seibert 2&3-mri
DESCRIPTION
biomedical imagingTRANSCRIPT
-
Nuclear Magnetic Resonance 1
1
Magnetic Resonance Imaging, Part I: Magnetization Basics, Pulse Sequences
and Contrast Mechanisms
J. A. Seibert, Ph.D.
Department of Radiology
UC Davis Medical Center
Sacramento, California
Learning Objectives
Review the basic physics of magnetic properties
Describe magnet types and peripheral components for MRI systems
Describe tissue magnetization, proton density, and relaxation parameters T1 and T2
Illustrate tissue contrast weighting and pulse sequences
Describe spatial localization with gradient magnetic fields, k-space matrix, and image reconstruction
2
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Nuclear Magnetic Resonance 2
3
Data
Storage
Digitizer &
Image
Processor
Host
Computer
Operating
Console
Pulse Prog
Measurement
Control
RF Transmitter
and Receiver
Shim Power
Supply
Gradient
Power Supply
Patient
Table
Magnet
Clock
Gradient
Pulse Prog
Magnet
0.3 - 3 Tesla (up to 60,000 earths field)
Components
MRI The components
4
Bar magnet
S
N
Current-carrying coiled wire
e-
e-
Dipole magnetic field
Magnetic Properties
Unit of magnetic field strength: T (tesla)
1 T = 10,000 G ; concentration of magnetic lines
Earths magnetic field = 0.5 G = 0.05 mT
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Nuclear Magnetic Resonance 3
Magnet Types
6
Magnet Components
Superconducting Air-core System
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Nuclear Magnetic Resonance 4
7
Magnetic Field Gradients
8
Phased array coil
Matching
Coil
MRI surface coils
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Nuclear Magnetic Resonance 5
9
Magnetic characteristics of elements
Electron orbital and molecular structures
Diamagnetism: paired electrons
Depletes magnetic field
Paramagnetism: unpaired electrons Augments magnetic field
Ferromagnetism: produces magnetic field due to molecular structure -- super paramagnetism
Susceptibility: the extent of material magnetization in a magnetic field
10
Magnetic Susceptibility
Paramagnetic agent: augments local magnetic field
Diamagnetic agent: depletes local magnetic field
Change in magnetic micro-environment causes change
in magnetic properties of local spins
Diamagnetic:
Paired electron spins
Paramagnetic:
Unpaired electron spins
water molecule
magnetic field lines
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Nuclear Magnetic Resonance 6
11
Magnetic Properties of the Nucleus
Protons & neutrons exhibit magnetic properties Non-integer quantum spin
Pairing of protons and neutrons: nuclear magnetic moment
For an even number of P and N in the nucleus, moment = 0
N=even P=odd or N=odd P=even, moment is non-zero
The magnetic moment of a single atom is not observable
Characteristic Neutron Proton
Mass (kg) 1.67410-27 1.67210-27
Charge (Coulomb) 0 1.60210-19
Spin Quantum Number
Magnetic Moment (joule/Tesla) -9.6610-27 1.4110-26
Magnetic Moment (nuclear magneton) -1.91 2.79
12
Magnetic Properties of Elemental Nuclei
Nucleus Spin
Quantum #
% Isotopic
Abundance
Magnetic
Moment
Relative Physiological
Concentration
Relative
Sensitivity
1H 99.98 2.79 100 1
13C 1.1 0.69 -- 0
17O 5/2 0.04 1.89 50 910-6
19F 100 2.63 410-6 310-8
23Na 3/2 100 2.22 8010-3 110-4
31P 100 1.13 7510-3 610-5
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Nuclear Magnetic Resonance 7
13
Protons are magnetized in strong field
Magnetic Resonance Imager
15,000 Gauss (1.5 T)
Protons magnetized in strong magnetic field
14
Magnetic Field and sample magnetization
Larmor Equation: Bo
Activation Energy, E = Precessional Frequency
Antiparallel spins
Higher energy
Parallel spins
Lower energy
Bo
E Net
sample
magnetic
moment
No magnetic field External magnetic field
Thermal energy agitates and randomizes spins in the sample
Under external field B0, protons organize in low (parallel) and high (anti-parallel) quantization energy levels
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Nuclear Magnetic Resonance 8
15
Precession: wobble of magnetization vector
Proton precessional
frequency dependent
on Bo
Bo
0 radians/s
Larmor Equation: Bo
Precessional frequency is proportional to applied magnetic field strength
Spinning top
Gravity
16
Gyromagnetic Ratio, (MHz / T)
Constant value, dependent on element
Allows selective excitation by adjusting RF frequency
Nucleus (MHz / T)
1H 42.58
