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70 terms

Quality control of radiographic equipment & accessories - 70

STUDY
PLAY
A brighter image
does not affect resolution
A parallel line-type resolution test pattern is used to test
the resolution capability of intensifying screens.
A quality control program includes checks on which of the following radiographic equipment conditions?
1. Reproducibility
2. Linearity
3. Positive beam limitation/automatic collimation
A radiographic rating chart is used to
determine if the selected mA, exposure time, and kVp are within safe tube limits
A slit camera is used to measure
focal spot size
A slit camera, as well as a star pattern or pinhole camera, is used to test
focal spot size
A spinning top device can be used to evaluate
1. timer accuracy.
2. rectifier failure.
A spinning top is
a metal disc with a small hole in its outer edge that is placed on a pedestal about 6 inches high. An exposure is made (eg, 0.1 second) while the top spins.
A synchronous spinning top test is used to test
timer accuracy or rectifier function in three-phase equipment.
A technique chart is therefore strongly recommended for use with AEC; it should indicate
the optimum kV for the part, the photocells that should be selected, and the backup time that should be set.
A technique chart should be prepared for each AEC x-ray unit and should contain the following information for each type of examination:
1. Photocell(s) used
2. Optimum kVp
3. Backup time
A three-phase timer can be tested for accuracy using a synchronous spinning top. The resulting image looks like a
solid arc, with the angle (in degrees) representative of the exposure time
An anode cooling curve identifies
how many HU the anode can accommodate and the length of time required for adequate cooling between exposures.
An exposure that can be made safely with the large focal spot may not be safe for
use with the small focal spot of the same x-ray tube.
An image's spatial resolution refers to its
recorded detail
As the target angle decreases,
the actual focal spot can be increased while still maintaining a small effective focal spot.
As the x-ray tube filament ages, it becomes progressively thinner because of evaporation. The vaporized tungsten is frequently deposited on the window of the glass envelope. This may
1. act as an additional filter.
2. reduce tube output.
3. result in arcing and tube puncture.
Bakelite
the forerunner of today's plastics
Because a full-wave-rectified unit produces 120 x-ray photon impulses per second, in 0.1-second the film should record
12 dots (if the timer is accurate)
Because the heat load an anode will safely accept varies with the size of the focal spot and the type of rectification,
these variables must be identified
Because three-phase equipment produces almost constant potential rather than pulsed radiation,
the standard spinning top cannot be used. An oscilloscope or synchronous spinning top must be employed to test the timers of three-phase equipment.
Because three-phase, full-wave-rectified current would expose a 360° arc each second, a 1/12-second exposure should expose a
30° arc. Anything more or less indicates timer inaccuracy.
Congruence of the x-ray beam with the light field is tested using
radiopaque objects
Delivery of a large amount of heat to a cold anode can cause
cracking if the anode does not have sufficient time to disperse the heat
Delivery of large exposures to a cold anode or the use of exposures exceeding tube limitation can result in
1. cracking of the anode.
2. rotor bearing damage.
Excessive heat can also be conducted to the
rotor bearings, causing increased friction and tube failure.
Exposures in excess of safe values deliver sufficient heat to cause
surface melts, or pits, on the focal track.
HU for 3-phase, 12-pulse equipment are determined by
multiplying mA x time x kVp x 1.41
HU for three-phase, six-pulse equipment are determined by
multiplying mA x time x kVp x 1.35
If exactly one half of the expected arc appears, one should suspect
rectifier failure
If less than the minimum response time is required for a particular exposure, the radiograph will exhibit
excessive density
In order for the projected light beam to be the same size as the x-ray beam,
the focal spot and the light bulb must be exactly the same distance from the center of the mirror.
In which of the following examinations would a cassette front with very low absorption properties be especially important?
