Radiographic equipment

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The most commonly used types of AEC devices are the

1. ion chamber.
2. photomultiplier tube.

AECs were originally developed to

achieve more consistent and reproducible film densities

This consistency reduces the number of

retakes, thereby reducing patient exposure dose.

The ion chamber

is positioned between the table and the cassette

the photomultiplier

is located below the cassette

the actual focal spot

is always larger than the effective, or projected/apparent, focal spot

The actual focal spot is a

finite area on the anode that is bombarded by electrons from the filament.

Because of the anode's bevel,

the effective focal spot is foreshortened and therefore is smaller than the actual focal spot

The anode heel effect has to do with

the variation in intensity of the x-ray beam as it is emitted from the focus.

The anode heel effect is more apparent with

small anode angles

The x-ray tube in a computed tomographic (CT) imaging system is likely to be associated with

1. a pulsed x-ray beam.
2. high production of heat units.
3. high-speed anode rotation.

The CT x-ray tube must have

a very high short-exposure rating and must be capable of tolerating several million heat units while still having a small focal spot for optimal resolution.

To help tolerate the very high production of heat units

the anode must be capable of high-speed rotation.

The x-ray tube produces

a pulsed x-ray beam (1-5 ms) using up to about 1000 mA.

The collimator assembly has two parts:

The prepatient, or predetector, collimator is at the x-ray tube and consists of multiple beam restrictions so that the x-ray beam diverges little.

This reduces

patient dose and reduces the production of scattered radiation, thereby improving the CT image.

The postpatient collimator, or predetector collimator

confines the exit photons before they reach the detector array and determines slice thickness.

What is the device that directs the light emitted from the image intensifier to various viewing and imaging apparatus?

Beam splitter

The light image emitted from the output phosphor

of the image intensifier is directed to the TV monitor for viewing and sometimes to recording devices such as a spot film camera or cine film.

The light is directed to these places by a

beam splitter or objective lens located between the output phosphor and the TV camera tube

The majority of the light will go

to the recording device, while a small portion goes to the TV so that the procedure may continue to be monitored during filming

All the following x-ray circuit devices are located between the incoming power supply and the primary coil of the high-voltage transformer

A. the circuit breaker.
B. the kilovoltage selector.
C. the autotransformer.

All circuit devices located before the primary coil of the high-voltage transformer are said to be on the

primary, or low-voltage, side of the x-ray circuit.

The timer, circuit breaker, autotransformer, kilovoltage selector switch, and (prereading) kilovoltage meter are all located

in the low-voltage circuit.

The rectifiers

are placed after the secondary coil of the high-voltage transformer and before the x-ray tube

Advantages of battery-powered mobile x-ray units include their

1. ability to store a large quantity of energy.
2. ability to store energy for extended periods of time.

There are two main types of mobile x-ray equipment:

capacitor discharge and battery-powered.

capacitor discharge units are

light and therefore fairly easy to maneuver,

the battery-powered mobile unit is

very heavy (largely because it carries its heavy-duty power source)

the battery-powered mobile unit

is capable of storing a large mAs capacity for extended periods of time. These units frequently have a capacity of 10,000 mAs, with 12 hours required for a full charge.

Circuit devices that will conduct electrons in only one direction are

1. valve tubes.
2. solid-state diodes.

Rectifiers

change AC into unidirectional current by allowing current to flow through them in only one direction.

Valve tubes

are vacuum rectifier tubes found in older equipment.

Solid-state diodes

the types of rectifiers used in today's x-ray equipment

Rectification systems are found

between the secondary coil of the high-voltage transformer and the x-ray tube.

Resistors

such as rheostats or choke coils, are circuit devices used to vary voltage or current.

Rare earth phosphors that may be used in intensifying screens include

1. gadolinium oxysulfide.
2. lanthanum oxybromide.

Rare earth phosphors are not scarce

they are difficult to separate from other materials with which they are combined in the earth

Rare earth phosphors are much more efficient than

calcium tungstate in absorbing x-ray photons and converting their energy into fluorescent light

Cesium iodide is

the phosphor of preference for the input phosphor of an image intensifier.

