Resolution & Voxel Size
Basic definition & disambiguation
Resolution and voxel size are related, but often confused concepts. The resolution of a CT scan is often incorrectly reported as the voxel size.
Voxel size is defined as the real-world distance that each voxel represents in a CT dataset. It is typically a parameter set (directly or indirectly) by the scanner operator. Since the majority of CT scans are acquired with isotropic voxels, the voxel size is often reported as the length of one side of the voxel (e.g. 50 microns). Smaller voxel size is often correlated with greater resolution, but several other factors affect resolution, as described below.
Image resolution is a metric that defines the smallest resolvable features in an image. An image is said to have higher resolution when it distinguishes smaller features. Resolution is formally defined as the size of the smallest identical sharp-edged features (and gap between them) that can be discriminated. In this context, discrimination is defined as a 20% or greater difference in the intensity of the gap and the features on a line profile through the features.
For example, if a line profile in an image shows two objects 10µm in width, separated by a center-to-center distance of 20µm (resulting in a 10µm gap between them), falls by 20% between the lines, then the resolution of the image is said to be 10µm. Resolution is determined by (1) size and number of detector or detector elements (2) focal spot of the X-ray and (3) distance between the source, object and detector. To characterize the resolution of an imaging system, it is typically necessary to image a resolution phantom.
More Information
The Shannon sampling theorem indicates that the highest resolution of an imaging system is limited by twice the sampling rate, which is to say, that the image resolution can never be better than twice the reconstructed voxel size. But that is only an upper bound to resolution. Resolution in a CT imaging system will be affected by focal spot size (penumbra effect), the geometrical magnification, and the point spread function of the detector. Resolution may be confounded further by sample motion blur during acquisition.