DESS (Double Echo Steady State) was originally known as FADE. It collects both the gradient echoes acquired in FISP (S+) and PSIF (S-) sequences and combines them. This increases the signal efficiency sufficiently to allow isotropic knee imaging with reasonable scan times. Phase encoding gradients are balanced to maintain the transverse steady state signals. The frequency encoding gradient is left on for the period of both the echoes, and is incompletely balanced to avoid dark banding artefacts otherwise associated with long TR fully balanced steady state sequences.
The contrast of DESS is quite unique. There is a strong fluid signal but fat is bright and other soft tissues appear similar to the short TR FISP image.
The PSIF echo is very sensitive to motion but this is not a major problem in orthopaedic applications. The parameters suggested are an application, using the current Vision software, of work by Hardy et al to optimise SNR and contrast between cartilage and joint fluid. Hardy suggests better results will be achieved with a lower bandwidth.
DESS Knee Parameters
de3d_9b130.xkc
TR 26.7 mSec TE 9 mSec Flip 60 degrees
128 mm sagittal slab 128 partitions
Matrix 256 x 256 FOV 250 mm 5/8 Phase A-P
1 Acquisition Scan Time 9:08 Resolution 1 x 0.98 x 0.98 mm
Water Excitation 3D DESS 128 mm sagittal slab 128 partitions
Matrix 256 x 256 FOV 250 mm 5/8 Phase A-P
1 Acquisition Scan Time 9:08 Resolution 1 x 0.98 x 0.98 mm
The water excitation variant gives very strong bone/cartilage contrast and can be helpful in identifying regions of chondro-malacia. This protocol is slice selective.
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