slides - Brainmapping.ORG

Introduction to MRI
Daniel B. Ennis, Ph.D.
Requirements for MRI
UCLA – Radiology – DCVI
Requirements for MRI
Dipoles to Images
• NMR Active Nuclei
– e.g. 1 H in H20
• Cryogen
– Liquid He and N2
• Magnetic Field (B0)
– Polarizer
• RF System (B1)
– Exciter
• Coil
– Receiver
• Gradients (GX, GY, GZ)
– Spatial Encoding Y-grad X-grad
Cryostat
Z-grad
Body Coil (B1)
Main Coil (B0)
µ Magnetic Moment
M z
M xy
S (t)
⇥
S k
I ( x)
Bulk Magnetization
Transverse Magnetization
Received Signal
k-space signal
Image
} B 0
} B 1
} Coil
} Gradients
} FFT
Radiology
Image Adapted From: http://www.ee.duke.edu/~jshorey
Radiology

Dipoles to Images
Main Field – B0
µ Magnetic Moment
M z
M xy
S (t)
⇥
S k
I ( x)
Bulk Magnetization
Transverse Magnetization
Received Signal
k-space signal
Image
} B 0
} B 1
} Coil
} Gradients
} FFT
Radiology
Main Field (B0) - Principles
Magnetic Dipoles & Larmor
• B0 is a strong magnetic field
– 1.5T, 3.0T, 7.0T, etc.
– Z-oriented
B 0 = B 0 k
• B0 forces M to precess
– Larmor Equation
• B0 generates M
– More B0, more M
⇥ =
M =
B
NX
total
n=1
µ n
Radiology
Radiology
Movie from Don Plewes

Bulk Magnetization
Zeeman Splitting
M =
NX
total
n=1
µ n
N
S
N
S
S
N
S
N
N
S
N
S
N
N
S
S
N
N
S
N
S
N
S
S
N N
N
E =+ 1 N N N
2
B 0
E = 1 2
B 0
N
S S
N N
S S
S
N
S
N
S
N
S S
S
N
S
N
S
B 0 is o
B 0 is on
}
}
Radiology
Ntotal=0.24x10 23 spins in a 2x2x10mm voxel
Radiology
N = Spin-Up State, Low Energy
N = Spin-Down State, High Energy
N
S
N
S
Zeeman Splitting
N N ⇥
⇥ hB 0
N total 2KT
= 42.58 ⇤ 10 6 Hz/T
h = 6.6 ⇤ 10 34 J · s [Planck’ Constant]
T = 300K (room temperature)
K = 1.38 ⇤ 10 23 J/K [Boltzmann Constant]
B 0 = 1.5T
RF Pulses – B1
N ⇥ N ⇤
⌅ 42.58 ⇤ 106 · 6.6 ⇤ 10 34 · 1.5
N total 2 · 1.38 ⇤ 10 23 · 300
Radiology
⌅ 4.5 ⇤ 10 6
~4.5ppm @ 1.5T
09

Dipoles to Images
B1 Field - RF Pulse
µ Magnetic Moment
M z
M xy
S (t)
⇥
S k
I ( x)
Bulk Magnetization
Transverse Magnetization
Received Signal
k-space signal
Image
} B 0
} B 1
} Coil
} Gradients
} FFT
• B1 is a
– radiofrequency (RF)
• 42.58MHz/T (63MHz at 1.5T)
– short duration pulse (~0.1 to 5ms)
– small amplitude
•

Dipoles to Images
Coils
µ Magnetic Moment
M z
M xy
S (t)
⇥
S k
I ( x)
Bulk Magnetization
Transverse Magnetization
Received Signal
k-space signal
Image
} B 0
} B 1
} Coil
} Gradients
} FFT
13
Radiology
Coils
Faraday’s Law of Induction
“The induced electromotive force or EMF in any closed circuit is equal to
the time rate of change of the magnetic flux through the circuit.”
--http://en.wikipedia.org/wiki/Faraday's_law_of_induction
Time-varying
Magnetic Field
Loop of
Wire
Voltage
Radiology
Radiology

NMR Signal Detection
8-Channel Head Coil
Each coil element has a unique sensitivity profile.
• Coil only detects Mxy
• Coil does not detect Mz
• Coil must be properly oriented
Faraday’s Law
of Induction
✓
V (t) / sin
M xy
Radiology
Radiology
Dipoles to Images
Gradients – Gx, Gy, & Gz
µ Magnetic Moment
M z
M xy
S (t)
⇥
S k
I ( x)
Bulk Magnetization
Transverse Magnetization
Received Signal
k-space signal
Image
} B 0
} B 1
} Coil
} Gradients
} FFT
17
Radiology

