Why is the background of PSIR image gray ?

[simonqueric]

From what I understand of MRI physics, gray levels correspond to different tissues in volumes.

However, in PSIR volumes (for instance, sub-mon005_ses-M0), the background is gray :

How is it possible ? Isn't the background supposed to be dark (like in T2/T1 contrasts )?

[jcohenadad]

https://mriquestions.com/phase-sensitive-ir.html