Why do we not measure FID but the gradient recalled echo?

[aelster]

Dear Harry1,

The FID shown in the image you cited was one I recorded several years ago from a uniform phantom without the use of any spatial encoding gradients. The oscilloscope was triggered to begin recording at several hundred microseconds after the RF pulse, so any noise/oscillations early in the FID were excluded.

The main reason we generally don't typically measure the FID for routine imaging is that (unless special methods are used) we need to apply gradients to localize the signal in space. A test-tube NMR laboratory spectrometer doesn't require spatial localization and so an FID can be used there.

You may wish to read the "Advanced Discussion" from the following Q&A http://mriquestions.com/free-induction-decay.html

ADE