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143 terms

Radiography Essentials Chapter 6

X-ray Circuit and Tube Heat Management
STUDY
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anatomically programmed radiography (APR)
Computerized controls that are programmable so that exposure factors from a conventional technique chart can be made available from within the computer.
automatic exposure control (AEC)
An electronic circuit within the x-ray machine that automatically terminates the exposure time when a predetermined quantity of x-rays has been detected.
autotransformer
Single-coil transformer.
back-up time
A manually set time that insures that an exposure will be terminated should the automatic exposure control fail.
diode
An electronic device that permits current to flow in one direction only.
electric ground
Can absorb or "drain off" any number of excess electric charges.
electronic timer
More sophisticated devices designed for use with three-phase and high-frequency generators; more capable of ultrashort exposure times; time settings available will be expressed in decimals; most accurate.
exposure switch
Closes the circuit, allowing current to flow through the primary side of the step-up transformer.
exposure timer
A device that terminates the exposure by opening a switch after a preset time has elapsed.
full-wave rectification
In this form of rectification, four diodes are employed in the circuit so the current can be "redirected" during the negative half of the electric cycle so that the current will flow in the same direction during both halves of the cycle; allows x-rays to be formed during the whole cycle of current.
half-wave rectification
In this form of rectification, diodes (one or two) are added to prevent the damage that might occur if the tube fails to provide adequate self-rectification.
heat unit (HU)
How maximum heat capacity of the anode is rated.
high-frequency x-ray generator
Employs special "inverter circuits"; provides a less expensive means of increasing the efficiency of x-ray production; can produce a near constant beam of x-rays.
ionization chamber
An enclosed volume of dry air connected in a circuit; located between the patient and the IR.
kVp meter
Measures the voltage output from the autotransformer; predicts the voltage across the x-ray tube.
line meter
A voltmeter, wired to measure the incoming voltage.
line voltage compensator
Ensures that the outgoing voltage is accurate, regardless of fluctuation in the incoming line; located on the primary (input) side of the autotransformer.
mA selector
A device called a rheostat.
low-voltage circuit
All of the devices of the ____ are located at the control console, with exception of the step-up transformer.
milliammeter (mA meter)
Measures the current flowing in the high-voltage circuit during an exposure; connected at the midpoint of the secondary side of the step-up transformer.
phototimer
Photomultiplier tubes that are sensitive to light.
rectification
Makes the current "right" for x-ray production by ensuring that it flows only in the right direction, in this case from filament to target.
rectifier
A unit used to change the AC into direct current (DC) so that it flows in only one direction.
rheostat
Varies the resistance in the circuit; permits control of the amperage in the circuit.
rotor switch
Necessary to make an exposure; also called "Prep", "Ready," or "Standby"; two functions: when activated it starts the rotating anode which creates the electrons and when held in the "on" position for a few seconds, it will indicate that the tube is ready for an exposure.
self-rectification
The x-ray tube itself controls the direction of current flow; current flows only during the positive portion of the electric cycle, when the anode is positive and the cathode is negative.
single-phase generators
Produce a pulsating current that alternates from positive to negative during each electric cycle resulting in a pulsating x-ray beam.
synchronous timer
Controlled by a small electric motor rotating at 60 revolutions/sec; time settings available will be expressed in fractions in multiples of 1/60 sec.
three-phase current
Alternating current is generated in three overlapping cycles that produce the waveform; generates x-rays more efficiently than single-phase current.
tube rating chart
Establishes the maximum tube capacity for a single exposure.
full-wave rectification
Employs four diodes.
half-wave rectification
Employs two diodes.
three
The x-ray circuit is divided into ____ sections or subcircuits.
low-voltage circuit, filament circuit, and high-voltage circuit
Subcircuits of the x-ray circuit.
low-voltage circuit
The subcircuit between the alternating current (AC) power supply (1) and the primary (input) side of the high-voltage (step-up) transformer (11).
control console
The unit where the operator sets all the exposure techniques, such as kVp, mA, and T/s.
main switch, line meter, line voltage compensator, autotransformer, kVp selector, kVp meter, contacts for autotransformer, exposure switch, and exposure time
Located in or on the control console.
220-V
Power supply to the x-ray machine used by most outpatient facilities.
main switch
Controls the power to the control console.