13C 10.7
17O 5.8
19F 40.0
23Na 11.3
31P 17.2
1T = 42.58 MHz
1.5T = 63.86 MHz
3 T = 127.74 MHz
For 1H Precessional Frequency with
Magnetic Field Strength
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Nuclear Magnetic Resonance 9
17
Sample magnetic moment, M
Mo
Group of protons net magnetized sample
Bo
Parallel
Anti-
Parallel
A group of protons exhibits an observable magnetic moment from the excess protons in the parallel state
18
B0
Laboratory Frame Rotating Frame
z
y '
x '
y
x
z
Frame of Reference
x y axes rotate at Larmor frequency x y axes stationary
Applied magnetic field B0 is directed parallel to the z-axis
x and y axes are perpendicular to z
Precessing
moment is
stationary
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Nuclear Magnetic Resonance 10
19
y
x
B0
M0
Mz
Mxy
z
Magnetization Vectors
Mxy Transverse Magnetization: in x-y plane
Mz Longitudinal Magnetization: in z-axis direction
M0 Equilibrium Magnetization: maximum vector along z-axis
Cartesian
Coordinates
Mxy vector rotates in the transverse plane at the Larmor frequency
20
Resonance and Excitation
B0
Resonance frequency
42.58 MHz / T Field strength (T)
Equilibrium Absorbed energy Excited proton Return to Equilibrium
Absorbed RF pulse Emitted RF pulse
Mz
Mxy
Mz Mxy
Mz Mz
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Nuclear Magnetic Resonance 11
21
Mz -- Longitudinal Magnetization
Applied field
B0
Excited spins
occupy anti-
parallel energy
levels
Time of B1 field
increasing
Equal numbers of parallel and
antiparallel spins
Mz = 0
Mz negative
More antiparallel than parallel
RF energy (B1)
applied to the
system at the
Larmor Frequency
Equilibrium --
more spins parallel than antiparallel
x y
z
Mz positive
B B1 at Larmor Frequency
x'
y'
z
22
A Magnetic field variation of electromagnetic RF wave
Clockwise
rotating
vector
Counter-
clockwise
rotating
vector
Time
Am
plit
ude
Mz
B1
C B1 off resonance
x'
y'
z
Mz B1
B1
B1 B1
B1
B1
B1
Direction of
torque on Mz
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Nuclear Magnetic Resonance 12
Excitation: Flip Angles 23
z
y'
x'
Mz
Mxy B1
Mz M0
Small flip angle Large flip angle z
y'
x'
Mz
Mxy
Mz
M0
B1
Mxy
x'
z
90 flip
B1 y' y'
-Mz x'
z
180 flip
B1
Common flip angles
Free Induction Decay (FID) 24
Equilibrium 90 RF pulse Dephasing Dephased Mxy = zero Mxy large Mxy decreasing Mxy = zero
x'
y'
z Rotating frame
Time
Laboratory frame
x
y
z
90
Rotating Mxy vector
induces signal in antenna
x
y
z
Time
+
-
FID
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Nuclear Magnetic Resonance 13
25
T2 decay
T2* decay
Mxy maximum
Mxy decreasing
Mxy zero
Time
Mxy
37%
t=0 t=T2
100%
Time
Mxy
T2 and T2* decay
T2: Intrinsic magnetic field variations
T2*: Intrinsic and extrinsic magnetic field variations
M t M exy
t
T( )
02
When t=T2, then e-1=0.37, and Mxy=0.37 M0
26
Return to Equilibrium: T1 relaxation
Mz
90
pulse
63%
t=0 t = T1
100%
0% Time
Mz
Mz
Mxy
Mz
Mxy
Mz
M t M ez
t
T( ) ( )
011
Spin-lattice relaxation
When t=T1, then 1-e-1=0.63, and Mz=0.63 M0
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Nuclear Magnetic Resonance 14
27
Indirect measurement of T1 90 excitation / 90 readout
Equilibrium
x y
z
0% Mz 90 pulse
x y
z
x y
z
x y
z
long
x y
z
medium
x y
z
short Delay time
90 pulse
(readout)
z
x y
Longitudinal
recovery x
y
z
100% Mz
Delay time (s)
100%
0%
Mz recovery
Resulting FID
28
T1 relaxation mechanisms
Large molecules: low tumbling frequency spectrum: little spectral overlap, inefficient coupling, long T1
Moderate sized molecules (e.g., proteins, lipids): intermediate frequency spectrum, large overlap, short T1
(efficient release of energy back to the lattice)
Small molecules: wide frequency spectrum, little spectral overlap, poor spin-lattice resonance, longer T1
Water: long T1; however, add water-soluble proteins and Gd contrast with hydration layer, much shorter T1
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Nuclear Magnetic Resonance 15
29
Molecular tumbling frequency spectrum
Rela
tive
Am
plit
ud
e
Low High
Small, aqueous:
Long T1
Large, stationary:
Longest T1
Medium, viscous:
Short T1
0
Frequency
Molecular size &
characteristics
T1 spin-lattice relaxation
0
Higher B0
Higher B0 causes longer T1
30
Long
Short
T1
T2
Molecular motion:
Molecular size:
Molecular interactions:
slow fast
large small
bound free
intermediate
intermediate
intermediate
