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Understanding T2 Relaxation in NMR

May 5, 2025

Lecture Notes: Nuclear Magnetic Resonance and T2 Relaxation

Recap from Previous Lecture

  • Discussion on nuclear magnetic resonance (NMR) process.
  • Placement of protons in a magnetic field results in alignment and processing at a set frequency.
  • Application of a perpendicular radiofrequency pulse causes protons to resonate in phase.
  • Flipping the net magnetization vector to 90 degrees results in maximum transverse magnetization and loss of longitudinal magnetization.

Key Concepts of Today's Lecture

Relaxation Processes in NMR

  • T2 Relaxation (focus of today's lecture): Loss of transverse magnetization.
  • T1 Relaxation: Regaining longitudinal magnetization.

T2 Relaxation: Spin-Spin Relaxation

  • Loss of transverse magnetization due to dephasing of spins (protons) after stopping the radiofrequency pulse.
  • Spin-spin interactions cause dephasing; spins interact and transfer energy.
  • Transverse Decay refers to loss of signal from loss of phase alignment.
  • T2 relaxation varies with the type of tissue due to different molecular structures.

Tissue Examples

  • Fat: Long triglyceride chains lead to more interactions, faster signal loss.
  • CSF (Cerebrospinal fluid): Freely moving molecules, slower signal loss.
  • T2 relaxation curves depend on tissue type and show signal loss over time.

T2* Decay vs T2 Relaxation*

  • T2 Decay* includes loss due to spin-spin interactions and magnetic field inhomogeneities.
  • T2* Decay is faster than T2 relaxation due to these inhomogeneities.
  • Inhomogeneities arise from MRI scanner imperfections, metallic objects, and spin interactions.

Compensating for T2* Decay*

  • Use of a 180-degree radiofrequency pulse to re-phase spins.
  • Applying a 180-degree RF pulse after 90-degree flip can recover phase alignment (echo).
  • Spin Echo Sequence improves signal recovery and compensates for magnetic field inhomogeneities.

Time to Echo (TE) and Signal Contrast

  • Adjusting TE time affects signal contrast:
    • Short TE: High signal, low contrast.
    • Medium TE: Optimal contrast between tissues (fat, muscle, CSF).
    • Long TE: Low signal, difficult to differentiate tissues.
  • TE adjustments allow visualization of T2 relaxation differences.

Conclusion

  • Understanding T2 relaxation and its implications for MRI imaging.
  • Next lecture will cover T1 Relaxation and its application in imaging.

Note: The understanding of T2 and T2* concepts is crucial for interpreting MRI images and understanding tissue-specific signal behavior. Adjusting parameters like TE can significantly affect the quality and diagnostic capability of MRI scans.*

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