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X‐Ray Generator QC
Generator Fundamentals
Generator Function
• Converts electrical energy from power supply
to useable x‐rays
• Controls key beam properties:
•Quantity
•Quality
•Intensity
•Duration
• Recall main components of x‐ray circuit
– Primary autotransformer (PAT)
– Primary high tension transformer (PHTT)
– Secondary high tension transformer (SHTT)
– Primary filament heating transformer (PFHT)
– Secondary filament heating transformer (SFHT)
PAT
• Compensates for voltage drops across mains
by maintaining volts/turn ratio.
PHTT
• Controls voltage input to primary side of high
tension transformer thereby controlling
voltage gain on secondary side
– kV selectors
– Exposure timer switch
– Voltage meter (calibrated to kV)
– Voltage Drop under load compensator
– Space charge compensator
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SHTT
• Provides rectified high voltage across X‐ray tube
• Contains rectifier circuitry
• mA meter
PFHT
• Controls current and voltage to filament heating transformer
(thermionic emission)
– mAs, focal spot selectors
– Part of space charge compensation and VDUL
SFHT
• Always operating at low level current to
maintain cathode filament at certain
temperature
• Boosted to appropriate current, based on mAs
selction, during preparation phase of exposure
Overload Protection
• Circuitry monitors mAs/voltage/focal spot
selection to ensure no excessive power
loading occurs.
• Exposure locked if this is the case.
Steps to Taking an Exposure
• Power Up
– Power distributed to tube locks, movement
controls, table, collimator
– Tube filaments given standby power
– Voltage line compensation occurs if necessary
• Select exposure factors
– Bucky selection
– mA/s
– kVp (space charge compensator may adjust
current to SFHT if low kV selected)
– Focal spot size
– Set SID
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• Exposure Prep.
– Circuits prevent exposure untill certain conditions
met:
•Anode rotation rate is appropriate
•Filament boost circuit has heated filament to approx
2000 degrees
•Voltage applied to Ionization chambers (AEC)
•Grid oscillation initiated
• Exposure
– Circuit closes exposure begins
– Exposure terminated after sufficient
time/mAs/dose reached
– X‐ray machine reverts to pre‐preparation state
Some Examples of Problems
• Line Voltage Compensation failure
– kVp variability which affects beam quality
• Voltage Drop Under Load Compensation
failure
– Non‐congruence between kVp selected and actual
kVp across tube at high mAs
– Beam quality affected
• Space Charge Compensation failure
– Low mAs at low kVp
– Beam quantity affected
• Filament Thinning
– Inherent filtration increase HVL affected
– Higher mAs than that selected
– Quantity of beam affected
kVp Related Tests
• Reproducibility
– Ability to produce same output for given settings
• Accuracy
– How closely actual output meets stated output
• Half Value Layer
– Beam spectrum modified to improve risk/benefit
Reproducability
• An X‐ray generator should produce the same
intensity of radiation whenever the same set
of technical factors are used.
• Reproducability testing evaluates this aspect
of performance through monitoring of the
variance in x‐ray output over a series of
identically controlled exposures.
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• Method
– A series of exposures are taken in succession with
the same technical factors and radiation output is
measured by dosimeter.
– Variance in the exposure levels is then calculated
and compared to standards
• Reproducability failure can lead to
inconsistent image quality and unnecessary
retakes
kVp Accuracy
• Due to the significant effects of kVp on image
quality and patient dose, selected kVp setting
and actual kVp as produced by the machine
must be accurate.
• Failure in accuracy can affect image quality
and patient dose
• Method
– A series of exposures is made at different kVp
levels and, with the aid of a kVp meter, a series of
actual kVp levels is determined.
– Comparison is made with reference to standards.
Which would you rather work with?
• Machine that is often
accurate but doesn't
always reproduce
results
• Machine that is
innacurate but
reproduces results
consistently
Half Value Layer
• Low keV photons
– Attenuated entirely within patient
– Never have probability of reaching IR.
– can increase a patient’s skin dose by as much as
90%
• Inherent and added filtration remove low
energy photons – beam hardening
• Photons reaching patient should have some
probability of reaching IR
Half Value Layer
• The amount of aluminum thickness necessary
to reduce a beam’s intensity to one half of its
initial value.
– Indicative of the keV spectrum within a beam
– Higher HVL – higher average beam energy
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Filtration And The Heterogeneous
Beam
Change in average photon
energy as beam passes through
attenuator
• Method
– Aluminum sheets of unit thickness are
cumulatively added between a collimator and
radiation detection device while successive
radiation measurements are made.
– Results are graphed and the HVL thickness of
aluminum is determined
– Comparison is made to standards
Half Value Layer
• There is a minimum for HVL but no Maximum
• What would be the effect of a very high HVL
– On Patient dose?
– On Image quality?
– On Tube life?
• Why do mammo units usually operate as close
to minimum HVL as possible?
– For maximum absorption