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Related Concept Videos

Diffusion01:12

Diffusion

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Diffusion is the passive movement of substances down their concentration gradients—requiring no expenditure of cellular energy. Substances, such as molecules or ions, diffuse from an area of high concentration to an area of low concentration in the cytosol or across membranes. Eventually, the concentration will even out, with the substance moving randomly but causing no net change in concentration. Such a state is called dynamic equilibrium, which is essential for maintaining overall...
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Diffusion01:21

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Diffusion is a type of passive transport. In passive transport, a substance tends to move from an area of high concentration to an area of low concentration until the concentration is equal across the space. For example, take the diffusion of substances through the air. When someone opens a perfume bottle in a room filled with people, the perfume is at its highest concentration in the bottle and is at its lowest at the edges of the room. The perfume vapor will diffuse, or spread away, from the...
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Water and Mineral Acquisition02:34

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Specialized tissues in plant roots have evolved to capture water, minerals, and some ions from the soil. Roots exhibit a variety of branching patterns that facilitate this process. The outermost root cells have specialized structures called root hairs that increase the root surface, thus increasing soil contact. Water can passively cross into roots, as the concentration of water in the soil is higher than that of the root tissue. Minerals, in contrast, are actively transported into root cells.
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Nervous Tissue: Myelin01:25

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The myelin sheath is a multilayered lipid and protein covering that insulates the axon of a neuron, enhancing the speed of nerve impulse conduction. Axons without this sheath are referred to as unmyelinated. Two types of neuroglia, Schwann cells in the peripheral nervous system (PNS) and oligodendrocytes in the central nervous system (CNS) are responsible for producing myelin sheaths.
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Behavior of Gas Molecules: Molecular Diffusion, Mean Free Path, and Effusion03:48

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Although gaseous molecules travel at tremendous speeds (hundreds of meters per second), they collide with other gaseous molecules and travel in many different directions before reaching the desired target. At room temperature, a gaseous molecule will experience billions of collisions per second. The mean free path is the average distance a molecule travels between collisions. The mean free path increases with decreasing pressure; in general, the mean free path for a gaseous molecule will be...
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States of Water01:23

States of Water

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Water exists in any one of the three classical states: solid (ice), liquid (water), and gas (steam or water vapor). The state of water depends on i) the intermolecular forces that draw molecules together and ii) the kinetic energy that leads to movements that pull them apart.
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Updated: Feb 15, 2026

Spectral Reflectometric Microscopy on Myelinated Axons In Situ
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Axon Diameter Mapping From Myelin Water Diffusion MRI.

Hong-Hsi Lee, Kwok-Shing Chan, Dmitry S Novikov

    IEEE Transactions on Medical Imaging
    |February 13, 2026
    PubMed
    Summary
    This summary is machine-generated.

    This study introduces a novel diffusion MRI method to measure myelinated axon diameter noninvasively. Simulations confirm its accuracy for in vivo applications, paving the way for clinical use.

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    Area of Science:

    • Neuroimaging
    • Biophysics
    • Medical Physics

    Background:

    • Diffusion MRI measures water diffusion within tissues.
    • Myelin water diffusion is sensitive to the microstructural environment of myelinated axons.
    • Accurate in vivo measurement of axon diameter is crucial for understanding neurological conditions.

    Purpose of the Study:

    • To develop and validate a diffusion MRI method for noninvasive measurement of myelinated axon diameter.
    • To establish the theoretical framework for myelin water diffusion under specific MRI pulse sequences.
    • To assess the feasibility of in vivo application using numerical simulations.

    Main Methods:

    • Proposed a theory for myelin water diffusion using diffusion MRI with wide gradient pulses and the Gaussian phase approximation.
    • Employed Monte Carlo simulations on cylindrical models mimicking myelin sheaths.
    • Evaluated diffusion signals using spherical mean diffusion and assessed performance at various signal-to-noise ratios (SNR).

    Main Results:

    • The developed theory accurately estimates axonal diameters, with a weighting towards outer calibers.
    • Simulations demonstrated the method's applicability at SNR > 20 on a high-performance MRI scanner (Connectome 2.0).
    • The technique measures restricted diffusion of myelin water to infer axon diameter.

    Conclusions:

    • The proposed diffusion MRI protocol enables noninvasive in vivo measurement of myelinated axon diameters.
    • The method shows promise for adaptation to clinically available high-gradient MRI scanners.
    • This technique offers a new tool for studying white matter microstructure in health and disease.