repetition time for the T1w MPRAGE

[Kerem Tezcan]

Dear all,

I am looking at the acquisition parameters for the T1 weighted MPRAGE sequence (the protocol is here https://www.humanconnectome.org/storage/app/media/documentation/s1200/HCP_S1200_Release_Appendix_I.pdf, page 24).

As far as I understand from this paper (https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0096899, Eqn. 1) the repetition time (TR) for the MPRAGE sequence is given by inversion time (TI), delay time (TD), number of RF pulses (turbo factor, N) and the echo spacing (tau) as 

TR = TI + TD + N*tau.

When I put these numbers in from the protocol from the above document, it yields:

TR = 1000ms + TD + 256*7.6ms = 2945ms + TD.

But TR is given as 2400ms in the protocol, so the delay time TD has to be negative...

I could not make sense of this unfortunately... Is the TI given to the k-space center rather than to the first RF pulse? What am I missing?

Any help is appreciated.

Thanks in advance.

[Pablo Velasco]

Yes, the TI is the time from the center of the inversion pulse to the acquisition of the ky = 0 (or kz = 0, depending on the looping).  That would be, roughly, N/2+1 (give or take 1).

That would give you TR = 1000ms + 129*7.6ms + TD = 1980.4ms + TD

[Kerem Tezcan]

Dear Pablo, dear all

Thanks a lot for the answer.

Another question: the protocol states reordering is linear (Appendix I, page 26, link above). I believe this refers to how the k-space is acquired, right?

If yes, my question is: is the acquisition in the read-out direction always in the same direction. i.e. left->right or does it alternate?

Graphically, assuming left-right here is the readout direction and up-down is the phase encoding direction:

1)

------------>

------------>

------------>

------------>

------------>

 or 2)

------------>

<------------

------------->

<------------

------------->

Thanks a lot again!

[Glasser, Matthew]

I believe it was case #1.  For HCP lifespan; however, it was case #2, which is why we elected not to do readout distortion correction (which only works if they are all in the same direction).

Matt.

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