Part III Physics: Medical Physics Option Magnetic Resonance Imaging

1
Part III Physics Medical Physics
OptionMagnetic Resonance Imaging

• Dr T A Carpenter
• http//www.wbic.cam.ac.uk/tac12

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Lecture Content

• Lecture I
• Overview of Nuclear Magnetic Resonance
• Excitation and Signal detection
• One pulse and Two pulse experiments
• Hardware

3
Lecture Content

• Lecture II
• How does NMR become MRI
• Effects of Magnetic Field Gradients
• Imaging pulse sequences
• contrast
• examples

4
Useful Web Sites
Rochester Institute http//www.cis.rit.edu/htbook
s/mri/mri-main.htm
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PD- T2 weighted with full coverage
TR 6055, TEeff 20 120, ETL 10, 512x240,
35.84 x 16.8. 27 5mm slices. Scan time 5mins gt360
acquired this year Work horse scan for
screening Could contrast, better deep grey
matter than FSE at 1.5T
7
PD- T2 weighted - Pituitary Tumour
TR 6055, TEeff 20 120, ETL 10, 512x240,
35.84 x 16.8. 27 5mm slices. Scan time 5mins
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PD- T2 weighted - Glioma
TR 6055, TEeff 20 120, ETL 10, 512x240,
35.84 x 16.8. 27 5mm slices. Scan time 5mins
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NMR History

• 1921 Compton electron spin
• 1924 Pauli Proposes nuclear spin
• 1946 Stanford/Harvard group detect first NMR
  signal
• mid -50 to mid 70s NMR become powerful tool for
  structural analysis
• mid-70 first superconducting magnets

12
NMR History

• 1976 Lauterbur First NMR image of sample tubes
  in a chemical spectrometer
• 1981 First commercial scanners lt0.2T
• 1985 1.5T scanner
• 1986 Rapid developments in SNR, resolution
  etc
• 1998 Whole body 8T at OSU

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Nuclear Zeeman Effect
Application of strong magnetic field B0 lifts
degeneracy of nuclear spin levels
DE
For spin 1/2 DE g h B0 g Gyromagnetic
ratio (constant of nucleus) For hydrogen g 42.5
Mhz/T
14
Population Difference
Given by Boltzman Statistics na exp(
-ghBo/kT ) nb population difference is small lt1
in 106 NMR is very insensitive
15
Semi-Classical Model
Gyroscopic motion of magnetic moment about B0
B0
m
Use classical mechanics(Larmor) w0 - g B0
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Ensemble Average
M
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Rotating Frame
Consider precessing moment in a frame of
reference rotating at the larmor frequency around
B0
w gBo
y
Y
X
x
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Rotating Frame
Classical treatment of M
Effect of RF in laboratory Frame
Y
Equivalent to sinusoidal Brf
X
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Rotating Frame
Classical treatment of M
B0
Effect of RF in rotating Frame
Y
X
X
Y
Brf
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Rotating Frame
Classical treatment of M
B0
Effect of RF in rotating Frame
Y
X
X
Y
Brf
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Rotating Frame
Classical treatment of M
B0
Effect of RF in rotating Frame
Y
X
X
Y
Brf
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Rotating Frame
Classical treatment of M
B0
Effect of RF in rotating Frame
Y
X
X
Y
Brf
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Rotating Frame
Classical treatment of M
B0
Effect of RF in rotating Frame
Y
X
X
Y
Brf
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Rotating Frame
Classical treatment of M
B0
Effect of RF in rotating Frame
Y
X
X
Y
Brf
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Rotating Frame
Classical treatment of M
B0
Effect of RF in rotating Frame
Y
X
X
Y
Brf
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Signal Detection
rotating Frame
B0
X
X
Y
Y
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Fourier Transformation
FT
Sampling frequency 2 expected frequency spread
(Nyquist)
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Effect of RF pulses
B0
z
z
90o degree pulse
x
x
B1 (rf)
y
y
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Effect of RF pulses
B0
z
z
90o degree pulse
x
x
B1 (rf)
y
y
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Effect of RF pulses
B0
z
z
180o pulse (inverting pulse)
x
x
B1 (rf)
y
y
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Effect of RF pulses
B0
z
z
180o pulse (inverting pulse)
x
x
B1 (rf)
y
y
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Effect of 180o RF pulses
B0
z
z
180o degree pulse
x
x
B1 (rf)
y
y
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Effect of 180o RF pulses
B0
z
z
180o degree pulse
x
x
B1 (rf)
y
y
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Effect of 180o RF pulses
B0
z
z
180o degree pulse
x
x
B1 (rf)
y
y
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Effect of 180o RF pulses
B0
z
z
180o degree pulse
x
x
B1 (rf)
y
y
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Effect of 180o RF pulses
x
x
B1 (rf)
y
y
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Effect of 180o RF pulses
x
x
B1 (rf)
y
y
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Effect of 180o RF pulses
x
x
B1 (rf)
y
y
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Effect of 180o RF pulses
x
x
B1 (rf)
y
y
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Effect of 180o RF pulses
x
x
y
y
x
x
y
y
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Effect of 180o RF pulses
x
x
y
y
x
x
y
y
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Two Pulse sequences (I) 90? 90 Saturation
recovery
Two Pulse sequences (I) 180? 90 Inversion
recovery
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T1 Spin Lattice Relaxation Time

