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Where does the MRI signal come from? This section explores the basic physics of magnetic resonance imaging.

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Small MRI Signal

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Why is the MRI signal so small?

Body tissues have a low magnetic susceptibility (poor "magnetisability").

The MRI signal is contaminated by external noise.

Thermal energy equalises the measured populations of the Zeeman eigenstates.

After an MRI signal is generated, it decays away very quickly.

INCORRECT. This answer to the question is incorrect, though the statement that body tissues have a low magnetic susceptibility is not incorrect.

Magnetic susceptibility is not a nuclear phenomenon; it is a bulk property of a substance produced principally from the movement of electrons. The MRI signal, however, is purely a nuclear phenomenon. Magnetic susceptibility is relevant in MRI to certain image artefacts, and the creation of microscopic magnetic field gradients which increase T2*.

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INCORRECT. The MRI signal is contaminated by RF noise, but this is not the principal reason why the MRI signal is so small.

The main noise source in MRI is the patient in the scanner. External RF noise is blocked by a Faraday cage built into the walls and the door (and the window) of the scanning room.

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CORRECT. Thermal motion of the spins at body temperatures used in MRI tends to remove any population difference between the two Zeeman eigenstates. Classically, we can say that the populations of the parallel and antiparallel spin states are almost equal. The equilibrium distribution is predicted by the (classical) Boltzmann distribution

Nup/Ndown = exp(-ΔE/kBT)

where kB is the Boltzmann constant (1.38066x10-23 J K-1), ΔE is the energy difference between the Zeeman eigenstates and T is the absolute temperature.

The excess of spins parallel to the external magnetic field is given by

spin excess ≈ Nhω0/2kBT

where N is the total number of spins present in the sample, h is the Planck constant over 2π, and ω0 is the angular frequency of precession of protons in an external magnetic field (the Larmor frequency).

For example: With typical B0 = 1.5T, the spin excess at body temperature (310K) is 1 part in 106 (or 0.000001). This means that there is only a very slight polarisation of the spin angular momentum vector along the direction of the external magnetic field—a small net magnetisation vector. When this magnetisation is moved into the transverse plane for measurement, the oscillating magnetic field which is generated is a very small signal, even with Avagadro numbers of protons in a few grams of tissue.

INCORRECT. The MRI signal does decay away, but not usually so fast as to prevent signal measurement.

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Further reading on this topic:
Books: MRI From Picture to Proton p137, Q&A in MRI p25, MRI: Physical Principles & Sequence Design p4, 89

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