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Anisotropy induced by macroscopic boundaries: surface-normal mapping using diffusion-weighted imaging.

Evren Ozarslan1, Uri Nevo, Peter J Basser

  • 1Section on Tissue Biophysics and Biomimetics, Laboratory of Integrative and Medical Biophysics, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland, USA. evren@helix.nih.gov

Biophysical Journal
|December 11, 2007
PubMed
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Magnetic resonance imaging (MRI) can map tissue boundaries using edge-enhancement. This technique exploits diffusion-weighted imaging to determine orientations normal to macroscopic surfaces for noninvasive assessment.

Area of Science:

  • Magnetic Resonance Imaging (MRI)
  • Diffusion-Weighted Imaging (DWI)
  • Biomedical Engineering

Background:

  • Macroscopic boundaries in MRI cause diffusion-related signal intensity increases, known as edge-enhancement.
  • This effect is particularly relevant in diffusion-weighted imaging (DWI) protocols sensitive to magnetic field gradients.
  • The orientation of diffusion gradients influences edge-enhancement, creating potential for mapping boundary normals.

Purpose of the Study:

  • To investigate the exploitation of diffusion anisotropy for mapping orientations normal to macroscopic boundaries.
  • To explore the phenomenon of 'remote-anisotropy' where boundary information can be detected even without a boundary within the imaging voxel.
  • To demonstrate the feasibility of using edge-enhancement for noninvasive assessment of tissue and organ boundary integrity.

Related Experiment Videos

Main Methods:

  • Utilized diffusion-weighted imaging (DWI) protocols where signal attenuation is primarily driven by magnetic field gradients.
  • Exploited diffusion anisotropy to map the direction normal to macroscopic boundaries.
  • Employed diffusion tensor imaging (DTI) to obtain the eigenvector corresponding to the smallest eigenvalue for surface-normal mapping, avoiding phase cancellations.

Main Results:

  • Simulations indicated that the hypothesized anisotropy is within observable limits, even in voxels without direct boundary presence (remote-anisotropy).
  • Identified conditions where phase cancellations could lead to edge detraction, and methods to avoid this effect.
  • Experimental validation on geometric constructs and real tissue confirmed the feasibility of mapping orientations orthogonal to macroscopic surfaces.

Conclusions:

  • The edge-enhancement mechanism in DWI can be effectively utilized to map orientations normal to macroscopic boundaries.
  • This technique, including remote-anisotropy, offers a novel, noninvasive method for assessing tissue and organ boundary integrity.
  • The smallest eigenvector from diffusion tensor imaging provides a convenient tool for generating surface-normal maps.