Relaxation time
Comparison of T1 and T2
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Nuclear Magnetic Resonance 16
31
T1 and T2 characteristics
T1 and T2 relaxation constants for several tissues (Note: values are estimates)
Tissue T1 0.5 T (ms) T1 1.5 T (ms) T2 (ms)
Fat 210 - 230 240 - 260 80
Liver 340 - 370 480 - 500 40
Muscle 370 - 450 800 - 900 45
White Matter 500 - 550 750 - 800 90
Gray Matter 600 - 650 900 - 920 100
CSF 1400 -1800 2400 - 3500 160
32
Pulse Sequences
Pulse sequences
excitation
relaxation
echo formation
data acquisition
Image contrast generated from characteristics of the tissues based upon differences of
T1
spin (proton) density
T2
Other (contrast agents, magnetization transfer, blood flow)
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Nuclear Magnetic Resonance 17
33
Pulse Sequences
Standard pulse sequences
Spin echo
90 excitation, 180 refocusing pulses
Inversion recovery
180 inversion pulse and 90 readout
Gradient echo
induced gradient echoes with variable flip angle
34
Pulse sequence parameters
Pulse sequences are a combination of excitation, relaxation, echo formation and
data acquisition
TR: time period between initial RF excitations
TE: delay between excitation and echo formation
TI: inversion time (for inversion recovery) to manipulate spin lattice recovery curves and null signal from specific tissues
Flip angle: amount of excitation by RF pulse
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Nuclear Magnetic Resonance 18
35
TE / 2
180
Rotating frame
Spin Echo Pulse Sequence
Mxy
90
Excitation
FID signal gradually decays
with rate constant T2*
After 180 pulse, echo
reforms at same rate
Spin echo peak amplitude
depends on T2
TE Echo
36
Multiple Spin-Echo: T2 vs T2*
Subsequent 180 pulses produce echoes
Extrinsic magnetic field inhomogeneities are cancelled
Fitting the peaks of consecutive echoes: true T2
90
pulse
180
pulse
180
pulse 180 pulse
T2* decay
T2 decay
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Nuclear Magnetic Resonance 19
37
Spin Echo Pulse Parameters
S f v e eHTR T TE T ( ) / /1 1 2
Signal intensity to spin density, flow, T1 and T2 relaxation
Manipulating TR (T1 dependency) and TE (T2 dependency)
generates tissue signal S
Differences between T1, T2 and spin density are isolated by
sequencing the timing of excitation with relaxation; this generates
contrast
38
Spin Echo Pulse Parameters
Signal
out
90 180
RF
pulses
TE / 2
90
TR
TE
TR: Time of Repetition
TE: Time of Echo
TE/2: Time of 180 refocusing pulse
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Nuclear Magnetic Resonance 20
39
T1 Contrast Weighting
Longitudinal recovery (T1)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 1000 2000 3000 4000 5000
Time (ms)
Rela
tive
Sig
na
l In
ten
sity
TR
Mz
Fat
Gray
CSF
White
Transverse decay (T2) Mxy
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 100 200 300 400 500
Time (ms)
Image
intensity
TE
Fat
Gray
CSF
White
Short TR Short TE
T1 contrast weighting: Short TR maximizes Mz differences;
short TE
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Nuclear Magnetic Resonance 21
41
Longitudinal recovery (T1) Transverse decay (T2) Mxy
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 100 200 300 400 500
Time (ms)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 1000 2000 3000 4000 5000
Time (ms)
Rela
tive S
ignal In
tensity
Fat
Gray CSF
White
TR
Image
intensity
TE
Mz
Fat Gray CSF
White
Proton Density Weighting
Long TR Short TE
Spin density contrast weighting. Long TR reduces Mz recovery
(T1) differences; short TE (
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Nuclear Magnetic Resonance 22
43
Longitudinal recovery (T1) Transverse decay (T2) Mxy
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 100 200 300 400 500
Time (ms)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 1000 2000 3000 4000 5000
Time (ms)
Rela
tive S
ignal In
tensity
Fat
Gray CSF
White
TR
Signal
intensity
TE
Mz
Fat
Gray
CSF
White
Long TR Long TE
T2 Contrast Weighting
T2 contrast weighting. Long TR reduces Mz (T1) differences;
Long TE (>80 ms) increases T2 differences.
44
T2 Contrast Weighting
Long TR minimizes T1
relaxation differences of the
tissues
A second echo allows for T2
decay to occur, so a T2 W
image is typically acquired in
concert with a PD W image.
Long TE allows T2 decay
differences to be manifested.
