TR not equal to scan time per volume

1

Hi all,

I noted during fMRI acquisition (EPI) that the scan time per volume (calculated as total scan time divided by total number of volumes) is around 40 ms longer than the actual TR. Can anyone clarify the reason for this? We have a delay in TR of 0, while Averages is set to 1.

Thanks in advance!
Christine

2

Hi,

We may need a little bit of context here to be able to help you.
Are you reading the total scan time on the MRI console? Which system are you working on? Where do you get the information of the actual TR? Which sequence parameters are you using?

3

Hi,

Yes, I’m reading the scan time and TR from the console (but I’ve also checked the TR value against the protocol). The scanner is a Siemens MAGNETOM Vida. The full fMRI protocol may be found here: protocol.pdf - Google Drive

4

Thank you for providing this information. What you mean is that if you multiply the number of volumes by the TR, you get approx. 29min27s while the protocol on the PDF is saying 29min35s?

It can be easily explained:
First of all, the introduction is on: This adds 4s (I think) to the protocol. It is a time at the beginning of the scan when the scanner makes a little noise with its gradients to warn the subject that more noise will be coming.
Then, there is a time dedicated for the dummy and calibration scans which is taken into account in the total scanning time but not part of the acquisition itself.

1 Like
5

Hi @jsein,

Thank you so much for your help. Yes, I was indeed referring to that discrepancy. I got 40 ms in my estimate because the last time we used this particular protocol we were only scanning for a little over 6 minutes.

You mentioned that the dummy and calibration scans are not part of the acquisition itself… is there any way how one can ensure that these scans are not included with the DICOM output? I’m still a bit of a newbie I’m afraid :slight_smile:

6

Hi @cfarr ,

The dummy and calibration scans are not included in the saved DICOM images in general for Siemens sequence. For some specific research sequence, these scans can be saved, but always in a separate series. I think that you can safely assume that the bold images you get from your Siemens scanner will not include any dummy not calibration scans.

7

Thanks again for your help! It is much appreciated.