Gradients
MRI Instrumentation
• Gradients are a:
– Small
•

X-Gradients
X+Z-Gradients
Z
Z
Z
Radiology
X
B 0 B 0 B 0 B 0 + B 0
Radiology
X
X
X+Z-Gradients
Possible Slice
Spin Isochromat
Group of spins with
the same resonance
frequency.
k-space
Z
Radiology
X
24

What is k-space
1D k-space
• Spatial Frequency Mapping
– Each echo measures some of the spatial
frequencies that comprise the object
– k-space has units of cm -1 or mm -1
– Audio signals have units of Hertz (s -1 )
• A line of k-space is filled by an echo
• 2D FT of k-space produces the image
time
-orspace
Any signal/image can be decomposed into a
summation of sine waves of appropriate amplitude.
Radiology
Radiology
1D k-space
1D k-space
time
-orspace
time
-orspace
Any signal/image can be decomposed into a
summation of sine waves of appropriate amplitude.
Any signal/image can be decomposed into a
summation of sine waves of appropriate amplitude.
Radiology
Radiology

1D k-space
1D k-space
time
-orspace
time
-orspace
Any signal/image can be decomposed into a
summation of sine waves of appropriate amplitude.
Any signal/image can be decomposed into a
summation of sine waves of appropriate amplitude.
Radiology
Radiology
Fourier Representation
What is k-space
k-space
image space
time
-orspace
➠ FFT
FFT
➠
low
frequency
high
k-space is the raw data collected by the scanner.
Radiology

Center
What is k-space
Contrast
What is k-space
➠ FFT
Edges
Edges
Contrast
Information
➠ FFT
Points in k-space represent different patterns in an image.
Radiology
Radiology
k-space
k-space spikes
image space
k-space and Field of View
ky
kx
FFT ➠
➠ FFT
ky
FOV =
1 k
kx
FFT ➠
A k-space spike creates a banding artifact.
Radiology
Radiology
Uniformly skipping lines in k-space causes aliasing.

k-space and Resolution
ky
kx
FFT ➠
ky
Image Contrast
kx
FFT ➠
Radiology
Acquiring fewer phase encodes decreases resolution.
34
Why Image Contrast
Why Image Contrast
Visual Area
of the Thalamus
Optic
nerve
Optic
chiasm
Optic
tract
Retina
Visual Cortex
The human visual system is more sensitive
to contrast than absolute luminance.
Radiology
Radiology

1952 Nobel Prize in Physics
“for their development of new methods for nuclear magnetic
precision measurements and discoveries in connection therewith“
Bloch Equations with Relaxation
Felix Bloch
b. 23 Oct 1905
d. 10 Sep 1983
Edward Purcell
b. 30 Sep 1912
d. 07 Mar 1997
DCVI
Bloch Equations
d M ~
dt = M ~ ⇥ B ~ Mxî+M yĵ (M z M 0 ) ˆk + Dr 2 M ~
T 2 T 1
{
Precession
• Precession
{
Transverse
Relaxation
{
– Magnitude of ~M unchanged
~M
Longitudinal
Relaxation
– Phase (rotation) of changes due to
• Relaxation
– T1 changes are slow O(100ms)
– T2 changes are fast O(10ms)
– Magnitude of M can be ZERO
• Diffusion
– Spins are thermodynamically driven to
exchange positions.
{
~B
Diffusion
DCVI
Longitudinal & Transverse Relaxation
M z (t) =M 0 z e
Radiology
{
Initial Condition
t
T 1 + M 0
⇣1 e
M xy (t) =M 0 xye t/T 2
{
Initial Condition
Return to Equilibrium
General solutions to the Bloch equations with relaxation
in the rotating frame during free precession.
⌘
t
T 1
{
Return to Equilibrium

T1 & T2 Relaxation
T1 and T2 Values @ 1.5T
M 0 xy
M 0
Tissue T1 [ms] T2 [ms]
gray matter 925 100
white matter 790 92
A.U.
Mz
Mxy
muscle 875 47
fat 260 85
kidney 650 58
liver 500 43
M 0 z
CSF 2400 180
Time [ms]
Radiology
Radiology
T1 Relaxation
T1 Relaxation
• Longitudinal or spin-lattice relaxation
• Typically, (10s ms)

