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 70 kVp 200 mA 0.25 second
Single Phase Exposure
Heat Units
70 kVp
200 mA
0.25 second
70 X 200 X 0.25 = 3500 heat units ?

4. ? Heat Units 60 kVp 100 mA 0.1 second
Three Phase Exposure
Heat Units
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 Voltage Waveform

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?
Can’t 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)
1F SFS 3400 RPM
3F SFS 3400 RPM
1F LFS 3400 RPM
3F LFS 3400 RPM
1F SFS 9600 RPM
3F SFS 9600 RPM
1F LFS 9600 RPM
3F LFS 9600 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 at 3400 rpm?
Safe at 9600 rpm? No No
No Exposure Allowed
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 .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 almost always 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 with 50,000 HU
Fluoro for 3 minutes x
~105,000 HU x
3 minutes

21. Cooling Start on cooling curve with current heat units
110,000 for this example
Cool for 2 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 2 5 10 20 30 1
37,000
24,000
16,000
10,000
7400
25,000
17,000
12,200
8,000
6,200 3
19,000
13,600
7,000
5,300 4
15,500
11,400
8,600
6,000
4,500
Exposures per second
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 2 5 10 20 30 1
37,000
24,000
16,000
10,000
7400
25,000
17,000
12,200
8,000
6,200 3
19,000
13,600
7,000
5,300 4
15,500
11,400
8,600
6,000
4,500 13
Exposures per second
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 +
arcing

29. Tube Insert Damage Bearing Damage Filament break
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
Don’t 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
Don’t 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
Don’t smoke that tube

33. Reducing Tube Wear: Raise 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
Don’t 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
Don’t 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
Don’t smoke that tube

36. Oil Leaks May be accompanied by air bubble in housing
Eventually causes high voltage arcing
Requires immediate service attention