Download Nuclei, Excitation, Relaxation

Outline
4.1 Principles of MRI
• What nuclei are MR active?
- Hydrogen (fat & water)
- Other Nuclei
• Why are they MR active?
Nuclei, Excitation, Relaxation
- Mass Number
• How do they behave in the magnet?
• Excitation
- RF excitation
• Radiofrequency Pulses
Carolyn Kaut Roth, RT (R)(MR)(CT)(M)(CV) FSMRT
CEO Imaging Education Associates
[email protected]
www.imaginged.com
- Larmor Frequency
• Relaxation
-T1
-T2
Slide # 2
What Do We Image?
Objectives
Upon completion of this course, the attendee
should…
1. Learn the various nuclei that are MR active
2. Understand why certain nuclei are MR
active
3. Realize how nuclei behave in the presence
of the magnetic field.
4. Understand Excitation
5. Understand Relaxation
6. Learn T1 & T2 weighted imaging
• What do we image … with CT?
–Soft tissues?
–Bones?
• What do we image with MRI?
T1
Slide # 3
What’s in an Atom?
Axial CT
–Soft tissues?
–Bones?
PD
T2
Slide # 4
The Nucleus, What Counts?
Mass Number
Atoms have a nucleus
P+
N
}
P+ P+
N
N
Protons ( + positive charge)
Neutrons (neutral)
Orbiting the nucleus
Nucleus
e-
Electrons ( - negative charge)
e-
P+
}
N
P+ P+
N
N
Number of Protons
Plus (+)
Number of Neutrons
Nucleus
e-
P+
Atomic Number
Number of Protons
Electron
Electron
Electron shell
Electron shell
Slide # 5
Slide # 6
1
What Elements are MR Active?
•
•
Proton Imaging
Periodic Table
Elements
Unique Atomic Structure
Odd Mass Number
Hydrogen
Phosphorous
Others?
Mass Number
P+
N
P+
Nucleus
e-
Number of Protons
Plus (+)
Number of Neutrons
=1
Atomic Number
Electron
P+
Electron shell
Slide # 7
Number of Protons
=1
Slide # 8
Hydrogen Imaging
Why do fat & water appear differently?
• The human body is roughly 75% water
• Hydrogen in Water
Water is H20
– Hydrogen in Water
O
H H
H2 0
– Hydrogen in Fat
CH3
T1 Image
Water is dark
Fat is bright
Water Molecule
H2O
• Hydrogen in Fat
PD Image
water / bright
fat / bright
C
H H H
Sagittal Cervical & Thoracic Spine
T1 Image
T2 Image
Fat is bright
Fat is bright
Water is dark
Water is dark
T2 Image
water /bright
fat / darker
Slide # 9
Fat Molecule
CH3
Slide # 10
Before there was imaging… Spectroscopy
Suppression of fat and/or water…
Fat
• Each chemical has a
different Frequency like
(fat & water)
Water
H20
•The location of the peak tells what
chemical
•The area under the peak tells how
much of that chemical
• The difference in frequency
is known as Chemical Shift
• MR Spectrum displays
Chemical Shift
• The study of the spectrum
is known as Spectroscopy
Water
H20
Silicone
Implant
Fat
CH3
Fat
CH3
chemical shift = 3.5 ppm
chemical shift
MR Spectrum
Slide # 11
Sagittal T2 breast unsuppressed
MR Spectrum
3.5 ppm…
@ 1.5T = 220 H
@ 3.0T = 440 Hz
@ 0.3T = 73 Hz
Slide # 12
2
Suppression of fat & water
Silicone Suppression
Silicone
Silicone
Silicone
Implant
MR Spectrum
MR Spectrum
Silicone
Implant
Water
H20
Water
H20
Silicone suppressed
Silicone appears dark
Fat and water suppressed
Silicone appears bright
Fat
CH3
Fat
CH3
chemical shift
100 hz @ 1.5T
Sagittal T2 breast unsuppressed
chemical shift
224 hz @ 1.5T
Sagittal T1 breast unsuppressed
Slide # 13
Slide # 14
Spectrocopy for Therapy Monitoring
1 week pre TX
Day 1
Choline / Citrate Imaging
Day 42
Day 70
Choline / Citrate
Choline
Choline
chemical shift
224 hz @ 1.5T
chemical shift
100 hz @ 1.5T
Choline
Choline
Choline/Citrate Image
Superimposed on H image
pre Brachytherapy
• Hydrogen Imaging
(shown in black & white)
• Choline / Citrate Imaging
(shown in red)
Increased in prostate
cancer
Reduced after treatment
with Brachy-therapy
Brachy-therapy also
known as radium seeds
Choline/Citrate Image
Superimposed on H image
post Brachytherapy
Slide # 15
Slide # 16
Hydrogen for MR Imaging
Tiny Proton Magnets
P+
P+
Nucleus
e-
• Moving charged particles,
like positively charged protons,
make magnetic fields known as
the magnetic moment (μ).