While this sequence has
high contrast, the signal
decay reduces the overall
signal and therefore the SNR
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Nuclear Magnetic Resonance 23
45 45 45
90 180 180 90
10 ms 45 ms 2500 ms 0
TR
TE1= 20 ms TE2= 70 ms
Multiple Echo, Variable TE
Spin Echo Image Contrast Weighting
1st echo 2nd echo
Parameter T1 contrast Spin density
contrast
T2 contrast
TR (ms) 400-600 1500-3500 1500-3500
TE (ms) 5-30 5-30 60-150
46
90 readout
180 refocusing pulse
180 excitation
RF
pulses
TR
180 excitation
TE/2
TI TE
Signal
out
Inversion Recovery Pulse Sequence
Initial 180 RF pulse inverts Mz to -Mz;
At Time of Inversion (TI), 90 RF pulse converts Mz into Mxy 180 refocusing pulse (TE/2) produces an echo at TE
S f v e eHTI T TR T ( ) / /1 2 1 1
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Nuclear Magnetic Resonance 24
47
Inversion Recovery
Inversion Recovery (T1)
Time (s)
180
pulse
Mz
90
pulse
TI
Inversion Time
White
Gray CSF
Signal intensity is dependent on T1 (with short TE)
T1 relaxation range is doubled
Contrast is dependent on TI
RF energy deposition is relatively high
-Mz
Time (ms)
TE
Transverse decay (T2) Mxy Image
intensity
Fat
White Gray
CSF
Fat
48
STIR: Short TI Inversion Recovery
Magnitude of Mz (T1)
Time (s)
Mz
Time (ms)
Transverse decay (T2) Mxy
Image
intensity
bounce point
TE
Fat
CSF
Gray
Inversion Time, TI
CSF
Fat
Gray
STIR
Short TI: Fat suppression; contrast appears T2-like Signal null (Mz=0) occurs at: TI = ln(2) T1; note: ln(2)=0.693
Fat suppression (T1 fat 260 ms, 1.5 T) TI = 0.693260 ms=160 ms
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Nuclear Magnetic Resonance 25
49
STIR T1
STIR (fat suppression) vs. T1
TR 5520; TI 150; TE 8 TR 750; TE 13
50
FLAIR Fluid Attenuated Inversion Recovery
Magnitude of Mz (T1)
Time (s)
Mz
Time (ms)
Transverse decay (T2) Mxy
Image
intensity
bounce point
TE
Inversion
Time, TI
FLAIR
CSF
Fat
Gray
Long TI: Fluid suppression; CSF signal is diminished
T1 CSF (3500 ms @ 1.5 T) requires TI = ln(2) * 3500 2500 ms
Very long TR (>7000 ms) allows significant Mz recovery
Fat
Gray
CSF
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Nuclear Magnetic Resonance 26
51
FLAIR image
T1 T2 FLAIR
TR 10000; TI 2400; TE 150 TR 549; TE 11 TR 2400; TE 90
52
Proton Density T2 FLAIR T1
Image Contrast Comparisons
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Nuclear Magnetic Resonance 27
53
/
Gradient magnetic fields allow selective excitation of
spins dependent on location
Larmor
frequency
+
-
0
null Slightly lower
magnetic field
Slightly lower
precessional frequency
Slightly higher
magnetic field
Slightly higher
precessional frequency
Signal localization for imaging
Gradients used to change local magnetic field in a known way
Selective excitation, detection, acquisition to create MR image
54
Coil pair
Distance along coil axis
Center of coil pair
Superimposed
magnetic fields
Magnetic field
variation
Magnetic field
Coil
current + Coil
current -
Linear change
in magnetic field
--a gradient
Magnetic field
Magnetic Field Gradients
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Nuclear Magnetic Resonance 28
55
X
B0
Y
Superimposed gradients
Z-axis
Net gradient = Gx2 + Gy
2 + Gz2
Individual gradients
Y-axis
X-axis Z
Magnetic Field Gradients
Peak amplitude: 10 to 50 mT/m
1 to 5 G/cm
Slew rate: time to reach peak
~5 250 (mT/m) / ms
56
B0
Slice-select gradient
Gradient coils
Excited slice of tissue
Excitation / Localization of Slice
z
x
y
Narrow-band RF
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Nuclear Magnetic Resonance 29
57
B0
Slice-select gradient
Gradient coils
Excitation/Localization of Slice
z
x
y
Narrow-band RF, slightly higher frequency
Excited slice of tissue
58
Slice Thickness
Slice Thickness, z
Variable RF Bandwidth
Fixed Gradient
Narrow bandwidth small z
Wide bandwidth large z Low gradient large z
High gradient small z
0
Bandwidth
Small
Narrow
Large
Wide
RF
Fre
qu
en
cy
Large
Low
Fixed RF Bandwidth,
Variable Gradient
Slice Thickness, z
fixed
0
Small
High
Gradient Strength
gradient fixed
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Nuclear Magnetic Resonance 30
59
Frequency Encode Gradient
Net frequency at
gradient location
Body cross-section
determined by SSG
Receiver coil
Fourier Transform
(position decoder)
Digitization
Composite
frequency
signal
signal
amplitude
Spatial position Line integral of signal amplitude
0 -f max +f max
Frequency Encode Gradient (FEG)
applied during echo formation
+
-
y
x
60
Demodulated MR Signal
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
1 cycle/cm
