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T2 Relaxation, Spin-spin Relaxation, Free Induction Decay, Transverse Decay | MRI Physics Course #4
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T2 Relaxation, Spin-spin Relaxation, Free Induction Decay, Transverse Decay | MRI Physics Course #4

Introduction to Nuclear Magnetic Resonance

In this section, the speaker introduces the process of nuclear magnetic resonance (NMR) and explains how protons align with an external magnetic field.

NMR Process

  • Protons are placed within an external magnetic field.
  • Protons align with the magnetic field at a set frequency.
  • A perpendicular radiofrequency pulse is applied, causing the protons to resonate in phase with each other and fan out from the longitudinal magnetization vector.
  • This results in the gain of transverse magnetization and loss of longitudinal magnetization.

T2 Relaxation - Loss of Transverse Magnetization

The speaker discusses T2 relaxation, which involves the loss of transverse magnetization. They explain terms such as spin-spin relaxation and transverse decay.

Spin-Spin Relaxation

  • The loss of transverse magnetization is caused by spins going out of phase with each other after the radiofrequency pulse is stopped.
  • Spins interact with each other, leading to dephasing.
  • This process is also known as spin-spin relaxation or T2 relaxation.

Transverse Decay

  • As spins dephase, they lose their net transverse magnetization vector.
  • This results in a loss of signal measured in MRI machines.
  • The interaction between spins causes different energy levels to transfer energy, leading to out-of-phase spins.

Behavior of Different Tissues during T2 Relaxation

The speaker explains how different tissues behave during T2 relaxation and demonstrates it using examples of fat and cerebrospinal fluid (CSF).

Fat vs. CSF

  • Fat consists of long chains where molecules can easily bump into each other, causing spins to interact and dephase quickly.
  • CSF, on the other hand, has free-moving molecules that are less likely to interact with each other, resulting in slower dephasing.

T2 Relaxation Curves

  • T2 relaxation curves depend on the type of tissue through which the spins are spinning.
  • Fat shows a rapid loss of signal due to significant dephasing, while CSF maintains more phase coherence and retains signal for a longer time.

T2 Star Decay and Magnetic Field Inhomogeneities

The speaker introduces T2 star decay and explains that it is not solely caused by spin-spin relaxation. They mention the influence of magnetic field inhomogeneities.

T2 Star Decay

  • The measurable decay observed in MRI machines is referred to as T2 star decay.
  • It includes both loss of signal due to spin-spin relaxation and magnetic field inhomogeneities.

Magnetic Field Inhomogeneities

  • Loss of signal can also occur due to variations in the magnetic field within the imaging region.
  • These inhomogeneities contribute to the overall decay observed during T2 relaxation measurements.

Timestamps have been associated with relevant bullet points.

New Section

This section discusses T2 star decay and the factors that contribute to magnetic field inhomogeneities, causing the loss of transverse magnetization.

T2 Star Decay and Magnetic Field Inhomogeneities

  • T2 star decay refers to the time it takes for transverse magnetization to decay.
  • In an ideal world, we would want a T2 value representing 63% loss in transverse magnetization vector due to spin-spin interactions, not magnetic field inhomogeneities.
  • Three mechanisms contribute to magnetic field inhomogeneities: imperfections in the MRI scanner's magnetic field strength, substances within the patient (such as metal or calcium), and de-phasing of spins disrupting the local magnetic field.
  • Protons at different locations within an inhomogeneous magnetic field experience different magnetic field strengths, leading to varying rates of de-phasing and loss of transverse magnetization.
  • The T2 star effect occurs due to these differences in magnetic field strength between protons.
  • The T2 star relaxation curve happens faster than the T2 relaxation curve.

New Section

This section explains how compensation for T2 star decay is achieved during image production.

Compensation for T2 Star Decay

  • To produce an image, a 90-degree RF pulse perpendicular to the main magnetic field is applied, resulting in T2 relaxation and loss of transverse magnetization.
  • In reality, spin-spin interactions and local magnetic field inhomogeneities cause additional loss of transverse magnetization known as T2 star decay.
  • A 180-degree RF pulse can be applied after spins have de-phased but before they fully re-phase with each other. This helps re-phase spins with one another.
  • By re-phasing spins using a 180-degree RF pulse, net transverse magnetization is regained, compensating for the T2 star decay.
  • The re-phasing of spins leads to an increase in transverse magnetization vector and a maximum net transverse magnetization.

New Section

This section discusses the de-phasing and re-phasing of spins within a voxel during image production.

De-phasing and Re-phasing of Spins

  • After applying a 90-degree RF pulse, spins within a voxel start to de-phase with each other, resulting in loss of transverse magnetization.
  • Differences in spin-spin interactions and magnetic field strengths cause varying rates of de-phasing among spins.
  • Over time, spins within the same voxel will start to re-phase with each other and gain longitudinal magnetization.
  • By applying a 180-degree RF pulse, faster de-phasing spins can be re-phased with slower ones.
  • When the faster spin catches up with the slower spin, they become in phase due to the 180-degree flip.
  • Waiting for the same period between the 90 and 180-degree pulses allows spins to fully re-phase, leading to maximum net transverse magnetization.

The transcript provided does not contain enough information for further sections.

New Section

In this section, the speaker discusses the concept of transverse magnetization and its relationship to spin-spin interactions or T2 relaxation. They mention that a pulse sequence called spin echo sequences can help regain T2 relaxation and account for local inhomogeneities in the magnetic field.

Transverse Magnetization and T2 Relaxation

  • Transverse magnetization is influenced by spin-spin interactions or T2 relaxation.
  • Spin echo sequences can be used to regain T2 relaxation and correct for magnetic field inhomogeneities.
  • Different tissues have different T2 relaxation times, which can be visualized by plotting their signals over time.
  • CSF has a longer T2 relaxation time compared to muscle due to hydrogen protons' ability to move freely.
  • The time to Echo (TE) can be adjusted to sample tissues at different stages of T2 relaxation.
  • A shorter TE results in higher signal intensity, while a longer TE leads to lower signal intensity.
  • Contrast between tissues increases with longer TE, allowing better differentiation on MRI images.

New Section

This section focuses on how changing the time to Echo (TE) affects signal intensity and contrast between different tissues during MRI imaging.

Impact of Time to Echo on Signal Intensity and Contrast

  • Sampling tissue with a short TE immediately after switching off the RF pulse results in high signal intensity for all tissues (muscle, fat, CSF), but no contrast between them.
  • Increasing TE decreases signal intensity but enhances contrast between tissues:
  • Muscle appears darker than fat and CSF.
  • Fat has lower signal intensity than CSF but higher than muscle.
  • CSF maintains a bright signal value even with prolonged TE, increasing contrast with other tissues.
  • Waiting too long without an appropriate TE causes loss of transverse magnetization and no detectable signal.

New Section

This section highlights the importance of T2 relaxation differences between tissues in creating contrast during MRI imaging. It also mentions that the next topic will focus on T1 relaxation.

Importance of T2 Relaxation Differences for Contrast

  • Changing TE helps highlight differences in T2 relaxation times between different tissue types.
  • Contrast between tissues is based on their varying T2 relaxation properties.
  • Prolonging TE further reduces signal intensity and diminishes contrast, making it difficult to differentiate tissues.
  • Understanding T2 relaxation differences allows for better visualization and differentiation of various tissue types in MRI images.

The transcript provided does not include any timestamps beyond 974 seconds (16 minutes and 14 seconds).

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