seibert 2&3-mri

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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 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

  • 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

  • Nuclear Magnetic Resonance 3

    Magnet Types

    6

    Magnet Components

    Superconducting Air-core System

  • Nuclear Magnetic Resonance 4

    7

    Magnetic Field Gradients

    8

    Phased array coil

    Matching

    Coil

    MRI surface coils

  • 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

  • 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

  • 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

  • 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

  • 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

  • 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

  • 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

  • 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

  • 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

  • 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

  • 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

  • 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)

  • 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

  • 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

  • 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

  • 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

  • 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 (

  • 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

  • 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

  • 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

  • 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

  • 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

  • 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

  • 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

  • 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

  • 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

  • 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

  • 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

  • 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

  • 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

  • Nuclear Magnetic Resonance 41

    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

  • Nuclear Magnetic Resonance 42

    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

  • Nuclear Magnetic Resonance 43

    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

  • Nuclear Magnetic Resonance 44

    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

  • Nuclear Magnetic Resonance 45

    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

  • Nuclear Magnetic Resonance 46

    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

  • Nuclear Magnetic Resonance 47

    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

  • Nuclear Magnetic Resonance 48

    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

  • Nuclear Magnetic Resonance 49

    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

  • Nuclear Magnetic Resonance 50

    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

  • Nuclear Magnetic Resonance 51

    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

  • Nuclear Magnetic Resonance 52

    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

  • Nuclear Magnetic Resonance 53

    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

  • Nuclear Magnetic Resonance 54

    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

  • Nuclear Magnetic Resonance 55

    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

  • Nuclear Magnetic Resonance 56

    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

  • Nuclear Magnetic Resonance 57

    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

  • Nuclear Magnetic Resonance 58

    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

  • Nuclear Magnetic Resonance 59

    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.

  • Nuclear Magnetic Resonance 60

    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