APPLICATION OF STIMULATED ECHO SEQUENCE I N MRI EXPLORATION OF BRAIN AND SPINE

We describe in this paper how the STEAM sequence can be an efficient t ool to obtain images free of flow artifacts in anatomical situation wh ere the spin echo failed. The simplest way to eliminate flow artifacts is to exploit the dephasing induced by motion in magnetic field gradi ents and to reduce to zero the signal from moving tissues. This can be achieve by increasing the time elapsed between the spin excitation an d the signal observed. Because of T2 relaxation, such an increase resu lts in a signal decrease when the spin echo sequence is used. The STEA M sequence has the unique property that the time elapsed between obser vation and excitation can be increased without change in T2 value and so allows a good suppression of signals from the moving spins with sho rt TE. Our results demonstrate that, although the stimulated echo inte nsity is only half that of a spin echo taken at the same read out time , the advantages of STEAM imaging can compensate for this partial loss in signal to noise in some particular clinical situations. The influe nce of mixing time on contrast has been evaluated using thoracic spine imaging and it has been shown that contrast between spine and CSF can be significantly improved (+ 60%) when TM is increased (from 17 ms to 50 ms). In the same time, the contrast between spine and fat tissue d ecreases (40%). This last effect facilitates the adjustment of contras t window. Suppression of motion artifacts has first been evaluated wit h thoracic spine imaging, using a whole body coil. Suppression of arti facts was better than that obtained with a flow compensated spin echo sequence, especially in the case of kyphotic patients when a presatura tion band was inefficient. In a second step suppression of motion arti facts has been evaluated from posterior fossa examination after inject ion of a paramagnetic contrast agent. The images obtained with the sti mulated echo sequence show a dramatic reduction of signal from blood i n the lateral sinus, and therefore an increase of quality by eliminati on of motion artifacts.