Analysing Pulse Sequence Fragments

Because the Bloch simulator is based on a classical approach rather than a quantum mechanical approach to the NMR phenomena its use in analysing pulse sequence fragments is somewhat restricted. Nevertheless, as will be illustrated in the next three examples, in spite of these restrictions the Bloch simulator is a very powerful aid in visualizing what is happening in a pulse sequence fragment and consequently is an extremely valuable teaching and research tool. 

Example 1 Most discussions involving the Bloch model introduce the concept of the rotating frame. The concept of a rotating coordinate system is a familiar one because in real life positions and motion are referred to the earth, a coordinate system that is rotating. Similarly rather than refer the motion of the magnetization vectors to the fixed laboratory coordinate system, it is simpler to refer their motion to a rotating frame of reference which rotates at the NMR transmitter frequency of the nucleus under study. 

Close the dialog box with OK. The resulting graph should be similar to the one shown on the left hand side of this Check it. Use the scroll bars at the sides of the window to adjust the view and to get the three dimensional Impression of the sphere. 

By increasing pll from low power levels to higher power levels the soft pulse gradually approaches an ideal hard pulse which excites nuclei uniformly over a wide range of frequency offsets, i.e. sweep width. 

Example 3 Composite pulses are designed to overcome the imperfections of ideal hard pulses . The Bloch simulator is a suitable tool to illustrate the superiority of a composite 180° pulse inversion compared to a standard 180° pulse. The application of composite pulses in decoupling techniques is discussed in sections 5.3.2 and 5.5.3. In Check it 4.3.4.3 the composite 180° pulse 90x/180y/90x (the indices x and y denote the pulse phase) is demonstrated. These simulations reveal how effective a composite 180° inversion pulse is for different rf offsets. 

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