Glossary Terms  Home

S

Sampling Rate

The frequency in time with which a measurement is made.

Scalar

A quantity that has magnitude but not direction.

Scanner Drift

Slow changes in voxel intensity over time.

Shimming Coils

Electromagnetic coils that compensate for inhomogeneities in the static magnetic field.

Significance testing

The process of evaluating whether the null hypothesis is true.  Also known as hypothesis testing.

Slice

A single slab of an imaging volume. A slice has thickness defined by the strength of the gradient and the bandwidth of the electromagnetic pulse used to select it.

Slice Selection

The combined use of a spatial magnetic field gradient and an electromagnetic field pulse to excite spins within a slice.

Smoothness

The degree to which the time courses of nearby voxels are temporally correlated.

Spatial Frequency

The frequency with which some pattern occurs over space.

Spatial Gradients (G)

A magnetic field whose strength varies systematically over space. Note that since a given spatial location only experiences one magnetic field, which represents the sum of all fields present, spatial gradients in MRI act to change the effective strength of the main magnetic field over space.

Spatial Resolution

the ability to distinguish changes in an image (or map) across different spatial locations.

Specific Absorption Rate (SAR)

A quantity that describes how much electromagnetic energy is absorbed by the body over time.

Static Magnetic Field

the strong magnetic field at the center of the mri scanner whose strength does not change over time. The strength of the static magnetic field is expressed in Tesla (T).

Statistical Map (or Statistical Parameter Map)

In fMRI, the labeling of all voxels within the image according to the outcome of a statistical test

Subtraction

In experimental design, the direct comparison of two conditions that are assumed to differ only in one property, the independent variable.

Superconducting Electromagnets

A set of wires made of metal alloys that have no resistance to electricity at very low temperatures. By cooling the electromagnet to near absolute zero, a strong magnetic field can be generated with minimal electrical power requirements.

Surface Coil

A radiofrequency coil that is placed on the surface of the head, very near to the location of interest. Surface coils have excellent sensitivity to signal from nearby regions but poor sensitivity to distant regions.