**Saia Unit 5 Flashcards
Battery/DC power
Provides electric potential
AC power
Provides electric potential
Capacitor
Temporarily stores an electric charge
Ammeter
Measures current
Voltmeter
Measures electric potential
Switch
Controls ON/OFF
Transformer
Increase or decrease voltage
Rheostat
Variable resistor
Diode/rectifier
Electrons flow in only one direction
Ground
A neutral object ready to receive electrons
Rectifier
Converts AC to DC
Imaging system contains 3 systems
Operators console
High voltage-generator/section
X-ray tube
Components of the Operators console function on relatively
Low voltage and amperage for Personnel safety
High voltage generator/section function on relatively
High voltage and amperage making them unsafe to be located near humans
X-ray tube functions on
High voltage and amperage which is Insulated for safety
Resistor
Inhibits electron flow
The operators console allows the radiographer to control
The X-ray tube current (mAs), voltage (KVP), and exposure time
MAs= quantity
Refers to the number of X-rays in the beam
KVP= quality
Refers to the penetrability of the beam
Exposure time =
How long voltage is applied
Current will only flow when voltage is applied
Operators console also includes
AEC selection
Bucky selection
On/off switch
Line compensation
Adjusts the incoming voltage to the precise value that the system is designed to operate on - most operate on 220V however the power companies voltage can vary by as much as 5% thus the need for a compensator to keep voltage at a constant 220V
Auto transformer
Variable ratio transformer which supplies a precise voltage to the filament circuit and to the high voltage circuit.
Once the voltage has passed through the auto transformer
It is no longer susceptible to fluctuations or surges in the line voltage
KVP selector
The secondary side of the autotransformer ,the percentage of available voltage is selected by the technologist, usually in 2 control selectors, a major (steps of 10KV) and minor KV (steps of 1-2KV)
Selected voltage from secondary of auto transformer is delivered to
The high voltage(step up) transformer
High voltage transformer has a fixed ratio of 1000:1, every volt becomes a kilovolt
The filament circuit
High amperage is required for thermionic emission in the filament, it is necessary to step-down the voltage - as voltage decreases amperage increases.
MA selector (part of the filament circuit)
Resistors or a rheostat along the filament circuit which controls the amperage delivered to the filament
Most filaments operate at between
3-6 amps,
As amperage to filament increases thermionic emission increases
MA =
The number of electrons thermionically emitted per second
Exposure timers (5 different types)
Mechanical (wind up timer)
Synchronous (measures voltage pulses -capable of 1/120 of a second)
MAS (delivers a selected mAs w/o setting the “S”)
Electric (modern - capable of 1/1000 of a second)
AEC (automatic exposure control)
High voltage generator
Increases voltage to kilovolts, increases amperage for thermionic emission, converts AC to DC
A high voltage generator consists of 3 main parts
High voltage transformer - step up
Filament transformer - step down
Rectifier - converts AC to DC
The high voltage transformer
Reconfigures wattage from primary to secondary, wattage remains constant between primary and secondary sides of the transformer
The more turns on a transformer
The stronger the magnet, the stronger the magnet the higher the voltage
Watt
The unit of electrical power
- volts x amps = watts
The primary side of the transformer
Is the side of the transformer that is initially supplied with current
The secondary side of the transformer
The side of the transformer in which current is induced to flow
Transformers can change voltage and/or amperage between primary and secondary sides of the transformer but…
Wattage remains constant between primary and secondary sides of the transformer
Step up transformer
Voltage is increased from primary to secondary sides
Step down transformer
Voltage is decreased from primary to secondary
Transformer “stepping”
How voltage is affected from primary to secondary sides of a transformer
High voltage transformer
Step up transformer
2 iron cores, operates by mutual induction
Fixed ratio - approximately 1000:1
Primary side initially supplied w/volts and amps
Secondary side induced kilovolts and milliamps
Filament transformer
Step down transformer
2 iron cores, operates by mutual induction
Fixed ratio - approximately 1:3
Primary side initially supplied with volts and amps
Secondary side induced volts and amps
If grid ratio is greater than 1
Step up transformer
If grid ratio is less than 1
Step down transformer
Voltage rectification
Converting AC to DC
Why convert from AC to DC
Transformers only work on AC
X-ray tube is most efficient on DC
Solid state diode/rectifier
Uses material which is semi conductive
Current is conducted through a diode only in one direction, opposed in the other
Cathode is uniquely designed to emit electrons
But not receive
Anode is uniquely designed to receive electrons but
But not to emit
Filament current
Amperage delivered to filament for thermionic emission
Tube current
Electrons traveling from cathode to anode (mAs)
The higher the current voltage, the greater the
Amplitude of the wave
During the positive cycle of the unrectified waveform
Anode is energized with positive charge
Cathode is energized with negative charge
Electrons flow from cathode to anode
During the negative cycle of the unrectified waveform
Anode is energized with negative charge
Cathode is energized with positive charge
Electrons cannot flow from cathode to anode
Half wave rectification is accomplished with
2 diodes which only allow the positive cycle of AC waveform to be conducted through the circuit.
