phase shift for 2 halves of output, peaks fill troughs

3.5% voltage ripple

Medium/High Frequency

Transformer efficiency: V ~ NA By increasing frequency, cross sectional area reduced for same power (50kW in tube

head!) Frequency of invertor ranges from 5-100 kHz! Feedback loop controlled – during exposure if kV drops off, increase invertor frequency &

kV increases Timer accuracy Shorter exposures

(<10 ms)

Generator Type / High Voltage Waveform

Tube Limits & Rating Charts

Tube insert has power/load limit Function of heat produced in exposure HU = kVp x mA x time x correction factor

single phase generator – less efficient Correction factor cpg generator =1.4 70 kVp x 100 mA x 0.1 sec = 700 HU (single phase)

Joules = watts x seconds 1 W = 1 V x 1 A = 1000 V x 0.001 A = keV x mA! assume constant voltage, so divide by correction

factor! 70 kVp / 1.4 x 100 mA x 0.1 sec = 500 J (single phase) For cpg is 700 Joules

Question:

What is highest kVp can safely use to get 35 mAs

(350 mA & 100ms)?

Question:

What is highest kVp can safely use to get 35 mAs

(350 mA & 100ms)?

Answer:

Should not exceed 100 kVp

Falling Load

Integrates area under tube rating curve

Applies highest mA in shortest time, reduces mA as exposure continues

Expensive, not used as much with today’s high output tubes

Generator Efficiency Implications

Single phase seldom at peak voltage, so set higher kVp

Three phase higher average kVp Less ripple means more mR/mAs (shorter

exposure time) 5 mR/mAs single vs. 10 mR/mAs three phase

Ripple based on some multiple of 60 Hz High frequency more common now, smaller and

cheaper than CPG

Generator Power Rating

Tube power handling should match generator output

Rated in kilowatts under load (kVp x mA) @ 100 kVp

80 kW generator can produce 800 mA at 100 kVp (simultaneously) Polydoros 80s, Medio CP80

Small clinic may have 20kW, 200 mA at most Angio/Cardio generators 100 kW and greater CT not necessarily high instantaneous, but tube

and generator sustain for long periods