Magnetic Resonance Imaging Topic 3 (MRI ) ANDRE CAJES B. RRT Clinical Instructor.
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Transcript of 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
CLASSIFICATION OF MAGNETS
Magnets are classified according to the
origin of the magnetic property.
Natural Occurring magnets
Permanent magnets
Electromagnets
MAGNETIC STATES OF MATTERAll matter has magnetic properties. There are 4 types of magnetic properties
Nonmagnetic
Diamagnetic
Paramagnetic
Ferromagnetic
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
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.
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.
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.
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
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.