Basic MRI I

1
Basic MRI I

• Chapter 3 Notes

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Terms to know

• Resonant Characteristic-- able to have resonance
• Magnetizedmakes a net magnetization vector in a
  magnetic field
• Magnetizationprocess of forming a net
  magnetization vector
• Resonancecapacity to absorb and re-radiate
  radiofrequency energy

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Two ways to display MR signals

• MR images
• MR spectra

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Magnetic nuclei

• Have magnetic moment
• Act like small magnets
• Have an odd number of protons and neutrons
• Have the quantum property of spin
• Are sometimes called spins

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Magnetic or non-Magnetic?
Magnetic Magnetic Non-Magnetic
H-1 C-13 C-12
F-19 0-15 O-16
Na-23 N-15
P-31
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RF signal strength

• Most significant factor in determining image
  quality and the time it takes to acquire an image

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Relative signal strength

1. Tissue concentration of the element
2. Isotopic abundance
3. Sensitivity of the specific nuclide

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Tissue concentration

• For hydrogen nuclei, or protons, this is the
  proton density or PD
• For other nuclei, referred to as spin density

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Isotopic abundance

• H-1 99.985
• H-2 0.015 deuterium
• H-3 -- lt0.0011 tritium

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Sensitivity

• Signal strength produced by an equal number of
  nuclei
• H-1 is the strongest, considered 1.0 or 100
• See table 3 in your book.

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Why proton imaging is so good

• High tissue concentration
• Most abundant magnetic isotope of hydrogen (H-1)
  is magnetic!

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RF energy

• RF pulses (transmitted) usually 90 degrees or
  180 degrees, but can be of any angle
• RF signal (received) received by coils that act
  as antennae
• Stored in k-space
• Fourier transform (FT) to make images
• Received as echoes

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Nuclear magnetic interactions

• Nuclear alignmentonly a few of every million
  nuclei align with the magnet, but this is enough
  to form a net magnetization vector
• The rest are tumbling randomly, due to thermal
  activity.

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Precession and resonance

• The rate of precession is the same as the
  resonant frequency, called the Larmor Frequency

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Excitation

• Tip the net magnetization vector away from the z
  directionany component in the x-y plane
  generates a signal that can be detected.

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Relaxation

• The net magnetization vector re-forms, lines up
  with the z direction, and the individual spins
  dephase.

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Frequency is proportional to field strength

• Freq gyromagnetic ratio x field strength
• Freq gamma x B-naught
• Freq ?HB0
• Gyromagnetic ratio ?H 43.58 MHz/T
• Field strength B0