Neuroradiology
Constructive Interference in Steady-state Imaging in the Central Nervous System
Fabrício Guimarães Gonçalves, MD, MSc1 and Lázaro Luis Faria do Amaral, MD2
1. Neuroradiology Fellow, Montreal General Hospital, McGill University Health Center; 2. Neuroradiologist, and Chief, Neuroradiology Department, Hospital da Real Beneficiência Portuguesa de São Paulo
Abstract
Constructive interference in steady-state (CISS) imaging 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.
Keywords
Magnetic resonance imaging, fast-gradient echo sequence, steady-state imaging, constructive interference in steady state, longitudinal magnetization, transverse magnetization, central nervous system, cranial nerves, neurocysticercosis, spine
Disclosure: The authors have no conflicts of interest to declare. Acknowledgments: The authors would like to make a special acknowledgment to Michael McInnis for reviewing the material and for useful suggestions. Received: July 8, 2010 Accepted: October 11, 2010 Citation: US Radiology, 2011;3:23–7 Correspondence: Fabrício Guimarães Gonçalves, MD, MSc, Diagnostic Radiology, Montreal General Hospital, 1650 Cedar Avenue, Room D5 137, Montreal, Quebec, Canada, H3G 1A4. E:
goncalves.neuroradio@
gmail.com
Constructive Interference in Steady-state Imaging
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.1 To run this type of sequence, high-gradient-strength magnets are required to reach the peak of the gradient as rapidly as possible. CISS has different names according to different manufacturers: it is called fast imaging employing steady-state acquisition (FIESTA) by General Electric, true fast imaging with steady-state precession (FISP) by Siemens, balanced fast field echo (FFE) by Philips, and true steady-state free precession (SSFP) by Toshiba. CISS is mainly used in the assessment of the central nervous system, but it is also used when imaging the abdomen, the musculoskeletal system, and the breast. The last three areas are not within the scope of this article.
How Constructive Interference in Steady-state Images are Acquired
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)
© TOUCH BRIEFINGS 2011
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.
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