Transcript Chapter 14 X-Ray Circuit

Primary Circuit
1) Main Switch
Location – Between AC source and primary of
step-up transformer
Purpose – Completes external circuit to x-ray machine
2) Fuses – Protects machine from overloaded
circuit
3) Line Voltage Compensator
Location – Within primary circuit & attached to primary of
autotransformer
Purpose – Maintains constant voltage in primary
circuit
4) Autotransformer
Location – Between the AC source and primary
of the step-up transformer
Purpose – Allows control of kVp by varying voltage to primary of
step-up transformer
Principle of operation – Self-induction
The X-Ray Circuit
LOCATION FOR LINE VOLTAGE COMPENSATOR
Primary Circuit
5) Pre-Reading kilovoltmeter
Location – Between autotransformer and primary of step-up trans.
Purpose – Indirectly measures kVp selected/adjustment of line v.
Principle of operation – Connected to circuit in parallel & works
on motor principle
6) Exposure Switch
Location – Exposure switch is between autotransformer & primary
of step-up transformer
Purpose – Manually closes circuit between autotransformer &
step-up transformer
Connected in “series”
Special feature – “Deadman” switch
Primary Circuit
7) Exposure Timer
Location – Between autotransformer & primary of step-up trans.
Purpose – Terminates exposure at proper time by opening circuit
between autotransformer & step-up transformer
Types of Exposure Timers:
1) Mechanical Timer
2) Electronic Timer
3) mAs Meter
4) Automatic Exposure Control (AEC)
5) Back-up Timer
Electronic Timer
AEC Exposure Timers
Phototimer Operation
AEC Exposure Timers
Back-up Timer
Purpose – Stops exposure in case AEC fails
- Prevents overexposure to patient and tube overloads
- May be set automatically by machine or manually on some
equipment
Setting Manual Back-up Time
- Divide mAs/mA
- Time must be at least 1.5 times expected exposure time or
150% of required mAs value for manual setting
- mAs is limited to 600 mAs for exposures over 50 kVp
Example: An AEC calls for 200 mA at .5 S exposure, what back-up
timer setting should be used?
.5 X 1.5 = .75 S back-up time
If setting back-up timer using mAs
200 mA X .5 S = 100 mAs X 1.5 = 150 mAs back-up mAs
Primary Circuit
8) Filament Circuit – Supplies heating current to the filament.
- Supplies 3 – 5 amps at 6 – 10 volts
- This process is controlled by mA button
This circuit also consists of:
mA Selector
Location – Connected in series between the autotransformer
and step-down transformer
Purpose – Regulates amperage to filament circuit that ultimately
controls tube current.
- May use rheostat (variable resistance), choke coil
(self-inductance) or high frequency circuit or saturable
reactor (application of DC to iron core to primary, creating
impedance)
mA Selector in Filament Circuit
mA selector
8) Filament Circuit (continued) – Also contains:
Filament Stabilizer – Corrects for variation in line voltage
Space Charge Compensator – Maintains filament current
for different kVp selections.
Filament Ammeter – Measures filament current.
9) Primary Windings – Step-up transformer
Secondary Circuit
1) Secondary Coil of Transformer
Principle of operation – Mutual induction
Step-up transformer – Steps up voltage to tube, drives
electrons from cathode to anode
Step-down transformer – Steps voltage down and
steps up amperage to filament of tube
2) mA Meter – Measures average tube current
Principle of operation – Motor principle
Location – Connected in series to the secondary of
step-up transformer (includes connection to ground
to protect operator from being electrocuted)
mAs meter is used for very short exposures
3) Rectifier
Purpose – converts AC to DC to prevent reverse bias
Location – Between secondary of step-up transformer and
x-ray tube
4) Cables to x-ray tube – Conducts high voltage between
rectifier and x-ray tube
The X-Ray Cables
Shock Hazard Minimized in Three Ways:
1) Insulation
2) Wire sheath that is grounded
3) Secondary of high voltage transformer is grounded at its
midpoint to minimize amount of insulation needed
The X-Ray Cables
Consist of 3 Conductors:
Cathode end of cable – All 3 conductors attach to filament
(attach to the 2 filament wires)
Other end of wire connects to secondary of transformer and
filament circuit
Anode end of cable –
One wire attaches to anode
At the other end of the cable, all 3 conductors in the cable attach
to a single conductor that attaches to the secondary of the
transformer
The Secondary Circuit
The X-Ray Circuit
The Control Panel
Varies by machine, but may include some of the components
below.
Three Phase Generator Circuits
Consist of 3 single phase currents running 120° out of phase
with each other.
