BME 595 - Medical Imaging Applications Part 2: INTRODUCTION TO MRI Lecture 1 Fundamentals of Magnetic Resonance Feb. 16, 2005 - PowerPoint PPT Presentation

Loading...

PPT – BME 595 - Medical Imaging Applications Part 2: INTRODUCTION TO MRI Lecture 1 Fundamentals of Magnetic Resonance Feb. 16, 2005 PowerPoint presentation | free to download - id: 5381ec-YjQzN



Loading


The Adobe Flash plugin is needed to view this content

Get the plugin now

View by Category
About This Presentation
Title:

BME 595 - Medical Imaging Applications Part 2: INTRODUCTION TO MRI Lecture 1 Fundamentals of Magnetic Resonance Feb. 16, 2005

Description:

BME 595 - Medical Imaging Applications Part 2: INTRODUCTION TO MRI Lecture 1 Fundamentals of Magnetic Resonance Feb. 16, 2005 James D. Christensen, Ph.D. – PowerPoint PPT presentation

Number of Views:824
Avg rating:3.0/5.0
Slides: 43
Provided by: JamesChr2
Category:

less

Write a Comment
User Comments (0)
Transcript and Presenter's Notes

Title: BME 595 - Medical Imaging Applications Part 2: INTRODUCTION TO MRI Lecture 1 Fundamentals of Magnetic Resonance Feb. 16, 2005


1
BME 595 - Medical Imaging Applications Part 2
INTRODUCTION TO MRI Lecture 1 Fundamentals of
Magnetic Resonance Feb. 16, 2005
  • James D. Christensen, Ph.D.
  • IU School of Medicine
  • Department of Radiology Research II building,
    E002C
  • jadchris_at_iupui.edu 317-274-3815

2
References
Books covering basics of MR physics
E. Mark Haacke, et al 1999 Magnetic Resonance
Imaging Physical Principles and Sequence Design.
C.P. Slichter 1978 (1992) Principles of
Magnetic Resonance. A. Abragam 1961 (1994)
Principles of Nuclear Magnetism.
3
References
Online resources for introductory review of MR
physics
Robert Coxs book chapters online
http//afni.nimh.nih.gov/afni/edu/ See
Background Information on MRI section Mark
Cohens intro Basic MR Physics slides http//porkp
ie.loni.ucla.edu/BMD_HTML/SharedCode/MiscShared.ht
ml Douglas Nolls Primer on MRI and Functional
MRI http//www.bme.umich.edu/dnoll/primer2.pdf J
oseph Hornaks Web Tutorial, The Basics of
MRI http//www.cis.rit.edu/htbooks/mri/mri-main.ht
m
4
Timeline of MR Imaging
1972 Damadian patents idea for large NMR
scanner to detect malignant tissue.
1985 Insurance reimbursements for MRI exams
begin.
1973 Lauterbur publishes method for generating
images using NMR gradients.
MRI scanners become clinically prevalent.
1937 Rabi measures magnetic moment of nucleus.
Coins magnetic resonance.
1924 - Pauli suggests that nuclear particles may
have angular momentum (spin).
NMR renamed MRI
1990 Ogawa and colleagues create functional
images using endogenous, blood-oxygenation
contrast.
1946 Purcell shows that matter absorbs energy
at a resonant frequency.
1973 Mansfield independently publishes gradient
approach to MR.
1959 Singer measures blood flow using NMR (in
mice).
1946 Bloch demonstrates that nuclear precession
can be measured in detector coils.
1975 Ernst develops 2D-Fourier transform for MR.
5
Nobel Prizes for Magnetic Resonance
  • 1944 Rabi
  • Physics (Measured magnetic moment of nucleus)
  • 1952 Felix Bloch and Edward Mills Purcell
  • Physics (Basic science of NMR phenomenon)
  • 1991 Richard Ernst
  • Chemistry (High-resolution pulsed FT-NMR)
  • 2002 Kurt Wüthrich
  • Chemistry (3D molecular structure in solution by
    NMR)
  • 2003 Paul Lauterbur Peter Mansfield
  • Physiology or Medicine (MRI technology)

6
Magnetic Resonance Techniques
  • Nuclear Spin Phenomenon
  • NMR (Nuclear Magnetic Resonance)
  • MRI (Magnetic Resonance Imaging)
  • EPI (Echo-Planar Imaging)
  • fMRI (Functional MRI)
  • MRS (Magnetic Resonance Spectroscopy)
  • MRSI (MR Spectroscopic Imaging)
  • Electron Spin Phenomenon (not covered in this
    course)
  • ESR (Electron Spin Resonance)
  • or EPR (Electron Paramagnetic Resonance)
  • ELDOR (Electron-electron double resonance)
  • ENDOR (Electron-nuclear double resonance)

7
Equipment
4T magnet
RF Coil
B0
gradient coil (inside)
Magnet
Gradient Coil
RF Coil
8
Main Components of a Scanner
  • Static Magnetic Field Coils
  • Gradient Magnetic Field Coils
  • Magnetic shim coils
  • Radiofrequency Coil
  • Subsystem control computer
  • Data transfer and storage computers
  • Physiological monitoring, stimulus display, and
    behavioral recording hardware

9
(No Transcript)
10
Main Magnet Field Bo
  • Purpose is to align H protons in H2O (little
    magnets)

11
Common nuclei with NMR properties
  • Criteria
  • Must have ODD number of protons or ODD
    number of neutrons.
  • Reason?
  • It is impossible to arrange these nuclei
    so that a zero net angular
  • momentum is achieved. Thus, these nuclei
    will display a magnetic
  • moment and angular momentum necessary for
    NMR.
  • Examples
  • 1H, 13C, 19F, 23N, and 31P with
    gyromagnetic ratio of 42.58, 10.71,
  • 40.08, 11.27 and 17.25 MHz/T.
  • Since hydrogen protons are the most abundant in
    human body, we use
  • 1H MRI most of the time.

