| T O P I C R E V I E W |
| weissman |
Posted - Mar 11 2003 : 3:09:08 PM
Dear Forumers,
I am currently running an event-related study on the 1.5 T using the standard spiral sequence. When creating the average time courses for each trial type, I'm noticing that the hemodynamic response peaks 1 TR earlier than is usual in my prior studies (between 2.5 and 3.75 seconds, rather than between 3.75 and 5.0 seconds), which used the same pulse sequence. I've checked to be sure that my onset times are correct and they are. I've also checked that the number of disacarded images is correct, and, using a stopwatch, that the first stimulus in each run onsets exactly when I think it does. So, I am a bit confused.
One possibility is that the HRF peaks early in my study. Another is that the 1.5 T is discarding one image less than we are entering in the user CVs (we enter 6, but maybe it discards only 5). Either would account for my results.
I'm wondering whether anyone has any insight into this issue or has experienced similar anomalies recently.
Thanks in advance!
Daniel Weissman
[Moved by moderator to its own topic.]
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| 2 L A T E S T R E P L I E S (Newest First) |
| charles.michelich |
Posted - Jun 18 2003 : 6:21:20 PM
Daniel and Simon,
For Allen's spiral pulse sequences (sss and invspiral) the number of disdaqs is stored with the data. If you look at the BXH files for each of your runs, you will see <disdaqs>#</disdaqs> where # is the number of disdaqs. You can use this to confirm that you entered the correct number of disdaqs at the scanner. Hopefully this will help you isolate the source of your unexpected timing.
Have a great day,
Chuck
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| tonev |
Posted - Jun 18 2003 : 2:07:43 PM
Daniel--We may be having a similar problem with our data. We too are using a spiral sequence on the 1.5T. As with your data, ours may be off by 1 or 2 TRs as well. However, it is not the peak that looks off (unlike you, I don't have any prior data that would tell me when the peak should be), rather, the HRF seems to begin ascending prior to the presentation of the stimulus. I too, have checked the timing again and again without finding any problems but I am certainly willing to accept that I may be comitting some simple error (e.g., specifying the incorrect number of disdaqs or dynamic timepoints per run, incorrectly specifying the tstatprofile paradigm file, etc.).
Any insight would be appreciated.
Cheers,
Simon |
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