radiographic

2.  Heart and Brain of X-ray System

3.  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)

4. p
s
p
s
N
N
V
V

V i 
V i
p p s s

10.  Battery storage
 Capacitor discharge
 Constant 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

11.  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

12.  1) Voltage in two circuits proportional to
number of turns in the two coils
 2) Power (Energy) is conserved:
p
s
p
s
V
V
N
N

 As Power (watts) is voltage x current:
 Therefore as voltage increases by turns ratio,
current decreases p p s s V I  V I
s
p
p
s
I
I
N
N


13. Bushberg

14.  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

15.  Step down transformer drops voltage
 10 V @ 3-5 A
 Filament current (A) indirectly controls tube current (and output
X-ray intensity)

16.  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

17.  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

18.  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

19.  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.
peak
peak
RM S   0.707
2
. . .

20.  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

21.  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)

23. Generator Type / High Voltage Waveform

24.  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

25. Question:
What is highest kVp can
safely use to get 35 mAs
(350 mA & 100ms)?

26. Question:
What is highest kVp can
safely use to get 35 mAs
(350 mA & 100ms)?
Answer:
Should not exceed 100 kVp

27.  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

28.  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

29.  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