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222 terms

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.
The most practical and efficient type is the
collimator
Because aperture diaphragms and flare cones have a fixed aperture size and shape,
their beam restriction is not as efficient as that of the variable-size collimator.
Aperture diaphragms, cones, and cylinders may be placed on a collimator track so that
the illuminated crosshairs are visualized.
collimator assembly contributes approximately
1.0 mm Al equivalent to the added filtration of the x-ray tube (because of the plastic exit portal and silver-coated reflective mirror),
The input phosphor of the image intensifier tube functions to convert
x-rays to light
Very close to the input phosphor, separated by a thin transparent layer, is the
photocathode
What is the relationship between kilovoltage (kV) and the half-value layer (HVL)?
As kV increases, the HVL increases
The HVL of a particular beam is defined as
that thickness of a material that will reduce the exposure rate to one half of its original value.
The more energetic the beam (the higher the kV),
the greater the HVL thickness required to cut its intensity in half.
it may be stated that kV and HVL have a direct relationship:
As kV increases, HVL increases.
When using the smaller field in a dual-field image intensifier,
1. a smaller patient area is viewed.
2. the image is magnified.
3. the image is less bright.
When a dual-field image intensifier is switched to the smaller field,
the electrostatic focusing lenses are given a greater charge to focus the electron image more tightly.
The focal point, then,
moves further from the output phosphor (the diameter of the electron image is therefore smaller as it reaches the output phosphor), and the brightness gain is somewhat diminished.
Hence, the patient area viewed is
somewhat smaller and is magnified.
However, the minification gain has been reduced and the image is somewhat
less bright
As the anode rotates, it constantly turns a new face to the incoming electrons; this is the
focal track
A backup timer for the AEC serves to
1. protect the patient from overexposure.
2. protect the x-ray tube from excessive heat.
When an AEC is installed in the x-ray circuit, it is calibrated to produce radiographic densities as required by
the radiologist
The master density is generally set on
normal to produce the required densities. In special cases, when this produces excessive or insufficient density, the master density may be adjusted to plus or minus density.
In which type of equipment does kVp decrease during the actual length of the exposure?
Condenser discharge mobile equipment
Which of the following circuit devices operate on the principle of self-induction?
1. Autotransformer
2. Choke coil
The principle of self-induction is an example of
the second law of electromagnetics (Lenz's law), which states that an induced current within a conductive coil will oppose the direction of the current that induced it.
self-induction is a characteristic of
AC only
The fact that AC is constantly changing direction accounts for
the opposing current set up in the coil.
Two x-ray circuit devices operate on the principle of self-induction:
The autotransformer operates on the principle of self-induction and enables the radiographer to vary the kilovoltage. The choke coil also operates on the principle of self-induction; it is a type of variable resistor that may be used to regulate filament current.
The high-voltage transformer operates on the principle of
mutual induction.
The part of a CT imaging system made of thousands of solid-state photodiodes is the
detector array
A CT imaging system has three component parts:
a gantry, a computer, and an operating console
The gantry component includes
an x-ray tube, a detector array, a high-voltage generator, a collimator assembly, and a patient couch with its motorized mechanism
While the x-ray tube is similar to direct-projection x-ray tubes, it has several special requirements:
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 scintillation detector array is made of
thousands of solid-state photodiodes
hese scintillation crystal (cadmium tungstate or rare earth oxide ceramic crystals) photodiode assemblies convert the transmitted x-ray energy into
light
That light is then converted into
electrical energy and finally into an electronic/digital signal.
If the scintillation crystals are packed tightly together so that there is virtually no distance between them, efficiency of x-ray absorption is increased, and patient dose is
decreased
Detection efficiency is extremely high—approximately
90 %
The high-voltage generator provides high-frequency power to the CT x-ray tube, enabling
the high-speed anode rotation and the production of high-energy pulsed x-ray photons.
