Magnetization in field gradients

The effectiveness of slice selection by the dipolar-decoupled DANTE sequence is illustrated in Fig. 5.3.16 with ID images of a ferrocene cylinder without (a) and with 

Most imaging techniques can be understood within the vector model of the Bloch equations (cf. Section 2.2.1). For this reason, the magnetization response calculated from the Bloch equations is investigated for arbitrary rf input and arbitrarily time-dependent magnetic-field gradients. In particular, the response which depends linearly on the rf excitation is of interest not only for imaging the spin density Mo(r), but also for 

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Comparisons of estimated diffusivity values on zeolites from sorption uptake measurements and those obtained from direct measurements by nuclear magnetic resonance field gradient techniques have indicated large discrepancies between the two for many systems [10]. In addition, the former method has often resulted in an adsorbate diffusivity directly proportional to the adsorbent crystal size [11]. This led some researchers to believe that the resistance to mass transfer may be confined in a skin at the surface of the adsorbent crystal or pellet (surface barrier) [10,11]. The isothermal surface barrier model, however, failed to describe experimental uptake data quantitatively [10,12]. 

A simple way to view a pulsed field gradient experiment is to add up the twist acquired by the sample magnetization in each gradient pulse and make sure they add up to zero for the desired pathway. If the twist is not zero at the beginning of acquisition of the FID, there will be no observable signal. For example, in the INEPT experiment (Fig. 8.26)... 

The electric field gradient is again a tensor interaction that, in its principal axis system (PAS), is described by the tluee components F Kand V, where indicates that the axes are not necessarily coincident with the laboratory axes defined by the magnetic field. Although the tensor is completely defined by these components it is conventional to recast these into the electric field gradient eq = the largest component,... 

Motion, and in particular diffiision, causes a further limit to resolution [14,15]. First, there is a physical limitation caused by spins diflfiising into adjacent voxels durmg the acquisition of a transient. For water containing samples at room temperature the optimal resolution on these grounds is about 5 pm. However, as will be seen in subsequent sections, difhision of nuclei in a magnetic field gradient causes an additional... 

Figure Bl.14.7. Chemical shift imaging sequence [23], Bothx- andj -dimensions are phase encoded. Since line-broadening due to acquiring the echo in the presence of a magnetic field gradient is avoided, chemical shift infonnation is retained in tire echo.

The displacement of a spin can be encoded in a manner very similar to that used for the phase encoding of spatial infonnation [28, 29 and 30]. Consider a spin j with position /-(t) moving in a magnetic field gradient G. The accumulated phase, cpj, of the spin at time t is given by... 

Flow which fluctuates with time, such as pulsating flow in arteries, is more difficult to experimentally quantify than steady-state motion because phase encoding of spatial coordinate(s) and/or velocity requires the acquisition of a series of transients. Then a different velocity is detected in each transient. Hence the phase-twist caused by the motion in the presence of magnetic field gradients varies from transient to transient. However if the motion is periodic, e.g., v(r,t)=VQsin (n t +( )q] with a spatially varying amplitude Vq=Vq(/-), a pulsation frequency co =co (r) and an arbitrary phase ( )q, the phase modulation of the acquired data set is described as follows ... 

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