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Diffusion generated T1 and T2 contrast.

I Kaufmann1, N Seiberlich, A Haase

  • 1Department of Physics, EP5 (Biophysics), University of Würzburg, Würzburg, Germany. kaufmann@physik.uni-wuerzburg.de

Journal of Magnetic Resonance (San Diego, Calif. : 1997)
|March 5, 2008
PubMed
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The bright ring effect in MRI scans of porous samples like wood is caused by magnetisation diffusion. This inflow effect, where relaxed magnetisation moves from the sample into the surrounding medium, creates a signal increase and limits image resolution.

Area of Science:

  • Magnetic Resonance Imaging (MRI)
  • Biophysics
  • Materials Science

Background:

  • Porous organic samples in water media often exhibit a "bright ring" artifact in MRI.
  • This artifact is characterized by a decreased T1 value in T1 maps and disappears when the medium is removed or contrast agents are added.
  • The bright ring effect is contingent upon a reduction in T1 relaxation time of the medium as it permeates the sample.

Purpose of the Study:

  • To investigate the origin of the "bright ring effect" observed in MRI of porous organic samples.
  • To elucidate the role of magnetisation diffusion in generating signal increases at the sample-medium interface.
  • To understand the impact of this diffusion effect on the resolution of relaxation time maps.

Main Methods:

  • Utilized a computer model to simulate magnetisation diffusion between regions with differing relaxation constants.

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  • Analyzed the inflow of relaxed magnetisation from sample boundary regions into the surrounding medium.
  • Quantified the relationship between T1 values, diffusion time, and the width of the observed transition region.

    • Main Results:

    • The "bright ring" is an inflow effect caused by diffusion transporting relaxed magnetisation from the sample into the surrounding medium.
    • A "transition region" with increased signal is observed where short T1 values from the sample mix with long T1 values from the medium.
    • The width of this transition region is proportional to the diffusion displacement, calculated from T1 values and diffusion time.
    • This diffusion effect limits the achievable resolution in relaxation time mapping.

    Conclusions:

    • The "bright ring effect" in MRI of porous samples is a diffusion-driven phenomenon, not an intrinsic property of the sample's outer layers.
    • The observed signal enhancement arises from the interplay of differing T1 relaxation times and magnetisation diffusion across the sample-medium interface.
    • The diffusion effect has implications for image resolution, particularly in quantitative relaxation time mapping.