BIDS naming for B1+ efficiency

Hi BIDS community!

There is a convention for naming B1+ maps with the suffix TB1map (see Quantitative MRI - Brain Imaging Data Structure v1.9.0), however, this “map” is in arbitrary unit and it is more a multiplication factor to scale flip angle maps. It was likely designed to use these maps for correcting qMRI metrics (derived from methods such as MPM, MTsat, etc.), as defined here.

Other usage of B1+ maps include the characterization of RF coils, for which the B1+ efficiency is a popular metric. Its unit is T/V (more often given as nT/V), or T/sqrt(W). More details in ISMRM Best Practices for Safety Testing of Experimental RF Hardware.

I understand BIDS has a 80/20 philosophy, however the 7T community is getting bigger every year, and characterizing RF coil is an increasingly important activity, which would benefit from more standardized and reproducible practices. I am collaborating on multiple projects that involve creating open-access data, and researchers end up uploading the data (with cryptic file names) as a single ZIP file on OSF, whereas BIDS compliant datasets on OpenNeuro would be far more reusable.

On a related note, I find it confusing that the current TB1map BIDS suffix is unit-less, although the term “map” suggests it is a quantitative map. It says “arbitrary” on the website, but looking at the FA effective equation, it is in fact unit-less. The transmit B1 field does have a physical unit (tesla). I would rather call BIDS’ TB1map something like TB1scale (or similar).