acquisition time for a DWI sequence

[gautam]

Hi Everyone,
Second try, since I did not see my earlier post on the same topic appear in the forum.
I had asked about the technique for expressing the acquisition time of a SS EPI DWI sequence in terms of the parameters of the protocol, like TR, TE, #directions and b etc. I was wondering if
someone can guide me through the process?
Also, what is the strategy for estimating SAR for a specific sequence. Currently, I think that it is directly proportional to 1) number of RF excitations 2) amplitudes of the RFs and 3) strength of various gradients utilized in the sequence, all combined and divided by the total acquisition time, and the weight of the subject. Well, this is a deposition of energy, but it does establish an upper limit.
Would be nice to hear your thoughts on this.
Thanks,
G

[aelster]

Dear Gautam,

Concerning DWI acquisition times, it depends on the particular sequence used, so there is no general formula. For single-shot EPI acquisition during a single TR interval one shot corresponds to a single direction and single b-value. The TE is irrelevant. Thus the acquisition time is proportional to the number of slices, number of signals averaged, number of directions chosen, number of b-values desired, and TR. If parallel imaging is used, the acquisition time is reduced by the R-factor.

Concerning SAR, I will be opening up an entire Q&A section on this in the near future, and the topic is too complex to be covered in the forum. So you'll have to wait for now, or find your answer elsewhere.

Thank you for your questions.

Allen Elster

[Melvinchunter]

The time of acquisition for a conventional twist echo or gradient echo sequence is the result of the reiteration time, stage encoding steps, and number of midpoints. For example, with a one second TR, 128 phase steps, and two averages we would get an acquisition time of about 1 x 128 x 2= 256 seconds or 4 minutes and 16 seconds. The actual time will be slightly longer.