• Describes the return to equilibrium for spins
  from the excited state
• Spins loose heat to the rest of the world
• Requires fluctuating magnetic field near the
  Larmor frequency for an effective transfer of
  energy from a spin to surrounding lattice

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Two Pulse sequences (II) 90? 180 ? Spin
Echo sequence
x
y
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Two Pulse sequences (II) 90? 180 ? Spin
Echo sequence
x
y
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Two Pulse sequences (II) 90? 180 ? Spin
Echo sequence
x
y
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Two Pulse sequences (II) 90? 180 ? Spin
Echo sequence
x
y
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Two Pulse sequences (II) 90? 180 ? Spin
Echo sequence
x
y
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Two Pulse sequences (II) 90? 180 ? Spin
Echo sequence
x
y
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Two Pulse sequences (II) 90? 180 ? Spin
Echo sequence
x
y
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Two Pulse sequences (II) 90? 180 ? Spin
Echo sequence
x
y
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Two Pulse sequences (II) 90? 180 ? Spin
Echo sequence
x
y
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Two Pulse sequences (II) 90? 180 ? Spin
Echo sequence
x
y
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T2 and T2
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Spin-Spin Relaxation Time

• Static inhomogeneities refocussed by 180 pulse
• Time varying imhomogeneity are not
• T2 changes in disease give rise to diagnostic
  value of MRI

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Bloch Equations
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Superconducting Magnet
Helium vessel containing super-con coil
Vacuum
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Superconducting Magnet
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Shimming
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Other Magnet Types
Permanent magnet, e.g. light weight rare earth
magnets, lt0.3T
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Other Magnet Types
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Other Magnet Types
Electromagnet lt0.3T
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Special Superconducting Magnets

• Active Shielding
• Extra coils reduce stray field
• Improves siting

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4
10
0.5T wholebody
3T AS wholebody
2
5mT contour
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RF Coils
Remember Brf must be ? B0
Field is ? subject, can use solenoid.
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RF Coils
Remember Brf must be ? B0
Saddle coil, Brf is ? coil access. Efficiency
is low, and homogeneity is poor
Field is ? subject, cannot use solenoid.
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Lecture Content

• Lecture II
• How does NMR become MRI
• Effects of Magnetic Field Gradients
• Imaging pulse sequences
• contrast
• examples

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How to Make Images
Impose (separately)
dBzdx
dBzdy
dBzdz
X gradient Gx
Y gradient Gy
Z gradient Gz
Typical values are 10-100 mT/m
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How to make images
For a Z gradient
wz -g(B0 Gz.z)
-hz
hz
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How to make images
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Imaging Gradients

• Special coils (together with power supplies)
  provide linear variation in B0 in X, Y and Z
  directions

Z
B0
Z
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Imaging Gradients

• Special coils (together with power supplies)
  provide linear variation in B0 in X, Y and Z
  directions

X,Y
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Selection of Slice
Use Fourier relationship
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Selection of slice
Slice thickness adjusted by changing gradient
strength or slice bandwidth (longer pulse has
narrower frequency spread) Slice position
adjusted by changing the centre frequency of the
pulse
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k-space