Amplitude: 1.0
2 cycles/cm
Amplitude: 0.75
180 phase shift
3 cycles/cm
Amplitude: 0.5
Composite
waveform
Am
plit
ud
e
Distance (cm)
0 1 2 3 4 5 -5 -4 -3 -2 -1
1.0
0.5
0 1 2 3 4 5 -5 -4 -3 -2 -1
1.0
0.5
0 1 2 3 4 5 -5 -4 -3 -2 -1
1.0
0.5
0 1 2 3 4 5 -5 -4 -3 -2 -1
1.0
0.5
Corresponding Fourier Transformation
Frequency (cycles/cm) or position
0 0.5 1.0 1.5
Fourier Transformation
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Nuclear Magnetic Resonance 31
61
Projection / Reconstruction with rotating FEG
Spatial position Line integral of signal amplitude after
Fourier transformation
Rotating FEG, Fourier
Transformation, spatial
domain projection data
y
x
+
-
+
-
+
-
Reconstruction of image
data occurs by filtered
backprojection
62
Phase Encode Gradient
y
x #1
First
Last
#1
#256
null
Phase gradient amplitude
discretely applied # times
Resulting phase shift
after gradient removed
#128
-
+
#60
+
-
#256
-
+
+
-
#188
-
Nuclear Magnetic Resonance 32
63
RF
SSG
PEG
FEG
Echo
DAQ
Excite protons
Localize (z)
Localize (y)
Localize (x)
Generate echo
Acquire data
TR
90 180 90
TE
Spin-Echo Pulse Sequence
64
ky
kx
-f max, x +f max, x
-f max, y
+f max,y
0
0
k-space: Frequency Domain matrix
Low frequency signals
are mapped around
the origin of k-space,
and high frequency
signals are mapped in
the periphery. Ph
ase
va
ria
tio
ns
Frequency variations
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Nuclear Magnetic Resonance 33
65
RF pulses
Slice select gradient
Phase encode gradient
Frequency encode gradient
MR signal
Data acquisition
90 180
Frequency Domain k-space
y
x
z
Spatial Domain image space
2-D Fourier
Transform
ky
kx
Repeat with different PEG for each TR
64
-63
0
-63 64 0
0
127 0 127
128 X 128 acquisition matrix
Signal Acquisition
66
0
128
-127 -127 128 0
255 255
0
0
k-space characteristics Central area
Low frequencies
Peripheral area
High frequencies
-
Nuclear Magnetic Resonance 34
67
Axial Coronal Sagittal
MR Data Acquisition
. And any arbitrary plane
Various planes are chosen according to the gradient coils
energized during the pulse sequence
68
Perfusion and Diffusion Contrast
Perfusion: delivery of oxygen / nutrients / contrast
agents (gadolinium) to cells via capillaries; DCE MRI
Blood Oxygen Level Dependent (BOLD) imaging
"functional MR"
Blood metabolism in active areas: MR signal change
Oxyhemoglobin is converted to deoxyhemoglobin (paramagnetic agent) and the magnetic susceptibility
reduces T2* in the local tissues
Identification of high metabolic activity area is identified by repeated brain stimulus signals and statistical correlation
-
Nuclear Magnetic Resonance 35
Functional MR: Finger Tapping example
69
70
Perfusion and Diffusion Contrast
Diffusion relates to random motion of water molecules in tissues
The in vivo structural integrity of certain tissues (healthy, diseased, or injured) is measured with
Diffusion-Weighted Imaging (DWI)
Water diffusion characteristics are determined with apparent diffusion coefficient (ADC) maps
-
Nuclear Magnetic Resonance 36
71
Diffusion Weighted Imaging
High gradient strength
Image contrast is dependent on the rate of random, Brownian motion of water protons
Requires EPI-class scanner acquisition capability (50-100 ms image)
72
Parameters influencing image contrast
Patient determined:
T1, T2, spin density, flow
Operator determined
TR, TE
Pulse sequence (SE, IR, GRE)
Image display
Use of MR contrast agents
Summary: MR Image Contrast
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Nuclear Magnetic Resonance 37
73
Magnetic Resonance Imaging, Part II: Advanced Sequences, Artifacts, MRS, Safety
J. A. Seibert, Ph.D.
Department of Radiology
UC Davis Medical Center
Sacramento, California
Learning Objectives
Methods to decrease image acquisition time
Pulse sequence diagrams
Spatial resolution and SNR details
MR Angiography
MR Artifacts
MR Spectroscopy
MR Safety
74
-
Nuclear Magnetic Resonance 38
75
Acquisition time: 2D FT Spin Echo
Acquisition time = #PEG x TR x NEX
For 256 192, 256128, the PEG is typically along the small dimension to reduce acquisition time
Example: spin-echo sequence, 256192 image matrix and 2 averages, TR=600 ms
Image time = 0.6s 192 2 = 230.4s = 3.84m
Example: spin-echo sequence, 256256 image matrix, 1 average, TR=2500 ms
Image time = 2.5s 192 1 = 480.0s = 8.0m
76
TR
TE Repeat
#2
#3
#4
#5
#6
#1 Slice:
Multislice Acquisition
# slices = TR / (TE+C)
Multiple narrow-band excitations of different resonance frequencies
allows simultaneous data acquisition in the volume. Number of
slices is determined by the TR and machine characteristics.
