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Introduction to MRI (Magnetic Resonance Imaging)

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Phase shift between each row 360/# of rows. Use Gx gradient when sampling to create ... Principles of Magnetic Resonance Imaging, Zhi-Pei Liang, Paul C. Lauterbur ... – PowerPoint PPT presentation

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Title: Introduction to MRI (Magnetic Resonance Imaging)


1
Introduction to MRI(Magnetic Resonance Imaging)
  • SpeakerTsung-Hsueh Lee
  • AdvisorProf. Tzi-Dar Chiueh
  • DateMarch 21, 2005

2
Outline
  • Physic phenomena
  • Spatial Encoding
  • Image Construction
  • Fast scanning
  • MRI Hardware
  • Conclusion
  • Reference

3
  • Physic phenomena
  • Spatial Encoding
  • Image Construction
  • Fast scanning
  • MRI Hardware
  • Conclusion
  • Reference

4
Spinning
  • Spinning charged particle creates an
    electromagnetic field
  • Spin quantum number S of energy states 2S1

M0
5
Precession
  • Larmor Equation
  • Why hydrogen nucleus
  • 1. Large component of
  • human body
  • 2. Odd number of protons
  • (unpaired protons)

B0 field
6
Energy Level
  • Energy state is not always the same
  • For 1H B01.5T Energy State2
  • Precession frequency 42.58MHz/T1.5T64MHz

Nuclei Unpaired Protons Unpaired Neutrons Net
Spin (MHz/T) 1H 1 0 1/2 42.58 2H 1 1
1 6.54 31P 0 1 1/2 17.25 23Na 0 1
3/2 11.27 14N 1 1 1 3.08 13C 0 1
1/2 10.71 19F 0 1 1/2 40.08
7
RF Pulse
  • If the pulse F equals Larmor F Resonance
  • Rf pulse causes a flip angle
  • and also makes protons get in phase
  • Some protons will change energy state

?
MXY
8
900 pulse and 1800 pulse
  • 900 pulse lets MXYM0 and
  • Used to excite protons
  • Partial flip
  • 1800 inverts M0 and precession direction

9
T1 Relaxation Time
  • After RF pulse
  • 1. Spins go back to the lowest energy state
  • 2. Spins get out of phase
  • T1 also called spin-lattice relaxation time
  • Spins give energy to the surrounding lattice

10
T2 Relaxation Time
  • Due to
  • Interactions among individual spins
  • External magnetic field inhomogeneity

11
T2 and T2
  • Use 1800 pulse to refocus
  • Eliminate the effect of external magnetic field
  • T2 Relaxation Time or spin-spin relaxation
    time

12
T1 and T2
  • T2 usually much faster than T1

13
Pulse Sequence
  • TR - time to repeat 900 pulse
  • TE time to echo
  • Spin Echo (SE) Sequence

14
T1 Weighted Image
  • Very long TR T1 effect canceled
  • Short TR short TE T1 weighted image

15
T2 Weighted Image
  • Long TE T2 weighted image
  • Very short TR Signal intensity too small

16
Frequency Energy Wave Length
X-ray 1.73.61012MHz 30150keV 80400pm
Visible Light (Violet) 7.5108MHz 3.1 eV 400nm
Visible Light (Red) 4.3108MHz 1.8 eV 700nm
MRI 3100MHz 20200 meV 610m
17
  • Physic phenomena
  • Spatial Encoding
  • Image Construction
  • Fast scanning
  • MRI Hardware
  • Conclusion
  • Reference

18
Slice-Select Gradient
  • Use different RF to excite different slices
  • Thickness
  • Frequency range
  • Gradient

B0
19
Frequency and Phase Encoding
  • Use Gy gradient to create a phase difference
    among different rows
  • Phase shift between each row 360/ of rows
  • Use Gx gradient when sampling to create different
    precession F among different columns

20
Filling Data Space
21
Phase Encoding
  • How does phase encoding work?

22
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23
Example
1.
2.
Gy
3.
4.
Gx
24
Example
1cos(?1t00)
0.8cos(?2t00)
1
1cos(?1t900)
0.8cos(?2t1800)
0.8
0.8cos(?2t3600)
1cos(?1t1800)
1cos(?1t2700)
0.8cos(?2t5400)
25
Direction of Slices
26
  • Physic phenomena
  • Spatial Decoding
  • Image Construction
  • Fast scanning
  • MRI Hardware
  • Conclusion
  • Reference

27
Sample
  • Nyquist Law

28
Data Space and K Space
  • Maximal signal intensity will be in the center
  • Due to refocusing for each row
  • Due to different dephasing rate for each column

Different Phase Encoding
Sample
29
FT Process
  • Split signal into two parts, real and imaginary
  • 1st 1DFT for each row
  • Modulus
  • 2nd 1DFT for each column

30
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31
Another way to construct imaging
Select one slice Do many experiments with
different directions of readout gradient
32
Filtered back projection
33
  • Physic phenomena
  • Spatial Decoding
  • Image Construction
  • Fast scanning
  • Conclusion
  • Reference

34
Multi-Slice Imaging
  • TR much longer than TE
  • Put different excitation in that time interval

35
FSE
  • Fast Spin Echo
  • Echo Train Length (ETL)
  • Different TE for different echo
  • Choose refocus timing at the TE we want

36
GRE
  • Gradient Recalled Echo
  • Why not decrease TR?
  • Partial flip angle
  • 1800 pulse cant be used
  • Another way to refocus

37
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38
EPI
  • Echo Planar Imaging
  • One shot and Multi-shot
  • Signal decays rapidly because T2
  • FOV (Field of View) too big
  • Requirement is hard to achieve

39
  • Physic phenomena
  • Spatial Encoding
  • Image Construction
  • Fast scanning
  • MRI Hardware
  • Conclusion
  • Reference

40
MRI System
41
Magnet and Gradient Coil
0.015 0.3 Tesla Resistive 0.5 3 Tesla
Superconducting
42
Conclusion
  • MRI is a very powerful and complicated system.
  • There are already many advanced techniques.

43
Reference
  • MRI The Basics, Ray H. Hashermi, William G.
    Bradley
  • Principles of Magnetic Resonance Imaging, Zhi-Pei
    Liang, Paul C. Lauterbur
  • MRI Physics for Radiologist, Alfred L. Horowitz
  • Fundamentals of MAGNETIC RESONANCE IMAGING,
    Donald W. Chakeres, Petra Schmalbrock
  • MRI made easy program, Schering

44
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45
Spin Echo
900
1800
RF
slice
phase
readout
echo
signal
TE
46
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47
A
B
D
E
C
K Space
Image Space
Coherent detector Complex numbers I jQ
__________________
__________________
? DFT ?
__________________
__________________
__________________
__________________
__________________
Real numbers
48
Fourier transform af FID
F
time
frequency
F-1
49
Gradient Echopulse timing
a0
RF
slice
phase
readout
echo
signal
TE
50
(No Transcript)
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