What is Larmor frequency?

Larmor frequency is the specific radio frequency at which atomic nuclei (most importantly hydrogen protons) precess or rotate when placed in a magnetic field. It is the frequency at which these nuclei can absorb and emit RF energy, making it fundamental to MRI.

In MRI, the Larmor frequency is determined by the strength of the static magnetic field (B₀) and is given by the Larmor equation:

f = γ × B₀

where f is the Larmor frequency, γ is the gyromagnetic ratio of the nucleus (for hydrogen, ≈ 42.58 MHz/T), and B₀ is the magnetic field strength in Tesla.

As a result, hydrogen nuclei resonate at approximately 64 MHz at 1.5 Tesla, 128 MHz at 3 Tesla, and around 300 MHz at 7 Tesla. MRI systems must transmit and receive RF signals precisely at this frequency to excite the nuclei and detect the signals used to form images.