29 Constructive Interference in a Steady State (CISS)

When certain echo paths of a balanced-SSFP (b-SSFP) sequence (Case 27) are out of phase during the data acquisition, a destructive interference pattern is seen. The resultant signal voids occur predominately in areas, such as air-tissue interfaces, where tissues display relatively large differences in magnetic susceptibility. These differences degrade the homogeneity of the local magnetic field, leading to a rapid dephasing of encoded echoes. The inner ear is a good example of a complex tissue with areas of very different magnetic susceptibility in close proximity, being composed of air, bone, nerves, and vessels. For high-resolution b-SSFP techniques to have clinical utility in such areas of highly variable magnetic susceptibility, the CISS technique (constructive interference in a steady state) was developed.

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