Resident Physics Lectures Christensen, Chapter 2B Tube Ratings George David Associate Professor...
-
Upload
wendy-clark -
Category
Documents
-
view
218 -
download
1
Transcript of Resident Physics Lectures Christensen, Chapter 2B Tube Ratings George David Associate Professor...
Resident Physics LecturesResident Physics Lectures
• Christensen, Chapter 2B
Tube Ratings
George DavidAssociate ProfessorDepartment of RadiologyMedical College of Georgia
Heat UnitsHeat Units
• A unit of energy• Single PhaseSingle Phase Definition
Kilovoltage X tube current X exposure time kVp X mA X sec
• Three Phase (constant potential/high frequency) Definition
1.35 X Kilovoltage X tube current X exposure time 1.35 X kVp X mA X sec
Heat UnitsHeat Units
• 70 kVp
• 200 mA
• 0.25 second
Single Phase Exposure
• 70 X 200 X 0.25 = 3500 heat units
Heat Units
?
Heat UnitsHeat Units
• 60 kVp
• 100 mA
• 0.1 second
Three Phase Exposure
• 60 X 100 X 0.1 X 1.35 = 810 heat units
Heat Units
?
Heat is the EnemyHeat is the Enemy
X-Ray Tube Heat
Tube Rating ChartTube Rating Chart
• Indicates load limit tube can safely accept
• Based upon Tube construction High Voltage
Waveform
Tube, Target, & RatingsTube, Target, & Ratings
• surface area bombarded by electrons
focal spot size (actual) target angle anode diameter
• Melting point• Heat transfer• Anode rotation speed
Tube Rating ChartsTube Rating Charts
• single exposure
• multiple rapid exposure (angiographic) capability
Single Exposure Rating ChartsSingle Exposure Rating Charts
• Tube specific• Incorporated in virtually all
generators prevents illegal single exposures
• Better ratings (more heat allowed) for
Large focal spot High speed anode rotation
Typical Single-Exposure Tube Rating Chart
Typical Single-Exposure Tube Rating Chart
• shows maximum exposure time for single exposure at given kV & mA
ExampleExample
• What is the maximum exposure time at 90 kVp & 300 mA?
ExampleExample
• What is the maximum exposure time at 120 kVp & 400 mA?
?
Can’t do 120 kVp at 400 mA for any exposure time.
Single Exposure Rating ChartsSingle Exposure Rating Charts
• Actually 8 charts combining: generator
» single phase (1» three phase (3
focal spot» small
» large
anode speed» standard (3400 rpm)
» high (9600 rpm)
1SFS
3400 RPM
3SFS
3400 RPM
1LFS
3400 RPM
3LFS
3400 RPM
1SFS
9600 RPM
3SFS
9600 RPM
1LFS
9600 RPM
3LFS
9600 RPM
On-Board Tube Rating ChartsOn-Board Tube Rating Charts
• Checks to see if legal exposure at low-speed rotation.
• Automatically switches to high speed anode rotation as needed
• Locks out illegal exposures
Allow 3400 rpm
Exposure
Yes
Allow 9600 rpm
Exposure
Yes
No No No ExposureAllowed
Safe at3400rpm?
Safe at9600rpm?
Kilowatt RatingKilowatt Rating
• Ability of x-ray tube to make single exposure of reasonable duration (usually .1 sec.)
• Found on tube rating chart standard assumptions
» Use 0.1 sec. exposure time
» Three phase chart
» high speed rotor rotation
Kilowatt Rating (cont.)Kilowatt Rating (cont.)
• Units1 watt = 1 volt X 1 amp
1 watt = 1 kilovolt X 1 mA
1 kilowatt (kW) = 1 kilovolt X 1 mA / 1000
• kW rating for a standard 0.1 sec exposure
kW rating = kVp X mA / 1000 use maximum mA at given kVp @ .1 sec
Kilowatt Rating (cont.)Kilowatt Rating (cont.)• 100 kVp exposure
usually used• For a 100 kVp, .1
sec exposurekW rating = mA / 10Use maximum mA at 100
kVp, .1 sec.
• Each focal spot has its own kW rating
XInterpolate!
