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Recording-2025-07-31T21:56:09.449Z

Hydrogen as the MR-Active Nucleus

  • MRI exploits only hydrogen-1 (¹H) because

    • Most abundant nucleus in the human body (water, fat, metabolites)

    • Single proton → positive charge & intrinsic spin → acts as a tiny magnet

    • Spin generates a magnetic dipole (north–south poles) represented by a vector

Proton Orientation in and out of the Magnet

  • In free space (no external field)

    • Proton magnetic moments are randomly oriented → vectors cancel → no net magnetization

  • Inside the main magnet (B₀)

    • Moments align with or against B_0

    • Two populations

    • Parallel / spin-up / low-energy (with B_0)

    • Antiparallel / spin-down / high-energy (against B_0)

    • Always slightly more parallel than antiparallel → produces a residual vector: Net Magnetization Vector ((\vec M))

Net Magnetization Vector ((\vec M))

  • Represents patient magnetization

  • Strength proportional to the excess number of parallel protons

  • Higher field strength ⇒ larger (\vec M) ⇒ stronger MR signal

    • Early systems: $$