Spatial resolution (MRI)

Last revised by Andrew Murphy on 2 Apr 2020

In MRI, spatial resolution is defined by the size of the imaging voxels. Since voxels are three-dimensional rectangular solids, the resolution is frequently different in the three different directions.  The size of the voxel and therefore the resolution depends on matrix size, the field-of-view, and the slice thickness.

The matrix size is the number of frequency encoding steps, in one direction; and the number of phase encoding steps, in the other direction of the image plane. Assuming everything else is constant, increasing the number of frequency encodings or the number of phase steps results in improved resolution. The frequency encoding depends on how rapidly the FID signal is sampled by the scanner. Increasing the sampling rate results in no time penalty. Increasing the number of phase steps increases the time of the acquisition proportionately. This is why images that have fewer phase encodings than frequency encodings, e.g., 128x256 or 192x256 will be used.

The field of view is the size of the area that the matrix of phase and frequency encoding cover. Dividing the field of view by the matrix size gives you the in-plane voxel size; hence, increasing the field of view in either direction increases the size of the voxels and decreases the resolution. Decreasing the field of view improves the resolution.

The slice thickness determines the depth of the voxel. This is almost always the largest dimension of the voxel in 2D imaging. Therefore, the resolution perpendicular to the image plane is the poorest. This is related to the maximum strength of the z-gradient coils as well as time restraints limiting the number of slices available. 3-D imaging utilizing phase encoding in the z-direction is capable of smaller slice thickness than 2-D imaging but carries a time penalty proportional to the number of slices.

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