3 Ǿ may be rectified to provide with 6 pulses using 6
rectifiers, 6 pulses with 12 rectifiers or 12 pulses with 12 rectifiers
(3 Ǿ, 6 p = 13% ripple, 3 Ǿ 12 p = 3% ripple
Three Phase Generator Circuits
•To work properly must have 3 primary & secondary windings in
transformer (one for each current)
• Must have 3 autotransformers (one for each current)
• Primary windings must be in delta configuration
• Secondary may be arranged in delta or star (wye) configuration
Advantages and Disadvantages of 3Ǿ Vs 1Ǿ Power Generation
Disadvantages:
1) Possible power surges –
• Current never reaches 0 potential
• Circuit cannot be opened or closed at zero potential
2) Less image contrast
• Due to higher effective kVp generated
Advantages:
1) Higher tube rating with short exposures
•
More mA can be applied during short exposure time
2) Nearly constant potential (less ripple)
- 13% for 3 Ǿ, 6 pulse
- 4% for 3 Ǿ, 12 pulse
3) Higher effective kVp
1Ǿ
3Ǿ
x mAs
2/3 (6 pulse)
4) Higher mAs
x mAs
1/2 (12 pulse)
x kVp
- 12%
Conversion Factors When Changing From 1Ǿ to 3Ǿ
1Ǿ
x mAs
3Ǿ
2/3 (6 pulse)
x mAs
1/2 (12 pulse)
x kVp
- 12%
Example: If 30 mAs is required for a single phase exposure,
how much mAs will be required for the same density on the
image with a 3 phase, 6 pulse generator?
30 X 2/3 = 20 mAs
Example: If 100 kVp were used on an x-ray machine with single
phase generation, how much should be used on a three phase
machine for the same density? 100 X .12 = 12
100 – 12 = 88 kVp
High Frequency Generation
Changes 60 Hz to high frequency current for even less ripple!
Operational Steps of the High Frequency Generator
1) 1Ǿ or 3Ǿ AC current is supplied to machine
2) A DC chopper converts the AC wave to a high frequency
DC wave that is less subject to line voltage fluctuations
3) An inverter converts the DC waveform to a high frequency
AC wave that can be used by the transformer
4) Voltage from the secondary side of the transformer is then
changed to DC for application to the tube, rectified and smoothed
Operation of a High Frequency Generator
High Freq Inverter
DC Chopper
Advantages of High Frequency Generators
• Smaller size
• Shorter exposure times available
• High kVp and mA can be used with short exposure times
• Very little ripple (1% ripple)
• Less variation in line voltage
• kVp can be more easily calibrated and controlled
• Real-time monitoring of kV, mA & exposure time
• Error detection circuitry
Power Rating of X-ray Generators and Circuits
Rated in kW (typically 30 – 80 kW) for x-ray machines
1 Watt = Energy expenditure of 1 joule
For DC P = IV
P (Watts) = Power
I = Current intensity
V = Voltage
Since high frequency generators produce a nearly constant
electrical waveform the same formula for DC can be applied:
P = mA X kV
Generator Problems
A high frequency generator uses 100 mA at 80 kV for an exposure.
How much energy was consumed to produce this exposure?
What is the maximum power rating for an x-ray machine
when the maximum mA for 100 kV is 300 mA.
Find the maximum power rating if the maximum exposure
factors for a particular x-ray machine are 800 mA at 70 kVp.
Falling Load Generators
A generator that automatically starts the exposure at the
highest mA for a selected kVp curve and drops it during the
exposure based on maximum heat loading capacity of the tube.
1) A microprocessor automatically drops mA in small steps
based on the selected kVp curve.
2) Tube operates at near maximum rating to produce optimal
mAs at each point on kVp curve.
Two Types of Technique Selection:
1) One-knob selection - R.T. sets kVp (microprocessor sets mAs)
2) Two-knob selection – R.T. sets both kVp & mAs
(microprocessor controls exposure time by the mA it selects)
Falling Load Generators
Advantages
1) Reduction of exposure time when using high mA
2) Simplifies technique selection by R.T.
3) Takes advantage of maximum tube loading capacity
Disadvantages
1) Takes control away from R.T. in choosing technical factors
Mobile X-Ray Units
1) Battery Powered Mobile Units
• Uses nine, 12 Volt DC batteries connected in series – Powers
mobile unit and x-ray tube (recharged by 110 V AC)
• Circuit Operation
a. Inverter changes DC from battery to 1kHz AC
for transformer use
b) Step-up transformer increases voltage
c) Rectification system changes AC to DC for tube
operation
d) Microprocessor control of kVp and mAs for improved
accuracy
• Uses rotating anode
• Nominal focal spot size of .75 m.m. (varies by manufacturer)
• Allows selection of kVp & mAs (no exp. time selection)
Mobile X-Ray Units
2) Capacitor Discharge Unit
• Older type of mobile unit
• Operates by charging a capacitor immediately prior to exposure
to operate x-ray tube (does not drive unit)
Operation:
a) kVp/mAs values are chosen
b) A charger button is pressed immediately prior to exposure
to charge the capacitor
c) Exposure switch is depressed to start exposure
d) Exposure is terminated via a grid-controlled (triode) x-ray
tube
Capacitor Discharge Diagram
Grid-Controlled X-ray Tubes
• Act as switches to start and stop exposure
• Grid is negatively charged focusing cup insulated from filament
• 1 – 2 kVp is applied to cup to break mA current in tube
Operation
• Exposure is started by removing the negative charge from the
grid
• Exposure is terminated by restoring the negative charge
Advantage – Allows precise control of short exposures.
Grid-controlled X-Ray Tube
Wavetail Cutoff With Capacitor Discharge Units
• Process of stopping capacitor discharge at a pre-set point
on a discharge curve.