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RADT2010 > Topic 4: Generators > Flashcards

Topic 4: Generators Flashcards

1
Q

High tension circuit

A
  • step up transformer
  • mA selector is highlighted
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2
Q

Mains supply

A
  • mains power switch
  • line voltage compensator
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3
Q

primary subcircuit

A
  • kV selector (autotransformer)
  • Timing circuit and exposure control
  • primary of high tension (HT) transformer
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4
Q

High Voltage subcircuit

A
  • secondary of HT transformer
  • rectifiers
  • xray tube
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5
Q

Filament Subcircuit

A
  • Step down transformer
  • mA selector
  • focal spot selector
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6
Q

Transformers

A
  • can only operate with alternating current (AC)
  • x-ray tube needs DC therefore rectifiers are placed before the tube
  • if the voltage increases, the current decreases; step-up transformer; HT transformer
  • voltage decreases; current increases; step down transformer; filament transformer
  • autotransformer; kV selector
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7
Q

Transformer equations

A
  • Vp- primary voltage
  • Vs- secondary voltage
  • Np- # turns in primary coil
  • Ns- # tuns in the secondary coil
  • Is- secondary current
  • Ip- primary current
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8
Q

Generator ratings

A
  • kilowatt rating or power rating: statement of the power output of the unit
  • controlled by generator design
  • max. kV and mA that can be achieved at 100msec
  • industry-standard is to use 100kV and maximum mA at 100 msec
  • P=IV ; Watts= Amperes X volts
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9
Q

Calculating the KW rating

A
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10
Q

MC Question

typical KW Ratings

A
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11
Q

Ripple

A
  • used to distinguish voltage waveform for different types of generators
  • the amount of variation from peak voltage that occurs during x-ray production
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12
Q

Types of Ripple + precents

A
  • 1 phase= 100% ripple
  • 3 phase 6 pulse ripple = 14%
  • 3 phase 12 pulse = 4%
  • HF = 1%
  • CP= 0-5%

CP= constant potential generators: any generator with a ripple of <5%

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13
Q

Rectification

A
  • Changing AC to DC
  • changing the negative half wave to the postive direction
  • required because tubes run on DC but transformers operate on AC
  • Made of semiconductors
  • called PN junction diodes
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14
Q

What does rectification look like?

A
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15
Q

Half wave rectification

A
  • we’ve elimiated the negative on the waveform but we’ve not yet flipped it into the positive
  • full wave rectiifcation is better than half wave
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16
Q

Full wave rectification

A
  • waveform is now “flipped up” into the positive
  • still 100% ripple
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17
Q

Exposure Switching + timing

A
  • how the tube is turned on and off
  • must be done quickly to advoid timing errors
  • accuracy is important
  • terminology:
  1. phasse in time/ phase out time: the time it takes the time it takes the generator to get the stop message ( AKA interrogation). the time it takes for the eqipment to turn on and off
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18
Q

two basic methods of exposrue switching

A

Primary

  • exposure timer
  • least complicated
  • safer to switch low voltages

Secondary

  • at the tube
  • grid controlled tubes
  • used when high exposure rates are required
  • pulsed fluoroscopy

which one is used depends on the unit and the exam being completed

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19
Q

types of exposure timers

A
  • electronic
  • mAs timers
  • AEC
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20
Q

Electronic timers

A
  • consist of complex circuitry based on the time required to charge a capacitor through a variable resistance

How it works

  • higher the resistance, the longer it takes for charge to reach the capacitor; longer exposure time
  • time you then cahnges the timer resistance selector
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21
Q

electronic timers: when the exposure button is pushed

A
  • the exposure begins because the thyristor becomes a conductor
  • the capacitor begins to become charged
  • when the capasitor aquires a certian voltage (a preset refrence value) the SCR device is turned off and the exposure stops
  • the amount of time taken for the capacitor to become completely charged depends on the selected resistance. the greater the resistance, teh longer the time
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22
Q

mAs timers

A
  • a specfifc timer that monitors the mA and time and terminate the exposure after a certain mAs has been achieved
  • used in mobile units and falling load generators
  • rehab unit
  • positioned on the secondary side of high voltage step up transformer
  • also called mAs integrators
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23
Q

AEC

A
  • Consists of several circuit components that convert the x-ray beam to electrical current
  • This current acts on an electronic timer circuit to terminate the exposure
  • a component on the other side of the patient, generally below the dectector and waits until a certain charged is reached to turn off the dectector
  • AEC increases or decreases the time of the exposure only
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24
Q

possible test Q: how does kV help with a hyperstenic patient?

