Pseudo frequency? Related to phase shift

[nomisme]

I am reading a text about how MRI works and I came across a thing that I am totally confused.

The book mentions about that few particles side by side with their nuclei spinning at the same frequency but they all have a phase difference. Since MRI cannot measure the phase differences directly, it will try to encode the phase differences in term of so called pseudo-frequency. With each unique pseudo frequency the MRI will know where that particle coming from.

The thing I am confused is how can each nuclei share a common frequency yet they have a different frequency due to their phase differences?

Am I misunderstanding something?

 

[Baluncore]

I think the phase difference you are referring to is determined by the differences in distance from the nuclei to the antenna.

 

[marcusl]

They precess at the same frequency in a uniform magnetic field. Once a field gradient is applied, their frequencies differ slightly depending on position. The gradient means that the applied field changes a little bit, linearly with position. This results in the basic frequency being shifted, typically, by parts per million.