Origin of the MR Signal

Where does the MR signal come from?  

The MR signal is a small electrical current induced in the receiver coil by the precession of the net magnetization (M) during resonance. This is a manifestation of Faraday's Law of Induction, wherein a changing magnetic field induces a voltage in a nearby conductor. 

The Faraday-Lenz principle of magnetic induction. A voltage (V) is generated in the coil proportional to the rate of change of the magnetic field (dB/dt). The induced current opposes the applied field.

Note: The description above utilizes classic principles of electromagnetism to explain the origin of the MR signal as an induced current in the receiver coil. An alternative (and equivalent) explanation using quantum mechanics is also available in which a portion of the absorbed energy is radiated coherently by photons and transferred to electric energy in the RF coil.

Advanced Discussion (show/hide)»

Although not really an advanced topic and discussed in multiple prior Q&A's, it is worth reviewing briefly how the NMR phenomenon occurs and its signal is generated.

When placed in an external magnetic field (Bo), individual nuclei/protons begin to precess at the Larmor frequency (fo = γBo).

Each nucleus can be characterized by a wave function (Ψ) that is a superposition of low-energy (spin-up) and high-energy (spin-down) states.

At thermal equilibrium there is a tendency of the entire system (when viewed statistically) to prefer a distribution of slightly favoring the lower energy states. This results in a net magnetization (M) aligned with Bo.

If the system is irradiated with energy from a transverse magnetic field (B1) at the Larmor frequency, nuclear magnetic resonance will occur. The B1 field is typically turned on for only a few msec and is called an RF-pulse.

During stimulation into nuclear magnetic resonance, the spin system is rotated so that the net magnetization (M), originally aligned with Bo, now precesses around the Bo direction and develops transverse components (Mxy).

After the B1 field is turned off, M continues to precess around Bo. A receiver coil sensitive to magnetic flux changes in the transverse plan can then detect the M_xy components as they oscillate at the Larmor frequency. A small current is generated in this coil via the Faraday-Lenz induction principle.

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References
     Hahn EL. Free nuclear induction. Physics Today, November 1953, pp. 4-9.

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