Spin Density, T1, T2, T2* Relaxation and Bloch Equations

Hongyu An1, Weili Lin2

1 Washington University, St. Louis, Missouri, 2 University of North Carolina at Chapel Hill, Chapel Hill, North Carolina
Publication Name:  Current Protocols in Magnetic Resonance Imaging
Unit Number:  Unit B3.1
DOI:  10.1002/0471142719.mib0301s00
Online Posting Date:  May, 2001
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Abstract

This unit discusses the four primary sources of signal variation in a standard MRI sequence. T1 and T2/T2* relaxation reveal the time dependence of the longitudinal and transverse component of the magnetization, respectively. When imaging is performed, the effective spin density is considered as the number of spins per voxel. The variation of signal with voxel size is discussed. Modification of Bloch equations that govern the motion of the magnetization is discussed for all four cases.

     
 
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Table of Contents

  • Overview
  • Technical Discussion
  • Key References
  • Figures
  • Tables
     
 
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Materials

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Figures

  •  FigureFigure B3.1.1 (A) The regrowth of the longitudinal component of magnetization from the initial value Mz(0) to the equilibrium value M0. (B) The decay of the magnitude of the transverse magnetization from an initial value.
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  •  FigureFigure B3.1.2 The upper diagrams show a 90° tip of a set of spins (isochromats) into the transverse plane such that they all lie along the y-axis (fixed frame) at some instant in time, as shown in the middle figure. Precession of the individual spins in the x-y plane immediately follows (the recovery of longitudinal magnetization is ignored since the focus is on transverse magnetization dephasing effects). The lower diagrams show the same process in terms of the net transverse magnetization which decreases in magnitude during the precession because of the fanning out of the spins.
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  •  FigureFigure B3.1.3 The trajectory of the tip of the magnetization vector showing the combined regrowth of the longitudinal magnetization and decay of the transverse components. The initial value was along the y axis and the reference frame is the laboratory.
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Videos

Literature Cited

Key References
    Bloch, F.A. 1946. Nuclear induction. Phys. Rev. 70:460.

This early paper discusses the basic concepts of NMR.

    Bloch, F.A., Hansen, W.W., and Packard, M. 1946a. Nuclear induction Phys Rev. 69:127.

The fundamental NMR concepts are also provided in this paper.

    Bloch, F.A., Hansen, W.W., and Packard, M. 1946b. The nuclear induction expreiment Phys Rev. 70:474.

More basic NMR concepts are discussed in this article.

    Haacke, E.M., Brown, R.W., Thompson, M.R., and Venkatesan, R. 1999. Magnetic Resonance Imaging: Physical Principles and Sequence Design. John Wiley & Sons, New York.

This book covers the technical discussion here as well as other advanced materials in detail.

    Purcell, E.M., Torrey, H.C., and Pound, R.V. 1946. Resonance absorption by nuclear magnetic moments in a solid. Phys Rev. 69:37.

This is another excellent early paper introducing the basic concepts.

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