Stimulated Echo (STE)
What is a stimulated echo?
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Whereas a spin echo (SE) arises from the action of two RF pulses, a stimulated echo (STE) occurs from the action of three or more RF pulses. In the diagram (right) three RF pulses have miraculously produced five echoes! But from where did all these echoes arise?
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A train of three radiofrequency pulses (1,2,3) creates three primary spin echoes (A,B,C), one secondary spin echo (D), and one stimulated echo (E).
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By inspecting the timing of each echo, we see that echoes A, B, and C are primary "conventional" spin (Hahn) echoes produced by each possible pair of RF-pulses. Specifically, echo A results from pulses 1&2, echo B results from pulses 2&3, and echo C results from pulses 1&3.
Echo D is called a secondary spin echo and results from the spins in echo A being acted upon by RF-pulse #3 by the following mechanism: i) At the center peak of echo A spins in the transverse plane have once again been brought into phase. ii) Between the center of echo A and RF-pulse #3, these spins again dephase by T2* effects. iii) RF-pulse #3 flips over some of these spins allowing them to rephase and eventually refocus into Echo D. iv) Note that RF-pulse #3 is located exactly midway between Echoes A and D.
Echo E is the stimulated echo (STE). A spin-phase diagram similar to that in the last Q&A for the Hahn echo is used. We once again consider four spins (a,b,c,d) that have dephased after the initial 90°-pulse. Spins a and c are in local fields lower than Bo, are precessing slightly slower than the Larmor frequency, and are losing phase. Spins b and d are in local fields higher than Bo, precessing faster, and gaining phase. Moreover, we have picked the spin pairs (a-b) and (c-d) to have local field offsets of the same magnitude but opposite polarities, so they gain or lose phase relative to their partner at the same relative rates.
Echo D is called a secondary spin echo and results from the spins in echo A being acted upon by RF-pulse #3 by the following mechanism: i) At the center peak of echo A spins in the transverse plane have once again been brought into phase. ii) Between the center of echo A and RF-pulse #3, these spins again dephase by T2* effects. iii) RF-pulse #3 flips over some of these spins allowing them to rephase and eventually refocus into Echo D. iv) Note that RF-pulse #3 is located exactly midway between Echoes A and D.
Echo E is the stimulated echo (STE). A spin-phase diagram similar to that in the last Q&A for the Hahn echo is used. We once again consider four spins (a,b,c,d) that have dephased after the initial 90°-pulse. Spins a and c are in local fields lower than Bo, are precessing slightly slower than the Larmor frequency, and are losing phase. Spins b and d are in local fields higher than Bo, precessing faster, and gaining phase. Moreover, we have picked the spin pairs (a-b) and (c-d) to have local field offsets of the same magnitude but opposite polarities, so they gain or lose phase relative to their partner at the same relative rates.
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The second 90°-pulse tips the dephased spins into the vertical (xz-) plane. For understanding the STE we only need consider the z-components of the the spins at this step. The z-components do not precess but remain aligned ("locked") around the Bo direction. However, T1 relaxation occurs with a resulting net growth in components along the +z-axis. This feature allows STE's to have T1-weighting.
The third 90°-pulse flips the z-components back into the transverse (xy-) plane where precession occurs. Now each spin in a pair can catch up with its partner, rephasing along the −y-axis and creating the STE. Just like the Hahn echo not all the components come into phase at exactly the same time. So the STE is of lower amplitude and spread out more widely in time. |
Formation of a stimulated echo by three 90°-pulses. After the second pulse we consider the z-components of the spins that do not precess but grow in the +z-direction by T1 relaxation. These "stored" longitudinal components are flipped back into the transverse plane by the 3rd RF-pulse, then rephase and form an STE.
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References
Burstein D. Stimulated echoes: description, applications, practical hints. Concepts Mag Reson 1996;8:269-278.
Frahm J, Merboldt KD, Hänicke W, Haase A. Stimulated echo imaging. J Magn Reson 1985;64:81-93.
Frahm J, Bruhn H, Gyngell ML, et al. Localized high resolution proton NMR spectroscopy using stimulated echoes: initial applications to human brain in-vivo. Magn Reson Med 1988; 9:79-93.
Sattin W, Marcel TH, Scott KN. Exploiting the stimulated echo in nuclear magnetic resonance imaging. I. Method. J Magn Reson 1985;64:177-182.
Burstein D. Stimulated echoes: description, applications, practical hints. Concepts Mag Reson 1996;8:269-278.
Frahm J, Merboldt KD, Hänicke W, Haase A. Stimulated echo imaging. J Magn Reson 1985;64:81-93.
Frahm J, Bruhn H, Gyngell ML, et al. Localized high resolution proton NMR spectroscopy using stimulated echoes: initial applications to human brain in-vivo. Magn Reson Med 1988; 9:79-93.
Sattin W, Marcel TH, Scott KN. Exploiting the stimulated echo in nuclear magnetic resonance imaging. I. Method. J Magn Reson 1985;64:177-182.
Related Questions
Are stimulated echoes used for imaging? I've never heard of that.
Are stimulated echoes used for imaging? I've never heard of that.