T2 Relaxation
• Transverse or spin-spin relaxation
– Molecular interaction causes spin dephasing
• Typically, T2

T2 * Relaxation
T2 * Relaxation
1
T ⇤ 2
1
T ⇤ 2
= 1 T 2
+ 1 T 0 2
Irreversible
Losses
Reversible
Losses
Irreversible
Losses
Reversible
Losses
Radiology
= 1 T 2
+ B 0
Radiology
1
T ⇤ 2
T2 * Relaxation
= 1 T 2
+ 1 T 0 2
Irreversible
Losses
Reversible
Losses
+ 1
T D 2
Irreversible
Losses
+ ···
Percent Signal [a.u.]
100
75
50
25
T2 * vs T2
T2 – 125ms
T2 * – 90ms
T2*

What are echoes
What are echoes
• Two-sided NMR signals
– First half from re-focusing
– Second half from de-phasing
• Spin Echoes
– Arise from multiple RF-pulses
• Gradient Echoes
– Arise from magnetic field gradient reversal
• Line of k-space
48
Radiology
Why echoes
Pulse Sequences
• Free Induction Decay
– NMR signal immediate after an RF pulse
– Signal decays rapidly
• T2 * (

Pulse Sequence Definitions
• TR - Repetition Time
– Duration of basic pulse sequence repeating block
– At least one echo acquired per TR
• TE - Echo Time
– Time from excitation to the maximum of the echo
Spin Echo Imaging
Radiology
51
Spin Echo
Spin Echo
• Advantages
– All spins within voxel rephased
• Insensitive to off-resonance
– B0 inhomogeneity
– Intravoxel Chemical shift signal loss
– Susceptibility
– Great for T1, T2, ρ contrast
• Not T2*
– High SNR
• Disadvantages
– TR can be long
– SAR can be high
RF
Signal
90°
Some T2* signal losses are reversible.
Radiology
Radiology

Spin Echo
Spin Echo
RF
90°
180°
RF
90°
180°
TE
Signal
Signal
Radiology
Radiology
Spin Echo
Spin Echo - Contrast
RF
90°
180°
TR
RF
90°
180°
TR
Signal
TE
Signal
e
t
T ⇤ 2
TE
e
t
T 2
Radiology
Radiology

Spin Echo
Spin Echo - Refocusing
RF
90°
180°
TR
TE
Signal
Radiology
How do you adjust the TR
How do you adjust the TE
Radiology
http://en.wikipedia.org/wiki/File:HahnEcho_GWM.gif
Spin Echo Contrast
Spin Echo Parameters
Spin Density Short Long
T1-Weighted Short Intermediate
T2-Weighted Intermediate Long
A Echo / ⇢
⇣
⌘
1 e TR/T 1
e TE/T 2
Spin Echo Contrast
Spin Echo Parameters
Spin Density 10-30ms >2000ms
T1-Weighted 10-30ms 450-850ms
T2-Weighted >60ms >2000ms
ρ
T2
Long
TR
Short
T1
X
Radiology
Radiology
Short
TE
Long
Images Courtesy of Mark Cohen

Spin Echo - Contrast
Spin Echo - Variable TE T2 Contrast
TE=13ms TE=26ms TE=53ms
Radiology
http://en.wikipedia.org/wiki/File:HahnEcho_GWM.gif
Radiology
TE=106ms TE=145ms TE=172ms
Fast Spin Echo
Fast Spin Echo
180°
180° 180°
90°
• Advantages
RF
GSlice
– Turbo factor accelerates imaging
– Can be used with 2D slice interleaving
– Allows T2 weighted imaging in a breath hold
• Disadvantages
GPhase
GReadout
Signal
Echo-1
T2-decay
Echo-2 Echo-3
– High turbo factors (ETL>4):
• Blur images
• Alter image contrast
– Fat & Water are both bright on T2-weighted
• Water/CSF T2 is long
• Repeated 180s reduce spin-spin interaction
– This lengthens the moderate T2 of fat
– SAR can be high
Radiology
Radiology

