Why should `AcquisitionDuration` and `RepetitionTime` be mutually exclusive?

Hi,

In the context of anatomical data (T2starw), we need to access the total acquisition time (a.k.a. “TA”) in order to compute SNR per unit time measures.

Looking into the BIDS doc, I see there is a field AcquisitionDuration, which seems to be what I am looking for. So, I was about to update the JSON metadata and add this field (based on the DICOM and/or PDF printout of the sequence parameters), but then I realized that we cannot have both AcquisitionDuration and RepetitionTime declared in the JSON file, according to this doc (I’m looking into ‘Option C’):

I suspect this limitation was created with EPI scans in mind (from an fMRI perspective), but does it make sense for anatomical data, where it is more difficult to retrieve the total acquisition time only based on the TR (because other information would be needed, such as matrix size, partial Fourier, in-plane and/or through-plane acceleration, 2D vs. 3D acquisition scheme, elliptical scanning scheme, introduction scans for steady state, etc.).

Thanks for your help,
Julien

Hi Julien,

These fields are specifically BOLD-related, and the reason it’s so baroque is to reduce the number of ways an acquisition scheme (continuous or sparse) can be described and prevent conflicts of information. Personally I would have preferred a much simpler scheme (RepetitionTime XOR VolumeTiming + AcquisitionDuration XOR SliceTiming), but backwards compatibility rules and some cases were considered earlier than others, and we got what we got.

Due also to the tight coupling of RepetitionTime to functional imaging, BEP-001 had to create an entirely new RepetitionTimeExcitation and RepetitionTimePreparation for Anatomy imaging data.

If these alone do not cover your use case, the AcquisitionDuration definition does not seem overly-tied to the functional context, so it could probably be written a little more generally and made available to anatomical images. Would RTE+RTP+AD get you where you need to go?

Best,
Chris

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Thank you for the quick response @effigies.

RTE+RTP do not get me AD because of the many other confounding parameters that define AD mentioned in my first post. So I would indeed recommend to make the usage of AcquisitionDuration more flexible for anatomical data users. I suspect this field will become increasingly useful in the future, as the neuroimaging community is handling/sharing/comparing datasets from various centers and acquisition time is an important parameter from protocol optimization standpoint.

For my project, I will then use AcquisitionDuration (even if RepetitionTime is declared) as I understand there is no alternative.

Cheers

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