Transcript Presentation for Agilent April 2000

www.ecf.utoronto.ca/~joy
Introduction
Nuclear Magnetic Resonance (NMR) is a phenomenon discovered
about 60 years ago. Since then it has been used for many
biomedical engineering applications from medical imaging to
the molecular and tissue structure and function. Using NMR
one can measure NMR spectra, diffusion coefficients, electric
current, flow velocity, temperature, blood oxygenation, brain
function, muscle metabolism, reaction rates and much much
more.
The IBBME is the proud owner of a TeachSpin PS1-A NMR
spectrometer. This is a device that can (in its present state)
demonstrate many basic features of NMR but little else.
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BME 1450 Introduction to NMR
September, 2003
www.ecf.utoronto.ca/~joy
Problem
Identify a biomedical application of NMR of interest to your group
and find out:
1. What is problem that NMR helps to solve?
2. How is NMR is used to solve this problem in theory?
3. How is NMR is used to solve this problem in practice?
4. What are the specifications and price of the NMR equipment
required?
5. Why are the above specifications important?
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6. Could the TeachSpin PS1-A NMR spectrometer be modified (if
necessary) to meet these specifications? If so how and if not
why not. BME 1450 Introduction to NMR
September, 2003
www.ecf.utoronto.ca/~joy
BME1450 Intro to NMR
November 2002
The
Basics
The Details
www.ecf.utoronto.ca/~joy
Example of MRI Images of the Head
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Bone and air are
invisible.
Fat and marrow are
bright.
CSF and muscle are
dark.
Blood vessels are
bright.
Grey matter is darker
than white matter.
BME 1450 Introduction to NMR
September, 2003
www.ecf.utoronto.ca/~joy
MRI Imagers
GE 1.5 T Signa
Imager
5
BME 1450 Introduction to NMR
GE 0.2T Profile/i
imager
September, 2003
www.ecf.utoronto.ca/~joy
BME1450 Intro to NMR
November 2002
The
Basics
The Details
The Details
www.ecf.utoronto.ca/~joy
Magnetic Resonance (MR)
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An object in a magnetic field B0 will
become magnetized and develop a net
Magnetization, M.
Most of M arises from the orbital
electrons but a small part is the Nuclear
Magnetization.
The nucleus has a magnetic dipole
moment, , and angular momentum, J.
||/|J| = , the gyromagnetic ratio.
For Hydrogen  = 43 MHz/T.
J and

 Magnetization is “magnetic dipole moment per unit volume”.
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BME 1450 Introduction to NMR
September, 2003
The Details
MR: Precession
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The 1.5T magnetic, B0 field of
the MR Imager makes the
Hydrogen Nuclei precess
around it.
The precession rate,, is the
Larmor frequency.
fL =  B0 = 43*1.5 = 64MHz for
Hydrogen in water
+- 300Hz in other molecules.
Z
B0
J or 
|B0|••t
X
Y
The Details
Receive
Coil
Summary
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The magnetization,M, is the
density of nuclear magnetic dipole
moments.
If you tip M away from B0 it will
precess, at frequency B0,
producing a measurable RF
magnetic field.
The precessing M will induce an
RF voltage in the receive coil if it is
not perpendicular to B0
This signal is called the FID (free
induction decay)
Z
B0
J or  or M
|B0|••t
X
Y
The Details
B0
Excitation
coils
MR Excitation pulse
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You can tip M by applying
a circularly polarized RF
magnetic field pulse, B1, to
the sample.
If B1 is at the Larmor
frequency, B0 you get this.
M is now precessing about
two magnetic fields.
B1 is effective because it
tracks M.
B0
B1
Z |B |••t
1
J or  or M
|B0|••t
B1
|B0|••t
X
Y
The Details
www.ecf.utoronto.ca/~joy
The Rotating Frame
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It is much easier to visualize all
this if you observe it from a
frame of reference which is
rotating at the Larmor
frequency, fL=B0.
B1 appears motionless in this
rotating frame and B0 effectively
disappears and…
During the excitation pulse, M
precesses only about B1 at
frequency B1!!
BME 1450 Introduction to NMR
Z |B |••t
1
MZ
M
My’
B1
X’
Y’
Rotating
Frame
September, 2003
The Details
www.ecf.utoronto.ca/~joy
The Rotating Frame
When the excitation
pulse is over, M is
stationary in the
rotating frame.
 In the Lab frame,
however, it is still
precessing.

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BME 1450 Introduction to NMR
Z
MZ
M
My’
X’
Y’
Rotating
Frame
September, 2003
The Details
www.ecf.utoronto.ca/~joy
Magnitisation Relaxation (Decay)
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The transverse (M) and longitudinal (M||)
components of the magnitization change with time.
Two relaxation times T1 (longitudinal) and T2
Z
(transverse). T1  T2
M0
M0
M(t)
||
M||(t)
M(t)
Y
t
T2
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X
BME 1450 Introduction to NMR
September, 2003
The Details
www.ecf.utoronto.ca/~joy
Basic NMR Pulse Sequence
Rotation by Q degrees
RF
Excitation
10s
Flip angle
Time
FID
100 ms
5ms << T2 !!!
What flip angle gives biggest FID????
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BME 1450 Introduction to NMR
September, 2003
NMR Instrumentation
www.ecf.utoronto.ca/~joy
The sample
The Main Magnet
•Ideally B0 is uniform to 1or 2 ppm
•In the teach spin magnet it is not as
good
•B0 non-uniformity over a sample
means that it produces a range of RF
frequencies around Bomean
•FID decay in T2* < T2
•Spectral lines become blurred
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BME 1450 Introduction to NMR
Move the sample holder
to the most uniform spot
September, 2003
NMR Instrumentation
www.ecf.utoronto.ca/~joy
The CP Spin echo sequence
This sequence overcomes the T2*non-uniformity
effects allowing T2 to be measured.
90 degrees Flip
180 degrees Flip
RF
Excitation
FID
30 ms
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BME 1450 Introduction to NMR
September, 2003
NMR Instrumentation
www.ecf.utoronto.ca/~joy
Why CP Spin echo makes an echo
•This animation shows the
rotating frame coordinates.
•The two RF pulses (p/2 & p) are
along the rotating x axis.
•The arrows are magnetisation at
various points in the sample.
•Most arrows precess faster or
slower than the rotating frame.
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BME 1450 Introduction to NMR
http://www.physics.monash.edu.au/~chrisn/espin.html
September, 2003
NMR Instrumentation
The mixer
•The FID is amplified and then shifted down in
frequency in the “mixer”.
mixer
FID
~15 MHz
X
Mixer output DC
time
RF oscillator 15 MHz
NMR Instrumentation
www.ecf.utoronto.ca/~joy
An FID and four Echos
FID
Scope sweep
10ms / div
Four Echos
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BME 1450 Introduction to NMR
September, 2003