Physics Applied to Radiology RADI R250 Fall 2003 CH 9

1
Physics Applied to RadiologyRADI R250 -- Fall
2003 CH 9

• CIRCUITRY

2
X-Ray Machine

• Purpose
• provide a specific current (mA) voltage (kV) to
  the x-ray tube
• convert electrical energy to electromagnetic
  energy (x rays) in a controlled manner
• control the energy of the x-ray photons
• control the number of photons

3
X-Ray Machine Circuit
4
Main Subcircuits (1)
5
Main Subcircuits (2)
6
Transformer Side
7
Functional Position
8
Control Panel Functions

• 1. ON/OFF
• 2. Regulate incoming power
• 3. kVp selection
• 4. TIME selection
• exposure control
• 5. mA selection
• FSS selection

2.
4.
1.
3.
5.
9
Control Panel Circuit Elements

• POWER SUPPLY
• 60 Hz AC 120 to 480 V
• Panel Power On/Off
• AUTOTRANSFORMER
• 1. line compensation
• a. line meter
• b. primary side adjustments
• 2. kVp selection
• a. secondary side adjustments
• variable turns ratio
• 3. filament circuit power

2a.
1a.
2a.
3.
1b.
10
Control Panel Elements (cont.) on secondary
side of autotransformer

• mA selector
• precision resistors
• meter (in transformer)
• FSS selector
• kVp selector
• major/minor taps
• meter (pre-reading)
• time selector
• circuit types
• exposure switch

3a.
2a.
3b.
2b.
2a.
1a.
1b.
11
Timer Circuit Types (1Æ)

• mechanical
• spring action
• synchronous motor
• start/stop with AC cycle
• 60 Hz 60 start/stop positions
• shortest time 1/cycle 1/60 s
• impulse
• start/stop with AC pulse
• 60 Hz 120 start/stop positions
• shortest time 1/pulse 1/120 s

12
Timer Circuit Types (3Æ)
capacitor

• electronic timer

operation selecting time varying resistance in
rheostat to control time it takes to charge the
capacitor activating exposure also begins storage
of charge in the capacitor when a precise voltage
is reached in capacitor, the switch opens
terminates the exposure microprocessor computer
chip technology shortest time 1 ms .001 s
1/1000 s
rheostat
switch
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Functional Position
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Timer Comparison

• 1Æ
• mechanical shortest 1/4 s 250.0 ms
• synchronous motor shortest 1/60 s 16.6 ms
• impulse shortest 1/120 s 8.3 ms
• 3Æ
• electronic shortest 1/1000 s 1.0 ms

15
Timer Circuit Types (3Æ)(cont.)

• mAs timer
• type of electronic timer
• uses tube current (mA) to charge capacitor
• time to charge capacitor time of exposure
• on secondary side of HV transformer

16
Timer Circuit Types (3Æ)(cont.)

• Automatic Exposure Control (AEC) timers
• electronic timer
• detects radiation that has passed through the
  patient
• terminates exposure after set amount of radiation
  reaches sensing device
• based on light emitted or ionization caused

17
Functional Position
18
Transformer Section

• components
• step-up transformer
• mA meter
• rectification circuit
• step-down transformers
• location
• in box linked between control console x-ray
  tube
• immersed in oil for insulation cooling

1.
2.
3.
4.
19
High Voltage Transformer

• Fixed TR between 500 600
• Primary Side (power from autotransformers
  secondary output)
• PS-AT PP-HV
• 120V 10A 120V 10A
• Secondary Side
• V to kV
• e.g. if TR 500 PP-HV 120 V 10 A
• then VS-HV 500 x 120V 60 kV
• and IS-HV 10A 500 .02 A
• amplitude of V sine wave
• see text Table 7.1, page 173

20
mA Meter

• location
• midpoint of secondary coil of HV transformer
• face of meter may be on control console
• grounded
• safety
• places 0V in center of coil
• each wire out of HV transformer carries ½VT
• operates only during an exposure

21
Rectification

• purpose
• convert HV AC to pulsed DC for x-ray tube
• location
• between HV secondary coil x-ray tube
• types
• full wave -- most common
• half wave -- limited use
• self -- rarely

22
Rectification

• process of changing AC to pulsed DC
• rectifier
• device that allows I to flow in only one
  direction
• types valve tube solid state

23
Valve tube (diode) glass vacuum tube w/ 2
electrodes

• 1) negative electrode -- cathode
• thermionic emission of e-
• 2) positive electrode -- anode
• cold metallic plate

24
Valve Tube Operation

• a) AC with on anode - on cathode ( AC pulse)
• e- flow from cathode to anode I

• b) AC with on cathode - on anode (-AC pulse)
• e- drawn back to filament no I

25
Solid State Rectifier
p-type
n-type
p-nj

• 1) "n-type" material (donor) similar to
  cathode
• contains loosely bound e-
• 2) "p-type" material (acceptor) similar to
  anode
• spaces in molecular structure to accept e-
• 3) p-n junction
• union of the two types of materials

