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Physics Ch. 5 Rectification, Power, High Frequency Generators, Voltage Ripple
Terms in this set (54)
Control Panel Circuit Elements
1) Line Voltage Compensator
2) Autotransformer--Kv major and minor
4) Timing Circuitry
Line Voltage Compensator
Since PG&E can't always supply continuous 220 power for x-ray machines
-Is wired to the Autotransformer
-Function: Automatically MEASURE & ADJUSTS the voltage coming into the x-ray generator to 220 volts
-Fluctuations can vary the power by 5%
-Time of day
-Amont of power used in other parts of the dept.
-# of patients being done in a certain time period.
-Located in the operating console
-Kv is selected at the control panel
-Has a VARIABLE turn ratio to step up or step down the incoming line voltage to be sent to the high voltage TRANSFORMER
-is tapped off to provide the kilovoltage in multiple of ten
is tapped off to provide kilovoltage in increments of one or two
Voltmeter (Kv Meter)
-Reads the voltage coming off the autotransformer
-Meter is connected in PARALLEL
High Voltage Transformer
-Has a FIXED turn ratio
-Usually the TURN RATIO is btwn 500 and 600 on most modern x-ray machines.
-Oil acts as insulator and coolant
-The process of converting ALTERNATING current to DIRECT current
-A rectifier is the device that allows CURRENT to flow in ONE DIRECTION only! UNIDIRECTIONAL
Why is it important that a rectifier allows current to flow in ONE DIRECTION only?
Current will flow?
can damage the filament and destroy it?
From Cathode (-) to Anode (+)
Why is Voltage Rectification important?
During the negative swing of the waveform, VOLTAGE could flow back to the CATHODE from the ANODE.
-CALLED Inverse Voltage
-Can damage the tube
DC chopper is device that converts
DC to pulsed DC
-is a DIODE
-An electronic device that contain 2 ELECTRODES
-in SOLID STATE
-Semiconductor type made of Silicon
Half Wave rectification
x-ray output from a half-wave unit is
-A condition in which the VOLTAGE is NOT allowed to swing NEGATIVELY during the HALF of its cycle.
-1 or 2 diodes
-common in early x-ray machines
Half Wave rectification Drawback
HALF the supply of POWER is wasted and TWICE the exposure time required.
The (-) half of the cycle is REVERSE so that a (+) VOLTAGE is always directed across the x-ray tube.
-At least 4 diodes/ rectifiers
-Currents is passed through the tube at 120 pulses/second
Full-wave rectification heat load capacity of the tube is
increased so HIGHER mA and kV can be used.
-Full-wave rectification is preferred since no power is wasted
-Main advantage: the exposure for ANY GIVEN technique is cut in half.
If one of the four diodes is damaged in a FULL-WAVE rectification
Image would look like
-Full-wave rectification would not occur
-Half-wave rectification would occur
For film: light, underexposed
For digital: S# high, Lgm # low
Single phase Power
When the waveform in a single phase power is near 0 the x-rays produced are
Results in a PULSATING x-ray beam with VOLTAGE going from 0 to a MAXIMUM of 120 pulses/second
-low ENERGY and low PENETRABILITY so have little diagnostic value.
One way to overcome deficiency from single phase power is to
-generate three simultaneous voltage waveforms out of sync with each other.
-called a 3 phase power
Three phase power
-Multiple waveforms are superimposed on one another which results in a waveforms that maintains a NEAR CONSTANT high voltage
-There is no relatively zero point
-There can be 6 pulses per 1/60 of a sec OR
12 pulses per 1/60 of a sec
Difference between 3 phase, 6 pulse AND
3 phase, 12 pulse is
the number of diodes
Three Phase, Six pulse
-only rectified one
3 phase generators have
-3 autotransformers supplying one HIGH VOLTAGE transformer.
-The HIGH VOLTAGE has 3 primary and 3 secondary coils.
-Primary coils are configured in a delta pattern ^
-Secondary coils are configured in a star or Y (wye) pattern
High Frequency generators
-Capable of generating btwn 500 to 5000Hz instead of the usual 60Hz.
-most common today
-virtually no wasted power bc waveform never comes back down to 0, is more constant
-Few low energy x-rays made so lower radiation dose the patient.
High Frequency generators Advantages
-Smaller than 3 phase
-A variation in peak waveform
-the less variation the more Efficient and Powerful the energy of the beam.
Single (one) phase
-has a voltage ripple of 100%
-Voltage goes from 0 to 100% and back down to 0 again. Not very Efficient
3 phase, 6 pulse power produces
-voltage with 13% ripple
-waveform never goes down to 0
-VOLTAGE supplied to the x-ray tube never falls below 87% of the maximum value
3 phase, 12 pulse power has
-VOLTAGE never falls below 96%
High Frequency generators are
-Voltage ripple is less than 1%
-Main advantage is production of higher radiation Quantity and Quality so less technique is needed.
