Inhomogeneous Bi fields

An excellent 180° pulse results if p, is set to 5. Under such a pulse, magnetization components at frequencies centred at zero in a well-defined frequency band are inverted adiabatically, while components at any other frequency are returned to their initial condition at the end of the pulse. Therefore, such a pulse can be used with inhomogeneous Bi fields. Adiabatic pulses have been designed for excitation of narrow [Rosl] as well as wide [Hwal, Kupl, Shel] frequency bands at low rf power. 

If we apply the TOCSY spin lock at this point on the x axis, we will destroy the first and fourth terms (Bi field inhomogeneity) and lock the second and third terms. Because the TOCSY mixing sequence transfers coherence from in-phase to in-phase, only the third... 

The inhomogeneity in Bi, especially when using surface coils, can lead to a spatially dependent population of desired and undesired coherence pathways. Field gradient pulses in combination with shaped RF pulses lead to spatially selective excitation or refocusing and, in such cases, localization can be viewed as a type of coherence pathway selection. The inherent Bi gradients resulting from the inhomogeneous RF fields of surface coils have also been used for water suppression in in vivo experiments. ... 

Figure 8.46. The ROESY spin-lock. During this, magnetisation parallel to the B] field remains spin-locked whereas orthogonal components are driven about this (here in the xz plane) and eventually dephase through Bi field inhomogeneity.

PRESS, based on a double spin-echo, requires exact 90° and 180° pulses ideally. However PRESS too can be applied in the inhomogeneous surface coil Bi field, provided the out-of-voxel signal from nonideal pulses is removed using phase-cycling schemes and/or large spoiler gradient pulses. The same applies to the stimulated echo sequence STEAM. More complicated versions of PRESS may be implemented... 

Numerical optimisation approaches have been traditionally used in design of selective excitation and inversion pulses. This usually involved a variation of a small number of parameters that describe a subset of pulse shapes. Kobzar et al proposed to apply the optimal control theory to remove the restriction of the predetermined pulse shape and to obtain a general solution that meets the criteria of maximum rf-amplitude, maximum pulse duration, temporal digitisation of the pulse and compensations for Bi-field inhomogeneity. The application of the method allowed to improve most of the published selective pulses. The method was extended further to optimise coherence transfer steps in coupled spin systems.The reported optimised propagators maximised the... 

S. Ehni, B. Luy, A systematic approach for optimizing the robustness of pulse sequence elements with respect to couplings, offsets, and Bi-field inhomogeneities (COB), Magn. Reson. Chem. 50 (2012) S63—S72. 

The SIN defined by Equation 7.6 for a given NMR resonance is proportional to the square of the nuclear precession frequency (mo, rad/s), the magnitude of the transverse magnetic field (Bi) induced in the RE coil per unit current (/), the number of spins per unit volume (Ns), the sample volume (Vs), and a scaling constant that accounts for magnetic field inhomogeneities. The SIN is inversely proportional to the noise generated in the RE receiver and by the sample (Vnoise) as defined by the Nyquist theorem,... 

What does this mean for the effect of the spin lock on sample magnetization If the sample magnetization starts on the / axis, for example, the tilted spin-lock axis will destroy the component that is perpendicular to the spin-lock axis and retain the component that is on the spin-lock axis. This preserved component is locked because it is on the axis of the effective field and has no reason to precess around the z axis. So even if the spin is off-resonance, its magnetization does not precess around the z axis during the spin-lock period. Instead, the component that is not on the tilted spin-lock axis precesses around the spin-lock axis until it is destroyed by Bi inhomogeneity, and the component that is on the spin-lock axis is retained. 

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... 

---