ECSE4963 Introduction to Subsurface Sensing and Imaging Systems

1
ECSE-4963Introduction to Subsurface Sensing and
Imaging Systems

• Lecture 18 MRI II
• Kai Thomenius1 Badri Roysam2
• 1Chief Technologist, Imaging Technologies,
• General Electric Global Research Center
• 2Professor, Rensselaer Polytechnic Institute

Center for Sub-Surface Imaging Sensing
2
Recap

• Last time we discussed
• Relation of MRI with respect to other imaging
  modalities.
• MRI images
• Nuclear spin
• Larmor frequency
• Today
• MR Imaging physics
• How do we make images with the spins?

3
A Brief History of NMR

• 1923 Proposed that nuclei had magnetic properties
  (Pauli)
• 1946 NMR demonstrated (Bloch and Purcell.)
• 1950 The Chemical Shift Spin-Spin Coupling
  discovered (Proctor Yu)
• 1952 First commercial NMR system delivered.
• 1973 Application of Fourier Transform proposed
  (Ernst)
• 1975 2D NMR proposed (Jenner, Ernst, Freeman)

Were awarded the Nobel Prize
4
A Brief History of MRI

• 1973 Imaging demonstrated (Lauterbur)
• 1975 Fourier Transform Imaging (Ernst)
• 1976 EPI proposed (Mansfield)
• 1976 First reported Human Body Images (Damadian)
• 1980 Spin Echo Imaging (Crooks,Young)
• 1983 First commercial systems delivered
• 1985 Fast Gradient Imaging (Frahm)
• 1986 Magnetic Resonance Angiography (Laub,
  Dumoulin)
• 1986 Fast Spin Echo (Hennig)

5
Summary Magnetic Resonance Principles

• Some atomic nuclei have a property called spin
• Spin gives the nuclei a magnetic moment
• These moments are randomly oriented
• When the spins are placed in a magnetic field,
  they align either with or against the field.

6
Summary Magnetic Resonance Principles

• The two states are not equal in energy and
  therefore not equally populated.
• This results in a net polarization or
  longitudinal magnetization
• Transverse magnetization can be created by
  flipping the spins with a magnetic field
  applied in the rotating frame of reference (i.e
  an RF pulse)
• After the flip the spins return to the
  equilibrium condition through the T1 and T2
  relaxation mechanisms measured by the RF coil.

7
A Typical MRI System
Host Computer
Patient- Fred Blogs
Sequence- Fast Spin Echo
TR 2000 msec
TE eff 100 msec
Nex 1
Axial NP FC
ACQUISITION
OPERATOR CONSOLE
MEMORY
Array Processor
ADC
P
U
NETWORK
L
S
LASERCAM
E
T/R Switch
RECEIVER
ARCHIVE
C
O
N
TRANSMITTER
T
R
X GRADIENT
O
Magnet
L
AMPLIFIER
L
Gradient
E
Y GRADIENT
RF Coil
R
AMPLIFIER
Coils
Z GRADIENT
AMPLIFIER
8
Nuclei Interact with a Magnetic Field
With a magnetic field
With no magnetic field

Nuclei in random
Nuclei align to the applied
orientations
field
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Individual Nuclei Precess about the Applied Field
n
Bo
The precession frequency is given by the
Larmor Equation
g
n

B
o
p
2
10
In reality, there are many nuclear spins in a
sample
n
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The Rotating Frame of Reference
The Rotating Frame
The Laboratory Frame
Z?
Z
n
Y?
Y
X?
X
Bo
n
12
In the rotating frame of reference the net
magnetization can be represented by a single
vector, Mz
Net Magnetization
The Rotating Frame
Z?
Z?
Mz
Y?
Y?
X?
X?
Bo
n
n
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Energy absorption a classical physics view
Rotating Frame of Reference
Z?
Z?
Mz
Y?
Y?
X?
X?
Mxy
B0
B1
Before
After
An RF pulse applied at the Larmor frequency
creates a magnetic field in the rotating frame of
reference
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Resonance

• If you apply energy of the correct frequency to
  any system you get absorption
• The opera singer and the glass trick !!
• If you apply energy of the correct frequency to
  nuclei they resonate and absorb
• i.e.... some jump from the lower ground state
  to a higher of excited state
• The resonance frequency is called the Larmor
  frequency
• The equation for response is
• Larmor frequency gyromagnetic ratio
  magnetic field

