I want to map the intensities of the fMRI 4D volumes onto its surface in order to get the functional values on each vertex in each frame. 

I have done the following steps for achieving this:

1. ran recon-all on its anatomical volume.

2. register the fMRI series with the anatomical volume by using bbregister.

3. assign values from volumes to each surface vertex through the command mri_vol2surf.

I have got a result of mapping the 4D volumes intensities onto its surfaces but there are 35.5% vertices assigned 0 value in every time frame. 

I am wondering is the ratio reasonable and how could I improve the result? I guess resolution would be one of reasons causing the high 0 value ratio, my fMRI data is 2mm^3 and anatomical data is 1mm^3.

Many thanks for your help.

Best,

An