~32 kW
Anode Thermal Characteristics ChartAnode Thermal Characteristics Chart
• 2 charts in one cooling curve
in absence of heating
anode heating» for continuous
heat input (fluoroscopy)
Continuous Heating - FluoroscopyContinuous Heating - Fluoroscopy
• Fluoro almostalways single phase
• Find appropriate curve HU/sec = kVp X mA
• Follow from current heat to right for fluoro time
Continuous Heating - FluoroscopyContinuous Heating - Fluoroscopy
• Technique 100 kVp 6 mA 600 HU/sec
• Start with50,000 HU
• Fluoro for3 minutes
x
3 minutes
x ~105,000 HU
CoolingCooling
• Start on cooling curve with current heat units
110,000 for this example
• Cool for2 minutes
x
2 minutes
xx
~40,000 HU
Angiographic Rating ChartAngiographic Rating Chart• Provides maximum heat units
per exposure for given # of exposures per second total exposures
2 5 10 20 30
1 37,000 24,000 16,000 10,000 7400
2 25,000 17,000 12,200 8,000 6,200
3 19,000 13,600 10,000 7,000 5,300
4 15,500 11,400 8,600 6,000 4,500
Exposuresper
second
Total # of Exposures
Maximum Load in Peak kV X mA X sec.
ExampleExample• How many total exposures can be
done at 90 kVp 100 mAs 3 frames / sec.
2 5 10 20 30
1 37,000 24,000 16,000 10,000 7400
2 25,000 17,000 12,200 8,000 6,200
3 19,000 13,600 10,000 7,000 5,300
4 15,500 11,400 8,600 6,000 4,500
Exposuresper
second
Total # of Exposures
Maximum Load in Peak kV X mA X sec.
90 X 100 = 9000 (Maximum Load)
13
Tube Rating ConsiderationsTube Rating Considerations
• ability of tube to withstand multiple exposures during several hours of heavy use depends upon anode storage / cooling curves housing storage / cooling curves
• housing cooling can be improved with fans oil / water circulators
Tube DamageTube Damage
Warning
Anode DamageAnode Damage
• heat capacity exceeded melted spots on anode
• thermal shock (high mA on cold anode) can cause cracks in anode (tube death)
Protecting the AnodeProtecting the Anode• Tube warm-up
• Eliminates thermal shock from high mA exposures on cold anode
• Warm-up needed whenever tube cold once in the morning not sufficient if tube not used
for several hours
High Voltage ArcsHigh Voltage Arcs• electrons move from filament to tube housing
instead of to anode• can be caused by filament evaporation
deposition of filament on glass envelope as result of» high filament currents» long filament boost time
• reduce by not holding first trigger longer than needed
• very short exposure with instantaneously very high mA
Generator often drops off line
+
arcing
Tube Insert DamageTube Insert Damage
• Bearing Damage
prevents proper rotation of anode» anode can run too slow
» anode can stop results in thermal damage to anode (melted
spots)
• Filament break
renders one focal spot completely inoperative
Reducing Tube Wear: Lower mAReducing Tube Wear: Lower mA
• Both exposure are 50 mAs Same radiation to image receptor Same dose to patient
Don’t smoke
that tube
80 kVp
500 mA, 0.1 sec
80 kVp
100 mA, 0.5 secor
Reducing Tube Wear: Lower mAReducing Tube Wear: Lower mA
• Low mA reduces tube wear filament temperatures lower reduces filament evaporation
Don’t smoke
that tube
80 kVp
500 mA, 0.1 sec
80 kVp
100 mA, 0.5 secor
Reducing Tube Wear: Lower mAReducing Tube Wear: Lower mA
• use lowest mA (and largest focal spot) consistent with patient motion considerations
• Large focal spot allows higher mA to be used
Don’t smoke
that tube
80 kVp
500 mA, 0.1 sec
80 kVp
100 mA, 0.5 secor
Reducing Tube Wear: Raise kVpReducing Tube Wear: Raise kVp
• High kVp exposures require less heat units for same film density
higher kVp more penetrating
• High kVp also reduces patient exposure More penetrating beam
• BUT higher kVp reduces contrast• Use highest kVp consistent with
required contrast Don’t smoke
that tube
70 kVp
100 mAs
90 kVp
40 mAsor
Reducing Tube WearReducing Tube Wear• Reduce use of high speed anode rotation
use longer times instead of higher kV and/or mA
• High speed rotation greatly increases bearing wear generators automatically select high speed for high
combinations of kV & mA BUT longer exposure times
» increase exposure time & patient motion
use lowest mA consistent with patient motion considerations
Don’t smoke
that tube
Reducing Tube WearReducing Tube Wear• Reduce first trigger holding time
Reduces bearing wear» Reduces tube rotation time
Reduces filament evaporation» filament evaporation can lead to tube arcing
• Holding first trigger sometimes necessary synchronizing breathing for children
Don’t smoke
that tube
Oil LeaksOil Leaks
• May be accompanied by air bubble in housing
• Eventually causes high voltage arcing
• Requires immediate service attention