• Magnetic moment behaves
like a tiny bar magnet
N
P+
SS
Electron
Atom
Proton
Slide # 17
Slide # 18
3
The Magnetic Moment
• Bipolar magnets
•Magnetic
moments
•Bar magnets
• Two Poles
•North pole
•South pole
• Magnetic field lines
run from the south
pole to the north pole
Vector… Magnetic Moment
N
• The magnetic moment is
represented by a vector
P+
P+
S
Vector represents
magnetic moment (μ).
Magnetic field lines
Slide # 19
Slide # 20
Review Vectors
Vector Addition
Vector #1
To add vectors
• Take the tail of Vector #1
• Place near the nose of Vector #2
• The vector has two properties
Magnitude
the length of the vector
Direction
the “direction” to which it points
•The vector can be added to another vector
Vector sum
•The Vector sum is between
Vector #2
Vector #1
Vector #2
Vector
Vectors
Slide # 21
Vector addition
Slide # 22
Magnet to Magnet
Outline
• What nuclei are MR active?
What happens when two magnets are together
• Opposite Magnets (poles) attract
- Hydrogen (fat & water)
- Other Nuclei
• Why are they MR active?
S
S
N
N
S
S
- Mass Number
• How do they behave in the magnet?
• Excitation
S
S
- RF excitation
• Radiofrequency Pulses
N
S
S
N
• Like magnets (poles) repel
- Larmor Frequency
• Relaxation
-T1
-T2
Slide # 23
Slide # 24
4
Magnets in a Magnetic Field
Direction of the Main Magnetic Field
When the patient is
in the MR imager
• Some H protons
“attract” to the
magnetic field
- Align with Bo
- Low energy
• Some H protons
“repel” the magnetic
field direction
-Oppose Bo
-High energy
Bo
N
P+
P+
S
S
Opposite Magnets
(poles) attract
Like Magnets
(poles) Repel
Direction of the
magnetic field
Bo
N
S
S
S
S
Image courtesy of Hitachi Medical
Slide # 25
Slide # 26
Protons in a Magnetic Field
• There are
roughly 500,000
protons in a drop of
water
• When the patient
is placed within the
magnetic field,
protons either
align or
oppose Bo
N
Classical Method
Bo
Bo
Protons in the magnetic field
• Low energy
-Attract
- Align
• High energy
-“Repel”
- Oppose
N
S
S
Slide # 27
Precession
N
S
S
Slide # 28
Cartesian Coordinate System
precessional path
spin
• Remember, protons are moving
(spinning) charged particles
- Known as spins
• Protons, align at an angle to the
magnetic field
- The angle is 37’
• Because protons spin, on an angle,
they begin to wobble or precess
-Wobble or precess like a spinning
top
-Precess at a specific rate or
frequency known as the
Precessional Frequency or
Larmor Frequency
-Precess along a path known as the
precessional path
Slide # 29
Z axis
Bo
N
precessional path
Low energy
Parallel
Spin Up
X axis
S
S
Y axis
High energy
Anti-parallel
Spin Down
Slide # 30
5
Protons to Vectors
Thermal Equilibrium
Z axis
Bo
Z
Bo
Bo
X
X axis
are replaced by vectors
X
Y
on the Cartesian coordinates
Z
X
Y
Y axis
Protons
Z
Bo
Y
Immediately after the patient is placed within the
magnetic field, there are an even number of spins
in the high & low energy states
Slide # 31
After a few seconds, there are more spins in
the low energy state. This condition is
known as thermal equilibrium
Slide # 32
Vector Sum
Net Magnetization (Mz)
In this case, vector sum is zero
Bo
For example… vector #1 + #6 cancel
Z
In this case, vector sum is Non Zero
Bo
Vector #1
Z
For example… vector #1 + #6 cancel and Vectors #3 + #5
cancel
Vector #3
Vectors # 3 + #4 cancel
Vector #2
Vector #1
Vector #3
Vector #6
Vector #2
Vector #3
Vector #1
Vector #3
X
Vector #1
Mz
Vector #6
Vector #4
X
Vector #4
Vector #4
Vectors #2 + #5 cancel
Y
Vector #4
Vector #6
Vector #5
Vector #5
Vector #2
Y
Vector #5
The net magnetization is responsible for MR images
Slide # 33
Slide # 34
Mz and Field Strength
Outline
Bo
Z
• What nuclei are MR active?
