Hi, Francesco
This is related to MRI physics. Make a long story short. The meaning of TR in different MRI sequences are different. Most fMRI data use epi sequences and structural data use some 3D sequences (e.g., MPRage in Simense). They are not directly related.
Also, I would not assume the TR is the time to acquire a slice in structural data as many T1/T2 data use 3D sequences, which involve complex excitation regimes. You can do a simple math. A typical MPRage T1 scan takes ~5-6 mins. If you use TR in ms to multiply the number of slices, you should not get the total time of acquisition.
Ruyuan