Fundamentals of Magnetic Resonance

Imaging

- Hardware and Principle

Outline

History of Magnetic Resonance Imaging

MR Imaging Hardware System

Principle of MRI

Timeline of MR Imaging

1920 1930 1940 1950 1960 1970 1980 1990 2000

1924 - Pauli suggests

that nuclear particles

may have angular

momentum (spin).

1937 Rabi measures

magnetic moment of

nucleus. Coins

magnetic resonance.

1946 Purcell shows that

matter absorbs energy at a

resonant frequency.

1946 Bloch demonstrates

that nuclear precession can be

measured in detector coils.

1972 Damadian

patents idea for large

NMR scanner to detect

malignant tissue.

1959 Singer

measures blood flow

using NMR (in mice).

1973 Lauterbur

publishes method for

generating images

using NMR gradients.

1973 Mansfield

independently

publishes gradient

approach to MR.

1975 Ernst develops

2D-Fourier transform for

MR.

NMR renamed MRI

MRI scanners become

clinically prevalent.

1990 Ogawa and

colleagues create

functional images using

endogenous, blood-

oxygenation contrast.

1985 Insurance

reimbursements for

MRI exams begin.

Source: http://www.fonar.com/timelineofmri.htm

Nobel Prizes for Magnetic Resonance

1944: Rabi (Physics)

resonance method for recording magnetic properties of atomic nuclei

1952: Felix Bloch and Edward Mills Purcell (Physics)Basic science of NMR phenomenon

1991: Richard ErnstChemistry (High-resolution pulsed FT-NMR)

2002: Kurt WthrichChemistry (3D molecular structure in solution by NMR)

2003: Paul Lauterbur & Peter MansfieldPhysiology or Medicine (MRI technology)

Modern 3 Tesla

MRI unit (Philips)

Main magnet

body

Patient Couch

Bore of the magnet

Instrumentation (1)

RF Coil (for head)

Instrumentation (2)

Magnet RF Coil

Source: Joe Gati, photos

Gradient Coil

Main Components of a Scanner Static Magnet (1)

Permanent Magnet

open

C-Shape

Standing MRI

Advantage:

-Simple

-Comfortable

-Inexpensive

-No need to use liquid Helium

-Low maintenance cost

Disadvantage:

-Low field strength (normally

Superconductive Magnet

closed

cylindrical

Static Magnet (2)

Advantage:

-High-field (normally >1.5T)

-High stability

-High homogeneity

-Cost low

Disadvantage:

-Expensive

-Helium needed

-High maintenance cost

Magnetic Field Strength

Measured by Tesla (T) vs Gauss (G)

1T = 10,000G

Earths magnetic field ~ 0.3~0.7G

Clinical MRI typically between 0.7T and 3.0T. It is very strong!

High field MRI

Advantage:

1) High SNR

2) Short acquisition time

3) Enable advanced MR

imaging, such as

MRS, BOLD etc

1cm

5 A

1 Guass

Disadvantage:

1) High cost

2) High noise

3) High SAR

4) High artefact

Gradient Magnets RF Coils

"antenna" of the MRI system

broadcasts the RF signal

and/or

receives the return signal

loop of wire

depth of the image generally limited to about one radius

for spines, shoulders, small body parts

Commonly for the knee

Better homogeneity

Volume coil

two parallel circular coils

pelvis imaging and cervical

spine imaging

Provides the best RF homogeneity

Commonly used as a transceiver coil

e.g., head, knee

Start

Atom = nucleus + electrons

Nucleus = neutrons + protons

Atom number = # protons

Atom weight = #neutrons + # protons

About Atom: A Review

To differentiate

atoms

Same atom

number but

different atom

weight are

different isotopes

Spin

Protons (nuclear constituent of atom) have a property of angular

momentum known as spin

Motion of electrically charged particles results in a magnetic force

orthogonal to the direction of motion

The spin value depends on the atomic number and atomic weight of the

particular nucleus.

Why 1H?

Reasons for choosing 1H:

1)1H occupies the largest proportion

- 3*1022/ml in water

1) Gyromagnetic ratio is much larger than others, and thus the magnetic

resonance signal is the largest

3) Different forms in biological organ

- water

- fat

So by default, MRI is 1H imaging!

Protons Aligning within a Magnetic Field

In field free space

randomly oriented

Source: Mark Cohens web slides Source: Robert Coxs web slides Source: Jody Culhams web slides

when placed in a magnetic field (B0; e.g., our MRI machines) protons will either align

with the magnetic field

there is a small difference in the number of protons in the low and high energy

states with more in the low state leading to a net magnetization (M)

Inside magnetic field

oriented with or against B0M = net magnetization

M

Precession

Protons precess in external magnetic field. The precessional axis is

parallel to the external magnetic field.

Source: Mark A. Brown, Richard C. Semelka

The Nobel Prize in Physics 1944

Rabi predicted that the magnetic moments of nuclei

could be induced to flip their magnetic orientation if

they absorbed energy from an electromagnetic wave

of the right frequency. They would also emit this

same amount of energy in falling back to the lower

energy orientation, and Rabi would be able to detect

this transition from one energy state to the other. He

called this method molecular beam magnetic

resonance.

