Seibert 2&3-MRI

download Seibert 2&3-MRI

of 60

  • date post

    22-Nov-2015
  • Category

    Documents

  • view

    12
  • download

    3

Embed Size (px)

description

biomedical imaging

Transcript of Seibert 2&3-MRI

  • 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)

    10