The device that receives the remnant beam, converts it into light, and then increases the brightness of that light is the

image intensifier

The visual apparatus that is responsible for visual acuity and contrast perception is the

cones within the retina

Cones are also used for

daylight vision

the most desirable condition for fluoroscopic viewing is

to have a bright enough image to permit cone (daylight) vision, for better detail perception. The image intensifier accomplishes this

The intensified image is then

transferred to a TV monitor for viewing

Cine and spot film cameras

record fluoroscopic events.

Capacitor discharge mobile x-ray units

1. use a grid-controlled x-ray tube.
2. provide a direct current output.

capacitor discharge units consist of

a capacitor, or condenser, which is given a charge and then stores energy until the x-ray tube uses it to produce x-rays.

The charge may not be stored for extended periods, however, because

it tends to "leak" away; the capacitor must be charged just before the exposure is made.

capacitor discharge units

x-ray tube is grid-controlled, permitting very fast (short) exposure times.

Capacitors

discharge a direct current (as opposed to single- or three-phase pulsating current) in which the kilovoltage decreases by a value of approximately 1 kV/mAs

although the value at the onset of the exposure may be 20 mAs and 80 kVp, at the end of the exposure the kV value will be approximately

60

capacitor discharge units permit only limited mAs values, usually

30 to 50 mAs per charge.

The device used to change alternating current (AC) to unidirectional current is

a solid-state diode

Some x-ray circuit devices, such as the transformer and autotransformer, will operate only on

AC

The efficient operation of the x-ray tube, however, requires the use of

unidirectional current, so current must be rectified before it gets to the x-ray tube

The process of full-wave rectification

changes the negative half cycle to a useful positive half cycle

An x-ray circuit rectification system is located

between the secondary coil of the high-voltage transformer and the x-ray tube.

Rectifiers are

solid-state diodes made of semiconductive materials such as silicon, selenium, or germanium that conduct electricity in only one direction.

a series of rectifiers placed between the transformer and x-ray tube functions to

change AC to a more useful unidirectional current.

An incorrect relationship between the primary beam and the center of a focused grid results in

1. grid cutoff.
2. insufficient radiographic density.

The absorption of primary radiation is termed

cutoff and results in diminished radiographic density.

The high-voltage, or step-up, transformer functions to

increase voltage to the necessary kilovoltage

the high-voltage, or step-up, transformer decreases

amperage to milliamperage.

The amount of increase or decrease depends on

the transformer ratio, that is, the ratio of the number of turns in the primary coil to the number of turns in the secondary coil.

To determine secondary V,

Vs = Ns
```` `````````
Vp =Np

To determine secondary I:

Ns = Ip
``````` ````````
Np =Is

All of the following are components of the image intensifier

A. photocathode.
B. focusing lenses.
C. accelerating anode.

The input phosphor of an image intensifier

receives remnant radiation emerging from the patient and converts it to a fluorescent light image

Directly adjacent to the input phosphor is the

photocathode, which is made of a photoemissive alloy (usually a cesium and antimony compound).

The fluorescent light image strikes the photocathode and is converted to

an electron image

The electrons are carefully focused, to maintain image resolution, by the

electrostatic focusing lenses, through the accelerating anode and to the output phosphor for conversion back to light.

The advantages of capacitor discharge mobile x-ray equipment include

1. compact size.
2. light weight.

Condenser discharge mobile x-ray units do not use

batteries; this type mobile unit requires that it be charged before each exposure

A condenser (or capacitor) is a device that

stores electrical energy. The stored energy is used to operate the x-ray tube only.

The major disadvantage of the capacitor/condenser discharge unit is that

as the capacitor discharges its electrical charge, the kVp gradually decreases throughout the length of the exposure-therefore limiting tube output and requiring recharging between exposures

Excessive anode heating can cause vaporized tungsten to be deposited on the port window. This can result in

1. decreased tube output.
2. tube failure.
3. electrical sparking.

