# MRI Question, precessing nuclei :) Help would be appreciate (1 Viewer)

#### megabakedpotato

##### Member
Up to the second last dot point of my last topic in my summaries and I have "hit the wall" somewhat. How would you explain this?

How is the amplitude of the signal given out when precessing nuclei relax related to the number of nuclei present?

Like I understand how the more nuclei relaxing the more radiation given off, but why would it increase the amplitude and not just the intensity?

#### unknown88

##### Member
Amplitude..l think you are talking about Larmor frequency. They hydrogen have there own net spin and spin anti-parallel or parralel. When the external magnetic field hits them it will force the hydrogen nuclues to align with the external magnetic field. When they align this is called precessing and the rate they do this at is the larmor frequency. Radio frequencys are given out that correspond to the larmor frequency. These radio waves give energy to the hydrogen nuclues in which tehy absorb..moving to a higher energy state one the radio frequency is turned off, the hydrogen nucleus return to there original energy state (relax). Intensity is the number of nuclues not that amount of energy they have.

#### Danstar2

##### Member
Up to the second last dot point of my last topic in my summaries and I have "hit the wall" somewhat. How would you explain this?

How is the amplitude of the signal given out when precessing nuclei relax related to the number of nuclei present?

Like I understand how the more nuclei relaxing the more radiation given off, but why would it increase the amplitude and not just the intensity?

For anyone looking at this still, I'm pretty sure it means the amplitude of the induced AC current in the detector coils. Remember the Strength of EMF induced in the detector coils is proportional to the strength of the precessing magnetic field.
Thus, if there are more nuclei present in the precessing magnetic field, they will give the current a in the detector coils a greater amplitude.