## 7 Discussion
The pulsed gradient field G produces a variable electric field, which can create unpleasant peripheral nerve stimulation (PNS), especially in modern gradient coils with high output (intensity and scanning speed) [131; 132]. In fact, efficient gradient coils produce shorter cycles with higher resolutions, thus amplifying their output, which leads to a shorter imaging duration. These PNSs can trigger sensations of muscle compression, irritation, or numbness. The static B\({}_{0}\) field is normally safe for common field strengths around 1.5 T. Recently, due to improved performance, ultra-high-field (UHF) MRI scanners (above 7 T) have been introduced [133]. These UHF scanners induce low-frequency currents in the conductive tissues of the moving body inside or near the scanner [134]. These induced currents and their fields trigger uncomfortable sensations such as falling sensation, light flashes, loss of balance, or muscle tremps. In addition, the interaction between a strong B\({}_{0}\) field and living tissues creates magnetic induction effects due not only to the induced currents but also to Lorenz forces [135]. These forces correspond to charged particles moving through the static field and experiencing forces in a direction perpendicular to the motion. These forces depend on the speed of movement and field intensity and could therefore be important in tissues for UHF scanners. Different disorders could arise due to forces such as, for example, magnetic vestibular stimulation (MVS) [136]. A common reported significant side effect caused by these forces is dizziness, which can eventually lead to nausea [137]. They can also cause involuntary eye motion and other effects [138]. * Regarding MRI compatibility issues, conventionally, an MRI is shielded regarding external field exposure. We can largely typify an external object as MRI compatible if it behaves in an MRI-safe manner, not affecting image quality, and working as expected. An incompatible matter can perturb mainly the RF field as abovementioned, which alters the image. The image quality can be deteriorated for diverse reasons related to the scanner fine tuning, living tissue susceptibility discrepancies, body-embedded matters, and medical tools. In the last two cases, metallic materials, which present susceptibility variations, depend on the size, the shape, and the orientation, which are responsible for image alteration. The electrically conducted materials can behave between dielectric and electric conducting functions of EMF wave frequency. Magnetic, dielectric, or conductor materials are characterized, respectively, by the permeability \(\mu\) (or the susceptibility \(\chi\)), by the permittivity \(\varepsilon\), or by the conductivity \(\sigma\). In highly magnetic material, \(\mu_{\mathrm{r}}>>1\) and \(\mu_{\mathrm{r}}\approx\chi\) note that \(\mu=\mu_{0}\cdot\mu_{\mathrm{r}}\) and \(\chi=\mu_{\mathrm{r}}-1\). For non-magnetic material, \(\mu_{\mathrm{r}}=1\) and \(\chi=0\). The relative values of \(\sigma\) and \(\omega\cdot\varepsilon\) (\(\omega=2\,\pi\,\mathrm{f}\)) characterize dielectric vs. electrically conducting behaviors of electrically conducted materials. For low f, \(\sigma>>\omega\cdot\varepsilon\approx\sigma\) and for high f, \(\omega\cdot\varepsilon>>\sigma\approx\omega\cdot\varepsilon\) and \(\sigma\approx 0\). A fully MRI-compatible material has zero values for both \(\chi\) and \(\sigma\). The dielectric nature of matters does not affect the compatibility. Regarding the RF field distribution, the eventually introduced matters should have \(\mu_{\mathrm{r}}=1\) and \(\chi=0\), with high \(\omega\cdot\varepsilon\), or conductors with a trivial cross-section perpendicular to the RF field B\({}_{1}\). With such features, the RF field distribution would not be altered. The MRI compatibility check can be accomplished for existing image-guided MRI systems by using experimental means. This can be carried out by measuring the perturbations of the field resulting in the insertion of checked objects within or near the scaffold according to the case.