1/120 second of dead time between pulses
Full wave rectification is accomplished with
4 diodes for each phase of power, two diodes block the negative pulse, two redirect it so that it is traveling the same direction as the positive pulse
Cycles per second is measures in
Hertz
Each cycle has a positive and negative and positive pulse
(2pulses), 120 pulses per second
Have wave rectification
60 pulses per second, only the positive pulse of AC waveform used to make xray, 100% voltage ripple
Full wave rectification
120 pulses per second, both the positive and negative pulses used to make xray, 100% voltage ripple
3 phase power
3 independent voltage waveforms working together in phase for mor efficient way to use voltage. Each voltage waveform has 120 pulses per second
3 phase 6 pulse -
360 pulses per second. 13% ripple
3 phase 12 pulse -
720 pulses per second, 4% ripple
High frequency generator
Modern system of powering an xray system, more efficient usage of available voltage.
Creates “square” waveforms of extremely high frequency creating high efficient power.
Only 1% ripple - less mAs required, lower patient exposure.
Voltage ripple
How far the voltage waveform falls from its peak
Main breaker
This is where the alternating current comes from the power circuit
Exposure switch
When you push the button to start an exposure this switch closes to the start the exposure
Auto transformer
This is where you adjust the KVP for exposure
Timer circuit
This part of the circuit stops the exposure
High voltage step up transformer
Bumps the voltage up so that the X-ray tube has very high voltage to make the electrons have enough energy to form X-rays
Four diode rectification circuit
This makes the current only go in one direction through the X-ray tube
Filament circuit variable resistor
Adjusts the current going into the filament
Filament step down transformer
Steps the voltage down and therefore the current up.
X-ray tube
Where X-rays are created
Motor stator
This rotates the anode
X-rays are created on demand, the technologist controls
When X-rays are created (exposure switch)
Strength of X-rays created (KVP adjustment)
How many X-rays created (mAs adjustment)
How long X-rays created (exposure timer)
All are controlled at the operators console
X-rays are created by a conversion of energy to another
Electrical energy into electromagnetic energy through a series of energy conversions.
Electrical, thermal, potential, kinetic, electromagnetic
Voltage
In xray this is the Force/strength of electron propulsion
Amperage
In xray this is the number of electrons in motion
Voltage
Also known as potential difference or potential or electromotive force (emf)
The force that propels electrons
Amperage
Also known as current or impedance.
The number of electrons in motion
1 amp =
6.3 x 10^23 electrons per second
Density
The overall blackening of a film/image
Density is referred to as ? With digital imaging
Brightness
Controlling factors of density are
MA
Time
MAs
MAs
The number of X-rays in the polyenergetic beam
Contrast
The difference in adjacent shades across a radiographic image
Primary function of contrast is to
Make recorded detail visible
High contrast
Black and white, few grays
Low contrast
Image with many grays
Contrast is primarily controlled by
KVP.
As KVP increases there is a greater variety of
Xray energies in the primary beam
Energy deposited into the IR =
Density
More energy values =
More shades of density
Shades of gray
KVP =
Peak xray strength in the polyenergetic primary beam
- X-rays are created at all energy values up to the peak strength
Fluoroscopy was developed
By Thomas Edison in 1896 as a real time dynamic image produced on glass plate covered with a layer of phosphor ,material.
Flux gain
Few xray photons converted to many visible light photons
Image intensifier for fluoroscopy
Glass envelope (maintains vacuum) Input phosphor (cesium iodide) Photocathode (antimony) Focusing lenses (electrostatic - mutual repulsion) Anode (positive charge attracts electrons) Output phosphor (zinc cadmium sulfide)
Image intensifier for fluoro is like am X-ray tube which results in
1 xray photon is amplified to many light photons
The image is intensified
Input phosphor for fluoroscopy
Cesium iodide - efficient at converting xray energy to visible light
Needle like crystals
Xray “Coming in” to the fluoro tube
Output phosphor for fluoroscopy
Zinc cadmium sulfide - efficient at converting electron energy to visible light
Last stage of fluoro tube (last letter Z in alphabet - inc)
Minification gain
Result of many electrons leaving the relatively large input phosphor/photocathode impacting the relatively small output phosphor
As minification gain increases
The visible light image becomes brighter
Minification gain formula
Minification gain = input phosphor diameter^2/output phosphor diamter^2
Standard input phosphor/photocathode sizes are
6”, 9”, or 12”
Total brightness gain
Minification gain x flux gain
How much has the image been intensified by the image intensifier tube
ABC - automatic brightness control
Maintains a preset brightness level by automatically adjusting the exposure factors to compensate for varying subject
Fluoro imaging techniques
Very low mA (0.5-5mA)
Higher KVP utilized
SSD for fluoro
Fixed - 15”
Mobile - 12”
Quantum mottle can be a problem with fluoroscopy due to
Not enough X-rays (mAs)
Not enough photons hitting the input phosphor
Magnification tubes
Modern tubes can magnify images 1.5-4 times
As voltage increases the electrons are pushed closer to the input phosphor which causes the image to be magnified at the output phosphor
The TV camera with fluoro
A camera is placed adjacent to the output phosphor in order to capture and transmit the output intensified image resulting in the radiologist no longer has to view image from phosphor screen and or be in the path of the beam
Two basic types of fluoro cameras
Vidicon - general fluoro
Plumbicon - interventional fluoro
Fluoro splitter
Able to split the signal from output phosphor to multiple components
- monitor
- video
- digital video
- hard film
The smaller the mode, the more magnified the image
Mag mode increases scatter radiation
Changes in voltage to the electrostatic focusing lenses causes
The electrons to narrow or widen their stream
Common field size for angio
35/25/15 cm
Common field sizes for general
25/17cm
A conventional fluoro system often has multiple imaging devices
TV camera Spot films Cine camera Cassette All use the image as displayed on the output phosphor
Fluoro exposure should not exceed
10R/min for general fluoro
5 minute timer
Fluoro unit must alarm every 5 minutes to alert radiologist/surgeon of fluoro time
Magnification fluoro causes
Increased dose
Fluoro is the rad techs #1
Source of exposure
-scatter from patient
Use inverse square law!
