www.ecf.utoronto.ca/~joy september, 2003bme 1450 introduction to nmr 1 nuclear magnetic resonance...

September, 2003BME 1450 Introduction to NMR1

www.ecf.utoronto.ca/~joy

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.

Introduction



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?

6. Could the TeachSpin PS1-A NMR spectrometer be modified (if necessary) to meet these specifications? If so how and if not why not.

Problem

BME1450 Intro to NMRNovember 2002

The BasicsThe Details




Example of MRI Images of the Head

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.



MRI Imagers

GE 1.5 T Signa Imager

GE 0.2T Profile/i imager

BME1450 Intro to NMRNovember 2002

The BasicsThe Details




Magnetic Resonance (MR)

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

The Details

Magnetization is “magnetic dipole moment per unit volume”.

MR: Precession

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.

Y

Z

J or

X

B0

|B0|••t

The Details

Summary

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)

Y

Z

J or or M

X

B0

|B0|••t

The Details

Receive

Coil

MR Excitation pulse

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.

Y

Z

J or or M

X

B0

|B0|••tB1

|B0|••t

|B1|••t

The DetailsB0

B1

Excitationcoils



The Rotating Frame

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!!

Y’

Z

M

X’

B1

|B1|••t

My’

MZ

The Details

Rotating Frame



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.

Y’

Z

M

X’

My’

MZ

The Details

Rotating Frame



Magnitisation Relaxation (Decay)

The transverse (M) and longitudinal (M||) components of the magnitization change with time.

Two relaxation times T1 (longitudinal) and T2

(transverse). T1 T2

M(t)||

M0

tT2

Y

X

Z

M 0

M(t)M (t)||

The Details



Basic NMR Pulse Sequence

The Details

What flip angle gives biggest FID????

RF

Excitation

TimeFID

100 ms

Rotation by degrees

Flip angle

10s

5ms << T2 !!!



The Main Magnet

NMR Instrumentation

•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

The sample

Move the sample holderto the most uniform spot



The CP Spin echo sequence

NMR Instrumentation

This sequence overcomes the T2*non-uniformity effects allowing T2 to be measured.

RF

Excitation

FID

30 ms

degrees Flip degrees Flip



Why CP Spin echo makes an echo

NMR Instrumentation

http://www.physics.monash.edu.au/~chrisn/espin.html

•This animation shows the rotating frame coordinates.

•The two RF pulses (/2 & ) 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.

The mixer

NMR Instrumentation

•The FID is amplified and then shifted down in frequency in the “mixer”.

XFID~15 MHz

mixer

RF oscillator 15 MHz

Mixer output DC

time



An FID and four Echos

NMR Instrumentation

FID

Four Echos

Scope sweep

10ms / div

www.ecf.utoronto.ca/~joy september, 2003bme 1450 introduction to nmr 1 nuclear magnetic resonance...

Documents

joyimaging defibrilator

magnetic field b0

frequency b0

b0 field

t magnetic

frequency b1

magnetic fields

measurable rf magnetic