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Transcript
Principles of Imaging Science II (RAD 120)
X-ray Imaging System Circuitry
X-ray Imaging System
• Operating console
– Set x-ray tube current (quantity) and voltage (quality)
– Controls line compensation, kVp, mA and exposure time
(mAs) via digital meters
– AEC devices
2
X-Ray Circuit
3
1
Operating Console
• RT controls & meters are located within the low voltage
side (primary) of the x-ray circuit
• Reduce shock potential
4
Operating Console Line Compensation
• Incoming Line Voltage (Mains)
– Electricity supply in US is 60 Hz AC, with a nominal
rms of 200 – 240 volts
• Polarity reverses 120 times/sec
• Voltages constantly fluctuate as resistors are activated
in circuit in accordance with Ohm’s Law
– Can vary +5% affecting x-ray production
– Supplied in the form of a three-phase power cycle
5
Single-Phase Power
• A. Voltage drops to zero
with every change in
direction
– Frequency of sine wave
is determined by # of
cycles/sec (cps)
– 60 Hz X 2 directions =
120 changes/sec
• B. Full wave rectified
circuit (DC pulsating)
– Produces no x-ray
photons 120X/sec
7
2
Single-Phase Power 1o
• RMS V of a single-phase
sinusoidal wave is
approximately 70.7% of
peak voltage
• Calculate the rms voltage of
a single-phase sine wave
with 90 kVp peak?
• 63.6
• 65 kVp peak?
• 46
• Inefficient, solve by using
three-phase power
8
Three-Phase Power 3o
• Supplied by power co
• As each wave peak begins
to drop toward 0, voltage
is boosted back to peak by
next phase
• A. Sum of phasing never
drops to 0
• Produces 3 pulses/half
cycle
– 6 pulses/Hz &
pulses/sec
360
9
X-Ray Circuit
10
3
Autotransformer
– Single winding around an iron core
– Operates on self-induction principle
– Good for controlling voltage on low voltage side of
x-ray circuit
– Supplies precise voltage to the high voltage
(secondary) and filament circuits
– @220 volts delivered to primary side from
incoming line voltage
• Voltage compensation automatic on 1o side
– Follows the transformer law
11
Autotransformer
• A, A1: primary
connections that conduct
input power to
Autotransformer
• C: Increases voltage due
to proximity to end and
number of turns encased
by the connections
• E: Decreases voltage
12
Kilovolt Peak (kVp) Selector
– kVp selector on output side
• kVp major, kVp minor controls
• 220 volts delivered to primary side output voltage of
autotransformer is usually between 100 to 400 volts
• Output voltage is then delivered to input side of step up
transformer for x-ray tube operation
13
4
Milliamperage (mA) Selector
•
•
•
•
•
X-ray tube current is controlled
by the filament circuit
Thermionic emission is based
upon temp of the filament
measured in amperes (A)
Filaments operate at 3 – 6
amps, 6-12 volts
Fixed mA stations as resistors
Falling load generators
– Max mA, drops
14
Milliamperage (mA) Selector
• Voltage from mA station is
delivered to filament
transformer (Step-Down)
– Lower voltage, higher
current to filament
• mA meter measures x-ray
tube current
– Placed in the center on
output side of high
voltage transformer
– May be placed on control
console
15
Exposure Timers
•
•
•
•
Determines exposure duration
– Connected on the primary side of the high voltage transformer
Types:
Electronic: Most common, microprocessor controlled.
– Short time 1ms
– Good for multiple sequence imaging
mAs Timer: Electronic timer monitors tube current and is on the
output side of the high voltage transformer.
– Uses the shortest exposure time for mAs selected
– Used in falling load generators
•
•
•
•
•
Designed to work in 3 phase or high frequency generators
Kvp, mA regulated separately
Exposure begins at highest mA, then decreases
Permits better use of acceptable x-ray tube limits; less costly
AEC
16
5
Voltage Rectification
• Required for x-ray tube operation
• Process of changing alternating current (AC) to
pulsating direct current (DC)
• A rectifier functions by allowing current to
flow through it in one direction only
• Electron flow in the x-ray tube must be from
cathode to anode
17
Semiconductors
• Modern method of rectification
• N type & P type semiconductors are used
– N-type have loosely bound electrons that flow
easily between the atom’s conduction bands
• Silicon/Phosphorus, Silicon/Arsenic
– P-type have electron traps (positive holes) that
attract and hold electrons instead of allowing
them to move to another atom
• Silicon/Boron, Silicon/Gallium
18
Diode
• Joining of n-type and p-type semiconductors
• Electrons are attracted toward positive charge
and move through the n-type material to the
junction between the semiconductors.
Additional electrons move in to replace
electrons that migrated
• At the junction, electrons are attracted to the
positive holes keeping a continuous electric
potential in one direction only
19
6
Solid-State Semiconductor Diode
20
Types of Rectification
• Half-wave Rectification
– Single rectifier that
suppresses the negative
half of the alternating
cycle
• No steady flow of
current
• Energy loss in form of
heat
21
Neg Cycle
Types of Rectification
• Full-Wave Rectification
– Four rectifiers that are
arranged to allow
electron flow from
negative - positive
– Uses all the current flow
from the AC source
• Rise and fall of current
potential
• Rippling of current
produces lower energy
x-rays
Pos Cycle
22
7
Full-Wave Rectification
• Positive Half Cycle
– Diodes A & D permit
electron flow during the
positive half cycle
– Diode C cannot conduct
electrons
• Negative Half Cycle
– Diodes B & C permit
electron flow
– Diodes A & D block
electron flow
23
Three-phase generators
• Incorporates three out-ofphase currents to produce
a steadier DC, eliminating
ripple
• Three coils of wire are
wrapped around the
generator core.
24
High-frequency DC
• X-ray circuitry is designed
to increase the standard
60 cycle frequency to 505,000 cycles/sec
– Nearly constant potential
waveform
– Smaller in design
– Increased radiation
quality and quantity
– Lower patient dose
– Increase x-ray tube life
25
8
Voltage Ripple
26
X-ray Circuit Diagram
27
9