Lead aprons must be worn in fluoroscopy that have at least
0.5mm Pb equivalent
Fluoro Bucky slot covers and lead drapes =
At least 0.25mm Pb
High contrast
Few shades of gray, increased contrast, lower KVP, “short scale” contrast
Low contrast
Many shades of gray, decreased contrast, high KVP, “long scale” contrast
Grid conversion factors
No grid = 1
5: 1 = 2
6: 1 = 3
8: 1 = 4
12: 1 = 5
16: 1 = 6
Filtration a minimum of
2.5mm Al/eq
The purpose of filtration
To remove weaker xray from the primary beam
Half value layer
The amount of filtration required to lower xray intensity to 1/2 of its original value
Collimator illuminance
Brightness of collimator bulb and field,
At least 15 foot candles (160 lux) at 40”
Xray to light field
Collimator to xray beam alignment
+/- 2%
Semiannual
Nine penny test
Test done for xray to light field (diameter of penny is .8 inches)
Positive beam limitation
Auto collimation of light field to IR size. Can be smaller, can NEVER be larger!
+/- 2%
Semiannual
Focal spot size is tested by
Pinhole camera, star pattern, slit camera
+/- 50%
Annual
KVP accuracy is measured with
Voltmeter and radiation meter
+/- 10%
Annual
Exposure timer accuracy
+/- 5% greater than 10ms
+/- 20% less than 10ms
Exposure linearity
Using the same mAs but differing combinations of mA and S
i.e. 100mA @ 1/2S same as 200mA @ 1/4S
AEC backup timer is set to self terminate if…
600 mAs reached
If the AEC backup timer is manually set, it should be set to
150% of expected mAs
AEC optical density versus change
+/- .30D
Film illuminator standard is
15 Watt daylight bulb
Fluoro rate ESE shall not exceed
10R/min
IR exposure rate ESE shall not exceed
20R/min
Repeat analysis goal
To minimize patient exposure
Wire mesh test is used to evaluate
Contact between intensifying screen and film
Annually
Speed uniformity is tested to assure
That all like speed intensifying screens respond to the same xray stimulation by releasing the same amount of visible light
+/- 10%
Annual
Darkroom fog can be no greater than
.08 (xray) or .05 (mammo)
Semiannual
Sensitometry
Measuring the response of film to exposure and processing
Daily
Developer is sensitive to changes in
Temperature, oxidation, concentration, contamination
Sensitometer
Device that emits varying intensities of light in a star pattern on a film
Pemetrometer
Aluminum step wedge that is exposed to radiographic film and developed to measure xray penetration
21 step sensitometer is
Most common (41% difference between steps) Also a 11 step (100% difference between steps)
Densitometer
Reads/measures image density, measures how much light is emitted to film and how much is transmitted
Optical density
Incident light striking the film to the intensity of light transmitters through film
The higher the optical density, the less light transmitted
Therefore the greater density
Density difference of contrast indicator
Found by subtracting the Dmin from Dmax
+/-.15OD
Hyporetention (fixer=hypo)
Must be less than .05Gr/m^2
Emulsion turns brown in storage due to inadequate washing of film in processor
Quarterly
+/-2%
Collimator dial accuracy (2% of SID)
SID accuracy (2% of SID)
Xray to light field
Positive beam limitation
+/-5%
REPRO
Exposure reproducibility
AEC reproducibility
+/-10%
KVP accuracy mR/mAs (from installation) Exposure linearity AEC density steps Film illuminators (speed uniformity)
+/- 50%
Focal spot size
Daily
Sensitometry
Quarterly
Hyporetention
Semiannual
Collimator dial accuracy
Xray to light field
Positive beam limitation
Darkroom fog
Annual
Filtration Half value layer Collimator illuminance SID accuracy Focal spot size KVP accuracy mR/mAs Exposure timer accuracy Exposure linearity Exposure reproducibility Protective apparel Film illuminators Film screen contact Speed uniformity
