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I am a cognitive science graduate student who began to study MRI with a little physics background. Please forgive me if I don't use proper verbs or prepositions.

I think I understood slice selection and frequency encoding. Let's assume that we want to get axial images. In slice selection, gradient magnetic field is applied in z-axis so that signals of protons which match the certain resonance frequency are measured. In frequency encoding, gradient magnetic field is applied in x-axis so that signals in a plane can be distinguished vertically by Fourier transformation.

But my question is that how do they yield different results when gradient magnetic field is applied? If we pick a certain slice using gradient, then shouldn't it be the same for frequency encoding? How can all of protons in a plane be measured in frequency encoding, while it is impossible in slice selection?

Thank you in advance.

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  • $\begingroup$ I've just read mriquestions.com/frequency-encoding.html and I think your contention is completely correct. I think I agree that some explanations available online are complete nonsense because of the contention you raise. I'll research a bit more and see if I can find the correct answer. Otherwise I hope someone more knowledgeable comes along. $\endgroup$
    – AXensen
    Apr 5, 2023 at 9:32

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I've done some reading since I left my comment, and I think I've successfully discerned the meaning of the pseudo-explanations given to medical students.

With frequency encoding, the gradient is applied during readout, not during the excitation. So you start by using a gradient in one direction, say $z$, so that only protons with that $z$ value receive the 90 degree pulse. Then they start oscillating at the Larmor frequency. But then you add an additional gradient in another direction $x$. The protons that were excited are still excited, but now they're emitting a response signal whose frequency varies in $x$. So you can differentiate between signals coming from different $x$ positions by looking at a specific frequency component of your response signal.

"Slice selection" seems to be the name of the technique where do the first "during excitation" gradient. "frequency encoding" seems to be the name of the technique where you differentiate between lines within a plane by giving each line a different frequency.

This explanation is definitely still not a complete explanation of MRI. I haven't yet understood how phase encoding works, but hopefully this helps anyway.

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