Vaporized tungsten acts as

an additional filter, thereby reducing tube output.

The tungsten deposit may also

attract electrons from the filament, creating sparking and causing puncture of the glass envelope and subsequent tube failure.

Double-focus x-ray tubes have two

filaments

A double-focus tube has two

focal spot sizes available

These focal spots are actually two

paths available on the focal track.

When the small focal spot is selected,

the small filament is heated, and electrons are driven across to the smaller portion of the focal track

When the large focal spot is selected

the large filament is heated, and electrons are driven across to the larger portion of the focal track.

Which of the following functions to increase the mA?

Increase in heat of the filament

The x-ray tube filament is made of

thoriated tungsten.

When heated to incandescence (white hot), the filament liberates electrons-a process called

thermionic emission

It is these electrons that will become the

tube current (mA). As heat is increased, more electrons are released and mA increases.

The x-ray imaging system that uses a flat panel detector built into the x-ray table is

direct digital radiography

In which of the following portions of the x-ray circuit is a step-down transformer located?

Filament circuit

Transformers are used to

change the value of alternating current (AC).

Transformers operate on the principle of

mutual induction

The secondary coil of the step-up transformer is located

in the high-voltage (secondary) side of the x-ray circuit.

The step-down transformer, or filament transformer, is located

in the filament circuit and serves to regulate the voltage and current provided to heat the x-ray tube filament.

The rectification system is also located

on the high-voltage, or secondary, side of the x-ray circuit.

The primary parts of the cathode include the

1. filament.
2. focusing cup.

The typical diode x-ray tube consists of

a positive electrode (the anode) and a negative electrode (the cathode)

Electrons are

released from the cathode's filament, directed toward the anode by the cathode's focusing cup, and delivered at very high speed to the anode's focal track.

The functions of a picture archiving and communication system (PACS) include

1. acquisition of digital images.
2. storage of digital images.

A parallel-plate ionization chamber receives a particular charge as x-ray photons travel through it. This is the operating principle of which of the following devices?

Automatic exposure control

A device used to ensure reproducible radiographs, regardless of tissue density variations, is the

phototimer

A photostimulable phosphor plate is used with

computed radiography (CR)

CR does not use

traditional intensifying screens or film. Rather, the CR cassette contains a photostimulable IP that functions as the image receptor (rather than as film emulsion)

The batteries in battery-operated mobile x-ray units provide power to

1. the x-ray tube.
2. machine locomotion.
3. the braking mechanism.

The image intensifier's input phosphor is generally composed of

cesium iodide

Cesium iodide is much more efficient in this conversion process than was the phosphor previously used:

zinc cadmium sulfide

How is the thickness of the tomographic section related to the tomographic angle?

The greater the tomographic angle, the thinner the section

The procedure whose basic operation involves reciprocal motion of the x-ray tube and film is

tomography

Accurate operation of the AEC device is dependent on

1. positioning of the object with respect to the photocell.
2. beam restriction.

If collimation is inadequate and a field size larger than the part is used, excessive scattered radiation from the body or tabletop can cause the AEC to

terminate the exposure prematurely, resulting in an underexposed radiograph.

Exposures less than the minimum response time of an AEC may be required when

1. using high mA.
2. using fast film-screen combinations.

The minimum response time, or minimum reaction time, is

the length of the shortest exposure possible with a particular AEC.

If less than the minimum response time is required for a particular exposure, the radiograph will exhibit

excessive density.

This problem becomes apparent when making exposures that require

very short exposure times, such as when using high milliamperage and fast film-screen combinations

To resolve this problem, the radiographer should

decrease the mA rather than the kVp, in order to leave contrast unaffected.

Which of the following techniques is used to evaluate the dynamics of a part?

Fluoroscopy

Stereoscopy

is a technique used to produce a radiographic third dimension.

Tomography

produces sectional images of body parts by blurring superimposed structures above and below the section, or level, of interest.

