Post on 23-Jun-2015
description
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
sspp
s
p
s
p
iViV
N
N
V
V
Single Phase
3 Phase
Voltage Ripple
I a kVp2
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
sspp IVIV
s
p
s
p
V
V
N
N
p
s
s
p
I
I
N
N
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