Download Magnetic Resonance TOPIC 3

Magnetic
Resonance
Imaging
Topic 3 (MRI )
ANDRE CAJES B. RRT
Clinical Instructor
OBJECTIVES:
Discuss the following Terms:

Magnet

Magnetic Field

Classification of
Magnet

Magnetic State of
Matter

Magnet Used in MRI

General style of
magnets employed
•
Hydrogen Atomic Nucleus
•
Proton Spin
•
Precision
•
Radio Frequency
•
Relaxation Time
MAGNETS

An object that is surrounded by a magnetic field
and that has the property, either natural or
induced, of attracting iron or steel.

To obtain a magnetic resonance (MR) signal from
tissues, a large static magnetic field is required.

The primary purpose of the static magnetic field
(known as “Bo” field) is to magnetize the tissue.
MAGNETIC FIELD
• A condition found in the region around a magnet
or an electric current, characterized by the
existence of a detectable magnetic force at every
point in the region and by the existence of
magnetic poles.
•
A vector quantity consisting of both a north and
south pole; it exerts an induction force on
ferromagnetic and paramagnetic substances.
Bipolar or Dipolar Magnets
 Always has a north or south pole
Bipolar and Dipolar Magnets
CLASSIFICATION OF MAGNETS
Magnets are classified according to the
origin of the magnetic property.
 Natural Occurring magnets
 Permanent magnets
 Electromagnets
CLASSIFICATION OF MAGNETS
Natural Occurring
Permanent
CLASSIFICATION OF MAGNETS
Electromagnets
MAGNETIC STATES OF MATTER
All matter has magnetic properties.
There are 4 types of magnetic properties
 Nonmagnetic
 Diamagnetic
 Paramagnetic
 Ferromagnetic
MAGNETIC STATES OF MATTER
Nonmagnetic
 Unaffected by magnetic field
 Example:

Glass

Rubber

Wood
MAGNETIC STATES OF MATTER
Diamagnetic
 Weakly repelled from both poles of a
magnetic field.
 Example:
Gold
 Diamonds
 Lead
 Silver

MAGNETIC STATES OF MATTER
Paramagnetic
 Weakly attracted to both poles of a
magnetic field.
Example:
 Gadolinium (excellent contrast agent
for MRI)
 Tungsten
 Aluminum
MAGNETIC STATES OF MATTER
Ferromagnetic
 Can be strongly magnetized
Example:
 Iron
 Nickel
Gauss & Tesla

Is the unit of the strength of a magnetic field.

Gauss is the smaller unit of measurement
compared with tesla.

1 tesla is equals to 10,000 Gauss

The earth’s magnetic field is about 0.5Gauss
MAGNET ROOM

The major component of MR system in the
magnet room is the magnet itself.

This magnet is large enough to surround the
patient and any antennas that are required
for radio wave transmission and reception.
MAGNETS USED IN MRI
Types of magnet used in MRI
 Permanent
 Resistive
 Superconductive
RESISTIVE MAGNETS

Are simple, although
large, electromagnets.

Earliest types of
magnets used in MRI

They consist of coils of
wire.
RESISTIVE MAGNETS
 A magnetic field is produced by passing an
electric current through the coils.
 The electrical resistance of the wire
produces heat and limits the maximum
magnetic field strength of resistive magnets.
 The heat produced is conducted away from
the magnet by cooling system.
RESISTIVE MAGNETS

Field Strength = up to 0.3 Tesla

They generally do not exceed 0.15
Tesla

Can be turned off when not in use

Temperature sensitive
Superconductive (cryogenic) Magnets

Are also electromagnet.

Most are solenoid in design

However, their wire loops are cooled to very low
temperatures with liquid helium and liquid
nitrogen (cryogens) to reduce the electrical
resistance.
SUPERCONDUCTIVE MAGNETS

This permits higher magnetic field strengths
with superconductive magnets than with
resistive magnets.

Capable of achieving high field strengths

Clinical MRI
 0.5 to 1.5 Tesla
SUPERCONDUCTIVE MAGNETS

Major advantage

High field strength, which results in inherently high signalto-noise ratio (SNR)

Major disadvantage

High cost associated with acquisition, siting, and
maintenance
QUENCH

Sudden and violent loss of superconductivity
PERMANENT MAGNETS

Consist of blocks or slabs
of naturally occurring
ferrous material

It has a constant field
that does not require
additional electricity or
cooling to low
temperatures.
Permanent Magnets

↑ amount of material = ↑ field strength

Field strength= 0.06 to 0.35 Tesla

Permanents magnets have the advantage that
their magnetic field does not extend as far away
from the magnet (fringe field) as do the other
magnetic field of other types.
What is Fringe Field?

The portion of the magnetic field extending
away from the confines of the magnet that
cannot be used for imaging but can affect
nearby equipment or personnel.
General style of magnets employed

Vertical field (open MRI)

Conventional Horizontal field
Magnet design

Vertical field magnet design uses 2 magnets,
one above and one below the patient

The frame, which supports the magnets, also
serves to return the magnetic field.
Note:
 Regardless of the style or type of magnet used,
the B0 field must be stable and homogeneous,
particularly in the central area of the magnet
(Isocenter) where imaging takes place

Field strength and homogeneity can be
increased by reducing the gap between the
two magnets.
Nuclear Magnetism

The name nuclear in NMR refers to the
nucleus of the atom.

Certain nuclei have properties that cause
them to display magnetic properties.

Hydrogen is the most abundant in the
human body therefore used in clinical MRI
Hydrogen used in MRI

Consist of single proton

Proton has mass, (+) charge & spins on its axis

Spinning motion of a positive charge particles
will create a magnetic field around the proton

Proton’s magnetic field is often termed
“magnetic moment”
Continue

Hydrogen is considered magnetically active.

Abundant with a large magnetic moment
and exist in 2 molecules: water & fats.
PROTON SPIN

Define proton spin..

Anything placed within the coil will become slightly
magnetised, which causes the protons to align
along the same axis.

Each proton aligns in one of two stable directions:
Spin-up, which is in the same direction as the field
or spin-down, in the opposite direction to the field.
PROTON SPIN
PROTON SPIN
The diagram below illustrates the difference between
(A) spin-up
(B) spin-down
Precession

Is a change in the orientation of the
rotation axis of a rotating body.
PRECESSION


Due to the influence
of B0, the hydrogen
nucleus “wobbles” or
precesses (like a
spinning top as it
comes to rest)
The axis of the
nucleus forms a path
around B0 known as
the “precessional
path”
Precessional Path
B0
Hydrogen
nucleus
Gyroscope
PRECESSION

The speed at which hydrogen precesses depends
on the strength of B0 and is termed the
“precessional frequency”

The precessional paths of the individual hydrogen
nucleus’ is random, or “out of phase”
Relaxation

The term returning to equilibrium is called
relaxation and can be thought of as two-step
process.

T1 relaxation or Longitudinal relaxation

T2 relaxation or Transverse relaxation
Radio Frequency

RF – electromagnetic radiation lower in energy
than infrared; the RF used in MRI is in the form
of a burst of RF energy (pulse) in the 10 to 200
MHz range.

Brief burst of RF electromagnetic energy
delivered to patient by RF transmitter.
END…