The Basics of NMR

Chapter 6




The 90-FID Sequence

In the 90-FID pulse sequence, net magnetization is rotated down into the X'Y' plane with a 90o pulse. The net magnetization vector begins to precess about the +Z axis. The magnitude of the vector also decays with time.


When this sequence is repeated, for example when signal-to-noise improvement is needed, the amplitude of the signal after being Fourier transformed (S) will depend on T1 and the time between repetitions, called the repetition time (TR), of the sequence. In the signal equation below, k is a proportionality constant and is the density of spins in the sample.

S = k ( 1 - e-TR/T1 )

The Spin-Echo Sequence

Another commonly used pulse sequence is the spin-echo pulse sequence. Here a 90o pulse is first applied to the spin system. The 90o degree pulse rotates the magnetization down into the X'Y' plane. The transverse magnetization begins to dephase. At some point in time after the 90o pulse, a 180o pulse is applied. This pulse rotates the magnetization by 180o about the X' axis. The 180o pulse causes the magnetization to at least partially rephase and to produce a signal called an echo.



S = k ( 1 - e-TR/T1 ) e-TE/T2

The Inversion Recovery Sequence

An inversion recovery pulse sequence can also be used to record an NMR spectrum. In this sequence, a 180o pulse is first applied. This rotates the net magnetization down to the -Z axis. The magnetization undergoes spin-lattice relaxation and returns toward its equilibrium position along the +Z axis. Before it reaches equilibrium, a 90o pulse is applied which rotates the longitudinal magnetization into the XY plane. In this example, the 90o pulse is applied shortly after the 180o pulse. Once magnetization is present in the XY plane it rotates about the Z axis and dephases giving a FID.



S = k ( 1 - 2e-TI/T1 )


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