Topic 4: Generators Flashcards
High tension circuit
- step up transformer
- mA selector is highlighted
Mains supply
- mains power switch
- line voltage compensator
primary subcircuit
- kV selector (autotransformer)
- Timing circuit and exposure control
- primary of high tension (HT) transformer
High Voltage subcircuit
- secondary of HT transformer
- rectifiers
- xray tube
Filament Subcircuit
- Step down transformer
- mA selector
- focal spot selector
Transformers
- 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
Transformer equations
- Vp- primary voltage
- Vs- secondary voltage
- Np- # turns in primary coil
- Ns- # tuns in the secondary coil
- Is- secondary current
- Ip- primary current
Generator ratings
- 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
Calculating the KW rating
MC Question
typical KW Ratings
Ripple
- used to distinguish voltage waveform for different types of generators
- the amount of variation from peak voltage that occurs during x-ray production
Types of Ripple + precents
- 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%
Rectification
- 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
What does rectification look like?
Half wave rectification
- 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
Full wave rectification
- waveform is now “flipped up” into the positive
- still 100% ripple
Exposure Switching + timing
- how the tube is turned on and off
- must be done quickly to advoid timing errors
- accuracy is important
- terminology:
- 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
two basic methods of exposrue switching
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
types of exposure timers
- electronic
- mAs timers
- AEC
Electronic timers
- 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
electronic timers: when the exposure button is pushed
- 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
mAs timers
- 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
AEC
- 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
possible test Q: how does kV help with a hyperstenic patient?
incerases the time only
Types of generators
- Single phase
- Three phase - 6 pulse and 12 pulse
- High Frequency
- Falling Load
- Battery Powered
- Capacitor Discharge
- Mains Dependent Mobile (Rad & Fluoro)
Single phase generator
- 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
three phase generator
- 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
*
three phase generators: types
- 6 and 12 pulse types
- number of pulses dependent on design of HT transformer
- 12 pulse is more complex but has a lower ripple
High Frequency
- 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
Falling Load
- 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
Battery Powered
- Use of series of batteries to produce radiation and drive mobile unitNickel-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
Capacitor Discharge
- 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
Direct Mobile Radiography
- 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
Direct Power Mobile Fluoroscopy
- 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
Ideal generator characteristics
- 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
QC testing
- kV
- Timer
- Repeatability
- Linearity
- Output
- Waveforms
- AEC
- mA is a very invasive type of testing that engineers do
waveforms
- evaluate the actual output waveform to determine if problems exist
- excellent source of information regarding a unit
waveforms: equipment
Storage oscilloscope
Detector
Camera to record waveform (newer units have printers or USB ports)
waveforms: information gained
- 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?
waveforms: how to read them
- Y axis: intensity
- X axis time
- total time
- time scale
timer accuracy
- 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)
Timer accuracy: Moterized spin top
- Spins at 1 rps
- 1 sec = 360 degrees
- Time = # of degrees in arc/360 degrees
Timer Accuracy: Manual Spin top
- Metal top with one hole
- Spin at 1rpsn1 sec = 120 dots
- Time = # of dots/120
Timer Accuracy: Acceptance limits (SC35)
- the loading factors must not deviate from the selected value by more than 10% + 1ms
- the timer must have a minimum time capability of 1/60sec (0.016 sec) or 5mAs
kV Accuracy
- 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)
kV accuracy: Acceptance limits (SC 35)
loading factors must not deviate from the selected value by more than 10% for X-ray tube voltage
reproducibilty is often evaluated as well
mA + mAs: Acceptance limits (SC 35)
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
Output
- 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:
- Quick check on the consistency of output of a particular unit
- Assure that the average radiation output is consistent in like systems
- Quick indicator of drift
- Show a change in beam filtration
- Linearity and reproducibility tests
- Half value layer measurements
Output: equipment + procedure
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)
Repeatability/ Reproducibility
- 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)
repeatability/ reproducibilty : Acceptance limits (SC 35)
- coeffcient of variation = S/X must not be greater than 0.05
- 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
- % variance is 5% or less
- all exposures are within 15% of the average
Linearity (mA, time, mAs)
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
linearity: Acceptance limits (SC 35)
% 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%
Reciprocity
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
Reciprocity: Acceptance limits (SC 35)
adequate reciprocity excist when the variance is less than 0.1 or % variance less than 10%
AEC testing
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
AEC: Density Controls + back up time
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)
AEC: minmum exposure time + technologist controlled parameters
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
AEC testing: Acceptance limits (SC35)
within manufacturers specfication
variation should not exceed 20%
AEC reproducibility
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
AEC: response to changes in tissue thickness
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.
AEC: Feild sensitivity
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
AEC: kV Compensation
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
AEC: mA Saturation
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
AEC: Density control
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
AEC: Back up timer check
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
AEC: Minimum exposure time check
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
