Radiography Essentials Chapter 6

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X-ray Circuit and Tube Heat Management

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.

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