12
Angular Momentum
J mwmvr
magnetic moment m g J where g is the
gyromagnetic ratio, and it is a constant for a
given nucleus
13
A Single Proton
m
There is electric charge on the surface of the
proton, thus creating a small current loop and
generating magnetic moment m.
The proton also has mass which generates
an angular momentum J when it is spinning.
J



Thus proton magnet differs from a magnetic bar
in that it also possesses angular momentum caused
by spinning.
14
Protons in a Magnetic Field
Bo
Parallel (low energy)
Anti-Parallel (high energy)
Spinning protons in a magnetic field will assume
two states. If the temperature is 0o K, all spins
will occupy the lower energy state.
15
Protons align with field
Outside magnetic field
  • spins tend to align parallel or anti-parallel to
    B0
  • net magnetization (M) along B0
  • spins precess with random phase
  • no net magnetization in transverse plane
  • only 0.0003 of protons/T align with field

randomly oriented
Inside magnetic field
longitudinal axis
Mz
Mxy 0
M
transverse plane
Transverse magnetization
Longitudinal magnetization
16
Net Magnetization
Bo
M
17
The Boltzman equation describes the population
ratio of the two energy states N-/N e E/kT
  • Larger B0 produces larger net magnetization M,
    lined up with B0
  • Thermal motions try to randomize alignment of
    proton magnets
  • At room temperature, the population ratio is
    roughly 100,000 to 100,006 per Tesla of B0

18
Energy Difference Between States
19
Energy Difference Between States
  • D E h n
  • D E 2 mz Bo
  • n g/2p Bo
  • known as Larmor frequency

g/2p 42.57 MHz / Tesla for proton
Knowing the energy difference allows us to
use electromagnetic waves with appropriate energy
level to irradiate the spin system so that some
spins at lower energy level can absorb right
amount of energy to flip to higher energy level.
20
Basic Quantum Mechanics Theory of MR
Spin System Before Irradiation
Bo
Lower Energy
Higher Energy
21
Basic Quantum Mechanics Theory of MR
The Effect of Irradiation to the Spin System
Lower
Higher
22
Basic Quantum Mechanics Theory of MR
Spin System After Irradiation
23
Precession Quantum Mechanics
Precession of the quantum expectation value of
the magnetic moment operator in the presence of a
constant external field applied along the Z
axis. The uncertainty principle says that both
energy and time (phase) or momentum (angular)
and position (orientation) cannot be known with
precision simultaneously.
24
Precession Classical
  • m Bo torque
  • dJ / dt
  • J m/g
  • dm/dt g (m Bo)

m(t) (mxocos gBot myosin gBot) x (myocos
gBot - mxosin gBot) y mzoz
25
A Mechanical Analogy of Precession
  • A gyroscope in the Earths gravitational field
    is like magnetization in an externally applied
    magnetic field

26
Equation of Motion Block equation
T1 and T2 are time constants describing
relaxation processes caused by interaction with
the local environment
27
RF Excitation
On-resonance Off-resonance
28
RF Excitation
  • Excite Radio Frequency (RF) field
  • transmission coil apply magnetic field along B1
    (perpendicular to B0)
  • oscillating field at Larmor frequency
  • frequencies in RF range
  • B1 is small 1/10,000 T
  • tips M to transverse plane spirals down
  • analogy childrens swingset
  • final angle between B0 and B1 is the flip angle

Transverse magnetization
29
Signal Detection via RF coil
30
Signal Detection
Signal is damped due to relaxation
31

Relaxation via magnetic field interactions with
the local environment
32
Spin-Lattice (T1) relaxation via molecular motion
Effect of temperature
Effect of viscosity
T1 Relaxation efficiency as function of freq is
inversely related to the density of states
33
Spin-Lattice (T1) relaxation
34
Spin-Spin (T2) Relaxation via Dephasing
35
(No Transcript)
36
Relaxation
37
Relaxation
38
T2 Relaxation
Efffective T2 relaxation rate 1/T2 1/T2
1/T2 Total dynamic static
39
Spin-Echo Pulse Sequence
40
Spin-Echo Pulse Sequence
41
Multiple Spin-Echo
42
HOMEWORK Assignment 1 1) Why does 14N have a
magnetic moment, even though its nucleus contains
an even number of particles? 2) At 37 deg C in
a 3.0 Tesla static magnetic field, what
percentage of proton spins are aligned with the
field? 3) Derive the spin-lattice (T1) time
constant for the magnetization plotted below
having boundary conditions MzM0 at t0
following a 180 degree pulse M0 at t2.0 sec.
About PowerShow.com