Mammography
Linearity means
that a given mAs, using different mA stations with appropriate exposure time adjustments, will provide consistent intensity
Loss of anode rotation would cause
one large melt on the focal track, as the electrons would bombard only one small area.
magnesium
the lightest structural metal
PBL is
automatic collimation and must be accurate to 2% of the SID. Light-localized collimators must be available and must be accurate to within 2 percent.
Reproducibility means
that repeated exposures at a given technique must provide consistent intensity
Selection of the optimum kV for the part being radiographed is essential-
no practical amount of mAs can make up for inadequate penetration (kV), and excessive kV can cause the AEC to terminate the exposure prematurely
surface melts, or pits result in
roughening of the anode surface and decreased tube output
the anode is not heard rotating, the radiographer should
not make an exposure
The backup timer functions to
protect the patient from overexposure and the x-ray tube from overload.
The bulb's emitted beam of light is deflected by a mirror placed at an angle of
45° in the path of the light beam.
The cassette front should be made of a sturdy material with a low
atomic number, because attenuation of the remnant beam is undesirable.
The collimator assembly includes
a series of lead shutters, a mirror, and a light bulb
The device used to test the accuracy of the x-ray timer is the
spinning top
The effect of the input screen's phosphor layer is similar to the effect of the phosphor layer thickness in intensifying screens; that is,
as the phosphor layer can be made thinner, recorded detail increases
The focal spot should be tested periodically to evaluate
its size and its impact on recorded detail; this is accomplished using a slit camera, a pinhole camera, or a star pattern
The minimum response time of an automatic exposure control (AEC)
is its shortest possible exposure time
The mirror and light bulb function to
project the size, location, and center of the irradiated field.
The number of HU produced during a given exposure with single-phase equipment is determined by
multiplying mA x time x kVp
The problem may become apparent when using
fast film-screen combinations or high milliamperage, or when imaging small or easily penetrated body parts.
The product of mAs and kVp determines
HU
The smaller the focal spot,
the more limited the anode is with respect to the quantity of heat it can safely accept
the smaller the input phosphor diameter, the greater the
spatial resolution
The speed of the imaging system has no impact on the use of
a radiographic rating chart.
The spinning top test may be used to test timer accuracy in
single-phase equipment.
The total number of HU that an exposure generates influences
the amount of stress (in the form of heat) imparted to the anode
the tungsten deposit may actually attract electrons from the filament, creating
a tube current and causing puncture of the glass envelope.
To determine how quickly an x-ray tube will disperse its accumulated heat, the radiographer uses an
anode cooling curve
To test the congruence of the light and x-ray fields,
a radiopaque object such as a paper clip or a penny is placed at each corner of the light field before the test exposure is made. After processing, the corners of the x-ray field should be exactly delineated by the radiopaque objects.
Tungsten may vaporize as a result of
anode abuse
What causes pitting, or many small surface melts, of the anode's focal track?
Repeated, frequent overloading
Which formula would the radiographer use to determine the total number of HU produced with a given exposure using three-phase, six-pulse equipment?
mA x time x kVp x 1.35
Which of the following information is necessary to determine the maximum safe kVp, using the appropriate x-ray tube rating chart?
1. mA and exposure time
2. Focal spot size
Which of the following will improve the spatial resolution of image-intensified images?
1. A very thin coating of cesium iodide on the input phosphor
2. A smaller-diameter input screen
Which of the following would be appropriate cassette front materials?
1. Magnesium
2. Bakelite
With full-wave-rectified current and a possible 120 dots (pulses) available per second, one should visualize
12 dots at 1/10 second, 24 dots at 1/5 second, 6 dots at 1/20 second, and so on
With single-phase, full-wave-rectified equipment (120 pulses/sec), for example,
12 dots should be visualized when using the 1/10-second time station. A few dots more or less indicate timer inaccuracy. If the test demonstrated exactly six dots, rectifier failure is strongly suspected.
With three-phase equipment,
a special synchronous spinning top (or oscilloscope) is used and a solid black arc is obtained rather than dots. The length of this arc is measured and compared with the known correct arc.