MRI 1.pdf

MRI terms Hayder Jasim Taher PhD of Medical Imaging Outline of my presentation  MRI.  Magnet.  Magnetic Field.  Classification of Magnet.  Magnetic State of Matter.  Magnet Used in MRI  General style of magnets employed  MRI Magnetic resonance imag...

MRI terms Hayder Jasim Taher PhD of Medical Imaging Outline of my presentation  MRI.  Magnet.  Magnetic Field.  Classification of Magnet.  Magnetic State of Matter.  Magnet Used in MRI  General style of magnets employed  MRI Magnetic resonance imaging ( is an imaging technique used primarily in medical settings to produce high quality images of the inside of human body MRI is based on the principles of nuclear magnetic resonance ( a spectroscopic technique used by scientists to obtain microscopic chemical and physical information about molecules The technique was called magnetic resonance imaging ( rather than nuclear magnetic resonance imaging ( and it produced an image of the NMR signal in a thin slice through the human body  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  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 Magnets used in MRI types of magnet used in MRI  Resistive  Superconductive  Permanent  RESISTIVE MAGNETS Resistive magnets :  Are simple, although large, electromagnets.  Earliest types of magnets used in MRI  They consist of coils of wire.  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  RESISTIVE MAGNETS  RESISTIVE MAGNETS Field Strength = up to 0.3 Tesla  They generally do not exceed 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  In the absence of the super cooled environment, the current in the magnet coils will experience resistance, and the static field will be loss  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  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  Most important Q1/ What are the most important characteristics of permanent magnet? Q2/ What are the advantage of permanent magnets? Q3/ What are the disadvantage of permanent magnets? Q4/ What are the most important characteristics of resistive electromagnets? Q5/ What are the advantage of resistive electromagnets? Q6/ What are the disadvantage of resistive electromagnets?