Gradient artifacts

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. 

Artifacts may be roughly categorized into those due to inherent limitations (e.g. pulses cannot excite unlimited bandwidths even if all hardware components work perfectly) and those that result from improper set-up of the experiment or nonideal functioning of the NMR spectrometer system. In this chapter we will mainly focus on the latter two. These artifacts are more likely to appear in multiple-pulse experiments. Quite often, they are avoided by clever programming of the experiments (e.g. interleaved acquisition of data for NOE spectra, use of pulsed-field gradients instead of phase-cycling). 

However, the validity of a similar assumption has to be questioned, in case the skin has previously been treated with a topically applied formulation [126], Opinions differ, whether the distinct curvature of the steady-state stratum corneum concentration gradient, reported in literature, may be an artifact of a wrong depth scale, since such a behavior cannot be reasonably explained by the established diffusion theory. 

A linearization of the steady-state concentration gradient could be demonstrated by relating the depth to the weight of the tissue, removed per piece of adhesive tape. However, large errors, especially, within the first tapes, cast doubt over these findings [127, 128], The procedure is time-consuming and artifacts, due to absorption and desorption of moisture, formulation excipients, or sebaceous lipids, are likely. 

The use of actively shielded magnetic field gradients has made the use of pulsed field gradients possible. The use of pulsed field gradients reduces experiment time, minimizes artifacts, and allows for further solvent suppression. 

---