This is the overall gradient echo imaging sequence,
so this prephase and frequency encoding gradient generate gradient echo.
The time from the RF pulse center to the echo center from here to here.
So this is called echo-time,
typically is denoted as TE.
The time from one RF pulse center to the next RF pulse center,
so in this figure from here to here.
This is this time is called time to repeat,
typically denoted as TR,
time to repeat or repetition time,
either way, it's typically denoted as TR.
This is defined for RF pulses exciting the same slice location. This is important.
If we acquire data in multi-slice mode,
in that case we acquire one phase encoding line for
one slice and then for the next slice folder next RF pulse excitation,
we excite a different location.
Location of RF excitation can be changed by changing the modulation frequency,
or transmission frequency, or Larmor frequency.
Just changing the frequency, resonance frequency.
And then we can excite a different location.
This TR definition is not about just RF pulse centers,
but it's about the RF pulses exciting the same location.
Meaning the transmission frequency,
the resonance frequency should be the same to define a time-to-repeat (TR).
Please keep in mind that.
The multi-slice imaging, one
excitation to the next excitation will have different location.
It will take longer time for the one slice to be excited again.
The time to repeat for the multi-slice imaging is longer,
which improved a little bit beyond but I'm
trying to tell you the concept over time to repeat here.
Typically, the slice refocusing, pre-phasing,
and phase encoding gradients are applied simultaneously as shown here.
This slightly focusing, and pre-phasing,
and phase encoding they're all independent to each other.
So, we can apply for them at the same time event as shown here.
This will minimize this minimum possible echo time,
which is sometimes very desirable to minimize the signal decay.
That is sometimes desirable.
These events are typically applied at the same time.