Similar to the high-frequency x-ray tubes used in projection radiography, conventional 60-Hz full-wave rectified power is converted to
a higher frequency of 500-25,000 Hz.
The high-frequency generator is
small in size, in addition to producing an almost constant potential waveform.
The CT high-frequency generator is often mounted in the
gantry's rotating wheel
The collimator assembly has two parts:
The prepatient, or predetector, collimator is at the x-ray tube that 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.
Inaccurate indexing can result in
missed anatomy and/or double-exposed anatomy.
Which of the following will occur as a result of a decrease in the anode target angle?
1. Decreased effective focal spot size
2. Greater photon intensity toward the cathode side of the x-ray tube
Target angle has a pronounced geometric effect on the
effective, or projected, focal spot size
As the target angle decreases,
the effective (projected) focal spot becomes smaller.
This is advantageous because it will
improve radiographic detail without creating a heat-loading crisis at the anode (as would occur if the actual focal spot size were reduced to produce a similar detail improvement).
disadvantages
With a smaller target angle, the anode heel effect increases; photons are more noticeably absorbed by the "heel" of the anode, resulting in a smaller percentage of x-ray photons at the anode end of the x-ray beam and a concentration of x-ray photons at the cathode end of the radiograph.
Typical examples of digital imaging include
1. magnetic resonance imaging (MRI).
2. computed tomography (CT).
Special equipment is available for digital radiography (DR) or CR:
images produced by either a fan-shaped x-ray beam received by linearly arrayed radiation detectors or a traditional fan-shaped x-ray beam received by a light-stimulated phosphor plate.
Digital images can also be obtained in
digital subtraction angiography (DSA), nuclear medicine, and diagnostic sonography.
Analog images are conventional images;
they can be converted to digital images with a device called a digitizer.
Pluridirectional tomography refers to
conventional tomographic equipment that is capable of several x-ray tube movements.
What device converts electrical energy to mechanical energy?
motor
The stator and rotor are
the two principal parts of an induction motor.
A generator converts
mechanical energy into electrical energy.
If a radiograph exposed using an AEC is overexposed because an exposure shorter than the minimum response time was required, the radiographer generally should
decrease the mA
using the master control's minus-density adjustment involves decreasing the
exposure time (and this is not possible)
Fractional-focus tubes, with a 0.3-mm focal spot or smaller, have special application in
magnification radiography
Magnification radiography may be used to
demonstrate small, delicate structures that are difficult to image with conventional radiography.
Because object-image distance (OID) is an integral part of magnification radiography, the problem of
magnification unsharpness arises.
The use of a fractional focal spot (0.3 mm or smaller) is essential to the maintenance of
image sharpness in magnification films
Radiographic rating charts
should be consulted, as the heat load to the anode may be critical in magnification radiography.
The long exposures typical of image-intensified fluoroscopy and tomography make the use of a fractional focal spot
generally impractical and hazardous to the anode.
Components of digital imaging include
1. computer manipulation of the image.
2. formation of an electronic image on the radiation detector.
Anode angle will have an effect on the
1. severity of the heel effect.
2. focal spot size.
3. heat load capacity.
As the anode angle is decreased (made steeper),
a larger actual focal spot may be used, while still maintaining the same small effective focal spot. Because the actual focal spot is larger, it can accommodate a greater heat load.
with steeper (smaller) anode angles,
the anode heel effect is accentuated and can compromise film coverage.
Dedicated radiographic units are available for
1. chest radiography.
2. head radiography.
3. mammography.
Dedicated chest units are available that will
transport a piece of unexposed film from the magazine into position between a pair of intensifying screens, make a phototimed exposure, and transport the exposed film to the automatic processor.
Which of the following contribute to inherent filtration?
1. X-ray tube glass envelope
2. X-ray tube port window
inherent filtration is usually the equivalent of
0.5 mm Al.
It is important to remember that as aluminum filtration is added to the x-ray tube, the HVL
increases