A

incerases the time only

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25
Q

Types of generators

A
  • Single phase
  • Three phase - 6 pulse and 12 pulse
  • High Frequency
  • Falling Load
  • Battery Powered
  • Capacitor Discharge
  • Mains Dependent Mobile (Rad & Fluoro)
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26
Q

Single phase generator

A
  • 100% ripple
  • shortest possible exposure time
  • not currently seen in practice
  • rectitified (0-100)
  • low kilowatt rating
  • only one paggage of voltage from teh line into the unit
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27
Q

three phase generator

A
  • three seperate sources of AC that are ‘out of phase’ with each other by 120 degrees
  • ripple 4-14%
  • higher effective energy of the beam due to the three seperate sources of AC
  • Exposure times as low as 1msec
  • large space requirements
  • popular in the 80s
    *
28
Q

three phase generators: types

A
  • 6 and 12 pulse types
  • number of pulses dependent on design of HT transformer
  • 12 pulse is more complex but has a lower ripple
29
Q

High Frequency

A
  • standard 60Hz is coverted to a much higher frequency: up to 25kHz
  • ripple very close to zero
  • smaller in size because the technology can be smaller
  • it is possible to fit transformers in tube housing (some mobile units i.e. Mobillet)
  • choice for new purchases
30
Q

Falling Load

A
  • special type of 3 phase or HF generator in which the mA decreases during the exposure
  • exposure at the high mA setting
  • allows for the lowest possible exposure time
  • pretty rare, older technology
  • special type of 3 phase
31
Q

Battery Powered

A
  • Use of series of batteries to produce radiation and drive mobile unit—Nickel-cadmium batteries that are rechargeable
  • Output similar to 3 phase or HF
  • Operator chooses kV and mAs or mA and time
  • Most common type of mobile in NA
32
Q

Capacitor Discharge

A
  • Radiation generated from the discharge of capacitors that must be charged prior to each exposure
  • Use grid controlled x-ray tubes to prevent leakage radiation after exposure
  • Collimator designed to automatically close after exposure to prevent leakage
  • Voltage drops during exposure 1kV/mAs
  • Limited to use with low mAs exposures
  • Low output applications
  • soon as the exposure is over the collimator cones close automatically
33
Q

Direct Mobile Radiography

A
  • needs to be plugged into the wall (mains dependent)
  • usually HF with larger tube head for transformers
  • total until is smaller and easier to move
  • hybrid: battery to drive; plugged in to expose
  • the newest are hybrid
34
Q

Direct Power Mobile Fluoroscopy

A
  • Mains dependent only or hybrid design that also uses battery for high output during image acquisition
  • Multiple framing rates
  • Rotating anode tubes
  • Pulsed fluoro options
  • C-arm
35
Q

Ideal generator characteristics

A
  • Efficient at converting electrical energy to x-radiation
  • Maximum dose per mAs
  • High power output
  • Low Ripple
  • Small size / weight
  • Low minimum exposure time
  • ReliabilityCost
36
Q

QC testing

A
  • kV
  • Timer
  • Repeatability
  • Linearity
  • Output
  • Waveforms
  • AEC
  • mA is a very invasive type of testing that engineers do
37
Q

waveforms

A
  • evaluate the actual output waveform to determine if problems exist
  • excellent source of information regarding a unit
38
Q

waveforms: equipment

A

Storage oscilloscope

Detector

Camera to record waveform (newer units have printers or USB ports)

39
Q

waveforms: information gained

A
  • is teh output stable during the exposure?
  • is the ripple consistent with the generator type?
  • are there any voltage spikes indicating problems?
  • is the time accurate?
40
Q

waveforms: how to read them

A
  • Y axis: intensity
  • X axis time
  • total time
  • time scale
41
Q

timer accuracy

A
  • To determine if the unit is delivering the same time of exposure as indicated on the control console

Equipment

  • Digital timing device
  • Oscilloscope
  • Motorized spin top
  • Manual spin top (Single phase only)
42
Q

Timer accuracy: Moterized spin top

A
  • Spins at 1 rps
  • 1 sec = 360 degrees
  • Time = # of degrees in arc/360 degrees
43
Q

Timer Accuracy: Manual Spin top

A
  • Metal top with one hole
  • Spin at 1rpsn1 sec = 120 dots
  • Time = # of dots/120
44
Q

Timer Accuracy: Acceptance limits (SC35)

A
  1. the loading factors must not deviate from the selected value by more than 10% + 1ms
  2. the timer must have a minimum time capability of 1/60sec (0.016 sec) or 5mAs
45
Q

kV Accuracy

A
  • To assure that the x-ray generator is producing the kV as indicated on the control console
  • A major cause of kV variation is calibrationnSome generators maintain their calibration better than others.
  • It is important to note that a change in kV may not be noticed because changes in mA or time will often compensate for changes in kV

Equipment

  • kV meter (filtered ionization chamber)
46
Q

kV accuracy: Acceptance limits (SC 35)

A

loading factors must not deviate from the selected value by more than 10% for X-ray tube voltage

reproducibilty is often evaluated as well

47
Q

mA + mAs: Acceptance limits (SC 35)

A

loading factors must not deviate from the selected value by more than:

  • 20% for the tube current
  • 10% +0.2 mAs for current-time product (mAs)
  • mA can only be measured by an engineer
  • we can work with mAs
48
Q