- Hydrogen (fat & water)
- Other Nuclei
Z
Bo
Mz
• Why are they MR active?
Mz
Vector #2
Vector #3
Vector #1
Vector sum
Net magnetization
Mz – magnetization along the “Z” axis
Vector #6
Vector #2
Vector #5
Vectors # 2 + #4 Add to form the Net Magnetization (Mz)
1.5T Image
Vector #4
3.0T Image
Vector #2
Vector #3
Vector #1
Vector #4
Vector #5
X
- Mass Number
• How do they behave in the magnet?
• Excitation
X
- RF excitation
• Radiofrequency Pulses
- Larmor Frequency
Vector #6
Vector #5
Y
Vector #6
Y
As field strength increases, more spins in line, greater net magnetization, higher image signal.
Slide # 35
•Relaxation
-T1
-T2
Slide # 36
6
How is excitation achieved?
Precession
• Remember, protons are moving (spinning)
charged particles, Known as spins
• Protons, align at an angle to the magnetic field
• Because protons spin, on an angle, they begin
to wobble or precess
-Wobble or precess like a spinning top
-Precess at a specific rate, or frequency
known as the
• Alignment
–Protons in the magnetic field
–Thermal equilibrium
• RF Pulse
–Larmor Frequency
• Resonance
precessional path
Precessional Frequency
or the Larmor Frequency
spin
• This frequency describes the energy that
keeps the spins in thermal equilibrium
• It is this energy that can “knock” the spins from
thermal equilibrium (excite the spins)
Slide # 37
Slide # 38
Precessional frequency
Wobbling Top & Precession
• We can determine the energy required to “excite” the spins
• In order to calculate this energy we need several components
•The magnetic moment of the proton
•The spin angular momentum of the proton
•The field strength of the magnet
precessional path
wobbling path
Precessional Frequency
• This frequency describes the energy that keep the spins in
thermal equilibrium
• It is this energy that can “knock” the spins from thermal
equilibrium
spin
spin
spin
A Top wobbles because of
A proton precessses (in the magnet) because of
•The weight of the top
• The magnetic moment of the proton
•The rate of spin (how fast it spins)
• The rate of spin (spin angular momentum)
• The gravity of the earth
• The magnetic field strength
Slide # 39
Slide # 40
Units of Measure for Frequency
Larmor Frequency
In order to calculate the precessional (Larmor) frequency
•The magnetic moment of the proton
•The spin angular momentum of the proton
Precessional frequency
Gyro-magnetic ratio = γ
Magneto-gyric ratio = γ
•The field strength of the magnet
Magnetic field strength = Bo
This is known as the Larmor equation
ω = Β γ
O
Larmor or Precessional
Frequency = ωο
spin
Precessional Path
The Larmor Equation
calculates the precessional frequency
• Precessional Frequency or Larmor frequency
- The rate at which the spins wobble, or precess
• Wobble or precess in cycles per second
-One cycle is once around the processional path
-One cycle is one sine wave
-One cycle per second = I Hertz (Hz)
O
- MHz , megahertz = 1,000,000 cycles per second
Magnetic field strength = Βο
Gyro-magnetic ratio = γ
Magneto-gyric ratio = γ
One cycle
Slide # 41
Slide # 42
7
Larmor Equation
Gyromagnetic Ratio
The gyro-magnetic ratio is constant for each chemical.
A trick to remember the Larmor Equation
One can imagine that a proton
wobbles pretty rapidly, hence
whoa!
Whoa Boy!
