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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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

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

Key to Nuclear Magnetic Resonance
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Nuclear Magnetic Resonance

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

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Nuclear Magnetic Resonance

• MAGNETIC MOMENT ALIGNS IN B0

No B0 random motion
B0 alignment
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Nuclear Magnetic Resonance

• RESONANCE REQUIRES CONSIDEDERATION OF SPECIFIC
  PRECESSIONAL FREQUENCY
• OF ATOMIC SPINS

Larmour Equation ? ?B0
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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
(falls within the range of radio waves)
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Resonance Excitation

• Energy absorption happens if spinning nuclei are
  hit with radiation at of the same frequency of
  the spin
• Leads to misalignment with B0
• Also leads to phase coherence.

This is Excitation
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Relaxation Free Induction Decay (FID)

• Remove the RF and the spins will loose their
  energy.
• Realign with B0

Loose phase coherance
Energy decays slowly
FID
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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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Image Formation
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Spatial Localisation of Pixels
Y
X
Z
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Image Formation
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Resolution
Few pixels Short scan time
Many pixels Long scan time
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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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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
Loose phase coherance T2 decay
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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