15
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Energy emission a classical physics view
Laboratory Frame of Reference
Z
Y
I
X
Mxy
B0
Rotating transverse magnetization induces
currents in a pick-up coil
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Energy emission a classical physics view
Laboratory Frame of Reference
Z
Y
I
X
Mxy
B0
NOTE Only transverse magnetization
can be detected!
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Why the Proton is used for Imaging

• The proton has the highest gyromagnetic ratio of
  all the natural nuclei
• Therefore the strongest signal
• Protons are present all over the body
• Water, Fats (lipids)
• There are a lot of protons
• Each cubic mm of water has about 2 x1022 protons
• i.e. 20,000,000,000,000,000,000,000

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Relaxation

• After RF excitation, magnetization returns to
  equilibrium
• This is called RELAXATION
• Different tissues relax at different rates
• Liquid like tissues, e.g. CSF, relax slowly
• Soft materials like fat relax quicker
• Hard materials like bone relax too fast to be
  seen
• In general pathological tissue relaxes slower
  that normal tissue
• Relaxation is described in terms of two
  relaxation times
• T1 is also called Spin-lattice or
  Longitudinal relaxation
• T2 is also called Spin-spin or Transverse
  relaxation

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T1 describes the return of longitudinal
magnetization to equilibrium
o
90 flip
1
Mz
0
Time
t0
-1
(-t / T1)
Mz(t) M0 - M0 e
o
180 flip
1
Mz
0
Time
t0
-1
(-t / T1)
Mz(t) M0 - 2M0 e
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(No Transcript)
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T2 describes the loss of phase coherence in the
transverse magnetization
o
90 flip
t
t
t0
t
3t
1
MXY
(-t / T2)
Mxy(t) M0 e
0
Time
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Both processes occur simultaneously
T1
o
90 flip
t
t
t
t3
t0
t0
T2
t
t
t
t3
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How do we Select a Slice to be Imaged?

• Nuclei are only excited by an RF pulse having the
  correct frequency.
• Magnetic field gradients make the Larmor
  frequency depend on position.
• A limited bandwidth RF pulse applied
  simultaneously with a magnetic field gradient
  will excite only those spins whose Larmor
  frequency is within the bandwidth of the RF
  pulse.
• Thus, by applying a selective RF pulse, only the
  spins within a slice will be excited. Spins
  whose Larmor frequencies are out of the band will
  not be excited.

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A Field Gradient Makes the Larmor Frequency
Depend upon Position
1.500 T
1.501 T
B0
63,872,000 Hz
63.861,000 Hz
Z
Gradient in Z
B(Z)
B
G



Z
o
Z
g
B
n

p
2
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A Field Gradient Makes the Larmor Frequency
Depend upon Position
1.501 T
63,872,000 Hz
X
B0
63.861,000 Hz
1.500 T
Gradient in X
B(X)
B
G



X
o
X
g
B
n

p
2
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Slice Selection in MRI
D
Bo
D
Bo
Z
Z
Excitation
Frequency

• A narrow frequency band or a strong gradient
  defines a thin slice.

• A broader frequency band or a weaker gradient
  defines a thicker slice.

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Three Dimensional Imaging

• Three dimensional imaging can be accomplished by
  adding another phase encoding dimension.
• During three dimensional data acquisition, every
  collected data point carries information for the
  entire 3D image.
• 3D imaging can provide isotropic voxels.

Three slices from a three dimensional MRI data set
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A Typical MRI System
Host Computer
Patient- Fred Blogs
Sequence- Fast Spin Echo
TR 2000 msec
TE eff 100 msec
Nex 1
Axial NP FC
ACQUISITION
OPERATOR CONSOLE
MEMORY
Array Processor
ADC
P
U
NETWORK
L
S
LASERCAM
E
T/R Switch
RECEIVER
ARCHIVE
C
O
N
TRANSMITTER
T
R
X GRADIENT
O
Magnet
L
AMPLIFIER
L
Gradient
E
Y GRADIENT
RF Coil
R
AMPLIFIER
Coils
Z GRADIENT
AMPLIFIER
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Three types of magnets are used in MRI

• Permanent
• Resistive
• Superconducting

Resistive Magnet
Permanent Magnet
0.2 Tesla
0.2 Tesla
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Superconducting magnets
0.5 Tesla open
0.7 Tesla open
1.5 Tesla
3.0 Tesla
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Permanent Magnets

• Permanent magnets use a block magnetic material
  as the source
• Useful up to about 0.2 Tesla
• Usually have a box design
• Low stray field
• Easy to site
• Relatively heavy
• Sensitive to temperature changes

D
DD
DD
B0
DD
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Permanent Magnets

