specify hemisphere in TSV for discrete segmentations

[Remi-Gau]

not sure if it makes sense but having an optional hemi column in those TSV could prove useful to standardize them

https://bids-specification.readthedocs.io/en/latest/05-derivatives/03-imaging.html#common-image-derived-labels

for example fmriprep (I suspect relying on Freesurfer labels) has some inconsistencies in labelling

• Left-Cerebral-Exterior
• Visual_V3d_l
• Cbm_Left_I_V_med
• ctx-lh-bankssts

[effigies]

We're constrained by backwards compatibility, so these integer values cannot be made either LH or RH specific. If we assume that we have INT16 images (so 15 bits of non-negative values), we could say something like bits 13/14 (0-indexed) indicate LH/RH.

Bits 13-14	Hemisphere
0 (00)	None
1 (01)	lh
2 (10)	rh
3 (11)	???

Bits 12-0	Structure
1	Gray matter
2	White matter
...	...

Which would lead to:

Binary value	Decimal value	Structure	Hemisphere
0000 0000 0000 0001	1	Gray matter	None
0000 0000 0000 0010	2	White matter	None
...	...	...	...
0010 0000 0000 0001	8193	Gray matter	lh
0010 0000 0000 0010	8194	White matter	lh
...	...	...	...
0100 0000 0000 0001	16385	Gray matter	rh
0100 0000 0000 0010	16386	White matter	rh
...	...	...	...
0110 0000 0000 0001	24577	Gray matter	???
0110 0000 0000 0010	24578	White matter	???
...	...	...	...

The advantage is that we can fully utilize the space and permit up to 8192 structures. The disadvantage is that the addition of 8192 or 16384 is not mentally convenient. Losing the orthogonality of the hemisphere bits and structure bits, we could say (struct + 10000 = lh_struct) and (struct + 10000 = rh_struct), which would be easy enough for humans and computers to perform.

Decimal value	Structure	Hemisphere
1	Gray matter	None
2	White matter	None
...	...	...
10001	Gray matter	lh
10002	White matter	lh
...	...	...
20001	Gray matter	rh
20002	White matter	rh
...	...	...

---

All this is a bit to the side of the naming convention issue. But it might be useful to have in our minds a textual decomposition that mirrors the numeric decomposition, e.g. {hemi}-{basestruct} <-> HEMI_START_VAL + BASE_STRUCT_VAL.