Main field magnets

The real power of MRI is the ability to exploit the inherent NMR properties of different tissues and tissue pathologies. The main MR parameters are known as longitudinal-relaxation time-constant (Tl), describing the time for the magnetization to recover to its equilibrium along the main-field axis after RF perturbation transverse-relaxation time-constant (T2), describing the... 

Fig. 20. Schematic illustration of a main-field switching interval. The single pulser interval Swt (switching time), nested between a previous interval Pand a next interval N, is actually composed of three distinct phases (1,2,3). During phase (1) the magnetic field B is actively driven along a linear ramp from its previous value Bp to a close vicinity of its next value During the subsequent phase (2) it settles to its final...

Here k is the proportionality constant between field and current (B = kI), Vp and Htv are the maximum positive and maximum negative power supply voltage, L and R are the inductance and resistance of the magnet, B is the instantaneous main field value, Su = (kIL)Vp and Sd = (kIL)Vn are the up-going and down-going slewing rates when B = 0, and a = R/kL. 

Wolfender et ah, and this coupled technique is treated more comprehensively in Chapter 1. The LC-NMR technique is by nature rather insensitive however, high-field magnets and recent improvements in solvent suppression, pulse field gradients, and probe technology have made it possible to achieve useful results on various flavonoid struc-tures. The detection limit with a 60 p.1 cell in a 500 MHz instrument for a compound with a molecular weight of around 400 amu may typically be around 20 p.g, and the information provided is hitherto mainly based on NMR spectra or correlation experiments. 

The nucleus of each atom in a molecule has a particular environment in which the external applied field is perceived with slight variations. The valence electrons create a small magnetic field, opposite to the main field, thus creating magnetic shielding. These local variations induce slight resonant frequency variations as compared to what would be seen in a vacuum (Fig. 9.1). 

These equations can be also expressed as functions of the strength H of the main magnetic field by using a relation of = yH, were y is the nuclear gyromagnetic ratio. Such expressions may be more useful for the usual broad-line NMR spectrometry in which the main field is slowly swept under a constant rotating subfield. 

As pointed out in Chapter I, the NMR absorption spectrum for polymers in the solid state is generally very broad. In such cases the NMR is usually observed as the so-called broad-line spectrum. In this, the resonance is recorded by slowly sweeping the main static magnetic field H modulated with a small amplitude and frequency under a constant high-frequency subfield rotating perpendicularly to the main field. Figure 5 shows schematically the principle of the measurement for the broad-line NMR... 

Near the center of the coils the total magnetic field of quadrupolar gradient coils and of the main field B, which is perpendicular to the coil axis, can be approximated by... 

In the center of a Maxwell pair the radial component of the magnetic field is half the longitudinal component. With the main field B pointing parallel to the coil axis, the total magnetic field is... 

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