X-ray tube life may be extended by

1. using low-mAs/high-kVp exposure factors.
2. avoiding lengthy anode rotation.
3. avoiding exposures to a cold anode.

X-ray tube life may be extended by using exposure factors that produce a minimum of heat, that is

a lower mAs and higher kVp combination, whenever possible.

When the rotor is activated,

the filament current is increased to produce the required electron source (thermionic emission). Prolonged rotor time, then, can lead to shortened filament life as a result of early vaporization.

Large exposures to a cold anode will

heat the anode surface, and the big temperature difference can cause cracking of the anode. This can be avoided by proper warming of the anode prior to use, thereby allowing sufficient dispersion of heat through the anode.

Disadvantages of moving grids over stationary grids include which of the following?

1. They can prohibit the use of very short exposure times.
2. They increase patient radiation dose.

moving grids function to

blur the images of the lead strips on the radiographic image

disadvantages of moving grids

1. their complex mechanism is expensive and subject to malfunction.
2. today's equipment makes possible the use of extremely short exposures, a valuable feature whenever motion may be a problem; grid mechanisms frequently are not able to oscillate rapidly enough for short exposure times, and as a result grid motion is "stopped" and lead strips are imaged.
3. patient dose is increased with moving grids. Since the central ray is not always centered to the grid because it is in motion, lateral decentering occurs (resulting in diminished density), and consequently an increase in exposure is needed to compensate (either manually or via AEC).

To maintain image clarity in an image intensifier system, the path of electron flow from the photocathode to the output phosphor is controlled by

electrostatic lenses

The input phosphor of an image intensifier receives remnant radiation emerging from the patient and converts it to

a fluorescent light image

Directly adjacent to the input phosphor is the

photocathode, which is made of a photoemissive alloy (usually a cesium and antimony compound)

The fluorescent light image strikes the photocathode and is converted to

an electron image

The electrons are carefully focused, to maintain image resolution, by the

electrostatic focusing lenses, through the accelerating anode and to the output phosphor for conversion back to light.

When the radiographer selects kilovoltage on the control panel, which device is adjusted?

Autotransformer

Because the high-voltage transformer has a fixed ratio, there must be a means of changing the

voltage sent to its primary coil; otherwise, there would be a fixed kVp

what makes these changes possible?

autotransformer

When kVp is selected on the control panel, the radiographer is actually adjusting the

autotransformer and selecting the amount of voltage to send to the high-voltage transformer to be stepped up.

The filament circuit supplies

the proper current and voltage to the x-ray tube filament for proper thermionic emission.

The rectifier circuit is responsible for

changing AC to unidirectional current.

Capacitor discharge mobile x-ray units use capacitors to power the

x-ray tube

Because capacitor discharge mobile x-ray units do not carry many batteries, it is much lighter and does not need a

motor to drive or brake it

Together, the filtering effect of the x-ray tube's glass envelope and its oil coolant are referred to as

inherent filtration

The glass envelope and oil coolant provide approximately

0.5 to 1.0 mm Al equivalent filtration, which is referred to as inherent because it is a built-in, permanent part of the tube head.

What equipment is mandatory for performance of a myelogram?

Tilting x-ray table

Myelography

requires that contrast media be instilled into the lumbar subarachnoid space and distributed via gravity to various levels of the subarachnoid space. This gravitational distribution is accomplished through the use of an x-ray table that is capable of angling or tilting during the procedure.

Star and wye configurations are related to

three-phase transformers

The terms delta and wye (or star) refer to

the configuration of transformer windings in three-phase equipment

Instead of having a single primary coil and a single secondary coil, the high-voltage transformer has

three primary and three secondary windings-one winding for each phase

Autotransformers operate on the principle of self-induction and have

only one winding.

Three-phase x-ray equipment often has

three autotransformers

The advantages of collimators over aperture diaphragms and flare cones include

1. the variety of field sizes available.
2. more efficient beam restriction.

There are three types of beam restrictors:

aperture diaphragms, cones and cylinders, and collimators.

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