±5%
How much the actual voltage to the x-ray machine can vary.
line voltage
Small variations in the incoming ____ may cause large variations in the kVp to the x-ray tube.
functions of the autotransformer
Provides the means for the kVp selection, it provides compensation for fluctuations in the incoming line voltage, and it supplies power to other parts of the x-ray circuit.
autotransformer's primary purpose
To vary the voltage to the primary side of the step-up transformer.
kVp selector
Located on the secondary (output) side of the autotransformer; varies the voltage.
output
Secondary side.
input
Primary side.
kVp meter
Prereading voltmeter.
secondary side
When the exposure switch closes the circuit, the current is induced to flow through the ____ of the transformer, creating voltage across the x-ray tube.
electron stream
Voltage causes the ____ to flow across the tube, producing x-rays.
filament circuit
The subcircuit of the x-ray circuit that is divided into two parts by the step-down transformer.
purpose of the filament circuit
To supply and control the heat required by the x-ray tube filament for thermionic emission of electrons.
filament circuit
Determines the mA through the x-ray tube.
primary side
The ____ of the filament circuit begins and ends with the contacts on the autotransformer.
step-down transformer
More coils on the primary side of the transformer than the secondary side.
step-up transformer
More coils on the secondary side of the transformer than the primary side.
filament circuit
Current flows from autotransformer to mA selector to primary side of step-down transformer and back to the autotransformer. Secondary side of step-down transformer conducts current through x-ray tube filament and back to the secondary side.
voltage
The step-down transformer reduces the ____ on the secondary side, providing an appropriate current to heat the filament.
mA selector
This setting determines the number of available electrons at the x-ray tube filament.
high-voltage circuit
This circuit begins and ends with the secondary side of the step-up transformer.
autotransformer, step-down transformer, mA selector, x-ray tube filament, and focal spot selector
Located in the filament circuit.
secondary side of the step-up transformer, x-ray tube, the milliammeter, and the rectifier unit
Located in the high-voltage circuit.
high-voltage circuit
Current flows in this circuit only during an exposure.
step-up transformer
High-voltage or high-tension transformer.
step-up transformer
Increases voltage by a set multiple.
transformer ratio
A set multiple by which a transformer always increases or decreases the incoming voltage.
at least 500:1
Transformer ratio for a high-voltage circuit.
"grounded"
The mA meter is ____.
electric ground
Provides safety from high-voltage shock when the meter is mounted in the control panel.
target
With self-rectification, the ____ of the x-ray tube prevents current from flowing from anode to cathode during the negative phase of the electric cycle.
self-rectification
____ results in prolonged exposure times and wasted energy.
diodes
Prevent "backflow" of current during the negative half of the electric cycle, and it is eliminated.
placed between the anode and the transformer
Placement if one diode is used in a half-wave rectification.
placed between the cathode and the transformer
Placement of second diode if two are used in a half-wave rectification.
half-wave rectification
No more effective than self-rectification.
60 pulses per sec, 1 pulse per cycle/Hertz
Half-wave rectification produces ____.
full-wave rectified
Most modern general-purpose x-ray machines are ____.
full-wave rectified current
Direct current.
constant
True direct current is ____.
pulsating
Rectified alternating current is ____.
120 pulses per sec, 2 pulses per cycle/Hertz
Full-wave rectification produces ____.
self rectification, half-wave rectification, and full-wave rectification
Produced by single-phase electric power.
single-phase current
60 electric cycles that produce 120 impulses (60 + and 60 -).
three-phase current
When this current is rectified, its waveform has the appearance of a "ripple" with no real low points.
40% to 50%
In a three-phase current, exposure times are decreased ____, because more x-rays are produced for the same kVp and results in lower patient dose.
"inverter circuits"
Convert rectified AC into a series of square pulses.
high-frequency generators
Produces a near-constant voltage waveform results in even less exposure time and lower patient dose compared to three-phase.
high-frequency generators
Nearly all new x-ray machines manufactured today use these.
"conventional" or computerized
Types of x-ray control panels.
"conventional" x-ray control panel
Operated by means of knobs and switches. Dials or meters indicate the settings.
computerized x-ray control panel
Have buttonlike controls and digital readouts; automatically perform some functions.
kVp, mA, exposure time, and filament/focal spot size and indicate the current settings
All control panels provide some means for selecting ____.
off/on switch
Controls the power to the control panel and the entire x-ray generator; corresponds to the main power switch in circuit diagram.
line voltage
Older x-ray machine control panels include a meter to monitor fluctuations in ____ and a control knob or button to adjust for ____ compensation; modern control units monitor ____ and compensate for fluctuations automatically.
mA stations
Several choices of mA on a conventional control panel.
