Constructive Interference in Steady State (CISS) is a modification of the TrueFISP sequence. The CISS sequence employs a robust T2-weighted 3D gradient echo technique, resulting in high-resolution isotropic images. It involves the execution of two consecutive runs of 3D balanced steady-state free precession with varying excitation levels, which are subsequently combined.

The image contrast in CISS is determined by the T2/T1 ratio of the tissue. Tissues with both long T2 and short T1 relaxation times exhibit high signal intensity on CISS images. Thanks to the elevated T2/T1 ratio, both water and fat display strong signals in this sequence. Notably, the CISS sequence excels at providing excellent contrast between cerebrospinal fluid (CSF) and other brain structures. Due to these qualities, the CISS sequence proves invaluable for the assessment of structures enveloped by CSF, such as cranial nerves.

CISS MRI image appearance

The easiest way to identify CISS images is by observing spaces filled with fat and fluids within the body (such as cerebrospinal fluid in the brain ventricles and spinal canal). Fluids typically appear as very bright areas, while fat appears as bright gray.

Tissues and their CISS appearance

Muscles – dark gray
Cranial nerves- dark gray
White matter – dark gray
Bone marrow – dark
Moving blood- bright
Gray matter – dark gray
Fluids – very bright
Bone – dark
Fat – bright gray
Air – dark


Axial CISS sequence used in IAMs imaging