• Permanent magnets use a block magnetic material
  as the source
• Useful up to about 0.2 Tesla
• Usually have a box design
• Low stray field
• Easy to site
• Relatively heavy
• Sensitive to temperature changes

NOTE The earths magnetic field is
approximately 0.5 Gauss or
0.00005 Tesla.
D
DD
DD
B0
DD
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Conductor carrying Magnets
Current

• Electrical current flowing in a loop
  generates a magnetic field
• Can use Resistive wire
• Fields lt0.5 Tesla
• Expensive to run
• Field drift a problem
• Most use superconducting wire
• Fields up to 8 Tesla (whole body)
• Expensive to make
• Very stable fields

Magnetic Field
Right hand rule
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Conductor carrying Magnets
Two-coil Helmholtz Design (Separation radius)
Overwound Solenoid Design
Four-coil Design
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Superconducting Magnets

• Superconducting wire
• As long as the temperature is maintained below a
  critical temperature and current, there is no
  resistance in the wire
• Tcritical NbSn ? 14oK _at_ 5 Tesla
• Tcritical NbTi ? 7oK _at_ 5 Tesla
• Low temperature frequently achieved with liquid
  helium
• Boiling temperature of liquid helium is 4.2
  degrees Kelvin
• Modern magnets use a cryocooler to maintain these
  temperatures
• As long as the superconducting wire is kept below
  its critical temperature, the current flows
  forever and the magnetic field is maintained
  without the addition of any power!
• If the superconducting wire exceeds its critical
  temperature or current, however, it will quench
  (i.e. become a normal wire with resistance).

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Superconducting Magnets

• During a quench all the energy in the magnet is
  converted to heat.
• The heat causes the liquid Helium to boil away.
• Recovery from a quench can take weeks.
• Magnets that dont survive a quench experience a
  black quench.

Quench of a high field laboratory NMR
spectroscopy magnet
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Magnet Uniformity

• Even with careful designs, the magnetic fields
  have non-uniformities.
• We have seen that this is critical for spatial
  resolution.
• To correct for these, a process called shimming
  has been developed.

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The Principle of Shimming
All magnets have some field inhomogeneity
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The Principle of Shimming
x gradient
y gradient
x2 gradient
x2-y2 gradient

• Well characterized field gradients can be
  created using shims
• Superconducting
• Resistive
• Passive

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The Principle of Shimming
x gradient
y gradient
x2 gradient
x2-y2 gradient
x gradient
y gradient
x2 gradient
x2-y2 gradient

Adding opposite non-uniformity's to a non
uniform field produces a uniform field!
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Field Strength affects Signal to noise

160
No Chemical Shift
140
e.g.. Head
120
Significant Chemical Shift
100
e.g.. Body
80
Signal to Noise
60
40
20
0
0
0.5
1.0
1.5
Field Strength (T)
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Comparison of 1.5 and 3T performance
1.5T 3T
Image courtesy of MGH
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Summary

• We have discussed
• Proton spin
• Next time
• MR Imaging physics
• How do we make images with the spins?

45
Homework Lecture 18

• Proposition MR Imaging works on a pulse-echo
  mechanism with the RF coil as transmitter and
  receiver.
• Discuss the pros and cons of this proposition.
• How is this similar/different from the
  ultrasound/OCT/Ground Penetrating Radar pulse
  echo mechanism?

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Acknowledgments

• Thanks to Dr. Charles Dumoulin of GE Global
  Research for the introductory slides.
• http//www.erads.com/mrimod.htm
• http//rad.usuhs.mil/rad/handouts/fletcher/fletche
  r/sld025.htm
• There are numerous sites on the web with
  excellent intros to MRI

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Instructor Contact Information

• Badri Roysam
• Professor of Electrical, Computer, Systems
  Engineering
• Office JEC 7010
• Rensselaer Polytechnic Institute
• 110, 8th Street, Troy, New York 12180
• Phone [phone]
• Fax [phone]
• Email roysam_at_ecse.rpi.edu
• Website http//www.rpi.edu/roysab
• NetMeeting ID (for off-campus students)
  128.113.61.80
• Secretary Betty Lawson, JEC 7012, (518) 276
  8525, lawsob_at_.rpi.edu

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Instructor Contact Information

• Kai E Thomenius
• Chief Technologist, Ultrasound Biomedical
• Office KW-C300A
• GE Global Research
• Imaging Technologies
• Niskayuna, New York 12309
• Phone [phone]
• Fax [phone]
• Email thomeniu_at_crd.ge.com, thomenius_at_ecse.rpi.edu
  
• Secretary Betty Lawson, JEC 7012, (518) 276
  8525, lawsob_at_.rpi.edu