Transcript Clinical Magnetic Resonance Imaging An Introduction (Part II)

Magnetic Resonance Imaging
Part 1
The Science Bit
Lynn Graham DCR Msc
Clinical Specialist in MRI
Particle Physics!
OUTLINE ( part 1)
 History + Local origins of MRI
 Fundamental Physics of MRI
 Tissue contrast + Versatility
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
Sir Joseph Larmor FRS.MA.DSC
Mathematician + Physicist
1857-1942
• Born 11th July 1857 at
Magheragall, Co Antrim
• Educated at RBAI, Belfast
• Graduated from Queens 1877
• Appointed Professor @ St Johns
College Cambridge 1903
• Knighted 1909
The Larmor Equation
L  B0
Key to Nuclear Magnetic Resonance
Clinical Magnetic Resonance Imaging
Nuclear Magnetic Resonance
NUCLEAR ATOMIC SPIN :
• +ve electric charge
• Intrinsic spin/ Precession
• nuclear magnetic moment
PRECESSION
Nuclear Magnetic Resonance
• MAGNETIC MOMENTS ALIGN WITH B0
No B0: random motion
B0 : alignment
+
Nuclear Magnetic Resonance
Out of phase
In phase
“OUT OF” PHASE
“IN” PHASE
The Larmor Equation
L  B0
L = Larmor frequency (MHz)
B0 = magnetic field (Tesla)
= gyromagnetic ratio
 L proportional to B0
Key to Nuclear Magnetic Resonance
NMR – CLINICAL MRI
Fat + Water = 99% body tissue
H
H
H
H
C
C
C
C
H
H
O
H
H H
H
Fatty acid chain
Water
H+: ALIGNMENT + PRESCESSION
NMR – CLINICAL MRI
• Apply the Larmor equation
L  B0
H1 @ 1 T :  = 42.58 MHz T-1
@ 1.5 T Larmor frequency = 63.87 MHz
Electromagnetic Spectrum
Precessional Frequency of H+
Resonance + Excitation
• Energy in the form of an RF pulse
• Leads to misalignment with B0: antiparallel
• Also leads to phase coherence.
This is Excitation
Resonance / Excitation
RF pulse
B0
Relaxation
• Remove the RF and the spins will loose their
energy.
• Realign with B0 : “relax”
• Loose phase coherance : “decay”
Energy loss is variable
Relaxation / Decay
B0
Excitation + Relaxation = MR Signal
Bo
NMV
+
90 RF pulse
Current induced in RF coil due to
alternating B field = MRI signal
Particle Physics : almost done!
Image Formation
X
Y
Z
Spatial Localisation
B0 +
B0
Gradient
B0 -
Image Formation
XX
Phase
Y
Z
Frequency
Slice
Pixel Mapping
Each line of data
is stored as the Image is built up
gradually
Fourier transform decodes data
+ forms the image
Phase
Frequency
X
Z
Y
Resolution
Few pixels
Short scan time
Many pixels
Long scan time
The MR effect!
Differing MR Images
• T1
• Fluid dark
• T2
• Fluid bright
Relaxation :
Free Induction Decay (FID)
• The spins will loose their energy in two ways:
Energy decays slowly
Relaxing back to B0
Loose phase coherance
T1 Recovery
T2 decay
T1 Recovery
Realign with
B0
T2 Decay
In phase
Out of phase
Loose phase
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!
Typical T1 + T2 values for tissues (@1.5T)
Tissue
Distilled water
Cerebro Spinal Fluid
Gray Matter
White Matter
Fat
Muscle
Liver
Kidney
T1 value
3000
2400
900
780
260
750
500
760
T2 value
3000
160
100
90
80
50
40
30
Pulse Sequences
• Pre-set sequences
of excitation,
relaxation and
signal organization
that vary tissue
contrast and image
quality.
T1 and T2 weightings
• Sag spine T1W
• Fluid dark
• Sag spine T2W
• Fluid bright
Tissue differentiation
• > 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.
Tissue contrast : Versatility of MRI
T1 SE
T2 SE
T1 SE
+ gad
GE brain
Tissue contrast : Versatility of MRI
FLAIR
Black blood
Fat sat
orbits
Angio
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