Lecture Notes: T1 Relaxation

Introduction

• Follow-up to T2 relaxation lecture
• T1 relaxation also known as spin-lattice relaxation
• T2 relaxation involves spin-spin interaction causing dephasing (transverse decay)
• T1 relaxation involves spins interacting with the lattice to gain longitudinal magnetization

Key Concepts

• Lattice: Structural components like macromolecules and proteins
• Longitudinal Recovery: Re-alignment of spins with the main magnetic field

Process of T1 Relaxation

• Spins move from transverse to longitudinal plane
• Example scenario: MRI machine with tissues (fat and CSF)
  • Application of RF pulse causes net magnetization vector to flip to 90 degrees
  • On turning off B1 pulse, T2 and T1 relaxation occur independently

Tissue Interaction

• Fat vs. CSF:
  • Fat: Quick gain of longitudinal magnetization due to more lattice
  • CSF: Slow T1 relaxation due to fewer structural components
• Interaction with lattice affects T1 relaxation rate
• Analogy: People tripping over chairs representing interaction with lattice

Graphical Representation

• Plot showing fat regains longitudinal magnetization faster than CSF
• Y-axis: Net longitudinal magnetization
• T1 contrast helps differentiate tissues based on relaxation times

T1 Time Constant

• Time to gain 63% of longitudinal magnetization
• Key Difference:
  • T2 time constant involves loss of transverse magnetization
  • T1 time constant is longer in CSF compared to fat

T1 vs. T2 Relaxation

• T2 Relaxation: Loss of signal, transverse decay
• T1 Relaxation: Gain of signal, longitudinal recovery
• T2* relaxation involves extra decay due to magnetic field inhomogeneities
• T1 relaxation differences in magnetic field strength average out*

Measuring Relaxation

• T2 relaxation: Direct measurement of transverse magnetization
• T1 relaxation: Cannot directly measure longitudinal magnetization
• Time to Echo (TE) and Time to Repetition (TR) important for measurement

T1 Weighted Images

• Short TR time highlights T1 differences
• TR influences T1 contrast in tissues
• Longer TR allows tissues to regain longitudinal magnetization
• Proton Density Weighted Image: Negates T1 and T2 differences with specific TR and TE times

Conclusion

• Understanding T1 and T2 times crucial for MRI imaging
• Next talk will cover weighting of MRI images for T1 and T2 contrasts

Additional Notes

• Images show T1 and T2 contribution
• TR and TE times manage the weighting of images