300
X-ray equipment in outpatient facilites often has a maximum mA of ____.
mA
Filament and focal spot are chosen automatically when the ____ is selected.
falling-load generators
Provide variable mA that is computer controlled according to the selection of the desired mAs and focal spot size.
reproducibility of exposure, mAs reciprocity, and mA and exposure time linearity
Quality control check tests regularly performed on the mA settings to ensure accuracy.
APR manual mode
The radiographer sets the mAs and the kVp.
APR automatic mode
The radiographer enters the examination and the body part measurement, and the computer selects the correct exposure factors.
depends on back-up timer (may be synchronous or electronic)
Automated timer's minimum exposure time.
17 msec (1/60 sec)
Synchronous timer's minimum exposure time.
1 msec (0.001 sec)
Electronic timer's minimum exposure time.
synchronous and electronic timers
Two types of manually set timers used for x-ray exposure control.
phototimers and ionization chambers
Two types of automated exposure controls.
phototimer
The exposure is terminated after a specific quantity of light has been detected.
ionization chamber
The exposure is terminated when a specific quantity of current has been measured in this circuit.
3
Both phototimers and ionization chambers usually have ____ detectors so that limited operators can select the location or locations within the radiation field where the radiation quantity will be measured.
central detector
Always in the center of the IR at the central ray.
greater
The exposure timer is set to a time that is ____ than the anticipated exposure time because if the back-up time setting is not long enough, the back-up timer will terminate the exposure prematurely.
150%
X-ray operators should voluntarily set the backup time at ____ more than the anticipated exposure.
600 mAs
Generators must terminate the exposure at ____ for exposures above 50 kVp.
2000 mAs
Generators must terminate the exposure at ____ for exposures below 50 kVp.
kVp major selector
Changes kVp by 10 kVp at a time.
kVp minor selector
Changes kVp by increments of 1 or 2 kVp at a time.
±5%
Maximum variability of the kVp.
rotor switch and exposure switch
Necessary to make an exposure.
exposure
Premature release of either the rotor switch or the exposure switch will abort the ____ before it is complete.
check line voltage; adjust if needed
First step in setting control panel and making an exposure; traditional x-ray control.
select milliamperes (mA)
Next step (after checking line voltage) in setting control panel and making an exposure; traditional x-ray control.
select exposure time
Next step (after selecting mA) in setting control panel and making an exposure; traditional x-ray control.
select kVp major first, then minor
Next step (after selecting exposure time) in setting control panel and making an exposure; traditional x-ray control.
set Bucky switch
Next step (after selecting kVp) in setting control panel and making an exposure; traditional x-ray control.
activate and hold rotor switch
Next step (after setting Bucky) in setting control panel and making an exposure; traditional x-ray control.
on signal, activate and hold exposure switch
Next step (after activating rotor switch) in setting control panel and making an exposure; traditional x-ray control.
observe exposure indicator to validate exposure and to determine when it is complete
Next step (after activating exposure switch) in setting control panel and making an exposure; traditional x-ray control.
release rotor and exposure switches
Last step in setting control panel and making an exposure (traditional x-ray control).
primary cause of problems that may shorten tube life
The anode of the tube accumulates enormous heat during exposures.
tube life
The factors that affect ____ are controlled by the radiographer.
cool tube
A ____ should be warmed up before any large exposure; an excessive exposure on a ____ may cause the anode to crack and to fail.
three exposures - 30 secs apart at a setting of 200 mA, 0.5 sec, and 80 kVp
Warm-up procedure in the absence of an established one.
warm-up procedure
Must be repeated if the tube has been idle for more than an hour.
tungsten surface
A rapid series of large exposures or a single excessive exposure may damage the tube by melting the ____ of the focal track.
safe
If the point of the tube rating chart is below the curved line that represents the mA setting, the exposure is ____.
not safe
If the point of the tube rating chart is above the curved line that represents the mA setting, the exposure is ____.
kVp x mA x time
Formula for how heat units (HU) produced by an exposure is determined; used for single phase generators.
kVp x mA x time x 1.35
Three phase Six pulse generator - HU formula.
kVp x mA x time x 1.41
Three phase Twelve pulse generator - HU formula.
kVp x mA x time x 1.45
High frequency generator - HU formula.
heat units (HU)
Used to calculate tube cooling time using a cooling chart.
rotor switch
Do not activate the ____ before you are completely ready to make an exposure; increases wear on both bearings and filament, shortening tube life.
after
It is best to start the rotor ____ the patient is ready.
cause of fluctuation in line voltage
Changes in power demand in the neighborhood or in the building.