# Root Canal Obturation by Electrochemical Precipitation of Calcium Phosphates
## 2 Materials and Methods
### Characterization of Sealing Quality
#### 2.5.2. Imaging Analysis of Precipitate Density  
SEM imaging was performed as described above. X-ray imaging was performed using a conventional dental X-ray unit (Heliodent Plus, Dentsply Sirona, Bensheim) and intraoral image plates (VistaScan, Durr Dental, Bietgheim-Bissingen). Micro-CT measurement for sealing quality was performed using a Siemens Inveon PET/SPECT/CT (Siemens Healthcare GmbH, Erlangen, Germany). The X-ray tube voltage and current were set to 80.0 kV and 500 \(\upmu\)A, respectively. The resolution was 13.1 \(\upmu\)m at a total scan duration of 39.0 min. The Inveon Acquisition Workplace software was used for process controlling and monitoring. The image analysis was performed using the RadiAnt DICOM Viewer (Medixant, Poznan, Poland) to reconstruct 3D data [7; 42; 43].  
For X-ray micro-tomography characterization of material distribution within the root canal, i.e., porosity of filling, \(\upmu\)CT scans were performed on a ZEISS Xadia 500 Versa equipped with a 160 kV transmission tube and an indirect detection system made of a scintillator, switchable objective lenses (0.39\(\times\), 4\(\times\), 20\(\times\)) and a CCD camera with 2048 \(\times\) 2048 pixels. Each tooth was attached to the sample holder with the crown down with no specific preparation being needed. The samples were imaged at a source-to-sample distance of 17 mm and a sample-to-detector distance of 12 mm. The 4\(\times\) objective and a 2-fold detector binning (1024 \(\times\) 1024) were used for the acquisition, resulting in an effective pixel size of 4 \(\upmu\)m. The tomographic scans included 1801 radiographic projection images that were recorded over a sample rotation of 360\({}^{\circ}\) and with an exposure time of 2 s per image. The tube voltage and power were 80 kV and 6 W, respectively. Tomographic reconstruction of the acquired data and further data processing was carried out with the software X-AID (MITOS GmbH, Garching, Germany). The filtered back-projection algorithm for local tomographies was applied in combination with the beam hardening correction for cupping artefacts. The initial datasets were cropped to 850 \(\times\) 500 \(\times\) 500 voxels during reconstruction. Thus, the final volumes consist of 850 stacked images of dimensions 2 \(\times\) 2 mm and cover a total height of 3.4 mm. The gray values of the reconstructed imagesrelate to differences in X-ray attenuation of the respective materials and certain features like the pores within the filling can be segmented based on grayscale thresholds. In the first step, image noise was reduced by using a bilateral post-processing filter to enable better segmentation. Binary volumes masking the root canals were created the following: (i) Segmenting the tooth material by manual thresholding. (ii) Removing segmented structures of similar grey values within the root canals by applying morphological operations like the opening operation. (iii) Inverting the resulting images. (iv) Removing remaining regions outside of the root canals. Binary volumes representing the pores were obtained by Otsu thresholding on the grey value distributions within the root canals. The volume fractions of the pores within the root canals were determined by relating the voxels belonging to the pores in the root canals to the total number of voxels in the root canals. For the analysis, the sample volumes were divided into blocks of 50 images, corresponding to section of 200 \(\upmu\)m. Omitting the first and last sections, 15 values were evaluated over a range of 3 mm for each sample.