Acquiring a k-space line

[Inaki4]

I've been wondering for a while about how making an image dealt with the fact that acquiring several samples of the k-space implies that all of them are acquired at different times, so the T2* relaxation would cause that there is a distortion because different samples are affected by different decay of the transverse magnetization measured.

I read in the book Fundamentals of Medical Imaging by Paul Suetens (Page 74) that this indeed is true, but the acquisition is based on the assumption that the measuring time is much shorter than the relaxation time, so it can be neglected.

Still, I wonder:

1) If we are considering an spin-echo sequence we are trying to get a T2-weighted image by means of cancelling the effect of T2* at t=TE. But since we measure before and after t=TE, we are affected in those measures by T2*, so we would require that the measuring time TM is much shorter than T2*, that is, TM<<T2*. If we are considering a gradient-echo sequence, we also need TM<<T2*.

2) What is more, if we are dealing with an EPI sequence (gradient echo or spin-echo), this implies that the measurement time is increased by a factor K (the number of lines acquired within the same pulse). So we need K·TM<<T2*.

I don't know the typical values of the sampling time during the echo, but is it reasonable to make all these assumptions?

Iñaki Rabanillo Viloria