# Frequency encoding gradient in MRI

1. Oct 15, 2015

### BobP

Hi, I am studying the physics of MRI (from a conceptual NOT mathematical point of view...please don't answer with heavy maths).

I understand how we can obtain a signal from a specific slice along our Z-axis. I know that we then apply a graded magentic field (frequency encoded gradient) along the X-axis. This causes all protons along the X-direction of the patient to spin at a different frequency, according to their position along the X-axis. What I do not understand is why this x-gradient will only affect protons in our previously selected z-slice. We are working in a different plane (orthagonol to the z-axis). Why do we not get a signal from the entire body according to the frequency distribution caused by the FEG?

I hope my question is clear.
Thank you

2. Oct 15, 2015

### Staff: Mentor

You only get signals from spins that were previously excited. The spins outside of your excited slice are indeed affected by the gradient, but they don't contribute to the signal.

3. Oct 15, 2015

### BobP

Thank you for your fast reply. I do not understand why this is. Is there a non-mathematical way of explaining why this is the case. thanks

4. Oct 15, 2015

### Staff: Mentor

The RF coils only receive signal from transverse spins. Longitudinal spins do not induce any voltage in the coil. Is that clear to you?

5. Oct 15, 2015

### BobP

Oh, so because only the protons in the z-slice lie in the transverse plane, only those protons will produce a signal that is detected by the induction of a varying X-gradient.
Yes. I get it. thanks so much!

6. Oct 15, 2015

### Staff: Mentor

Yes, exactly.

7. Oct 15, 2015

### BobP

Thanks. You're an absolute star! I've been struggling to understand what's going on for a day now! so grateful