Gradient-echo techniques

Gradient echo techniques and phase velocity mapping are useful for demonstration and characterization of mural thrombus and for qualitative and quantitative assessment of aortic regurgitation associated with aneurysm of the ascending aorta. 

Noguchi Y, Murakami T, Kim T, et al (2002) Detection of hypervascular hepatocellular carcinoma by dynamic magnetic resonance imaging with double-echo chemical shift in-phase and opposed-phase gradient echo technique comparison with dynamic helical computed tomography imaging with double arterial phase. J Comput Assist Tomogr 26 981-987... 

There are a number of NMR methods available for evaluation of self-diffusion coefficients, all of which use the same basic measurement principle [60]. Namely, they are all based on the application of the spin-echo technique under conditions of either a static or a pulsed magnetic field gradient. Essentially, a spin-echo pulse sequence is applied to a nucleus in the ion of interest while at the same time a constant or pulsed field gradient is applied to the nucleus. The spin echo of this nucleus is then measured and its attenuation due to the diffusion of the nucleus in the field gradient is used to determine its self-diffusion coefficient. The self-diffusion coefficient data for a variety of ionic liquids are given in Table 3.6-6. 

Speck O, Chang L, DeSilva NM, Ernst T. Perfusion MRI of the human brain with dynamic susceptibility contrast gradient-echo versus spin-echo techniques. J Magn Reson Imaging 2000 12 381-387. 

Nowadays, self-diffusion coefficients are almost exclusively measured by NMR methods, through the use of methods such as the 90-8-180-8-echo technique (Stejs-kal and Tanner sequence) [10-12]. The pulse-echo sequence, illustrated in Figure 4.4-2, can be divided into two periods of time r. After a 90° radio-frequency (RF) pulse the macroscopic magnetization is rotated from the z-axis into the x-y-plane. A gradient pulse of duration 8 and magnitude g is appHed, so that the spins dephase. 

General overview of several studies of transport and intermolecular interactions in compressed supercritical fluids is presented. The unique aspects of the instrumentation used in these studies are emphasized. First, the results of NMR studies of self-diffusion in supercritical ethylene and toluene are discussed. These experiments used the fixed field gradient NMR spin-echo technique. Second, the novel NMR technique for the determination of solubility of solids in supercritical fluids is described. 

The MRA techniques discussed in the following base on gradient echo sequences and can generally be divided into flow-dependent and flow-independent ones. The main flow dependent techniques are time-of-flight MRA (TOF-MRA) and phase contrast MRA. CE-MRA, in contrast, is blood flow-independent, although flow phenomena can still influence... 

Table 18.2 summarizes the sequences and MR techniques which in our experience can be proposed (as mandatory or optional) in the diagnostic management of veno-occlusive disorders of the brain. Venous MRA can either be performed with the time-of-flight (TOF) or with the phase-contrast (PC) technique. In addition to the tomographic images, a flow sensitive gradient-echo sequence should be obtained if CVST is in question. As a fast screening examination we prefer a TOF 2D FLASH sequence (Table 18.2 Fig. 18.4, see 18.6c), oriented 90 degree to the flow direc-...

FIGURE 9.3 Pulse sequence for measurement of diffusion coefficient D by the pulsed field gradient spin echo technique. The gradient G is applied for a period 8 both before and after the 180° pulse, with separation A. 

Fig. 2. A typical timing diagram used with the oscillating-gradient technique to produce gradient echoes. (Adapted from Ref. 16.)...

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