Christian%20Schwarzbauer

1

MRI Physics in a Nutshell
Christian Schwarzbauer
2
MR images What do we see ?

• MRI images are usually based on the signal from
  protons
• A Proton is the nucleus of the hydrogen atom
• Hydrogen is the most common element in tissue
• The signal from protons is due to their spin

3
The nuclear spin

• Elementary property of an atomic nucleus
• Each spin carries an elementary magnetization
• Spins align in an external magnetic field
  (like a compass needle)

4
Macroscopic sample
5
Macroscopic sample
B0
M
6
Excitation
B0
M
radio waves
? ? B0
7
Precession and signal induction
M
? ? B0
123 MHz _at_ 3T
receiver coil
8
Longitudinal and transverse components
Mz
M
Mxy
9
Exication with different pulse angles
equilibriumstate
90o pulse(maximum signal)
30o pulse
180o pulse (no signal)
10
Relaxation
non-equilibrium state
relaxation
RF pulse
equilibrium state
equilibrium state
11
Relaxation
Two independent relaxation processes
relaxation
T1 longitudinal relaxation time (? 1 s)
T2 transverse relaxation time (? 100
ms)
12
Relaxation

• Transverse Magnetization vanishes quickly
  (short T2)
• Longitudinal Magnetization relaxes slowly
  (long T1)

13
Signal loss due to magnetic field inhomogeneities
t 0
? ? B0
t 20 ms
has higher frequency than
14
Effective transverse relaxation (T2)
Spin dephasing as a result of magnetic field
inhomogeneities
Transverse relaxation (T2)
Effective transverse relaxation (T2 lt T2)
15
Effective transverse relaxation
No inhomogeneities (T2 100 ms)
Moderate inhomogeneities (T2 40 ms)
Strong inhomogeneities (T2 10 ms)
16
T2 related signal dropouts
T2 reduction due to local field
inhomogeneities ? signal dropouts
reduced T2
normal T2 (about 40 ms)
EPI image
17
The principle of MRI
18
Slice selective excitation
? ? (B0 s Gs)
Gs
w gt w0
w w0
w lt w0

• Only spins in slice of interest have frequency
  w0
• RF pulse with frequency w0 excites only spins in
  slice of interest

19
Slice position
Gs
s1
s0
? ? (B0 s Gs)
20
Slice orientation
Gs
? ? (B0 s Gs)
21
Mulit-slice MRI
Gs
?4
?3
?2
?1
? ? (B0 s Gs)
22
Slice profile
Frequency (w)
? ? (B0 s Gs)
Position (s)
23
Slice profile
Frequency (w)
? ? (B0 s Gs)
Position (s)
24
Slice thickness (SLTH)
SLTH
SLTH Full width at half maximum of the slice
profile
25
Multi-slice MRI
Tissue in the inter-slice gap contibutes to the
signal of the adjacent slices
26
Spatial encoding

• Slice selective excitation
• Transverse magnetization precesses in the
  excited slice (? ? B0)

27
Spatial encoding

• Gradient pulse in x-direction

Gx
28
Spatial encoding
Gy

• Gradient pulse in x-direction

• Gradient pulse in y-direction

29
Spatial encoding

• Gradient pulse in x-direction

• Gradient pulse in y-direction

Signal
30
Image reconstruction and k-space (Simple example
3 x 3 matrix)
y
ky
x
kx
Object space(9 unknown parameters)
K space
31
Image reconstruction and k-space (Experimental
data 128 x 128 matrix)
FFT
K space (raw data)
Object space (image)
32
Conventional MRI (e.g. MP-RAGE)
33
Conventional MRI (e.g. MP-RAGE)
34
Conventional MRI (e.g. MP-RAGE)
35
Conventional MRI (e.g. MP-RAGE)

• Problem This sequence is rather slow
• K space is sampled line by line
• After each excitation one must wait for the
  longitudinal magnetization to recover

36
Echo-planar imaging (EPI)
37
EPI A technical challenge
Signal decay due to transverse relaxation
(Example T2 40ms)
Within 80 ms the signal has decayed to nothing
Complete image must be acquired in less than 80
ms (in general T 2 T2)
High temporal, but low spatial resolution
38
EPI at the CBU
Slice thickness 3 mm Inter-slice gap 0.75 mm
(25 ) Number of slices 32 (whole brain coverage)
Matrix size 64 x 64 Field of view 192 x 192
mm Spatial resolution (in-plane) 3 x 3 mm
Echo time (TE) 30 msRepetition time (TR) 2000
ms
39
Standard slice orientation
How many slices ?
120 mm
32
3 mm 0.75 mm
And the minimum TR ?
32 62.5 ms 2000 ms
120 mm
40
Coronal slice orientation
How many slices ?
180 mm
48
3 mm 0.75 mm
And the minimum TR ?
48 62.5 ms 3000 ms
180 mm