Title: Introduction to MRI (Magnetic Resonance Imaging)
1Introduction to MRI(Magnetic Resonance Imaging)
- SpeakerTsung-Hsueh Lee
- AdvisorProf. Tzi-Dar Chiueh
- DateMarch 21, 2005
2Outline
- 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
4Spinning
- Spinning charged particle creates an
electromagnetic field - Spin quantum number S of energy states 2S1
M0
5Precession
- Larmor Equation
- Why hydrogen nucleus
- 1. Large component of
- human body
- 2. Odd number of protons
- (unpaired protons)
B0 field
6Energy 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
7RF 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
8900 pulse and 1800 pulse
- 900 pulse lets MXYM0 and
- Used to excite protons
- Partial flip
- 1800 inverts M0 and precession direction
9T1 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
10T2 Relaxation Time
- Due to
- Interactions among individual spins
- External magnetic field inhomogeneity
11T2 and T2
- Use 1800 pulse to refocus
- Eliminate the effect of external magnetic field
- T2 Relaxation Time or spin-spin relaxation
time
12T1 and T2
- T2 usually much faster than T1
13Pulse Sequence
- TR - time to repeat 900 pulse
- TE time to echo
- Spin Echo (SE) Sequence
14T1 Weighted Image
- Very long TR T1 effect canceled
- Short TR short TE T1 weighted image
15T2 Weighted Image
- Long TE T2 weighted image
- Very short TR Signal intensity too small
16Frequency 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
18Slice-Select Gradient
- Use different RF to excite different slices
- Thickness
- Frequency range
- Gradient
B0
19Frequency 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
20Filling Data Space
21Phase Encoding
- How does phase encoding work?
22(No Transcript)
23Example
1.
2.
Gy
3.
4.
Gx
24Example
1cos(?1t00)
0.8cos(?2t00)
1
1cos(?1t900)
0.8cos(?2t1800)
0.8
0.8cos(?2t3600)
1cos(?1t1800)
1cos(?1t2700)
0.8cos(?2t5400)
25Direction of Slices
26- Physic phenomena
- Spatial Decoding
- Image Construction
- Fast scanning
- MRI Hardware
- Conclusion
- Reference
27Sample
28Data 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
29FT Process
- Split signal into two parts, real and imaginary
- 1st 1DFT for each row
- Modulus
- 2nd 1DFT for each column
30(No Transcript)
31Another way to construct imaging
Select one slice Do many experiments with
different directions of readout gradient
32Filtered back projection
33- Physic phenomena
- Spatial Decoding
- Image Construction
- Fast scanning
- Conclusion
- Reference
34Multi-Slice Imaging
- TR much longer than TE
- Put different excitation in that time interval
35FSE
- Fast Spin Echo
- Echo Train Length (ETL)
- Different TE for different echo
- Choose refocus timing at the TE we want
36GRE
- Gradient Recalled Echo
- Why not decrease TR?
- Partial flip angle
- 1800 pulse cant be used
- Another way to refocus
37(No Transcript)
38EPI
- 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
40MRI System
41Magnet and Gradient Coil
0.015 0.3 Tesla Resistive 0.5 3 Tesla
Superconducting
42Conclusion
- MRI is a very powerful and complicated system.
- There are already many advanced techniques.
43Reference
- 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(No Transcript)
45Spin Echo
900
1800
RF
slice
phase
readout
echo
signal
TE
46(No Transcript)
47A
B
D
E
C
K Space
Image Space
Coherent detector Complex numbers I jQ
__________________
__________________
? DFT ?
__________________
__________________
__________________
__________________
__________________
Real numbers
48Fourier transform af FID
F
time
frequency
F-1
49Gradient Echopulse timing
a0
RF
slice
phase
readout
echo
signal
TE
50(No Transcript)