Output

A
  • The output of an x-ray tube varies with technical factors used.
  • In order to compare output over time and between units it is valuable to accurately determine the exposure per mAs (mR/mAs)
  • These measurements are useful in the following situations:
  1. Quick check on the consistency of output of a particular unit
  2. Assure that the average radiation output is consistent in like systems
  3. Quick indicator of drift
  4. Show a change in beam filtration
  5. Linearity and reproducibility tests
  6. Half value layer measurements
49
Q

Output: equipment + procedure

A

equipment

  • dosimeter

procedure

  • Take exposures at different kV and mAs, dependent on test
  • Follow test protocols, i.e. SID, field size, etc.
  • Determine output (mR) or output per mAs (mR/mAs)
50
Q

Repeatability/ Reproducibility

A
  • The ability of an x-ray unit to produce consistent output exposure for the same technical factors

Equipment

  • dosimeter

Procedure

  • Take several exposures using the same technical factors and exposure conditions. Set technical factors each time by simulating normal operating conditions (change settings and expose between test exposures)
51
Q

repeatability/ reproducibilty : Acceptance limits (SC 35)

A
  1. coeffcient of variation = S/X must not be greater than 0.05
  2. all exposures must be within 15% of the average

Standard dev. Is the truest or best way of determining the information but not really applicable to tests

  1. % variance is 5% or less
  2. all exposures are within 15% of the average
52
Q

Linearity (mA, time, mAs)

A

The relationship between mAs (mA and time) and exposure should be linear when all other factors constant

equipment

  • dosimeter

Procedure

  • take sereval expsoures using constant kV and increasing mA, time or mAs
53
Q

linearity: Acceptance limits (SC 35)

A

% varience= [(Maximum - minimum)/ average] / 2 X 100

adequate linearity exists when teh varience is less than 0.1 or the percent variation less than 10%

54
Q

Reciprocity

A

nAt a constant kV and mAs, the radiation output must be consistent regardless of the mA and time factors used

Equipment

  • dosimeter

Procedure

  • take several exposures using constant kV and mAs, with a variation in mA and time
55
Q

Reciprocity: Acceptance limits (SC 35)

A

adequate reciprocity excist when the variance is less than 0.1 or % variance less than 10%

56
Q

AEC testing

A

To check the various response characteristics of automatic exposure control

  • Reproducibility
  • Response to changes in tissue thickness
  • Field sensitivity
  • kV compensation
  • mA saturation
  • Density control
  • Back up timer
  • Minimum response time
57
Q

AEC: Density Controls + back up time

A

density controls

  • +2, +1, N, -1, -2
  • changes the refernce voltage or resistance of the circuit
  • operator controlled to control image quality

Back up time

  • safety factor
  • 600 mAs maxiumum (SC35)
58
Q

AEC: minmum exposure time + technologist controlled parameters

A

Minimum Exposure Time

  • detector and circuit require a certain minimum time to work
  • body part may be too small for AEC

technologist controlled parameters

  • Detectors
  • Density setting
  • kV, mAnFocal spot
  • Backup time
  • SID and Collimation
  • Positioning
59
Q

AEC testing: Acceptance limits (SC35)

A

within manufacturers specfication

variation should not exceed 20%

60
Q

AEC reproducibility

A

An AEC should produce reproducible results

  • Position phantom on table top at 100 cm SID
  • Select midrange kV and mA
  • Select center detector and normal density
  • Make several exposures
61
Q

AEC: response to changes in tissue thickness

A

An AEC should be able to compensate for varying thicknesses of patients

  • Make several exposures of the phantom making it increasingly thicker - mAs (time) should increase to compensate.
62
Q

AEC: Feild sensitivity

A

The output should remain unchanged regardless of the detector field selected

  • Make exposures of phantom using each field separately
  • Make exposures of the phantom using all combinations of the fields
63
Q

AEC: kV Compensation

A

The AEC should provide a constant IR exposure over a wide range of kV

  • Make several exposures at 60, 80, 100, 120 kV at the mA station commonly used for AEC exposures
64
Q

AEC: mA Saturation

A

At constant kV the AEC should provide the same output regardless of the mA station selected

  • Make several exposures at a fixed kV but use of range of commonly used mA stations
65
Q

AEC: Density control

A

To determine the actual % change in density resulting from the various density controls

  • Expose the phantom at each of the density controls
  • Display results for equipment operators
66
Q

AEC: Back up timer check

A

to determine the actual backup time

  • Cover the detectors with thick sheets of leadnPlace a timing device on top of the lead
  • Make exposure in AEC modenRecord time of exposure
  • The back-up timer should terminate the exposure so the total exposure does not exceed 600 mAs
  • it is recommended that service engineers carry out this test due to undue strain on the system
67
Q

AEC: Minimum exposure time check

A

To record the shortest time at which the AEC will function

  • Place phantom over detectors and timing device on phantom
  • Progressively decrease the thickness of the phantom until the exposure time no longer changes as the thickness decreases
  • There are no minimum limits, however techniques for very thin patients should be at least 2 times the minimum

RADT2010 (2 decks)

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