In fMRI, a voxel refers to?

• A. A single measurement unit of electrical activity
• B. A small 3D volume element of the brain image data
• C. A color scale
• D. A scanner coil

Answer: (B)

In fMRI, a voxel is the smallest 3D element of the brain image data—a tiny cube that represents a volume of brain tissue. The brain image is built from many of these voxels arranged in a 3D grid, and for each voxel you obtain a time series of signal intensity as the scanner tracks changes in blood oxygenation levels related to neural activity. The size of a voxel determines spatial resolution (common sizes are around 2 mm on each side), and the voxel itself is the data unit, not a measurement of electrical activity, a color scale, or a scanner coil. Activation maps identify which voxels show significant changes, translating those data units into colors to visualize where activity occurs.