Summary of T1, T2 and Proton Density Weighting | MRI Weighting and Contrast | MRI Physics Course #6

Main Ideas and Concepts

• T1 and T2 Relaxation:

  T1 Relaxation (longitudinal recovery) and T2 Relaxation (transverse decay) occur at different rates in various tissues, contributing to contrast in MRI images. T1 Relaxation involves the recovery of longitudinal magnetization, while T2 Relaxation involves the loss of transverse magnetization.

• Weighting in MRI:

  MRI images can be manipulated to highlight either T1 or T2 differences through specific pulse sequence adjustments, resulting in T1-weighted or T2-weighted images. Proton Density Weighting is another type of imaging that emphasizes the density of hydrogen protons in tissues.

• Pulse Sequence Parameters:
  • Time of Echo (TE): The time between the RF pulse and the sampling of the transverse magnetization signal. Adjusting TE can highlight T2 differences.
  • Time of Repetition (TR): The time between successive RF pulses. Adjusting TR can highlight T1 differences.
• Image Contrast:

  A short TE with a long TR emphasizes T1 differences, while a long TE with a long TR emphasizes T2 differences. Proton density weighted images are created with long TR and short TE, focusing on the number of protons available for nuclear magnetic resonance.

• Examples of Tissue Relaxation Times:

  Different tissues have distinct T1 and T2 values. For example, CSF has a T2 time of approximately 160 ms and a T1 time of about 2000 ms.

• Practical Considerations:

  Understanding the relationship between TR and TE values helps identify the type of weighting in an MRI image.

  • Long TR: 2000 ms
  • Short TR: 300-600 ms
  • Long TE: 80-160 ms
  • Short TE: 10-30 ms

Methodology for MRI Weighting

• Creating a T1 Weighted Image:
  • Use a short TE to prevent T2 differences from affecting the image.
  • Use a short TR to capture T1 differences.
• Creating a T2 Weighted Image:
  • Use a long TR to allow for T1 recovery.
  • Use a long TE to emphasize T2 differences.
• Creating a Proton Density Weighted Image:
  • Use a long TR to allow for full recovery of longitudinal magnetization.
  • Use a short TE to minimize T2 effects, focusing on the density of protons.

Summary of Key Points

• T1 and T2 Relaxation processes are independent but simultaneous.
• Adjusting TE and TR allows for the creation of images that highlight specific tissue characteristics.
• Understanding the basic principles of MRI weighting is essential for interpreting images and recognizing tissue differences.

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