File talk:MR spin echo.ogv

This file incorrectly shows the spins changing rotation direction (clockwise to counter clockwise) after the inversion pulse. This does not happen, the video is incorrect and should be removed. — Preceding unsigned comment added by SBarnes (talk • contribs) 21:15, 2 January 2009 (UTC)

It makes sense if you are in the rotating frame reference system: then the rotation direction is only due to gradients and inhomogenheities, and can happen in either direction. I will change the description to clarify this. --IngFrancesco (talk) 19:41, 16 February 2009 (UTC)

Sorry to pick up the debate again after two years, but the above explanation is still incorrect. True, the rotation can happen in either direction, but it will never change direction. If the local inhomogeneity causes and increased rate of rotation (due to increased field) that will not suddenly change to a decreased rate of rotation (which would require an decreased magnetic field) after the 180 pulse. A change in the direction of rotation would only be caused by a change in the rate of rotation (so the delta to the rotating frame would change) and this could only be caused by a change in the local field strength experienced by that spin (see larmor equation). The pulse obviously does not change the local field and will therefore not change the direction of rotation. Any basic MR text book will describe this in detail, it is also covered here:

http://www.imaios.com/en/e-Courses/e-MRI/MRI-signal-contrast/180-RF-pulse

I still have that blender file you sent me somewhere :-) and I worked on it for quite a while 2 years ago, but I had never used Blender before and got frustrated. Maybe I should give it another go. If you want to work on it I would suggest slowing down the rotation so you can see what is happening, and giving each spin a separate color so you can tell them apart. I suddenly brought this up again because I just noticed the file is being used now.SBarnes (talk) 15:54, 5 April 2011 (UTC)