Contrast, parameters

Parameters for elements (basis liinctions in ah miiw methods usually derived from experimental data and empirical parameters in semi-empirical methods nsually obtained from empirical data or ah initu> calcii la lion s) are in depen den t of th e ch em -leal environment, [n contrast, parameters used in molecular mechanics methods often depend on the chem ical en viron-ment. 

Introducing Color into the Image Contrast Parameters 39... 

The contrast parameters entering into this comparison have been determined recently for the system under consideration here (see Fig. 1). The contrast A /jo, of the Br counterion was determined from their respective crystallographic radii. Hence, as already discussed in previous papers [14, 17, 18] the hydration shell of the ion is treated as bulk water. This may induce a small error if A /ion is small. The value calculated in this way for Br is A f ion =26 e/ion. For the contrast of the macroion we used the value Aprod = 25 e/nm3 [14]. 

Proper choice of tR and tE for suitable Ti and T2 weights is an important factor for determining image contrast in medical imaging. Due to less restrictions in measurement time and the use of rf power, a much wider variety of contrast parameters can be accessed in material applications of NMR imaging (cf. Chapter 7). 

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. 

The contrast parameters relevant to material characterization through NMR imaging are the intrinsic NMR parameters of the sample. They are referred to as the contrast parameters per se. They can be divided into chemical and physical parameters, and into molecular, mesoscopic, microscopic, and macroscopic parameters. A list of NMR parameters for contrast in NMR imaging is compiled in Table 7.1.1. 

Chemical contrast parameters are the chemical shielding and the indirect coupling. They relate to the chemical structure and are of molecular nature, although the distribution of the chemical shifts, for instance, also provides information about the physical nature of... 

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