CISS-MRI Sequence

High Resolution Brainstem Imaging

CISS (Constructive Interference Steady State)
CISS is a strongly T2 weighted GRE sequence. In essence it is a pair of True FISP sequences acquired with differing regimes of alternating the phase of the excitation pulses. Individually these True FISP sequences display very strong T2 weighting but are affected by dark phase dispersion bands which are caused by patient induced local field inhomogeneities and made prominent by the relatively long TR used. The different excitation pulse regimes offset these bands in the two sequences. Combining the images results in a picture free of banding. The image combination is performed automatically after data collection, adding some time to the reconstruction process.
The overwhelming power of the 3D CISS sequence is its combination of high signal levels and extremely high spatial resolution. CISS images yield the best detail available of the cisternal portions of cranial nerves. In combination with the isotropic MP-RAGE we believe they completely remove any need for contrast media in identifying acoustic neuromas.
The sequence has inherent flow compensation because of its perfectly balanced gradients. Compared to conventional FISP or GRASS it is quite insensitive to CSF pulsations. True FISP and CISS sequences require a very high level of control over gradient switching and shaping. CISS requires very high local field homogeneity so an excellent base magnet homogeneity is required, and all sequences must be preceded with a patient specific shim adjustment. Metal in the field will degrade the images substantially so patient preparation should include the removal of all head and neck jewellery, and metal from clothing. CISS is available in 2DFT and 3DFT implementations. 

Characteristics of 3D-CISS

• Prone to magnetic susceptibility artefacts
• Very high T2 contrast yields minimal subtle signal variations in soft tissue, but excellent tissue/fluid contrast
• Not affected by the blurring artefact of shorter T2 objects associated with long ETL T2 fast spin echo sequences
• Slow sequence and slow reconstructions

(avoid using odd numbers of partitions)
• Slab profile is poor

(use a slab 20% larger than required coverage)

CISS in application 
We currently use the parameters suggested by Siemens in the VISION VB25A software to provide a very high resolution anisotropic data set for imaging the CP angle and structures of the inner ear. This is not a novel approach but automatic combination of the images makes the technique easy to implement on a routine basis. 
After acquiring the scan, put the slices into MPR software to create axial images properly aligned with the acoustic nerves. At these slice thicknesses perfect positioning is impossible and presentation is greatly enhanced by an anatomically correct display. For optimum display of the VII and VIII cranial nerves from brainstem to middle ear angle, angle upwards from transverse to coronal almost parallel to the roof of the 4th.

CISS Sequence parameters 
ciss3d_2_6b195.ykc 
TR 12.25 mSec TE 5.9 mSec Flip 70⁰ 
1 transverse slab 32 mm thick 46 partitions 
Matrix 230 x 512 (60%) 200 x 150 mm FOV 
Frequency over-sampling Phase L-R 
1 acquisitions Scan time 4 min 20 sec 
Resolution 0.7 x 0.65 x 0.39 
Patient shim before sequence

CISS Protocol Variations: 
Imaging time can be reduced to 3 minutes and SNR improved by 20% by choosing a partition thickness of 1 mm. Display of the cisternal portions of cranial nerves is significantly better than can be achieved with TSE techniques, but the MIP reconstructions of inner ear structures are noticeably poorer. 
By selecting 32 1 mm partitions, 1 NEX FOV 250 x 187 mm (6/8) with a 0.5 mm square pixel, the SNR is virtually equal to the initial sequence but scan time is reduced to 5 minutes. The images look as good as the 8 minute images but cannot be placed in the MPR software as they contain too many data points. They can be used where scan time is an issue. Another good compromise is that suggested by Siemens for orbit imaging, increasing voxel volume slightly to 0.7 x 0.78 x 0.39 mm, to create a single NEX sequence at 6:42 that can still be MIPed. 

Other Alternatives 
CISS is currently unique to late model Siemens MRI systems but a number of other high resolution T2 weighted approaches have been reported. 
Customised sequences were investigated by Dr Schmalbrock et al using single small coils and SIGNA 3x and 5x. They suggested spatial resolution of .5 x .5 x 1 mm was achievable with the standard gradient system, but concluded that substantial SNR improvements were needed, through the use of better coils and shorter TE. They proposed 3DGRASS (25/7 50⁰) to yield isotropic 0.5 mm resolution in 14 minutes with a 3 inch (7.5 cm) surface coil. 
More recent work by Lee et al used a standard GE SIGNA, a phased array of two 3 inch coils and a customised sequence. They have suggested an overlapping interleaved set of 2 mm 2D FSE scans (see box) to yield images that can be used in MIP software, but would also be suitable for creating near axial MPRs. The sequence uses a flip angle of 160⁰ to attempt to reduce blurring of the shorter T2 tissues.

T2 FSE sequence parameters (SIGNA) 
FSE ETL=32 ESP 19 mSec 
3 sequences with 5 slices 2 mm thick interleaved to yield one slice each 1 mm 
TR 4000 mSec TE 100 mSec Flip 160⁰ 6 NEX BW +/-32kHz 
FOV 200x100 mm Matrix 512 x 512 In plane resolution approximately 0.2 mm x 0.4 mm 
Scan time 3 min x 3 
The same group has been developing a 1 mm 3DFSE T2 weighted sequence.