Knee 3D FISP-MRI Seqience

Knee 3D FISP 
3D transverse steady state sequences (FISP, GRASS, etc) have been used in the knee for many years. Our current sequence has anisotropic data because of a relatively thick partition. This restricts the quality of coronal and axial views but it still provides excellent display of the cruciate ligaments and menisci in near sagittal planes. The sequence is slice selective. Slice direction phase over-sampling is used to control aliasing. Its T1/T2 weighting provides good display of cartilage, muscle and ligaments. Meniscal tears and degeneration appear bright and the thin slices make extension to the articular surfaces easy to identify. The fat signal from marrow is not excessive. Bright fluid signal aids the identification of effusions and cystic collections.

fi3d_10b130.wkc 
TR 22 mSec TE 10 mSec Flip 400 Scan time 6:02 
Sagittal slab 96 mm 64 partitions 30% slab over-sampling 
FOV 160 x 160 mm 256 matrix 69% Frequency over-sampling 
Phase H-F 25% Phase over-sampling 
1 Acquisition scan time 6:02 Resolution 1.5 x 0.9 x 0.63 mm

Isotropic FISP ? 
Converting the 3DFISP sequence above to 1 mm isotropic voxels would require a slab thickness of 112 mm with 112 partitions, no slice direction phase over-sampling, and a 100% matrix. The scan time would become 10:33 and the relative SNR drops to 61%. The options to recover the lost SNR include reducing bandwidth to 50 Hz/pixel, using a more efficient coil, or changing to a more efficient sequence type.