-
Nuclear Magnetic Resonance 39
77 77 Data Synthesis
ky
kx
Fractional NEX:
Acquired data = matrix + 1 line
Synthesized mirror image data from
opposite quadrants
? Tradeoff?
2 Decreased acquisition time, but Loss of SNR by 2 or ~40%
kx
ky
Fractional Echo:
minimum TE reduced
Data
extracted
Mirror
image
FEG
min TE peak
peak min TE
78
RF
SSG
PEG
FEG
Echo
90 180
Echo train length (ETL) = 4
Effective TE = 16 ms
90
16 ms 32 ms 48 ms 64 ms
k-space
TR
Effective
TE
Fast Spin Echo (FSE): Multiple PEG per TR
Phase
encode
ordering:
early
echoes
mapped
to the
center of
k-space
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Nuclear Magnetic Resonance 40
79
90 readout
180 refocusing pulse
180 excitation
RF
SSG
PEG
FEG
Echo
180 excitation
TR
TE TI
Inversion Recovery Spin Echo
STIR (short TI and TR) and FLAIR (long TI and TR) imaging are
useful sequences for reducing tissue signals of fat and fluid
80
Gradient Recalled Echo Pulse Sequence
Rewinder gradient is needed to re-establish phase for next TR
SSG
PEG
FEG
Echo
Rewinder gradient
TE
TR
RF
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81
90 180
Gradient
Echo
PEG
FEG
SSG
RF
Effective TE
ky
kx
Echo Planar Image Sequence
PEG blips
The initial PEG-
FEG sets data
acquisition to
the upper left
Each PEG blip
ratchets to the
next line in the
k-space matrix
The oscillation
of FEG induces
echo formation
throughout TR
k-space 64x64 images in 50 ms!
(useful for functional imaging)
82
3D Fourier Transform Imaging
Broadband, nonselective RF pulse excites a large volume of spins
Two gradients are applied in the slice encode and phase encode
directions, followed by the frequency encode (readout) gradient
Image acquisition time:
TR #Phase Encodes (z axis) #Phase Encodes (y axis) # Avg
TR of 600 ms, T1 W, 1 avg requires 163 m (2.7 h) for 128128128
TR of 50 ms, gradient echo requires ~15 min for the same exam
A 3D FT (three sequential FTs) along column, row, and depth axis
High SNR compared to a similar 2D acquisition sequence
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83
3D data acquisition
Slice Encode (phase #1)
Phase Encode
(phase #2)
Frequency Encode
Isotropic Anisotropic
Time of scan: (1283 matrix)
TR #SEG #PEG NEX
600 ms TR, 1 NEX = 2.7 hr
50 ms TR, 1 NEX = 15 min
84
MR Spatial Resolution
Spatial resolution dependencies: FOV, gradient field strength, sampling bandwidth
Matrix: 128128, 128256, 192256, 256256, 512256, 512512, 5121024, 10241024 .
MR roughly 2 less than CT for similar FOV 25 cm FOV and 256256 matrix on the order of 1 mm
Small FOV (surface coils), pixel size of 0.1 to 0.2 mm
Typical slice thickness ~5 to ~10 mm
Partial volume averaging over slice thickness
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85
Signal to Noise Ratio (Image quality)
Proportional to
Volume (3D is the best)
NEX 1/2 (larger # excitations)
Bandwidth -1/2 (narrower bandwidth)
Tradeoff of spatial resolution versus SNR
Function of
receiver coil quality factor
slice gap and cross-excitation
magnetic field strength (B1.0 to B1.5)
reconstruction algorithm
86
RF Receiver Bandwidth
SNR proportional to BW-1/2
broad (16 kHz)
narrow (4 kHz)
RF bandwidth =
1 / dwell time =
1 /T
(8 kHz)
T
T
Signal
Noise
T Sample dwell time
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87
Unsaturated spins: high signal
Flow-Related Enhancement
Imaging
volume
Flow presaturation
Pre-saturated spins: equal signal
Presaturation
pulses
MR and Blood Flow
88
MR Angiography: TOF Images
Exploitation of blood flow enhancement by detecting moving blood
Unsaturated blood moves into imaged volume, producing a bright signal
Detectable range is limited by the eventual saturation of the tagged blood
2-D Time of Flight Images
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89
Volume of 2D Images
Maximum Intensity Projection: MIP
Projection angles
MIP images
Projections are cast through the image stack (volume)
The maximum signal along each line is projected
90 2D Projection Angiograms from MIP
2D Projection
Angiograms from MIP
Data volume
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91
MR Artifacts
Positive or negative signal intensities that do not accurately represent the anatomy
Can obscure or mimic pathological processes or anatomy
In some cases can help with the differential diagnosis by providing extra information
Origins:
Machine
Patient
Processing
92
Susceptibility
Magnetic susceptibility: induced internal magnetization relative
to external magnetic field, causing rapid T2* dephasing