Isidor Isaac Rabi

(1898-1988)

Austrian

For resonance method for recording the magnetic

properties of atomic nuclei

Larmor Equation

Frequency (rate) of precession is proportional to the strength of

magnetic field

: Gyromagnetic ratio

Unit of /2 pi : MHz/T

Resonance

frequency (MHz)Magnetic field (T)

Larmor frequency slightly depends on the molecular structure the

protons 1H belong to. Fat molecules are large and surrounded by many

electrons, which reduce the effective external field. This way the Larmor

frequency of fat is roughly 150 Hz lower at 1 T (220 Hz at 1.5 T) than that

of water.

Gyromagnetic Ratio

For the following scanners,

What is the resonance frequency of the following nuclei in

each of the magnetic fields?

1H23Na31P

Question

/2

(MHz/T)

Bo

= 0.7T Bo

= 3.0T Bo

= 7T

1H 42.57

23Na 11.26

31P 17.23

Phillips 3.0 Tesla

Clinical MRI

GE 0.7 Tesla

low field MRI

SIEMENS 7.0 Tesla

High field MRI

Question Net Macroscopic Magnetization (no B0)

when an external magnetic field is absence

= (0,0,0)

Net Macroscopic Magnetization (with B0)

The phenomenon of quantized energy states in the presence of an

external magnetic field is known as the Zeeman effect

The energy difference (E) between the two levels is exactly

proportional to the frequency v and thus the magnetic field B0:

Zeeman Effect

h (Planck's constant)

= 6.626 10-34 Js

low energy state (spin up)

high energy state (spin down)

Net Macroscopic Magnetization (with RF)

Before:

1) # low-energy protons are slightly more than # high energy protons

2) No net magnetization in the transverse plane -- the phase of transverse components

are random

After:

1) half of the different protons with low energy reversed their energy state no net

macroscopic longitudinal magnetization

2) The phase of the transverse component are consistent

Effect of a 90o Pulse Excitation

Bo

Coordinate System

Absorption of the RF energy of frequency causes M0 to

rotate away from its equilibrium orientation by an angle

Flip Angle

Break

Types of Relaxation

When the RF is turned off, the return to equilibrium iscalled relaxation

The protons immediately begin to realign themselves and return to their original equilibrium orientation

Longitudinal relaxation precessing protons are pulled back into alignment with main magnetic field of the scanner (B

o) reducing

size of the magnetic moment vector in the x-y plane

Transverse relaxation precessing protons become out of phase leading to a drop in the net magnetic moment vector (M

o)

Transverse relaxation occurs much faster than longitudinal relaxation

T1 decay describes the longitudinal magnetization returns to equilibrium.

Longitudinal Relaxation

T1 = time required for Mz to recover 63% of its original value

http://www.youtube.com/watch?v=A0dl4_wxr1c&list=PLCD41685D8499AAB1

Mz(t) = Mo(1 - e-t/T1).

Transverse Relaxation

T2 decay describes the return to equilibrium of the transverse magnetization, MXY

T2 = time required for 63% of the initial magnetization (Mxy) to dissipate

Mxy(t) = Mxyoe-t/T2

http://www.youtube.com/watch?v=7K-Dg5jmV-8&list=PLCD41685D8499AAB1

Summary of Relaxation

Energy emission

After break

MR signal

Tissue A

Mxy

Tissue B

MxyMR signal

Tissue B

MR signal

Tissue A

T2 Weighted Imaging

Tissue A

Mz

Tissue B

Mz

Tissue A

Mxy

Tissue B

Mxy

Tissue A

Mxy

Tissue B

Mxy

Signal

Tissue A

Signal

Tissue B

1. For Tissue A and B

PD are the same

Mz are the same

when 900 RF pulse is

on, Mz changed to 0

2. When 900 RF pulse

is on, for Mxy, A = B

3. After some time, for

Mxy, A > B

For T2: A > B

4. The MR signal: A > B

T1 Weighted Imaging

Tissue A

Mz

Tissue B

Mz

Tissue A

Mz

Tissue B

Mz

Tissue A

Mxy

Tissue B

Mxy

Signal

Tissue A

Signal

Tissue B1. For Tissue A and B

PD are the same

Mz are the same

when 900 RF pulse is

on, Mz changed to 0

2. When 900 RF pulse

is off, Mz gradually

recovered.

For Mz, A > B

Given T1: A > B

3. Apply another 900 RF pulse

Mz gradually recovered.

For Mxy, A > B4. MR signal: A > B

The Nobel Prize in Physics 1952

Felix Bloch

Switzerland Edward Mills Purcell

U.S.A

for their development of new methods for nuclear magnetic precision

measurements and discoveries in connection therewith

- Scientific principle of MRI

- determine the time evolution

of nuclear magnetization

- relaxation phenomena

- related problems of molecular structure

- measurement of atomic constants,

- nuclear magnetic behaviour at low

temperatures

PD=proton density

Summary

For samples in external magnetic field, the

sample is exposed to energy at the correct

frequency that will be absorbed.

A short time later, this energy is reemitted,

which can be detected and processed.

A brief summary video

http://www.youtube.com/watch?v=1CGzk-

nV06g