## 4 Discussion
In most of the papers regarding the rotary NiTi instruments, the apex transportation and preservation of the canals' natural shape were analyzed [7,8,24]. [PERSON] and [PERSON] in the research on Race instruments used for the preparation of L-shaped canals observed apex transportation outside [24]. [PERSON] et al. reported apex transportation in each instrument used (WaveOne Gold, ProTaper Next, ProTaper Universal) [7]. The average values of transportation for WaveOne Gold vs. ProTaper Next were negative--inside transportation. The results obtained for Pro Taper Universal elucidated that the transport tion was more pronounced than in ProTaper Next and WaveOne Gold. Authors proved that the differences in transportation between those pairs of instruments were statistically significant (\(p<0.05\)). No statistically significant difference in transportation was obtained in groups where canals were instrumented with NiTi instruments. Statistically significant differences were observed while comparing the K group with each group's rotary system. Observed changes may be caused by the different taper of the last instrument used. The presented results show possibility to use MR Imaging (called in literature as MR Microscopy) for visualization of the root canals topography [14]. The quality of the MR visualization does not depend on complication in shape of the canals. An important aspect of the MRI measurements is to obtain high contrast between the cavities and the tooth. This may be achieved by filling cavities with water or water solution of paramagnetic ions having high MR signal [13]. Another important aspect is maximal space resolution possible to achieve in reasonable time. In the used sequence (3D SE) increasing twice the 3D resolution requires four time longer experimental time (assuming that lower S/N does not require additional signal accumulation), while the necessary disk space increases 8 times [15]. For artificial teeth the time of the measurements is not the most critical problem. However, to implement this technique in vivo, it is necessary to decrease it substantially. Using the solution with the appropriate T1 and T2 values for filling cavities may decrease the total measurement time. However, more promising solution might be used for the fast acquisition techniques. Sequences based on the train of the echoes instead of the single echo are especially favorable. Another possibility--sequences based on the gradient echo, seems to be less useful because of their sensitivity to the magnetic susceptibility artefacts. Magnetic resonance microscopy/imaging is not yet used in clinical dental practice. It was not until the STRAFI technique that proton imaging of real hard dental tissues was possible, although the resolution of such registered images left much to be desired, and it was not possible to distinguish individual types of hard tissues. Therefore, a valuable and reasonable solution for imaging tooth structure seems to be the use of SPI and spin echo methods, which are available or possible to implement on standard equipment and allow imaging of both mineralized parts and dental pulp [15]. The SPI method was used in previous studies for caries detection in vitro [13]. Once the current difficulties in magnetic resonance imaging are overcome, it will be possible to introduce this type of visualization technique to the level of in vivo analysis. In the future, magnetic resonance microscopy may become a diagnostic tool supporting the work of a clinician. The results obtained indicate the possibility of using the spin-echo based magnetic resonance imaging technique for spatial mapping of external tooth topography and the course and structure of tooth root canals for therapeutic, diagnostic, and didactic purposes [35; 36]. Determination of parameters through the use of magnetic resonance imaging techniques may improve the planning and conduct of endodontic treatment in the future [37; 38].