Transcript Resident Physics Lectures

Resident Physics Lectures
• Christensen, Chapter 2B
Tube Ratings
George David
Associate Professor
Department of Radiology
Medical College of Georgia
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
Heat Units
Single Phase Exposure
• 70 kVp
• 200 mA
• 0.25 second
Heat Units
• 70 X 200 X 0.25
= 3500 heat
units
?
Heat Units
Three Phase Exposure
• 60 kVp
• 100 mA
• 0.1 second
Heat Units
• 60 X 100 X 0.1
X 1.35 = 810
heat units
?
Heat is the Enemy
X-Ray
Tube
Heat
Tube Rating Chart
• Indicates load limit tube can
safely accept
• Based upon
 Tube construction
 High Voltage
Waveform
Tube, Target, & Ratings
• surface area bombarded by
electrons
 focal spot size (actual)
 target angle
 anode diameter
• Melting point
• Heat transfer
• Anode rotation speed
Tube Rating Charts
• single exposure
• multiple rapid exposure
(angiographic) capability
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
Typical Single-Exposure
Tube Rating Chart
• shows maximum exposure time for
single exposure at given kV & mA
Example
• What is the maximum exposure time at
90 kVp & 300 mA?
Example
• 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 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
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?
Yes
Allow 3400
rpm
Exposure
No
Safe at
9600
rpm?
Yes
Allow 9600
rpm
Exposure
No
No Exposure
Allowed
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
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 @ .1 sec
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
~32 kW
Anode Thermal Characteristics Chart
• 2 charts in
one
 cooling curve
in absence of
heating
 anode heating
» for continuous
heat input
(fluoroscopy)
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
Continuous Heating - Fluoroscopy
• Technique
 100 kVp
 6 mA
 600 HU/sec
• Start with
50,000 HU
• Fluoro for
3 minutes
x
x
3 minutes
~105,000 HU
Cooling
• Start on cooling
curve with
current heat
units
 110,000 for this
example
• Cool for
2 minutes
~40,000 HU
xx
x
2 minutes
Angiographic Rating Chart
• Provides maximum heat units
per exposure for given # of
 exposures per second
 total exposures
Total # of Exposures
Exposures 1
per
2
second
3
4
2
37,000
25,000
19,000
15,500
5
24,000
17,000
13,600
11,400
10
16,000
12,200
10,000
8,600
20
10,000
8,000
7,000
6,000
Maximum Load in Peak kV X mA X sec.
30
7400
6,200
5,300
4,500
Example
• How many total exposures can be
done at
 90 kVp
90 X 100 = 9000 (Maximum Load)
 100 mAs
 3 frames / sec.
Total # of Exposures
Exposures 1
per
2
second
3
4
2
37,000
25,000
19,000
15,500
5
24,000
17,000
13,600
11,400
10 13
16,000
12,200
10,000
8,600
20
10,000
8,000
7,000
6,000
Maximum Load in Peak kV X mA X sec.
30
7400
6,200
5,300
4,500
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
Tube Damage
Warning
Anode Damage
• heat capacity exceeded
 melted spots on anode
• thermal shock (high mA on cold
anode)
 can cause cracks in anode (tube death)
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
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
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
Reducing Tube Wear: Lower mA
80 kVp
500 mA, 0.1 sec
80 kVp
or
100 mA, 0.5 sec
• Both exposure are 50 mAs
 Same radiation to image receptor
 Same dose to patient
Don’t
smoke
that tube
Reducing Tube Wear: Lower mA
80 kVp
500 mA, 0.1 sec
80 kVp
or
100 mA, 0.5 sec
• Low mA reduces tube wear
 filament temperatures lower
 reduces filament evaporation
Don’t
smoke
that tube
Reducing Tube Wear: Lower mA
80 kVp
500 mA, 0.1 sec
80 kVp
or
100 mA, 0.5 sec
• 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
Reducing Tube Wear: Raise kVp
70 kVp
100 mAs
or
90 kVp
40 mAs
• 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
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
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
Oil Leaks
• May be accompanied by air bubble in
housing
• Eventually causes high voltage arcing
• Requires immediate service attention