MRI- basic principles

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MRI- basic principles
2014-10-09 03:14:02
atoms protons neutrons electrons ions isotopes

atomic structure, motion in the atom, MR active nuclei, the hydrogen nucleus, alignment, precession, the larmor equation, resonance, the MR signal, the free induction decay signal (FID), relaxation, T1 recovery, T2 decay, pulse timing parameters
Show Answers:

  1. what are isotopes?
    atoms of elements  with the same number of protons but a different number of neutrons
  2. what is the atomic number of an element?
    the sum of the protons in the nucleus
  3. what is the mass number of an element?
    the sum of protons and neutrons in a nucleus
  4. what happens to a nucleus with an even mass number regarding it's spin? what's the disadvantage?
    • half spin in one direction and half in the other
    • nucleus has no net spin
    • no angular momentum
  5. what happens to a nucleus with an odd mass number regarding it's spin? what's the advantage?
    • spin directions are not equal and opposite, so the nucleus itself has a net spin or angluar momentum
    • these are known as MR active nuclei
  6. how are MR active nuclei characterized by?
    • their tendency to align their axis of rotation to an applied magnetic field
    • this occurs because they have angular momentum or spin and as they contain positively charged protons, they possess electrical charge.
  7. the law of electromagnetic unduction (set out by Michael Faraday in 1833) refers to what three individual forces?
    • motion, magnetism, and charge
    • states that if two of the these are present, then the third is automatically induced
  8. based on the law of electromagnetic induction (Michael Faraday 1833), active nuclei that have a net charge and are spinning (motion), automatically acquire a what? and can align with an external magnetic field
    magnetic moment
  9. what are some of examples of elements of MR active nuclei, together with their mass numbers?
    • hydrogen-1
    • carbon-13
    • nitrogen-15
    • oxygen-17
    • fluorine-19
    • sodium-23
    • phosphorus-31
  10. alignment of active nuclei is measured how?
    measured as the total sum of the nuclear magnetic moments and is expressed as a vector quantity
  11. are all of the magnetic moments of all nucleus in the body the same strength?
    • no, energy is specific to every nucleus
    • determines the sensitivity to magnetic resonance
  12. what is protium?
    • the isotope of the hydrogen nucleus
    • contains a single proton (atomic and mass number 1)
  13. the magnetic moment of each nucleus has vector properties. What are the vector properties?
    • size and direction and is denoted by an arrow.
    • the direction is either south or north
    • the length of the vector designates the size of the magnetic moment
  14. why is the net magnetization vector (NMV) larger at high field strengths than low field strengths resulting in improved signal?
    because there are fewer nuclei that have enough energy to join the high-energy population and align their magnetic moments in opposition to the stronger B^0 field.
  15. what is net magnetization vector (NMV)?
    • the net magnetic moment of the patient
    • difference in parallel or spin up and anti-parallel or spin down nuclei with the B^0 magnetic field
  16. what is precession?
    each hydrogen nucleus is spinning on its axis. the influence of B^0 produces an additional spin or wobble of the magnetic moments of hydrogen around B^0
  17. what is precessional path?
    the magnetic moments follow a circular path around B^0
  18. what is precessional frequency?
    • the speed at at which they wobble around B^0
    • MHz/sec
  19. what is the Larmor equation?
    precessional frequency = B^0 x gyromagnetic ratio

    • B^0- magnetic field strength of the magnet
    • gyromagnetic ratio- angular momentum and magnetic moment of each MR active nucleus. It is constant and is expressed as the precessional frequency of a specific MR active nucleus at 1T. MHz/T
  20. what is the gyromagnetic ratio of hydrogen at 0.5T, 1T and 1.5 T?
    1T= 42.57 MHz/T (constant)

    1.5T = 42.57 x 1.5 = 63.86 MHz/T

    0.5T = 42.57 x 0.5 = 21.28 MHz/T
  21. what is resonance?
    a phenomenon that occurs when an object is exposed to an oscillating perturbation that has a frequency close to its own natural frequency oscillation
  22. what is an RF pulse?
    • energy that is given to a nuclei at the same larmor frequency of the nuclei to create resonance
    • called excitation
  23. what is the result of giving an RF pulse at the same larmor frequency as its target causing resonance?
    • the NMV (net magnetization vector) cause to no longer lie parallel to B^0 but an angle to it. This is called the flip angle
    • this occurs because some of the low-energy nuclei are given enough energy via resonance to join the high-energy population, thus balancing occurs
  24. the magnitude of the flip angle depends on what?
    • the amplitude and duration of the RF pulse
    • usually 90 degrees to B^0
  25. what is the B^0's new term after a 90 degrees flip angle occurs?
    • B^0- longitudinal plane
    • 90 degrees plane- transverse plane
  26. what is relaxation?
    • the process by which hydrogen loses energy given by the RF pulse and re-aligns with the longitudinal plane or B^0
    • as relaxation occurs, the NMV returns to re-align with B^0 because some of the high-energy nuclei return to the low-energy population and align their magnetic moments in the spin-up direction
  27. what is recovery and decay?
    • recovery is the amount of magnetization in the longitudinal plane gradually increases
    • decay is at the same time, but independently, the amount of magnetization in the transverse plane gradually decreases