Magnetic Resonance Imaging Topic 3 (MRI ) ANDRE CAJES B. RRT Clinical Instructor

OBJECTIVES: Discuss the following Terms: Hydrogen Atomic Nucleus 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 4

Bipolar and Dipolar Magnets

CLASSIFICATION OF MAGNETS Magnets are classified according to the origin of the magnetic property. Natural Occurring magnets Permanent magnets Electromagnets 6

CLASSIFICATION OF MAGNETS Natural Occurring Permanent

CLASSIFICATION OF MAGNETS Electromagnets A magnet consisting essentially of a coil of insulated wire wrapped around a soft iron core that is magnetized only when current flows through the wire An electromagnet is a type of magnet whose magnetic field is produced by the flow of electric current.

MAGNETIC STATES OF MATTER All matter has magnetic properties. There are 4 types of magnetic properties Nonmagnetic Diamagnetic Paramagnetic Ferromagnetic 9

MAGNETIC STATES OF MATTER Nonmagnetic Unaffected by magnetic field Example: Glass Rubber Wood 10

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 12

MAGNETIC STATES OF MATTER Ferromagnetic Can be strongly magnetized Example: Iron Nickel 13

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. Whenever electrical current is applied to a wire, a magnetic field is induced around the wire. To produce a static field, (NOT ALTERNATING), direct current is required 18

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 19

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 21

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 In the absence of the super cooled environment, the current in the magnet coils will experience resistance, and the static field will be lost 22

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 Precessional Path 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” 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.

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