Magnetic Resonance Imaging Part 1 The Science Bit

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Magnetic Resonance Imaging Part 1The Science
Bit

• Lynn Graham DCR Msc
• Clinical Specialist in MRI

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Particle Physics!
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OUTLINE ( part 1)

• History Local origins of MRI
• Fundamental Physics of MRI
• Tissue contrast Versatility

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History Lesson

• Carl Fredrich Gauss. (1777-1855)
• German Physicist
• Findings led to a knowlegdge of magnetism and its
  quantifiation
• Gauss- unit of measurement of magnetism

• Nikola Tesla (1856 1943)
• Serbian Electrical Engineer
• Work in electromagnetic induction
• Tesla unit of measurement for Magnetic Field
  strength

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Sir Joseph Larmor FRS.MA.DSCMathematician
Physicist 1857-1942

• Born 11th July 1857 at Magheragall, Co Antrim
• Educated at RBAI, Belfast
• Graduated from Queens 1877
• Appointed Professor _at_ St Johns College Cambridge
  1903
• Knighted 1909

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The Larmor Equation
Key to Nuclear Magnetic Resonance
Clinical Magnetic Resonance Imaging
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Nuclear Magnetic Resonance

• NUCLEAR ATOMIC SPIN
• ve electric charge
• Intrinsic spin/ Precession
• nuclear magnetic moment

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PRECESSION
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Nuclear Magnetic Resonance

• MAGNETIC MOMENTS ALIGN WITH B0

No B0 random motion
B0 alignment
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Nuclear Magnetic Resonance
Out of phase
In phase
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OUT OF PHASE
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IN PHASE
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The Larmor Equation

• ?L Larmor frequency (MHz)
• B0 magnetic field (Tesla)
• gyromagnetic ratio

Key to Nuclear Magnetic Resonance
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NMR CLINICAL MRI
Fat Water 99 body tissue
H ALIGNMENT PRESCESSION
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NMR CLINICAL MRI

• Apply the Larmor equation

H1 _at_ 1 T ? 42.58 MHz T-1
_at_ 1.5 T Larmor frequency 63.87 MHz
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Electromagnetic Spectrum
Precessional Frequency of H
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Resonance Excitation

• Energy in the form of an RF pulse
• Leads to misalignment with B0 antiparallel
• Also leads to phase coherence.

This is Excitation
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Resonance / Excitation
B0
RF pulse
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Relaxation

• Remove the RF and the spins will loose their
  energy.
• Realign with B0 relax
• Loose phase coherance decay

Energy loss is variable
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Relaxation / Decay
B0
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Excitation Relaxation MR Signal
Bo
NMV
90 RF pulse
Current induced in RF coil due to alternating B
field MRI signal
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Particle Physics almost done!
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Image Formation
X
Z
Y
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Spatial Localisation
B0
B0
Gradient
B0 -
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Image Formation
XX
Y
Z
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Pixel Mapping
Each line of data is stored as the Image is
built up gradually Fourier transform decodes
data forms the image
Phase
Frequency
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X
Z
Y
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Resolution
Few pixels Short scan time
Many pixels Long scan time
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The MR effect!
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Differing MR Images

• T2
• Fluid bright

• T1
• Fluid dark

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Relaxation Free Induction Decay (FID)

• The spins will loose their energy in two ways

Energy decays slowly Relaxing back to B0

T1 Recovery
T2 decay
Loose phase coherance
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T1 Recovery
B0
Realign with
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T2 Decay
In phase
Out of phase
Loose phase
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Brownian Motion

• FAT
• Large , slow molecules
• Lots of bumps
• Fast energy loss
• Short T1 Short T2

• WATER
• Small , fast molecules
• Fewer bumps
• Slow energy loss
• Long T1 long T2

Mr Blobby Vs Speedy Gonzalez!
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Typical T1 T2 values for tissues (_at_1.5T)
Tissue T1 value T2 value
Distilled water Cerebro Spinal Fluid Gray Matter White Matter Fat Muscle Liver Kidney 3000 2400 900 780 260 750 500 760 3000 160 100 90 80 50 40 30
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Pulse Sequences

• Pre-set sequences of excitation, relaxation and
  signal organization that vary tissue contrast and
  image quality.

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T1 and T2 weightings

• Sag spine T2W
• Fluid bright

• Sag spine T1W
• Fluid dark

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Tissue differentiation

• gt 99 body tissues produce MR signal
• Each tissue has unique properties
• - molecular
  structure
• - number of H
  ions
• -
  moving/stationary
• Each tissue behaves differently in the MR
  environment
• ?
• Unique MR signals from normal abnormal
  tissues
• ?
• Excellent disease
  diagnosis.

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Tissue contrast Versatility of MRI
T1 SE
T2 SE
T1 SE gad
GE brain
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Tissue contrast Versatility of MRI
Fat sat orbits
FLAIR
Black blood
Angio
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Coming up Next !!!

• Clinical Applications of MRI
• MRI Equipment
• Safety issues of MRI
• Advantages Disadvantages of MR
• MRI vs Other imaging modalities ( CT/ USS)
• Clinical Images