Combining Shaped Pulses and Pulsed Field Gradients Excitation Sculpting

6 COMBINING SHAPED PULSES AND PULSED FIELD GRADIENTS EXCITATION SCULPTING  

It is rather tedious to move the spectral window every time we want to select a peak with a shaped pulse, but it is necessary as the center of the Gaussian excitation profile is at 

SHAPED PULSES, PULSED FIELD GRADIENTS, AND SPIN LOCKS 

The only problem is that we need to find this magic selective pulse that delivers a 180° rotation everywhere but the center of the spectral window. There are shaped pulses that do this, but it turns out that the simplest solution is a series of six hard pulses separated by equal delays. If we divide the 90° rotation into 13 small rotations of equal angle, the sequence is 

Now let s return to the 3-9-19 sequence. For the water peak, it s simple. As there is no evolution during the r delays, it is just a sequence of six pulses whose rotation is exactly balanced between ccw rotations (3-9-19) and cw rotations (19 — 9 — 3). The net rotation is zero and the water magnetization ends up on the +z axis, where it started. Water is not affected by the pulse train. The same is true if the offset is v0 — vr = 1/r 

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