• k-space is the raw data space before fourier
  transformation into the image
• 2D image will be represented by a 2D array of
  data points spread throughout k-space
• Differing the k-space trajectory will alter image
  contrast

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Image vs k-space
?(r)
S(k)
k(t) ?/2??G(t)dt
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Image vs k-space
?(r)
S(k)
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Image vs k-space
?(r)
S(k)
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Image vs k-space
?(r)
S(k)
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Image vs k-space
FT
?(r)
S(k)
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Information in k-space
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GE k-space trajectory
RF
G
S
G
R
G
P
S(t)
?(r)
S(k)
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GE k-space trajectory
RF
G
S
G
R
G
P
S(t)
-kr
kr
?(r)
S(k)
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GE k-space trajectory
RF
G
S
G
R
G
P
S(t)
-kr
kr
?(r)
S(k)
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GE k-space trajectory
kp
RF
G
S
G
R
G
P
S(t)
-kp
-kr
kr
?(r)
S(k)
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GE k-space trajectory
kp
RF
G
S
G
R
G
P
S(t)
-kp
-kr
kr
?(r)
S(k)
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GE k-space trajectory
kp
RF
G
S
G
R
G
P
S(t)
-kp
-kr
kr
?(r)
S(k)
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GE k-space trajectory
kp
RF
G
S
G
R
G
P
S(t)
-kp
-kr
kr
?(r)
S(k)
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GE k-space trajectory
kp
RF
G
S
G
R
G
P
S(t)
-kp
-kr
kr
?(r)
S(k)
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GE k-space trajectory
kp
RF
G
S
G
R
G
P
S(t)
-kp
-kr
kr
?(r)
S(k)
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GE k-space trajectory
kp
RF
G
S
G
R
G
P
S(t)
-kp
-kr
kr
?(r)
S(k)
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GE k-space trajectory
kp
RF
G
S
G
R
G
P
S(t)
-kp
-kr
kr
?(r)
S(k)
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GE k-space trajectory
kp
RF
G
S
G
R
G
P
S(t)
-kp
-kr
kr
?(r)
S(k)
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GE k-space trajectory
kp
RF
G
S
G
R
G
P
S(t)
-kp
-kr
kr
?(r)
S(k)
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GE k-space trajectory
kp
RF
G
S
G
R
G
P
S(t)
-kp
-kr
kr
?(r)
S(k)
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GE k-space trajectory
kp
RF
G
S
G
R
G
P
S(t)
-kp
-kr
kr
?(r)
S(k)
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GE k-space trajectory
kp
RF
G
S
G
R
G
P
S(t)
-kp
-kr
kr
?(r)
S(k)
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GE k-space trajectory
kp
RF
G
S
G
R
G
P
S(t)
-kp
-kr
kr
?(r)
S(k)
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GE k-space trajectory
kp
RF
G
S
G
R
G
P
S(t)
-kp
-kr
kr
?(r)
S(k)
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GE k-space trajectory
kp
RF
G
S
G
R
G
P
S(t)
-kp
-kr
kr
?(r)
S(k)
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SE k-space trajectory
kp
RF
G
S
G
R
G
P
S(t)
-kp
-kr
kr
?(r)
S(k)
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SE k-space trajectory
kp
RF
G
S
G
R
G
P
S(t)
-kp
-kr
kr
?(r)
S(k)
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SE k-space trajectory
kp
RF
G
S
G
R
G
P
S(t)
-kp
-kr
kr
?(r)
S(k)
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SE k-space trajectory
kp
RF
G
S
G
R
G
P
S(t)
-kp
-kr
kr
?(r)
S(k)
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SE k-space trajectory
kp
RF
G
S
G
R
G
P
S(t)
-kp
-kr
kr
?(r)
S(k)
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SE k-space trajectory
kp
RF
G
S
G
R
G
P
S(t)
-kp
-kr
kr
?(r)
S(k)
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Definitions
TR
RF
G
S
G
R
G
P
S(t)
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Definitions
TE
RF
G
S
G
R
G
P
S(t)
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Controlling contrast
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Proton Density
TR
111
T2 Contrast
TR
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PD- T2 weighted with full coverage
TR 6055, TEeff 20 120, ETL 10, 512x240,
35.84 x 16.8. 27 5mm slices. Scan time 5mins gt360
acquired this year Work horse scan for
screening Could contrast, better deep grey
matter than FSE at 1.5T
113
T1 Contrast
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Alzheimers
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T2 Contrast
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T2 and T2 of acute head injury
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Effect of Flip angle a
B0
X
Y
Brf
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Effect of Flip angle a
90o pulse Maximum signal but have to wait 5T1 for
recovery
B0
X
Y
Brf
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Effect of Flip angle a
B0
X
Y
Brf
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Effect of Flip angle a
Flip angle 30o detect M0sin a 0.5 M0 remaining
M0cos a 0.87 M0
B0
X
Y
Brf
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Contrast versus ?
Contrast versus TR
TR/TE/?
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Why does MRI take so long

