# Slices and aquisition of fMRI

Is a slice obtained with functional MRI (fMRI) always 3-dimensional, i.e., x-y-z? I thought that a typical fMRI scan's total imaged volume would be 4D, i.e., made up of 3D slices plus time.

• The sub-question 2 has just been answered on Bio.SE (biology.stackexchange.com/questions/34269/… - coincidence or what!) and I think it should not be cross-posted here. Could you perhaps elaborate more on subquestion #1 (i.e., remove subquestion 2 and focus on #1?)?
– AliceD
Commented May 16, 2015 at 10:38

I will focus on question #1:

Analysis of MRI scans is typically done using voxels (Fig. 1). Voxels have a volume defined by three dimensions (length, width and depth).

Voxel analysis. Source: Philips

The slice thickness plus the 2D in-plane resolution (pixel size) yields the voxel size. The time to scan a slice can be regarded to be negligible. Making a scan consisting of multiple slices takes time. Hence, the time dimension is not directly relevant for voxel size. Practically, voxel size depends mostly on slice size, as well as the capacity of the scanner. More Tesla's makes smaller voxels possible.

• Thank you for the explanation. So a voxel is a 3D pixel and a whole fMRI volume consists of voxels + time which makes it 4D. But does your answer imply that a slice is also 3D because a slice consists of voxels or is it indeed 4D with considering time? Commented May 16, 2015 at 11:26
• A slice can be considered to be made at once. Multiple slices takes time. in the end making one slice also takes time, but negligibly so. i have adapted my answer
– AliceD
Commented May 16, 2015 at 11:28
• I mean the time in a timeseries, i.e. imaging different volumes in sequences. Nevertheless, a slice is in 3D, right? Commented May 16, 2015 at 11:41
• Yes, slice is 3D
– AliceD
Commented May 16, 2015 at 11:42