Frequency = ωο
Slide # 43
42.6 MHz/T
40.1 MHz/T
17.2 MHz/T
Slide # 44
Calculating the Larmor Frequency
Radiofrequency Energy?
ωO = Β O γ
γ for 1H (hydrogen) = 42.6 MHz/T
If the Field strength (Bo) is 1.0 Tesla
γ for 1H (hydrogen) = 42.6 MHz/T
If the Field strength (Bo) is 1.5 Tesla
Then…
Then…
ωο = (1.5T) x (42.6 MHz/T)
ωο = 42.6 MHz (Megahertz)
Gyro-magnetic ratio = γ
Magneto-gyric ratio = γ
1H (hydrogen)
19F (fluorine)
31P (phosphorous)
Here’s the actual equation
O
ωο = (1.0T) x (42.6 MHz/T)
Magnetic field strength = Βο
Gyromagnetic ratio (γ) or the Magneto-gyric ratio (γ) for several chemicals
ω = Βγ
O
ωO = Β O γ
ωο = 63.9 MHz (Megahertz)
Do we use radiation in MR?
• Electromagnetic spectrum
• X-rays
–High energy
–Ionizing radiation
• MR Radiofrequency
–Low energy
–Non-ionizing
Ionizing 1022
Radiation
1020
Gamma rays
1018
Xrays
1016
Visible light 1014
1012
Microwave
1010
Cell phone 108
106
Computer
Monitor
Radiowaves
104
102
100
At 1.5T the frequency is roughly 64 MHz.
In most cities, channel 3 broadcasts at roughly 64 Mhz.
Slide # 45
Direct
Current
Hz
Slide # 46
Resonance
RF Transmitter Configurations
• Once the Larmor frequency
is calculated
• Spins can be excited by the
radiofrequency pulse – at the
Larmor frequency
• If the RF energy matches
the precessional frequency of
the spins…
• Resonance is achieved
Slide # 47
B1
RF energy
RF Transmitters
Slide # 48
8
Net Magnetization (Mz)
Excitation
Z
Bo
Z
In this case, vector sum is Non Zero
Z
Bo
RF
For example… vector #1 + #6 cancel and Vectors #3 + #5 cancel
Vector #3
Vector #1
Mz
Mz
Vector #6
Vector #2
Vector #3
Vector #1
Vector #4
Vector #5
Vectors # 2 + #4 Add to form the Net Magnetization (Mz)
Mxy
X
X
Vector #4
X
Vector sum
Net magnetization
Mz – magnetization along the “Z” axis
Vector #6
YAs the result of the RF pulse…
Net magnetization moves from Mz to Mxy
Spins achieve phase coherence
Some low energy spins
- absorb energy
- enter the high energy state
Y
Vector #5
Vector #2
Y
The net magnetization is responsible for MR images
Slide # 49
Slide # 50
Vector Sum
Excitation
Z
RF
Bo
Z
RF
Vector #3
Z
Vector #4
Mz
Vector #2
Vector #5
Mxy
X
Vector #1
Mxy
Vector #6
X
X
Vector sum
Net magnetization
Mxy – magnetization along the XY plane
Y
As the result of the RF pulse…
Net magnetization moves from Mz to Mxy
Spins achieve phase coherance
Some low energy spins
- absorb energy
- enter the high energy state
Y
Y
Slide # 51
Slide # 52
Image Contrast Parameters
Outline
• What nuclei are MR active?
- Hydrogen (fat & water)
- Other Nuclei
• Why are they MR active?
- Mass Number
• How do they behave in the magnet?