26
Solid State Rectifier Operation

• a) AC with on p-type - on n-type (AC pulse)
• e- move across junction to the spaces I

• b) AC with on n-type - on p-type (-AC pulse)
• e- drawn away from the junction no I

27
Rectification Process

• rectifiers placed in circuit to convert AC
  current to pulsed DC
• methods
• suppress unwanted part of the AC (half wave
  rectification)
• redirect unwanted part of AC (full wave
  rectification)

I
V
V
I
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Full-Wave Rectification

• Positive HV pulse Negative HV pulse

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Half-Wave Rectification

• Positive HV pulse Negative HV pulse

x-rays
30
Self Rectification

• Positive HV pulse Negative HV pulse

31
Step-Down (Filament) Transformer

• purpose
• adjust current to cathode filament to produce the
  heat required for the thermionic emission of e-
• control x rays
• IF heat
• heat e-
• e- x rays

32
Filament Transformer (cont.)

• operation
• primary current adjusted by precision resistors
  (Ohms Law)
• fixed TR .10 to .05
• secondary current to 3 to 5 A required for
  thermionic emission
• 2 selectable transformers
• small filament transformer
• large filament transformer

33
Voltage Ripple

• ripple
• voltage fluctuation in waveform
• formula

• minimum voltage (use decimal form of )

34
Root Mean Square Voltage

• 1Ø AC has a 100 voltage ripple
• maximum V ¹ average V
• VRMS "effective" voltage
• VRMS with same heating effect as a specific DC V
• Formula

VRMS .707 Vmax
35
Effective Voltage Problem

• What is the effective voltage for a 1Ø x-ray
  machine set at 75 kVp?
• VRMS ?? Vmax 75 kVp
• VRMS .707 Vmax
• .707 x 75 kVp
• 53.025 kV 53 kV

36
Filament Circuit vs. Tube Circuit
IT anode to cathode P from kVp mA
IF around filament P from V A
37
Circuit Summary

• 2 major subcircuits
• filament circuit
• thermionic emission of e- at filament
• high voltage circuit
• PD to accelerate e- from cathode to anode
• Other circuit parts
• timing method
• rectifiers
• meters

38
Single vs. Three Phase Power

• Single phase (1Ø)
• power supplied or used one cycle at a time
• unrectified

• 1 1 - pulse in each 1/60s cycle
• 200 voltage ripple

• rectified
• 1 or 2 pulses in each 1/60s cycle
• 100 voltage ripple

39
Single vs. Three Phase Power (cont.) Three phase
(3Ø )

• power supplied/used w/ a new cycle beginning each
  120
• results in three overlapping sine waves
• 3 pulses 3 - pulses occur in the length of
  time of one complete cycle (1/60s)
• each wave can be rectified (3Ø6p)
• pulses overlap reducing voltage ripple Vmin

40
3Ø6 Pulse Duration Problem

• What is the pulse duration a 3Ø6 x-ray machine?
• TP ?? p/c 6 f 60
• TP 1/p x 1/f
• 1/6 x 1/60
• 1/360 s 0.0028 s 2.8 ms

41
3Ø Voltage Ripple
12 - 15

• pulse overlap decreases ripple to
• 12-15 on 3Ø6p
• 3-5 on 3Ø12p
• effective voltage nearly maximum voltage

42
Voltage Ripple Problem

• What is the voltage ripple of a 3Ø6 machine that
  has a minimum voltage of 71 kV when machine is
  set at 84 kVp?
• ?? Vmax 84 kVp Vmin 71 kV
• (Vmax - Vmin) / Vmax x 100
• (84 - 71) / 84 x 100
• 15.47619 15

43
Minimum Voltage Problem

• What is the minimum voltage that would be a
  available in a 3Ø12 x-ray machine that has a 4.5
  voltage ripple when 75 kVp is set on the machine?
• Vmin ?? .045 Vmax 75 kVp
• Vmin Vmax - ( x Vmax )
• 75 kVp - (.045 x 75)
• 71.625 72 kV

44
X-ray Machine Power Rating

• kilowatt rating
• electrical power of the machine
• power formula adapted to compensate for
• dimensional prefixes used in x-ray machines
• effectiveness of 3? vs. 1?

45
Kilowatt Rating Problem

• What is the power rating of a 1Ø machine that
  can operate at 150 kVp and 500 mA?
• P1Ø ?? kVp 150 mA 500
• P1Ø (kVp x mA) / 1000 x .7
• (150 x 500) / 1000 x .7
• 52.5 kW 50 kW

46
3Ø comparison to 1Ø

• 3Ø more efficient than 1Ø
• 3Ø requires more complex circuitry
• 3Ø more expensive to install

47
High Frequency Power

• machine designed to have a low ripple (use 1Ø power
• 1Ø AC rectified then smoothed chopped into high
  frequency output (kHz)

48
Rectification Review

• Type rectifiers Ripple Wave Form (2
  cycles)
• 1ØSelf 0 100
• 1Ø1p 1 or 2 100
• 1Ø2p 4 100
• 3Ø6p 6 or 12 12-15
• 3Ø12p 12 3-5

