Overview of Magnetic Resonance and MRI Spin Echo Sequence

Introduction

• Speaker: Dr. Erin Gomez
• Topic: Magnetic resonance and basic MRI spin echo sequence

Protons in the Body

• Protons exist in fat, muscle, sugars, and water
• Hydrogen atoms are protons (1 proton, 1 positron, 1 electron)
• Protons act like bar magnets

MRI Scanner Basics

• MRI scanner functions as a giant magnet
• Generates a magnetic field (B0)
• Protons align parallel or anti-parallel to B0
• Net magnetization vector along the z-axis (patient's body)

Proton Precession

• Protons spin along their axes (precession or nuclear spin)
• Precession frequency governed by Larmor equation
  • Dependent on magnetic field strength and gyromagnetic ratio

Radio Frequency (RF) Pulses

• RF pulses influence protons
• Protons move to alternate planes and process in phase
• Change in longitudinal magnetization
• Excitation leads to anti-parallel orientation
• Synchronization as transverse magnetization

Recovery and Spin Echo Sequence

• Protons recover to original state after RF pulse
• Spin echo sequence involves:
  • 90-degree RF pulse flips net magnetization
  • Induces transverse magnetization
  • Longitudinal magnetization increases as protons dephase
  • Free induction decay generates an electrical signal

Key Terms

• T1 Time: 63% recovery of longitudinal magnetization
• T2 Time: 63% loss of transverse magnetization
• Each tissue has unique T1 and T2 times

Free Induction Decay and T2* Effects*

• Applies to 90-degree pulses
• Signal decays rapidly
• Dephasing speed is T2* constant
• Magnetic field inhomogeneity influences T2* effects

Combating T2* Effects*

• Additional RF pulse (180-degree refocusing)
• Aligns protons to minimize T2* effects
• Synchronizes proton procession to produce an echo
• Multiple 180-degree pulses generate multiple echoes*

Imaging Techniques

• Spin Echo Imaging: Using RF pulses to achieve echoes
  • Protons aligned with B0
  • 90-degree RF pulse creates transverse magnetization
  • 180-degree pulse rephases protons
  • Echoes captured by MRI scanner
  • TE: Time to Echo
  • TR: Time to Repetition

Conclusion

• Summary of magnetic resonance and MRI spin echo sequence basics