• Excitation
- RF excitation
• Radiofrequency Pulses
T1WI
Short TR
Short TE
Bright fat
PDWI
Long TR
Short TE
Bright fat & water
Slide # 53
T2WI
Long TR
Long TE
Bright water
- Larmor Frequency
• Relaxation
- Signal Induction
-T1
-T2
Slide # 54
9
RF Receiver Configurations
Faraday’s Law of Induction
• Drag a magnet across a conductor,
a voltage is created (induced) within the conductor
MR Signal
FID
Spine coil , linear array
TMJ coils (3”round)
RF
Receiver
coil
5” round linear coil Chest coil, volume array
• dB / dt = dV
Change of magnet divided by time = voltage
• ΔB / Δt = ΔV
Slide # 55
Slide # 56
Fourier Transformation
Converting MR Signal
prism
Water
Ft
White light
Fat
coil
FID
Spectrum
Time domain
Frequency domain
Light spectrum
Ft
Free Induction Decay (FID)
Slide # 57
Slide # 58
Chemical shift
• Each chemical has a
different Frequency like
(fat & water)
•Parts per million (PPM)
•Fat / water 3.5 ppm
•@ 1.5T = 224 Hz
•Varies with field strength
MR Spectrum
Imaging and Spectroscopy
Water
H20
Fat
CH3
chemical shift
MR Spectrum
Slide # 59
Slide # 60
10
Relaxation
Excitation Review
Bo
Bo
Z
Z
Z
Z
RF
RF
Mz
Mz
Mxy
Mxy
Y
Y
Y
X
X
X
As the result of the RF pulse…
Net magnetization moves from Mz to Mxy
Spins achieve phase coherance… all get together…
Some low energy spins
- absorb energy
- enter the high energy state… some get high
X
Y
As the result of the RF pulse…
Relaxation
Net magnetization moves from Mz to Mxy
get out of phase- get apart…T2
Spins achieve phase coherance… all get together…
return to longitudingl axis- some get low… T1
Some low energy spins
- absorb energy
- enter the high energy state… some get high
Slide # 61
Slide # 62
Relaxation…T2* Decay
T2* & T2 Decay
T2* decay
T2 decay
Equation for …T2*
T2 + T2’ = T2*
RF pulse
T2*
Mxy
coil
Axial T2* Brain Image
Partially dephased
In phase
Completely dephased
echo
FID
Axial T2* Brain
Mx,y = transverse magentization
Axial T2 Brain
Slide # 63
Slide # 64
Is a susceptibility artifact always a bad thing??
Spin Echo Imaging
TR
1800
T2*
900
900
T2 decay
Timing diagram
echo
FID
TE
echo
FID
180' RF pulse
Axial T2* Brain
Axial T2 Brain
Slide # 65
Axial GE abdomen Image
Axial SE abdomen Image
Slide # 66
11
Runners on the Race
Runners on the Race – Spin Echo
1800
900
1800
900
FID
FID
Echo
I’m the
fast
guy
Start
Thought
I was
winning
Start
I’m on
your
heels
Gotcha!
Inhomogenieties
Phase #1 start together
and get apart
Runners turn 180’
Phase #3 cross
Phase #4 starting line, together
get apartSlide
again# 68
Slide # 67
Runners on the Race – Gradient Echo
I’m the
fast
guy
Z
Mz
Start
Mxy
I’m on
your
heels
X
Inhomogenieties
Phase #1 start together
and get apart
Z
RF
FID
Start
Relaxation
Bo
450
Phase #2 after the 180
Turn around apart
Phase #2 runners
change places
Phase #3 cross finish
line, together
Slide # 69
T2 Relaxation
Phase #4
get apart again
Y
X
Y
As the result of the RF pulse…
Net magnetization moves from Mz to Mxy
Relaxation
Spins achieve phase coherance… all get together…
get out of phase- get apart…T2
Some low energy spins
return to longitudingl axis- some get low… T1
- absorb energy
- enter the high energy state… some get high
Slide # 70
T2 Decay
• T2 decay
• Transverse
• Spin spin
Slide # 71
• Exponential decay
•Decays in ½ lives
• in 1 T2 time 63% decay
•37% remains
•In 2 T2 times 81%
•In 3 T2 times 90%
•In 4 T2 times 95%
•In 5 T2 times 98%
Slide # 72
12
T2 Decay & Image Contrast
2 for 1 - Dual Echo Imaging (Multi Echo)
1800
1800
900
T2 times
Fat = 50 ms
Water = 200 ms
Proton density-TE1
Less T2 weighted
T2 decay
more T2 weighted
T2 decay
FID
Water H20
FID
T2WI-TE2
echo
echo
Fat CH3
echo
TE 1
TE 2
Slide # 73
Slide # 74
T1 Relaxation
T1 Recovery
•T1 recovery
• Spin lattice
•Longitudinal
Fat
CH3
• Exponential recovery
• Recovers in ½ lives
• in 1 T1 time 63% recovery
•37% remains
•In 2 T1 times 81%
•In 3 T1 times 90%
•In 4 T1 times 95%
•In 5 T1 times 98%
Mz
Water
H20
Mx,y
Slide # 75
Slide # 76
T1 Recovery and Image Contrast
Short & Long TR Imaging
Short TR
T1 times
Fat = 150 ms
Water = 2000 ms
1800
900
more T1 weighted
Fat
CH3
less T1 weighted
Mz
Long TR
Water
H20
1800
900
Mx,y
Slide # 77
Slide # 78
13
A Few Fun Facts about T1 & T2
A Few Fun Facts about TR & TE
We cannot
change….