• Answer
• Only one phase encode line acquired per
  excitation
• Spin Echo 2563s for T2, 2560.6s for T1
• Gradient Echo 25635ms (but have to do 3D
• Solution
• get more phase encode lines per excitation

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RARE/FSE/TurboSE
Pulse sequence
Multiple spin echos
RF
G
slice
G
1
G
2
Rx
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RARE k-space trajectory
kp
-kp
-kr
kr
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RARE k-space trajectory
kp
-kp
-kr
kr
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RARE k-space trajectory
kp
-kp
-kr
kr
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RARE k-space trajectory
kp
-kp
-kr
kr
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RARE k-space trajectory
kp
-kp
-kr
kr
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RARE k-space trajectory
kp
-kp
-kr
kr
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RARE k-space trajectory
kp
-kp
-kr
kr
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Echo Planar Imaging

• Fastest imaging method
• Typical AQ time is 30-100ms
• Low RF deposition
• Very fast gradient switching
• Highly demanding on MRI hardware
• B0 homogeneity
• gradient switching

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Why ?

• freeze involuntary patient motion
• visualization of dynamic process
• fast imaging minutes
• turbo imaging seconds
• More complex MRI experiments
• obtain multiple images vary some parameter e.g.
  TI
• reduce patient examination time

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GE-PEI k-space trajectory
kp
RF
G
S
G
R
G
P
S(t)
-kp
-kr
kr
?(r)
S(k)
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GE EPI k-space trajectory
kp
RF
G
S
G
R
G
P
S(t)
-kp
-kr
kr
?(r)
S(k)
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GE EPI k-space trajectory
kp
RF
G
S
G
R
G
P
S(t)
-kp
-kr
kr
?(r)
S(k)
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GE EPI k-space trajectory
kp
RF
G
S
G
R
G
P
S(t)
-kp
-kr
kr
?(r)
S(k)
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GE EPI k-space trajectory
kp
RF
G
S
G
R
G
P
S(t)
-kp
-kr
kr
?(r)
S(k)
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GE EPI k-space trajectory
kp
RF
G
S
G
R
G
P
S(t)
-kp
-kr
kr
?(r)
S(k)
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GE EPI k-space trajectory
kp
RF
G
S
G
R
G
P
S(t)
-kp
-kr
kr
?(r)
S(k)
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GE EPI k-space trajectory
kp
RF
G
S
G
R
G
P
S(t)
-kp
-kr
kr
?(r)
S(k)
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GE EPI k-space trajectory
kp
RF
G
S
G
R
G
P
S(t)
-kp
-kr
kr
?(r)
S(k)
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GE EPI k-space trajectory
kp
RF
G
S
G
R
G
P
S(t)
-kp
-kr
kr
S(k)
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GE EPI k-space trajectory
kp
RF
G
S
G
R
G
P
S(t)
-kp
-kr
kr
S(k)
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GE vs EP Imaging
Assume FOV 25cm AQ 10ms Matrix 256
time/sample 10-2/256 Bandwidth 25kHz
Gread 25 x 103/0.25 100 000Hz/m
2.5 mT/m
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GE vs EP Imaging
Assume FOV 25cm AQ 0.5ms Matrix 128
time/sample 5x10-4/128 Bandwidth 250kHz
Gread 250 x 103/0.25 1 000
000Hz/m 25 mT/m
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MRI at 3T
128x128 single shot, GE echo planar. X,Y,Z shim
only (30s) No template or navigator
correction Straight FFT after row reversal
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fMRI (functional MRI)
Monitor T2 or T2 contrast during cognitive
task eg acquire 20-30 slices every 4
seconds Design experiment to have alternating
blocks of task and control condition Look for
statistically significant signal intenisty
changes correlated with task blocks
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Resting
O2 glucose
oxyhaemoglobin
deoxyhaemoglobin
150
Activated
ATP
ADP
O2 glucose
O2
Blood flow
over-compensation
BOLD signal
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Effect of Intravascular Oxygenation level
T2 (and T2) reduced because of diffusion
through field gradients
deoxy
oxy
Blood vessel
Paramagnetic
Diamagnetic
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T2 curves activated and rest
resting
activated
oxyhaemoglobin
deoxyhaemoglobin
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Unilateral Finger Opposition (high res)
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Definitions

