Lecture Notes: MRI Spin Echo Sequence and Magnetic Resonance

Introduction to Magnetic Resonance

• Presenter: Dr. Erin Gomez
• Overview: Discusses magnetic resonance, focusing on MRI spin echo sequences.

Basics of Protons in MRI

• Protons exist in fat, muscle, sugars, and water in the body.
• Hydrogen atom: Consists of a proton, positron, and electron, acting like bar magnets.
• Orientation: Protons normally orient randomly but align in a magnetic field.

MRI and Magnetic Fields

• MRI Scanner: Functions as a giant magnet, producing a magnetic field (B0).
• In the magnetic field, protons align parallel or anti-parallel, creating a net magnetization vector along the z-axis.

Proton Precession

• Precession (Nuclear Spin): Protons spin around their axes.
• Larmor Equation: Describes precession frequency based on magnetic field strength and gyromagnetic ratio.

Influence of RF Pulses

• RF Pulses: External radiofrequency pulses can alter proton alignment and precession.
• Protons can be knocked into alternate planes, changing longitudinal to transverse magnetization.
• Excitation: Protons can be anti-parallel with extra energy.

Recovery and Dephasing

• Post-RF pulse, protons return to alignment and asynchronous precession.
• Longitudinal Recovery: Regaining original state of orientation.
• Transverse Decay: Loss of synchronized precession.

Spin Echo Sequence

• 90-degree Pulse: Initial RF pulse creates perpendicular net magnetization vector.
• Transition to transverse magnetization and synchronization during recovery.

Free Induction Decay (FID)

• Net magnetization vector spiral induces electrical signals (FID).
• T1 and T2 Times:
  • T1 Time: 63% recovery of longitudinal magnetization.
  • T2 Time: 63% loss of transverse magnetization.

Challenges of Free Induction Decay

• Limitations: Applicable to 90-degree pulses, rapid signal decay, and variability in magnetic field.
• T2 Effects:* Caused by magnetic field inhomogeneity, leading to signal dropout.*

Combating T2* Effects*

• 180-degree Refocusing Pulse: Realigns protons to counteract T2* effects.
• Echo: Synchronization releases energy, creating an echo.*

Imaging and Spin Echo

• Spin Echo Imaging: Uses multiple 180-degree pulses for repeated echoes.
• TR (Time to Repetition): Time between sequence repetitions.

Summary Diagram

• Depicts the sequence of events in basic spin echo:
  • Initial alignment and precession.
  • Application of RF pulses.
  • Rephasing and echo production.

Conclusion

• Overview of magnetic resonance and basic MRI spin echo sequence.