Flow artifact

Another type of flow artifact is due to voxel inflow and outflow problems large velocities make a spin leave its designated voxel in the time between signal encoding and signal readout. As the traveled distance d and the velocity v are related by the expression d = vt, it is obvious that the effect is more severe, the smaller the voxel size is. Slow velocities, on the other hand, may be masked by diffusive displacements. 

In order to avoid flow artifacts it may be advisable to replace the spatial encoding pulses (right-hand box) by velocity compensated pulses such as shown in Figure 2.9.4(e) for phase encoding. The amplitude of the Hahn spin-echo is attenuated by hydrodynamic dispersion. Evaluation of the echo attenuation curve for fixed intervals but varying preparation gradients (left box) permits the allocation of a hydrodynamic dispersion coefficient to each voxel, so that maps of this parameter can be rendered. 

The flow artifacts detected in the droplet size measurements are similar to those reported by Goux et al. [79] and Mohoric and Stepisnik [80]. In their work natural convection effects led to an increase in the decay of signal attenuation curves, causing over-prediction in the self-diffusion coefficient of pure liquids. In order to avoid flow effects in droplet size distributions, flow compensating pulse sequences such as the double PGSTE should be used. It has been demonstrated recently that this sequence facilitates droplet size measurements in pipe flows [81]. 

Proton density-weighted images seem to be as sensitive as FLAIR in detecting SAHs of less than 24 h. CSF flow artifacts in cisterns and ventricles are less likely to be observed with proton density than with FLAIR (Wiesmann et al. 2002). A recent report suggests that proton-density weighted sequences allow an accurate diagnosis of hyperacute SAH (Fiebach et al. 2004). 

Since MPC dynamics yields the hydrodynamic equations on long distance and time scales, it provides a mesoscopic simulation algorithm for investigation of fluid flow that complements other mesoscopic methods. Since it is a particle-based scheme it incorporates fluctuations, which are essential in many applications. For macroscopic fluid flow averaging is required to obtain the deterministic flow fields. In spite of the additional averaging that is required the method has the advantage that it is numerically stable, does not suffer from lattice artifacts in the structure of the Navier-Stokes equations, and boundary conditions are easily implemented. 

The shift of Pb-214 to a slightly higher size distribution compared to Pb-212 was also found using 1-ACFM and HVI impactors (Fig. 2). The higher flow rates of these impactors, as well as the ability to measure HVI activity by gamma spectroscopy, made us confident that this shift was real and not a data analysis artifact. 

The values for k+ and k were compared for temperature jump and stopped-flow conditions for DNA concentrations where the decay followed a mono-exponential function and no migration between DNA molecules occurred (see below).94 This report shows the importance of detecting fluorescence signals at the magic angle, which eliminated the fast components in the kinetics due to artifacts. The values for the association and dissociation rate constants obtained by both techniques are similar. 

System specification proceeds much as before but has an added element of user-interface design. The normal artifacts of system specification—a type model and operation specs—are now accompanied by prototypes and UI specs describing the screens, dialog flows across windows, information presented and required, and reports. These user-interface elements are kept consistent with the type model and are reviewed through scenarios. 

Figure 6 is a schematic representation of a DNA histogram. The ability of the flow cytometer to rapidly count several thousand nuclei contributes to the sensitivity of this technique for DNA analysis. However, problems due to sample quality, staining, and instrumental artifacts should be recognized and minimized to insure accurate interpretation of data (B2). 

FIGURE 64. 13C MAS NMR spectra of Cgo acquired at 150 K (a) Spectrum obtained when the gas stream is not laser-polarized (laser off), (b) Difference between the spectrum obtained when the gas stream is laser-polarized (laser on) and spectmm (A). This spectrum quantitatively represents the observed SPINOE intensity, (c) Difference between two successively recorded spectra obtained when the 129Xe flowing into the rotor is not laser-polarized. This demonstrates that the difference spectrum is free of artifacts. Reproduced by permission of Elsevier Science B. V. from Reference 70... 

The pKj values obtained for PCu(II) and PCu(I) are summarized in Table 9. Before discussing what appears to be a clear cut trend, the precision of individual pKa s should first be considered. It has for example been necessary to revise (downwards) the spinach PCu(II) pKa from 5.3 to 4.8 [121]. Plastocyanin is known to denature at or around pH 4.0 and in earlier work the lowest pH used was 4.5, which does not allow as accurate a fit to pK, values. By using a pH-jump method in which the final low pH is attained at the time of stopped-flow mixing (one reactant solution carrying substantially more buffer is allowed to control the pH), it is now possible with some confidence to include data down to pH 4.0. The other uncertainty is in the precision of the pKg for the PCu(I) remote site from a two pK fit. Again the data has to be free from artifacts introduced at... 

Gadella, B.M., Miller, N.G., Colenbrander, B., van Golde, L.M. and Harrison, R, A. 1999, Flow cytometric detection of transbUayer movement of florescent phospholipid analogs across the boar sperm plasma membrane elimination of labeling artifacts. Mol. Reprod. Dev.53 108-125. 

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