• Diffusion relates to the microscopic Brownian
  thermal motion of molecules
• Perfusion, classically is defined as that
  process that results in the delivery of nutrients
  to cells, normally expressed as ml/min/100g wet
  weight of tissue

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Effect of Diffusion on NMR

• Rms. of an ensemble is zero
• For a single molecule diffusion results in a
  gaussian distribution of displacements

r
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Diffusion and Spin echoes
d
d
D
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Diffusion and Spin echoes
I/I0 e -bD b g2g2d2(D-d/3)
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D and ADC
I/I0 e -bD b g2g2d2(D-d/3)
Log (I/I0)
H2O 2.1 x 10 -3 mm2s-1 DMSO 0.55 x 10 -3
mm2s-1 normal 0.71 x 10 -3 mm2s-1 ischaemic
0.55 x 10 -3 mm2s-1
b
159
Diffusion Weighted Imaging
RF
Gs
Gr
Gp
160
Diffusion Weighted Imaging
161
d
d
D
Typical Values ? 20, ? 50
Log (I/I0)
b
162
Practical Problems in Human DWI

• Gross Motion
• Head motion
• breathing
• Pulsitility
• CSF/brain pulsation
• Anisotropy
• D is direction dependant, especially white matter

163
Practical Problems in Human DWI

• Gross Motion
• Echo Planar Imaging
• navigator echoes
• Pulsitility
• gating plus navigator echoes
• Anisotropy
• Measure trace, Dxx Dyy Dzz
• Measure full tensor (all matrix elements)

164
Diffusion Weighted EPI (b1570 s/mm2)
READ
PHASE
SLICE
FOV 25cm, TE 118ms TY DW-EPI 128x128
interpolated to 256x256 Partial k-acquisition
(62.5) 4 interleaves, d 28ms D 66 ms
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Diffusion Weighted EPI (b1570 s/mm2)
166
Diffusion Weighted EPI (b1570 s/mm2)
167
MRI and O15 water PET
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Effect of Intravascular Gd
Tissue
Blood vessel
Tissue
170
Effect of Intravascular Gd
T2 (and T2) reduced because of difussion
through field gradients
Tissue
Blood vessel
Tissue
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Data Analysis

• Fit first pass of the bolus (avoid recirculation)
• Gamma variate, or (better) Monte Carlo
• Estimate arterial input function from large
  vessel signal
• rrCBV, rrCBF but absolute MTT

173
T2 weighted FSE images (3555/80/4)
rrCBV-map
map of the bolus delay (MTT image)
Perfusion weighted MRI of a patient with a high
grade stenosis (gt90) of the right internal
carotid artery leading to a terminal supply zone
infarction in the region of the middle cerebral
artery, from http//www.picker.com/mr/acr/perfusn
/perfusn.htm
174
Caution

• Numbers obtained are not for true perfusion (as
  measured by PET)
• Similar to dynamic CT, DSC measures
  micro-capillary flow
• However good correlation between PET and DSC (in
  pigs), in humans??

175
True Perfusion by MRI

• Arterial spin labeling
• EPISTAR, ASL, QUIPS
• label arterial blood on the way into brain
• subtract images with and without labelling
• difference is due to arterial water that has
  entered tissue, i.e. perfusion

176
Scanner Overview
Gradient Controller
Master Controller
X
Y
Z
RF Controller
RF Amplifier
DAC
Receiver
Gradient Coil
RF coil
preamp
Magnet