mri quiz 2

• • MAGNET     The earliest magnets were described 2000 years ago as naturally occurring black stones that attracted iron.

  These “leading stones,” or lodestones, were thought to be magic by the natives in a region of present-day western Turkey, then known as Magnesia in the Greek language
• MAGNET is a material or object that produces a magnetic field.
• ·       Magnets used for MRI are of three types:

  o    permanent,

  o   resistive and

  o   superconductive.
• ·       Permanent MRI magnets

  o   use permanently magnetized iron like a large bar magnet that has been twisted into a C-shape where the two poles are close together and parallel. In the space between the poles, the magnetic field is uniform enough for imaging.
• MAGNETIC FIELD

  ·       A magnetic field is generated by a moving charge (electrical current).

  ·       The direction of the magnetic field can either be clockwise or counter-clockwise with respect to the direction of flow of the current.
• ·       AMPERE’S LAW OR FLEMING’S RIGHT-HAND RULE

  o   determines the magnitude and direction of the magnetic field due to a current;

  If you point your right thumb along the direction of the current, then the magnetic field points along the direction of the curled fingers.
• When a magnet is moved in and out of a closed circuit, an oscillating current is produced, which ceases the moment the magnet stops moving. Such a current is called an INDUCED ELECTRIC CURRENT.
• ·       FARADAY’S LAW OF INDUCTION

  o   explains the phenomenon of an induced current.
• The change of magnetic flux through a closed circuit induces an electromotive force (EMF) in the circuit
• EMF is defined as the energy available from a unit of charge travelling once around a loop of wire
• The EMF drives a current in the circuit and is the result of a changing magnetic field inducing an electric field
• Imaginary lines of magnetic induction create a field effect
• The strength of the magnetic field is defined by placing an imaginary north pole in it and measuring the force on the pole
• This is similar to the definition of the electric field by its force acting on a positive charge
• ·       The tesla (T) is the standard international (SI) unit for the magnetic field.

  ·       An older unit still very much in use is the gauss (G);

  o   1 T equals 10,000 G.
• ATOMIC NUCLEUS small, dense region consisting of protons and neutrons at the center of an atom,

  ·       discovered in 1911 by Ernest Rutherford based on the

  1909 Geiger–Marsden gold foil experiment
• ·       An atom is composed of a:

  —      positively charged nucleus,

  —      with a cloud of negatively charged electrons surrounding it,

  —      bound together by electrostatic force.
• —      The NUCLEUS :

  o   makes up much less than .01% of the volume of the atom,

  o   but typically contains more than 99.9% of the mass of the atom, with a very small contribution from the electron cloud.

  o   Protons and neutrons are bound together to form a nucleus by the nuclear force.
• PROTON SPIN

  ·       Protons and neutrons spin about their own axis within the nucleus.

  ·       The direction of spin is random, so that some particles spin clockwise and others anticlockwise.

  ·       nucleus has an even mass number, the spins cancel each other out so the nucleus has no net spin.

  ·       nucleus has an odd mass number,the spins do not cancel each other out and the nucleus spins.
• The Larmor equation is used to calculate the frequency or speed of precession for the magnetic moments of a specific nucleus in a specific magnetic field strength
• The Larmor equation is simply stated as: ω0 = γB0
• ω0 = precessional frequency = ω0
• B0 = strength of the external field in tesla (T)
• γ = gyromagnetic ratio, a constant of proportionality
• The precessional frequency of the magnetic moments of a specific nucleus at 1T has units of MHz/T
• The precessional frequency or Larmor frequency is proportional to the strength of the external field and can be calculated for any type of MR active nucleus and field strength
• For nuclei of interest in clinical MR, protons have the highest Larmor frequency at any field strength
• Increasing the Larmor frequency results in higher MR signals
• Magnets used for MRI are of three types: permanent, resistive, and superconductive
• Permanent MRI magnets use permanently magnetized iron twisted into a C-shape where the two poles are close together and parallel
• A magnetic field is generated by a moving charge (electrical current)
• Ampere’s law or Fleming’s right-hand rule determines the magnitude and direction of the magnetic field due to a current
• Changing magnetic fields generate electric currents
• The tesla (T) is the standard international (SI) unit for the magnetic field
• The atomic nucleus consists of protons and neutrons at the center of an atom
• Protons and neutrons spin about their own axis within the nucleus
• Nuclei with an odd number of protons are MR active and act like tiny bar magnets
• The Larmor equation is used to calculate the frequency or speed of precession for the magnetic moments of a specific nucleus in a specific magnetic field strength
• Radiofrequency (RF) is composed of an oscillating electric field positioned 90° to an oscillating magnetic field
• A 90-degree RF pulse reorients the magnetization vector to a direction 90 degrees perpendicular to the direction it had prior to the pulse