Constructive Interference in Steady State Imaging in the Central Nervous System

Abstract

Constructive interference in steady state (CISS) imaging is a member of the family of fast gradient echo (GRE) sequences. The CISS sequence is particularly dependent on high gradient amplitude and slew rates. When inside a magnetic resonance imaging (MRI) scanner, the randomly moving protons in the patient's body align along the longitudinal z-axis.The z-axis is a magnetization vector parallel to both the long axis of the patient's body and the bore of the scanner. This magnetization force, represented as a vector along the positive side of the z-axis, is called longitudinal magnetization (LM). When a radiofrequency pulse (RFP) is applied, the magnetization vector is tipped to the transverse plane.This tipped magnetization vector has two components: the LM, which recovers, and the transverse magnetization (TM), which decays during each repetition time (TR) period in the x-axis.CISS is a 'green' sequence. The main recycling idea is not to waste the signal from the decaying TM in every RFP, because it is this signal that provides the T2 properties of a given tissue. Instead of being lost, the TM is reinforced into the transverse plane. The TM is recovered by applying an 180° phase shift during each TR period, after which the TM shifts in the transverse plane. The following RFP simultaneously tips a component of the residual TM back along the z-axis and a portion of the LM into the transverse plane (x-axis). After multiple TR periods have elapsed, this feeding of the LM into the TM, and vice versa, eventually establishes an equilibrium state of both the LM and the TM.It must be stressed that it is necessary to keep the TR shorter than the T2 relaxation time of the tissue; in this way, there is insufficient time for the TM to decay completely before the next RFP excitation. To achieve CISS images, residual TM is always necessary. Once the equilibrium is reached, two types of signal are produced: the first is a post-excitation signal, which consists of free induction decay arising from the most recent RFP, and the second is the echo reformation prior to excitation, when residual echo is refocused at the time of the subsequent RFP. Constructive Interference in Steady State Imaging in the Central Nervous System AbstractConstructive interference in steady state (CISS) is a fully refocused fast-gradient echo sequence that is mainly used in the assessment of the central nervous system. The most important advantages of steady-state imaging are short acquisition times, high signal-to-noise ratio, and better contrast-to-noise ratio. Owing to its cisternographic effect, CISS is useful in the assessment of the cranial nerves, and can also be used when studying cysts, cystic masses, and neurocysticercosis and in hydrocephalus cases. CISS has been shown to be useful in spinal imaging, especially in cases of arteriovenous malformation and when it is helpful to better characterize intra-and extramedullary cystic abnormalities. KeywordsMagnetic resonance imaging, fast-gradient echo sequence, steady-state imaging, constructive inte...