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