Magnetic Resonance Imaging Topic 3 (MRI )

ANDRE CAJES B. RRTClinical 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 MAGNETSNatural Occurring Permanent

CLASSIFICATION OF MAGNETSElectromagnets

MAGNETIC STATES OF MATTERAll 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 MATTERDiamagnetic

Weakly repelled from both poles of a magnetic field.

Example: Gold Diamonds Lead Silver

MAGNETIC STATES OF MATTERParamagnetic

Weakly attracted to both poles of a magnetic field.

Example: Gadolinium (excellent contrast agent

for MRI) Tungsten Aluminum

MAGNETIC STATES OF MATTERFerromagnetic

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 signal-

to-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 SPINThe 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”

Hydrogennucleus

B0

Precessional Path

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.

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