Magnetization filters parameter weights

A magnetization filter consists of modulated rf excitation, for example, from a sequence of nonselective rf pulses with given flip angles and given pulse separations. These parameters of the pulse sequence are adjustable, and they determine the characteristics of the filter, that is, they can be used to tune the filter transfer function. Therefore, they are referred to as extrinsic contrast parameters. They must be discriminated from the intrinsic contrast parameters, which are the NMR parameters specific of the sample under investigation and are related to the material properties [Manl]. Therefore, the parameter vectorp in the weight factor of eqn (7.1.2) is separated into two parts, a vector Pe of extrinsic parameters and a vectorPj(r) of intrinsic parameters. 

Filters are given by pulse sequences which, in most cases, generate longitudinal magnetization which differs from the thermodynamic equilibrium state in some specific way [99, 100]. Many different filters can be conceived and combined to produce parameter weights of spin density images. Basically, any pulse sequence can be considered a magnetization filter, but particularly useful ones are... 

The first approach is illustrated in Figure 29 for the simple case of spin-lock-filtered projections of a composite piece of rubber cut from the tread of a car tire. Such a filter imposes a Tip weight on the spin density (cf Fig. 29a). From the space distributed magnetization decays, the parameter map of can be obtained... 

Fig. 29. Filter-weighted and spin-lattice relaxation in the rotating frame Tip of an elastomer composite of SBR and natural rubber cut from the tread of a car tire, (a) A stack of one-dimensional images with variable duration tf of the spin-lock radio-frequency magnetic field, (b) A Tip parameter map obtained from the decay shown in (a). Reproduced from Ref 143, with permission from Wiley.

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