Inversion Recovery
Inversion Recovery
• Key Features
– Signal Preparation Block
• 180° RF Inversion Pulse
• TI – Inversion Time [ms]
– Signal Measurement Block
• Spin Echo or Gradient Echo
• Signal during imaging is dependent on
– T1 and TI
• TR is typically long (>2000ms)
– Better for 2D sequences
• Can null a single T1 species if
– TI=ln(2)T1=0.69T1
• Can be used for quantitative T1 mapping
62
Radiology
Inversion Pulses
Inversion Recovery
Radiology
Radiology

Radiology
180°
Inversion Recovery
Contrast
180°
Contrast
Relax
Imaging
TI
TR
Radiology
180°
Inversion Recovery
TE
180°
90°
180°
Contrast
TR
TI
Relax
Contrast
Radiology
180°
Inversion Recovery
180°
90°
180°
TR
TI
Mz
Contrast
Relax
TE
Contrast
Radiology
180°
Inversion Recovery
180°
90°
180°
TR
TI
Mz
Contrast
Relax
TE
Contrast

Basic Gradient Echo Sequence
RF
e
t
T ⇤ 2
Gradient Echo Imaging
Slice
Select
Phase
Encode
Free Induction Decay (FID)
Freq.
Encode
68
Radiology
Basic Gradient Echo Sequence
Basic Gradient Echo Sequence
e
t
T ⇤ 2
RF
RF
Slice
Select
Free Induction Decay (FID)
Slice
Select
Gradient Echo!
Phase
Encode
Phase
Encode
Freq.
Encode
Freq.
Encode
Radiology
Radiology

Basic Gradient Echo Sequence
TR
TE
RF
Basic Gradient Echo Sequence
TR
TE
RF
Slice
Select
Phase
Encode
Slice
Select
Phase
Encode
Wasted
Time
Freq.
Encode
Freq.
Encode
Radiology
Radiology
Gradient Echo + Spoiling
RF Phase
Cycling
RF
Slice
Select
Spoiler
Gradient
Spoiler
Gradient
Gradient Echoes & Contrast
Phase
Encode
Freq.
Encode
Radiology

Spoiled Gradient Echo Contrast
T2*-weighted Gradient Echo Imaging
Axial Shoulder
Axial Shoulder
Gradient Echo Parameters
Type of Contrast TE TR Flip Angle
Spin Density Short Long Small
T1-Weighted Short Intermediate Large
T2 * -Weighted Intermediate Long Small
Radiology
A echo / ⇢ 1 e TR/T 1
1 cos ↵e TR/T 1
sin ↵e TE/T⇤ 2
Contrast adjusted by changing TR, flip angle, and TE.
Radiology
TE=9ms
TE=30ms
Susceptibility Weighting (darker with longer TE)
Bright fluid signal (long T2* is brighter with longer TE)
Radiology
Spoiled GRE & Ernst Angle
Ernst = arccos
e TR
T 1
⇥
Produces the largest MRI signal for a given TR and T1.
Tissue T1 [ms] T2 [ms]
muscle 875 47
fat 260 85
MRI Signal [A.U.]
Radiology
Spoiled GRE & Ernst Angle
Fat
Muscle
Contrast
10° 20° 30° 40° 50° 60° 70° 80° 90°
Flip Angle

Spoiled GRE & Ernst Angle
90°
180°
Spin Echo EPI
90°
1° 5° 10° 20°
High Muscle Signal
High Fat Signal
RF
GSlice
GPhase
TE
TR
30° 45° 60° 90°
Highest Contrast
GReadout
Signal
T2*-decay
Off Resonance Effects Accumulate
Radiology
Radiology
Spin Echo EPI
• Advantages
– Can acquire data in a “single shot”
– Can be used with 2D slice interleaving
– Allows fast T2 * weighted imaging
• Disadvantages
– Single Shot EPI
• Ghosting
• Blur images
• Image distortion
• Alter image contrast
– Multi-shot EPI
• Slower than single shot
– Faster than SE
• Applications
– DWI, Perfusion, fMRI
µ Magnetic Moment
M z
M xy
S (t)
⇥
S k
I ( x)
Dipoles to Images
Bulk Magnetization
Transverse Magnetization
Received Signal
k-space signal
Image
} B 0
} B 1
} Coil
} Gradients
} FFT
Radiology
Radiology

Thanks
Daniel B. Ennis, Ph.D.
ennis@ucla.edu
310.206.0713 (Office)
http://ennis.bol.ucla.edu
Peter V. Ueberroth Bldg.
Suite 1417, Room C
10945 Le Conte Avenue
UCLA – Radiology – DCVI