## 2 Materials and Methods
### Magnetic Resonance Imaging (MRI)  
The measurements were accomplished using preclinical Bruker Biospec 9.4 T MR scanner, equipped with gradient coils of 60 mm ID and the maximum field gradient of 1 T/m. The quadrature RF birdcage coil (35 mm ID) was used for collecting signals. Before insertion into the MRI probe-head, the root canal models were rinsed with distilled water using a syringe with a needle to avoid fine debris and micro-bubbles distorting the MR image. The training blocs were placed in a syringe with an inner diameter of 12.7 mm, filled with water. Prior to the experiment, the distilled water was degassed to uniformly fill the cavities of the endo-block with the liquid and minimize the occurrence of air microbubbles. This enabled eliminating sources of artefacts originating from the difference in magnetic susceptibility between air and water or endo-block material. After degassing, the container (syringe) in which the measurement was carried out was sealed with parafilm. RARE pulse sequence was used for a pilot image made up of 32 layers with a thickness of 500 micrometers and in-plane resolution of 31 \(\times\) 31 micrometer. The image orientation was selected so that the plane of the layers was perpendicular to the upper edge of the training block, and the entire imaged area with a 16 \(\times\) 16 \(\times\) 16 mm field of vision included the syringe containing the interesting fragment of the model root canal. With repetition time (TR) of 5 s and 3 acquisitions (NA), the measurement time (TA) was 10 min. The obtained image allowed for positioning the imaging plane for the following high-resolution imaging, aimed at the accurate 3D mapping of the model root canal' curvature details.  
The main measurement was made using the RARE sequence, with slice thickness reduced to 100 micrometers. In order to improve the signal-to-noise ratio, NA = 25 was used. To minimize the measurement time, the field of view was limited to a cuboid 17 \(\times\) 16 \(\times\) 3 mm, with a resolution of 33 \(\times\) 31 \(\times\) 100 micrometers. Each 3D image consisted of 512 \(\times\) 512 \(\times\) 30 voxels. The imaging plane has been selected in such a way as to fully illustrate the course of curvature of the model root canal. For TR = 5 s and effective TE = 36 ms, TA was 1.5 h. Measurements were performed twice, before and after the preparation of training blocs with a selected type of endodontic tools.  
Imaging Data Analysis  
The obtained 3D imaging data were analyzed using Fijl [16]. First, the image was filtered using 3D median filter. This filter effectively eliminates noise without adding addi  
Figure 1: Example of resin-based endo-training block with “L-shaped” canal, before preparation by the rotary endodontic instrument (MTwo).  
tional values to the image, allowing keeping sharp edges between water-filled model of root canal and the surrounding solid resin. Thus, the image was binarized based on signal threshold obtained from the histogram. The binarized 3D images, before and after the endodontic development, were subtracted and added to each other and differential/summed 3D images were constructed. Then, based on the number of voxels of the differential image, after taking into account the volume of a single voxel (i.e., 31 \(\times\) 33 \(\times\) 100 micrometers), the volume difference was determined, corresponding to a volume of material removed during endodontic elaboration.