Bq inhomogeneity

As stated before, the initial perturbation should be maximal with respect to the equilibrium state. Since we are dealing with transverse magnetization here, this maximal perturbation is obviously a 90° pulse. However, it can be immediately noticed that signals collected after a simple read-pulse, decay exponentially according to a time constant which differs from the genuine T2 by a contribution due to the static induction Bq inhomogeneity ... 

Fig. 3. The basic Hahn sequence for the measurement of the transverse relaxation time T2. Any precession motion characterized by the frequency v in the rotating frame is refocused. This precession may arise either from chemical shift or from Bq inhomogeneity (symbolized by the shaded area, which has been strongly reduced for visualization purposes owing to the fast decay of the fid, it should in fact extend to the whole circle).

Suppose that a pulse Fourier transform proton NMR experiment is carried out on a sample containing acetone and ethanol. If the instrument is correctly operated and the Bq field perfectly uniform, then the result will he a spectrum in which each of the lines has a Lorentzian shape, with a width given hy the natural limit 1/(7tT2). Unfortunately such a result is an unattainable ideal the most that any experimenter can hope for is to shim the field sufficiently well that the sample experiences only a narrow distribution of Bq fields. The effect of the Bq inhomogeneity is to superimpose an instrumental lineshape on the natural lineshapes of the different resonances the true spectrum is convoluted by the instrumental lineshape. 

Note that increased bandwidth (leading to a higher noise floor) may be needed as Bq inhomogeneity becomes worse. As discussed below, Bq inhomogeneities may also lead to some signal loss. 

Shorter results in less signal. The static field of MR scanners must be very uniform to prevent signal loss and image artifacts. Typically, Bq inhomogeneity of MR scanners is about 10 parts per million (ppm) over perhaps a 40-cm-diameter spherical volume (dsv) for imaging. In spectroscopy, measurements of small fi uency shifts must be accurately made and Bo inhomogeneity is typically limited to perhaps 0.1 ppm over a 10-cm dsv. ... 

Small Bq inhomogeneities related to the magnet quality Spins dephase... 

The deuterium lock prevents changes in the static field (Bq) and radiofrequency (B,) by maintaining a constant ratio between the two. It therefore ensures long-term stability of the magnetic field. If the lock is not applied, a drastic deterioration in the shape of the NMR lines is expected, due to magnetic and radioffequency inhomogeneities, (a) With deuterium lock... 

An alternative approach for the elimination of zero-quantum coherence is based on its evolution during periods of spin-locking in inhomogeneous Bq or Bj fields (Titman et al., 1990 Davis et al., 1993). This approach is particularly attractive, because it does not rely on time-consuming phasecycling schemes and variation of z-filter delays. Consider an inhomogeneous rf field with amplitude I f (r) that depends on the position r in the... 

At this point, with Bi off, the individual nuclear magnetic moments that comprise M begin to dephase (relax transversely) because inhomogeneities in the magnetic field Bq... 

In inhomogeneous Bq and B fields the flip angle may be relatively well defined if the Bo gradient is strong. Then the B field is quite homogeneous over the sensitive... 

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