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Magnetic Resonance Diffusion Imaging: Introduction and Concepts 

Magnetic Resonance Diffusion Imaging: Introduction and Concepts
Chapter:
Magnetic Resonance Diffusion Imaging: Introduction and Concepts
Author(s):

Denis Le Bihan

DOI:
10.1093/med/9780195369779.003.0005
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date: 17 October 2018

This chapter introduces the key concepts for understanding how images of molecular diffusion can be obtained with MRI and how to interpret them, taking into account the diffusion MRI encoding process and its limitations. First, the main gradient-encoding schemes proposed to produce MRI maps of the diffusion process (either qualitative diffusion-weighted images or quantitative images of the apparent diffusion coefficient (ADC)) are introduced and compared. The different steps required to obtain meaningful diffusion images with MRI are then described, along with specific issues that arise in the context of MRI, such as gradient pulses interactions (b-factor), sensitivity to motion, background local magnetic gradients in tissues, or diffusion anisotropy. The nature of biological information embedded in diffusion images and how it relates to biologically relevant features, such as perfusion (intravoxel incoherent movement (IVIM) effect), diffusion restriction, molecular interaction with barriers or macromolecules, and hindrance, are also discussed. Finally, a conceptual representation of the diffusion process is proposed, as seen on an MRI temporal and spatial scale, based on a biphasic model with a fast and a slow diffusion pool bound to cell membranes. This representation may give an account of the observed variations in ADC with cell size, cell proliferation, and tissue orientation (anisotropy).

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