could any one help me explain how the rf B1 field in an NMR spectrometer works? Especially the mechanism it uses to flip the net magnetisation by an angle of 90 or 180 degrees?
Would be very gratefull for a link or and explanation to this.
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A magnetic moment precesses about the net field. It's precession rate is proportional to the strength of the net field. Now imagine that the rf field is circularly polarized (usually is in modern NMR or MRI) and has frequency omega which happens to be the frequency associated with the main field (the
usually large polarizing field). Then in a frame rotating at omega the rf field appears static and the main field appears to be zero. In this frame the magnetic moment then precesses about the field
due to the rf field. The stronger the rf field strength the greater the precession rate. So knowing that rate one can determine how long to apply rf field to achieve a given tip angle.
Note that this argument is only approximate because it assumes a well defined frequency omega. In a real experiment the frequency of the applied rf-field is not delta function-like in its distribution due to its finite lifetime - natural broadening.
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