Tissue-air interfaces, paramagnetic agents (gadolinium), tattoos, braces, rapid changes in tissue densities
Susceptibility is helpful in diagnosing the age of blood
hemorrhage
Gadolinium agent has paramagnetic characteristics to shorten
T2 and hydration layer to shorten T1
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Susceptibility Artifacts 93
Axial T2-W Spin Echo Axial T2*-W Gradient Echo
94
Actual
FOV
Ideal
Gradient Artifacts
System calculates position
based on linear gradients
Non-linear gradients cause
anatomical distortion
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95
RF coil artifacts
Signal variation occurs due to coil sensitivity
Loss of signal with distance, or enhanced focal signals due to proximity of coils to skin
Inadequate shimming, calibration
Coil close to skin Inadequate shimming for fat saturation
96
Bad pixel in k-space Resultant image
Equipment-dependent Artifacts K-space errors
Illustrates contribution of sinusoidal frequency to the image at kx=2, ky=3
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97
Motion Artifacts
FE
G
PEG FEG
PE
G
Chiefly along the PHASE ENCODE GRADIENT direction
98
Non-gated vs. gated acquisition
Velocity
ECG Signal
1
1 1
1 2 2
2
2 3 3
3 3 4
4 4
4 5
5 5 6
6
6
Non Gated
1 1 1 2 2 2
3 3 3 4 4
5 5 6 6
ECG Gated
Velocity
ECG Signal
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99
Chemical Shift Artifact
frequency
fat water
+ - PEG
+
-
PEG
+ - FEG
3-4 ppm difference, fat
~5 ppm difference, silicone
Chemical Shift
water
fat
silicone
-
FEG
+
water
fat
100
Chemical Shift Artifact
Resonance frequency variation tissue (water), fat, silicone causes a shift and misregistration perpendicular to the FEG
Chemical shift dependent on field strength (ppm) 3 ppm chemical shift (fat water) has a frequency difference: 0.5T: 21 MHz 3 ppm = 63 Hz
1.5T: 64 MHz 3 ppm = 190 Hz
Chemical shift is more severe for higher field strength magnets
Chemical shift dependent on gradient strength (BW):
25 cm (0.25 m) FOV, 256256 matrix, Gradient strength of 2.5 mT/m 0.25 m = 0.000625 T
0.000625 T 42.58 MHz/T = 26.6 kHz /256 pixels = 104Hz/pixel.
Gradient strength of 10 mT/m 0.25 m = 0.0025 T
0.0025 T 42.58 MHz/T = 106.5 kHz /256 pixels = 416 Hz /pixel.
Chemical shift is more severe for lower gradient strengths
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101
T1 W TR=450 ms TE=14 ms
Fat saturation T1 W TR=667 ms TE=8 ms
FE
G
Water
Fat
The left image is
T1 weighted
The right image
is T1 weighted
with chemical fat
saturation pulses.
In both images,
the FEG is
vertically
oriented.
102
Rectangular object:
1st + 3rd + 5th + 7th
Frequency synthesis of object (harmonics):
1st + 3rd + 5th
n = 128
n=256
Sharp boundary in MR image:
1st
Frequency synthesis of object
+ 3rd
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103
256 (vertical) 128 (horizontal) 256 256
ringing
Ringing (Truncation) Artifact
104
Wraparound Artifact
Sampling rate
-
+
A
A
B
B
C
FOV
C
C
Wrap-around
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105
Magnetic Resonance Spectroscopy
Measurement of tissue metabolites
Metabolic peaks identified by frequency shift (ppm)
MRS is included in conventional MR protocol
Sequences of 10-15 minutes (TE=135 or 270 ms)
Single voxel and multi-voxel acquisition methods
Metabolites of interest
Fat peak
Frequency
(ppm)
Water peak
Rela
tive
Am
plit
ud
e
0 1 2 3 4
106
MRI vs. MRS signals
MRI: signals derived from bulk protons in water/fat
MRS: signals derived from the amplitude of proton metabolites in targeted tissues, and separated by chemical shift, due to electron cloud shielding
Metabolites Shift (ppm) Properties / Significance
Lipids 0.9 1.4 Necrosis (in brain MRS)
Lactate 1.3 Anaerobic glycolysis
Alanine 1.5 Amino acid
N-acetyl-L-aspartate (NAA) 2.0 Presence of intact glial structures
Glutamine 2.2 2.4 Neurotransmitters
Creatine (Cr) 3.0 Energy metabolism
Choline (Cho) 3.2 Membrane turnover, cell proliferation
Myo-inositol 3.5 Glial cell marker
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107
MRS in-vivo localization methods
Single voxel: generate a cubic or rectangular volume
Advantage: high SNR
Multi voxel: multiple voxels in 1-, 2-, or 3-dimensions
Advantage: larger volume & data superimposed with image
Voxel selection:
STEAM: stimulated echo acquisition mode
3 orthogonal section-selective 90 pulses
Well defined, minimizes contamination from adjacent tissues
PRESS: point resolved spectroscopy
3 orthogonal pulses: 90, followed by two 180 pulses
Less well defined, but higher SNR
108