-Located between the Autotransformer and the High-voltage transformer (with one exception)
Types of timers
-Used on old portable and dental x-ray units
-works like an egg timer
-Minimum exposure time is 250ms (.25, 1/4 sec)
A small electric motor designed to turn a shaft 60 revolutions/minute (no Q on this)
-Minimum exposure time is 17ms (1/60 sec)
-Most sophisticated, complicated & accurate of all x-ray timers
-Allows a wide rage pf exposure intervals and is accurate to 1ms (.001)
-Capable of rapid sequence imaging so particularly suited for intervention imaging procedures.
-An electronic timer that measures the product of the mA & the exposure time and shuts off the exposure when the desired mAs is obtained
-Since the timer must monitor the tube current to function properly, it is located on the secondary side of the high-voltage transformer.
-is wired into the circuit following the high-voltage transformer
Automatic Exposure Control (AEC)
-A device that uses 3 detectors to measure the amount of radiation reaching the image receptor.
-Automatically terminates the EXPOSURE when the required amount of exposure has reached the IR.
-Tech sets kV, AEC sets the mAs
Photomultiplier Tube (PMT) (QUIZ)
-Light sensitive device
-Positioned under the IR
-x-rays go through the pt, through the IR, and through a fluorescent screen
-Screen fluoresces & gives off light
-converts the LIGHT energy into ELECTRICAL energy
-exposure is terminated when CHARGE received corresponds to the pre-selected optical density.
-Radiation sensitive device
-Most common, has 3 replaced PMT's " placed BETWEEN THE PATIENT AND THE IR.
-Very thin and filled with small compartments of gas
-Ionization occurs within the chamber when
x-rays react with the gas and a charge is produced.
-When the appropriate CHARGE has been reached based on the pre-set optical density, the exposure is terminated.
Back Up Timer (AEC systems)
-refers to the length of time the exposure will continue.
-Usually set at 1-2 seconds
-Role of back-up timer is to act as a safety mechanism in case of AEC failure
-Protects the patient from over exposure
-protects the tube from excessive heating
Proper operation of the AEC depends on several factors:
-the correct placement of the part over the detector
-Improper placement results in over or under exposure of the part
-Poor positioning is the most common error when exposing with an AEC
Proper detector selection
Detectors(s) selected should be superimposed by the anatomic structure being visualized
-Ex: PA chest uses two outside detectors
Accurate collimation why?
-Excessive scatter causes the AEC to terminate the exposure early.
-If film, it would come out as a light image
AEC the part must be
What else might affect proper operation of the AEC?
what is the alternative?
conducive to an exposure with an AEC.
-Barium, bowel gas, pathology
for AEC the correct
bucky must be selected.
-Most radiographic rooms contain an AEC device in a wall unit and in the x-ray table.
what will happen to the exposure if table bucky is selected and the patient is standing in front of the upright bucky for a PA chest?
How else can over exposure occur?
-When AEC is activated during cross-table examination or gurney work
-Most newer radiographic units are designed so an exposure cannot be made if AEC is selected but no cassette is in the bucky
kV Selection and the AEC
-The AEC device controls the DENSITY of the IR NOT the kV
-The kV Selected should reflect
--the optimum penetration of the part
--the optimum scale of contrast
-Using higher kV allows the AEC device to select a shorter exposure time
--Use of high kV must be balanced with scale of contrast
AEC may not work with low kV, Why?
-decrease kV= Increase absorption on tissues
-decrease x-rays reach detectors
-AEC devices are equipped with density controls
-allows the radiographer to fine tune the density produced by the unit
-Generally in the form of buttons on the control panel numbered -2,-1, +1, +2
-Each increment reflects an increase or decrease in exposure timer from average by 25%
-Routine use of (+) or (-) density selections indicates a problem with the CALIBRATION of the AEC device
Displays the mAs the AEC device used during the exposure
Whys is looking at the mAs readout important for technologist to notice?
-Become more knowledgeable of manual technical factors
-If image is sub-optimal due to exposure, gives the technologist a baseline for the repeat
-Helps with studies (air-con BE) where AEC and manual exposure are combined.
Minimum Response Time
-Refer to the SHORTEST exposure time that the system can produce.
-is LONGER than other types of radiographic timers of any AEC system
-Important to consider when radiographing pediatric and uncooperative patients
Anatomically Performed Radiography (APR)
-System that allows technologist to select a button on the control panel that represents an anatomic area.
-Preprogrammed set of exposure factors displayed
-Technologist can adjust technical factors or over ride and use the AEC
-AEC and APR are not related in there functions but are often combined on units
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