RF Power Levels for Shaped Pulses and Spin Locks

13 RF POWER LEVELS FOR SHAPED PULSES AND SPIN LOCKS 

The amplitude of an RF pulse can be expressed in units of telsa (Bi). This corresponds to the magnitude (length) of the B vector in a rotating-frame vector diagram. Pulse amplitude is most commonly expressed in terms of the frequency of rotation of sample magnetization as it precesses around the B vector (for on-resonance pulses) during the pulse. 

The inverse of this frequency of rotation (27t/yB in seconds) is the time it takes for the sample magnetization to rotate one full cycle under the influence of the B field. This is simply the duration of a 360° pulse, and one fourth of this time is the 90° pulse duration, 190. 

For example, a 10 jis hard pulse at a B field strength of 25 kHz will rotate the sample magnetization by = 10 x 10-6 s x 25 x 103 cycle/s = 0.25 cycle = 90°. So this pulse is a 90° pulse. A 10-ms soft pulse at a B field strength of 25 Hz will rotate the sample magnetization by = 10 x 10-3 s x 25 cycle/s = 0.25 cycle = 90°. So this is also a 90° pulse (Fig. 8.48). The area of the rectangular pulses is the same  

The soft pulse is 1000 times lower in amplitude compared to the hard pulse, but it is 1000 times longer than the hard pulse. Although both pulses deliver a 90° rotation when on-resonance, they have very different behavior off-resonance. The excitation profile of both pulses is a sine function  

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