MRS acquisition
Suppress signal from water (& fat) protons
H2O signal 10,000 to 100,000 times that of metabolites
CHESS: CHEmically Selective Saturation; 3 frequency selective pulses with dephasing signal
STIR: bounce point (0) signal from protons in bulk water
Downsides: loss of metabolite signal
Magnet & data acquisition requirements
Uniform and homogeneous magnetic field
High field strength, phased array surface coil
Pre-scanning for shimming, frequency tuning, optimizing suppression, setting transmitter & receiver gain
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MRS metabolites: measurement of tissue
chemistry
109
110
MRS examples
SNR: height of largest metabolite peak / bkgnd noise
Line width: based on homogeneity, frequency
Choline is elevated in tumors - a cell membrane component; increased cell turnover malignancy
Rad
ioG
rap
hic
s 2
006;
26:S
173S
189
MR Spectrum from anaplastic oligoastrocytoma Choline/Creatine ratio map
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111
MRI Siting
Tenants of MR siting: (1) protect local environment from the MR system, (fringe fields) (2) protect MR system from the local environment (RF noise)
Superconductive magnets produce extensive fringe fields
Magnetic shielding reduces fringe fields
Field strength below 0.5 mT is uncontrolled
Field strength above 0.5 mT needs controlled & restricted access
Electronic equipment (image intensifiers, gamma cameras, and color TVs) impacted by fringe fields of less than 0.3 mT
Faraday cage: copper lining in MRI room to attenuate external RF signals
112
Ra
dia
l d
ista
nce
(m
)
2
4
6
8
10
Axial distance (m)
Unshielded
12 0.3 mT
0.5
1
3
5
20
2
4
6
8
1
0.5
0.1 mT
0.3 Shielded
2 4 6 8 10 12 14
Fringe Fields, 1.5 T Magnet
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113
MRI Quality Control
Phantoms
Periodic Tests
Accreditation Minimum required standards; Continuing education; System Evaluation
1. High contrast (spatial) resolution
2. Slice thickness accuracy
3. RF bandwidth tuning
4. Geometric accuracy and spatial uniformity
5. Signal uniformity
6. Low contrast resolution (sensitivity)
7. Image artifact evaluation
8. Preventive maintenance logging and documentation
9. Review of system log-book and operations
114
MRI quality control phantom
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115
MR Safety
MR safety considerations
Strong magnetic fields
Radiofrequency energy (heating SAR)
Time varying magnetic gradient fields
Confined imaging device (claustrophobia)
Noisy operation (gradient coil activation and
deactivation, creating acoustic noise
Patient considerations: Implants, prostheses, aneurysm
clips, pacemakers, heart valves, etc
Localized heating, potential serious harm
Ferromagnetic materials brought into the imaging room
116
MR Safety
MR safety zones
Zone I: freely accessible to general public
Zone II: interface between Zone I and Zone III and where MR-screening questions take place
Zone III: restricted to Level 1 and Level 2 personnel and appropriately MR-screened
individuals
Zone IV: the MR scanner magnet room itself, always located within Zone III
MR labeling
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117
MR Safety
118
MR Safety
Personnel Definitions
Non-MR personnel are patients, visitors or facility staff who do not meet criteria for Level 1 or 2 MR
personnel
Level 1 MR personnel have passed minimal safety education efforts and can work in Zone III areas.
Example individuals are MR office staff, patient aides
Level 2 MR personnel are more extensively trained in the broader aspects of MR safety issues, e.g.,
potential for thermal loading, burns, neuromuscular
excitation, etc, from changing gradients. Example
individuals are MR technologists, radiologists, dept.
nursing staff.
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119
MR Bioeffects
Biological effects of magnetic fields
Low field strengths: no acute or chronic biological effects
High strength (> 4 T) dizziness and disorientation
High field strengths (>20T) enzyme kinetic changes
Common bioeffects are tissue heating caused by RF
energy deposition and/or by rapid gradient switching
120
Summary
Physics explains and demonstrates the complexity and the functionality of MRI
Only slightly less than two decades of clinical imaging has occurred; MRI is often the modality of
choice in many examinations
The future is extremely promising
advances in technology continue
image acquisition in a non-invasive and safe manner
specialized systems and capabilities