## Rf Power Levels For Shaped Pulses And Spin Locks

The amplitude of an RF pulse can be expressed in units of telsa (B1). This corresponds to the magnitude (length) of the B1 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 B1 vector (for on-resonance pulses) during the pulse.

The inverse of this frequency of rotation (2nlyB 1 in seconds) is the time it takes for the sample magnetization to rotate one full cycle under the influence of the B i field. This is simply the duration of a 360° pulse, and one fourth of this time is the 90o pulse duration, 190.

For example, a 10 |xs hard pulse at a B i field strength of 25 kHz will rotate the sample magnetization by © = 10 x 10-6 s x 25 x 103 cyclels = 0.25 cycle = 90°. So this pulse is a 90° pulse. A 10-ms soft pulse at a B1 field strength of 25 Hz will rotate the sample magnetization by © = 10 x 10-3 s x 25 cyclels = 0.25 cycle = 90°. So this is also a 90° pulse (Fig. 8.48). The "area" of the rectangular pulses is the same:

Pulse amplitude = B^tesla) a yB1/2n(Hz) = 1/(4 x t90)

"Area" = width x height = 10 jis x 25 kHz = 10 ms x 25 Hz = 0.25

90° pulses

Excitation profiles

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