49
Single vs. Three Phase Power (cont.) Three phase
(3Ø )

• power supplied/used w/ a new cycle beginning each
  120
• results in three overlapping sine waves
• 3 pulses 3 - pulses occur in the length of
  time of one complete cycle (1/60s)
• each wave can be rectified (3Ø6p)
• pulses overlap reducing voltage ripple Vmin

50
1Ø vs. 3Ø HV Transformers

• Each phase must have step-up transformer

51
3Ø Transformer Coils
Wye -- delayed output
Delta -- either side
Wye -- secondary only
52
3Æ Transformer Configurations

• simple 3Æ 6 pulse delta/wye (DU)
• primary secondary
• 15 voltage ripple

53
3Æ Trans. Configurations (cont.)

• complex 3Æ 6 pulse delta/wye/wye (DUU)
• primary secondary
• 12 voltage ripple

54
3Æ Trans. Configurations (cont.)

• 3Æ 12 pulse delta/wye/delta (DUD)
• primary secondary
• 2 secondary waves from each primary
• 3-5 voltage ripple

55
3Æ Rectification

• each secondary coil requires 2 rectifiers
• 3Ø6 with DU coils 6 rectifiers

56
3Æ Rectification (cont.)

• 3Ø6 with DUU coils 12 rectifiers

57
3Æ Rectification (cont.)

• 3Ø12 with DUD coils 12 rectifiers

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Ripple Factor ()

• variation in tube V during an exposure
• What is the ripple of the following waveform?

(Vmax-Vmin) / Vmaxx 100
(80kV-68kV)/ 80kV x 100 15 variation of kV
59
Vmin Problem

• An 3Ø12p x-ray generator has a ripple factor of
  5. What is the lowest kV on the x-ray tube
  during an exposure set for 300 mA, 75 kVp and 25
  ms?
• (assume 2 sig.fig.)
• (Vmax-Vmin) / Vmaxx 100
• Vmin Vmax-(/100) (Vmax)
• 75 kVp - (5/100)(75 kVp)
• 71.25 kV 71 kV

60
Generator Summary

• Type p/c p/s Wave Form (2 cycles)
• rectifiers Ripple

1ØSelf 1 60 0 100
1Ø1p 1 60 1 or 2 100
1Ø2p 2 120 4 100
3Ø6p 6 360 6 or 12 12-15
3Ø12p 12 720 12 3-5
61
Medium/High Frequency Generators

• 1Ø unit that increases frequency of power
• uses DC choppers to enable 6 kHz
• high frequency transformers efficiency
• rectified f 13 kHz
• kHz smoothed so PD to x-ray tube nearly constant
• ripple
• small in size

62
Comparison of X-ray Machines

• 1Ø 3Ø High freq.
• V ripple 100
• p/s 60 or 120 720 13,000
• X-ray Quantity X more highest
• X-ray Quality X higher 3Æ
• min time 1P (8 ms) 1 ms 3Æ
• machine X higher highest
• operation X lower ??
• generator size moderate largest smallest

63
Battery Powered Generators

• power from NiCd batteries
• batteries provide all power
• batteries must be recharged
• HV f 500-1000 Hz DC
• rectified f 1000-2000 HZ AC with 100 ripple
• smoothed to minimize ripple

64
Capacitor Discharge Generator

• capacitor bank charged by rectified high voltage
• during exposure capacitors provide kV across tube
• problems
• as capacitors drain kV drops (1 kV/mAs)
• any residual charge may shock operator

65
Falling-Load Generators

• operates at shortest time highest mA
• uses series of steps (mA t) to achieve mAs
• see Figure 7-23 (page 194)

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Miscellaneous Terms

• Interrogation time
• time to start producing radiation from the time
  the signal to begin occurs
• any delay that occurs at the beginning of an
  exposure
• "phase in" time
• 1Ø 1 pulse (8.3 ms)
• 3Ø 1 ms
• High freq. 0 ms (instantaneous)

67
Miscellaneous Terms (cont.)

• kilowatt rating (review)
• power output of the generator
• power that the tube can use
• power formula

68
Exposure Power Problem

• An x-ray machine is set for an exposure to use 90
  kVp, 300 mA and 1/10 s. What is the power used
  for this exposure? (3Æ 1Æ)
• P3Æ?? kVp 90 mA 300
• P3Æ (kVp x mA)/ 1000
• (90 x 300)/1000 27.0 30kW
• P1Æ?? kVp 90 mA 300
• P1Æ (kVp x mA)/ 1000 x .7
• (90 x 300)/1000 x .7 18.9 20kW

69
Machine Power Rating Standard

• kW ratings determined "under load"
• comparison of machine performance
• standard when comparing machines
• not highest kV highest mA on control
• "over load" conditions not acceptable
• Standard for rating
• use highest mA at 100 kVp

70
Power Rating Comparison

• highest mA _at_ three kVp levels
• machine A B
• kVp mA kW mA kW

150 800 120 500 75 125 1000 125 1000 125
100 1250 125 1300 130 standard for comparison
rating