Resident Physics Lectures

1
Resident Physics Lectures

• Christensen, Chapter 2B
• Tube Ratings

George David Associate Professor Department of
Radiology Medical College of Georgia
2
Heat Units

• A unit of energy
• Single 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

3
Heat Units
Heat Units
Single Phase Exposure

• 70 kVp
• 200 mA
• 0.25 second

• 70 X 200 X 0.25 3500 heat units

?
4
Heat Units
Heat Units
Three Phase Exposure

• 60 kVp
• 100 mA
• 0.1 second

• 60 X 100 X 0.1 X 1.35 810 heat units

?
5
Heat is the Enemy
X-Ray Tube
Heat
6
Tube Rating Chart

• Indicates load limit tube can safely accept
• Based upon
• Tube construction
• High VoltageWaveform

7
Tube, Target, Ratings

• surface area bombarded by electrons
• focal spot size (actual)
• target angle
• anode diameter
• Melting point
• Heat transfer
• Anode rotation speed

8
Tube Rating Charts

• single exposure
• multiple rapid exposure (angiographic) capability

9
Single 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

10
Typical Single-Exposure Tube Rating Chart

• shows maximum exposure time for single exposure
  at given kV mA

11
Example

• What is the maximum exposure time at 90 kVp 300
  mA?

12
Example

• What is the maximum exposure time at 120 kVp
  400 mA?

Cant do 120 kVp at 400 mA for any exposure time.
?
13
Single Exposure Rating Charts

• Actually 8 charts combining
• generator
• single phase (1F)
• three phase (3F)
• focal spot
• small
• large
• anode speed
• standard (3400 rpm)
• high (9600 rpm)

1FSFS3400 RPM
3FSFS3400 RPM
1FLFS3400 RPM
3FLFS3400 RPM
1FSFS9600 RPM
3FSFS9600 RPM
1FLFS9600 RPM
3FLFS9600 RPM
14
On-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

Safe at3400rpm?
Safe at9600rpm?
No
No
No ExposureAllowed
Yes
Yes
Allow 3400 rpm Exposure
Allow 9600 rpm Exposure
15
Kilowatt 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

16
Kilowatt Rating (cont.)

• Units
• 1 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 _at_ .1 sec

17
Kilowatt Rating (cont.)

• 100 kVp exposure usually used
• For a 100 kVp, .1 sec exposure
• kW rating mA / 10
• Use maximum mA at 100 kVp, .1 sec.
• Each focal spot has its own kW rating

X
Interpolate!
32 kW
18
Anode Thermal Characteristics Chart

• 2 charts in one
• cooling curve in absence of heating
• anode heating
• for continuous heat input (fluoroscopy)

19
Continuous Heating - Fluoroscopy

• Fluoro almostalways single phase
• Find appropriate curve
• HU/sec kVp X mA
• Follow from current heat to right for fluoro time

20
Continuous Heating - Fluoroscopy

• Technique
• 100 kVp
• 6 mA
• 600 HU/sec
• Start with50,000 HU
• Fluoro for3 minutes

105,000 HU
x
x
3 minutes
21
Cooling

• Start on cooling curve with current heat units
• 110,000 for this example
• Cool for2 minutes

x
x
40,000 HU
x
2 minutes
22
Angiographic Rating Chart

• Provides maximum heat units per exposure for
  given of
• exposures per second
• total exposures

Total of Exposures
Exposurespersecond
Maximum Load in Peak kV X mA X sec.
23
Example

• How many total exposures can be done at
• 90 kVp
• 100 mAs
• 3 frames / sec.

90 X 100 9000 (Maximum Load)
Total of Exposures
13
Exposurespersecond
Maximum Load in Peak kV X mA X sec.
24
Tube 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

25
Tube Damage
Warning
26
Anode Damage

• heat capacity exceeded
• melted spots on anode
• thermal shock (high mA on cold anode)
• can cause cracks in anode (tube death)

27
Protecting 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

28
High 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

29
Tube 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

30
Reducing Tube Wear Lower mA
80 kVp 500 mA, 0.1 sec
80 kVp 100 mA, 0.5 sec
or

• Both exposure are 50 mAs
• Same radiation to image receptor
• Same dose to patient

Dont smoke that tube
31
Reducing Tube Wear Lower mA
80 kVp 500 mA, 0.1 sec
80 kVp 100 mA, 0.5 sec
or

• Low mA reduces tube wear
• filament temperatures lower
• reduces filament evaporation

Dont smoke that tube
32
Reducing Tube Wear Lower mA
80 kVp 500 mA, 0.1 sec
80 kVp 100 mA, 0.5 sec
or

• use lowest mA (and largest focal spot) consistent
  with patient motion considerations
• Large focal spot allows higher mA to be used

Dont smoke that tube
33
Reducing Tube Wear Raise kVp
70 kVp 100 mAs
90 kVp 40 mAs
or

• 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

Dont smoke that tube
34
Reducing 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

Dont smoke that tube
35
Reducing 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

Dont smoke that tube
36
Oil Leaks

• May be accompanied by air bubble in housing
• Eventually causes high voltage arcing
• Requires immediate service attention