Magnetic Resonance Imaging, Vol. 13, No. 4, pp. 629-632, 1995 Copyright 0 1995 Elsevier Science Ltd Printed in the USA. All rights reserved 0730-725X/95 $9.50 + .oO 0730-725X(94)00128-C l Technical Note ON DOUBLING THE SIGNAL IN LOCALISED STIMULATED ECHO MEASUREMENTS G.S. PAYNE AND M.O. LEACH CRC Clinical Magnetic Resonance Research Group, Royal Marsden Hospital and Institute of Cancer Research, Surrey, England Stimulated echo (STEAM) localisation yields up to 50% of the full magnetisation. It is shown that given the good magnetic field homogeneity now obtained in clinical MRS, and with a slight modification to the gradient pattern, the localised signal may be nearly doubled, although at the expense of introducing a frequency-dependent ampli- tude for peaks in the final spectrum. A two-step phase cycle is also required. The principle is demonstrated on a clinical MR system, and the relative merits of the sequence are discussed. Keywords: ‘H MRS; Stimulated echo (STEAM); Sequence development; Local&d spectroscopy. The stimulated echo (STEAM)’ technique for local- ised MR spectroscopy yields up to 50% of the full sig- nal. We present a modified sequence in which the localised signal is nearly doubled, although at the ex- pense of introducing a frequency-dependent amplitude for peaks in the final spectrum. THE STIMULATED ECHO SEQUENCE 90 - 7 - 90 - TM - 90 - 7 - acquire The stimulated echo’ appears at time 7 following the third 90” pulse. Hence TE = 27. The evolution may be followed using vector notation for the x, y, z com- ponents. For an offset of w rad/s, (chemical shift plus that due to B. inhomogeneity), 90” flip angles, long T1 and T2, and full Tl decay during TM; After the third 90” pulse the magnetisation has equal components along +x. After a further time 7 all mag- netisation components are in the same half of the xy plane (Fig. l), and S a (M,) a (-cos2 ~7) = -$ In STEAM, each radiofrequency (RF) pulse is slice- selective and is applied in the presence of orthogonal B. field gradients. The signal in the resulting stimu- lated echo is therefore localised to the cuboid corre- sponding to the intersection of the three slices. A spoiler gradient is used to ensure total dephasing during TM. A MODIFICATION TO DOUBLE THE SIGNAL The Principle The relative signal in the stimulated echo is 50% pro- vided that total dephasing occurs between the first pair of 90” RF pulses. If little dephasing occurs here, then at the end of TM the magnetisation is along --z only for on-resonant spins (the case for off-resonant spins is considered below). Since the spins are in phase, the 90” pulse generates an FID immediately, rather than an echo (Fig. 2), with amplitude up to twice that of the stimulated echo. Since three 90” pulses are used this se- quence may still be made volume selective as before. RECENED 9/6/94;A~mm~~ 11/14/94. Department, Royal Marsden Hospital, Downs Road, Sutton, Address correspondence to Dr. G. S. Payne, Physics Surrey SM2 5PT, UK. 629