T1 recovery
T2 decay
unless we
change
Field strength
Temperature
or Add contrast
agents!
We can change
TR & TE
And…
TR goes with T1
TE goes with T2
Slide # 79
Slide # 80
A Few Fun Facts about T1
A Few Fun Facts about T2
T1 times at
1.5T
Are in the
neighborhood
of …
2000 ms for
water
150 ms for fat
T2 times at
1.5T
Are in the
neighborhood
of …
200 ms for
water
50 ms for fat
Slide # 81
Slide # 82
A Few Fun Facts about Image Contrast
We cannot
change….
T1 recovery
T2 decay
unless we
change
Field strength
Temperature
or Add contrast
agents!
Let’s Make a T1 Image
T1 times at
1.5T
Are in the
neighborhood
of …
2000 ms for
water
150 ms for fat
T2 times at
1.5T
Are in the
neighborhood
of …
200 ms for
water
50 ms for fat
We can change
TR & TE
And…
TR goes with T1
TE goes with T2
Slide # 83
T1 times at
1.5T
Are in the
neighborhood
of …
T1WI
2000 ms for
water
150 ms for fat
Short TR (500 ms)
Short TE (20 ms)
Bright fat
T2 times at
1.5T
Are in the
neighborhood
of …
200 ms for
water
50 ms for fat
We can change
TR & TE
And…
TR goes with T1
TE goes with T2
Slide # 84
14
Let’s Make a T2 Image
Let’s Make a PD Image
T1 times at
1.5T
Are in the
neighborhood
of …
T2WI
Long TR (4000 ms)
Long TE (100 ms)
Bright water
2000 ms for
water
150 ms for fat
Long TR (4000 ms)
Short TE (20 ms)
Bright fat & water
T2 times at
1.5T
Are in the
neighborhood
of …
200 ms for
water
50 ms for fat
We can change
TR & TE
And…
TR goes with T1
TE goes with T2
T1 times at
1.5T
Are in the
neighborhood
of …
PDWI
2000 ms for
water
150 ms for fat
T2 times at
1.5T
Are in the
neighborhood
of …
200 ms for
water
50 ms for fat
We can change
TR & TE
And…
TR goes with T1
TE goes with T2
Slide # 85
Slide # 86
What is a Pulse Sequence?
Image Contrast Parameters
Spin echo family
T1WI
Short TR
Short TE
Bright fat, short T1 time
PDWI
Long TR
Short TE
Bright fat & water
T2WI
Long TR
Long TE
Bright water, long T2 time
Slide # 87
Longer
Scan
times
Better
quality
Gradient echo family
Faster
Scan
times
lower
quality
T1Weighted Image
SE
(TSE) FSE
IR
Fast IR
PD Weighted Image
SE
(TSE) FSE
FLAIR
Fast FLAIR
Looks like PD
T2 Weighted Image
SE
FSE
STIR
Fast STIR
Looks like T2
(T1 FFE) GrE spoiled
TOF MRA
Enhanced MRA
(PD FFE) GrE
EPI Flair
T2* Weighted Image
(T2* FFE) GrE
PC MRA
EPI
Perfusion
Diffusion
Slide # 88
Outline
• What nuclei are MR active?
- Hydrogen (fat & water)
- Other Nuclei
• Why are they MR active?
4.1 Principles of MRI
Nuclei, Excitation, Relaxation
- Mass Number
• How do they behave in the magnet?
• Excitation
- RF excitation
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• Radiofrequency Pulses
- Larmor Frequency
• Relaxation
Carolyn Kaut Roth, RT (R)(MR)(CT)(M)(CV) FSMRT
CEO Imaging Education Associates
www.imaginged.com
[email protected]
-T1
-T2
Slide # 89
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