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Micro-Computed Tomography in Endodontic Research
Introduction
High-resolution micro-computed tomography is an innovative technology with several applications in endodontic research and education. Conventional X-ray computed tomography (CT) is an imaging modality that was first described by Hounsfield (1973). This technique produces a series of images through tomography or imaging by sections, which are then reconstructed three-dimensionally using computer software programs (Hounsfield, 1973).
The possibility of traditional CT application in endodontics to three-dimensionally reconstruct teeth was first explored by Tachibana and Matsumoto (1990). While these investigators were able to demonstrate anatomical configuration of teeth using CT, the spatial resolution of 0.6 mm was found to be insufficient to allow for detailed analysis of root anatomy and structures. The authors concluded that conventional CT offered only limited application in endodontics due to its high radiation dose, time consumption, cost, insufficient resolution, and inadequate computer software capability. However, some investigators (Velvart et al., 2001) still found traditional CT useful compared to periapical radiographs when planning for periapical surgery of mandibular molars and premolars. These authors concluded that CT imaging provided beneficial information on mandibular canals and their proximity to the lesion or root apex that is not available from dental radiographs. Further advancements in technology resulted in the development of newer versions of CT scanning such as cone beam computed tomography (CBCT). In clinical settings, CBCT imaging has surpassed conventional CT and has become increasingly popular for endodontic presurgical treatment planning and diagnosis. Detailed discussion of CBCT technology and its applications in clinical practice are beyond the scope of this chapter.
Other technological advancements allowed for the introduction of a miniaturized form of traditional CT, the micro-CT (Kak and Stanley, 1988) for use in nonclinical settings. Micro-CT applies comparable principles to those of conventional CT, but the three-dimensional reconstructions of small objects, such as teeth, are developed to a resolution of within a few microns (<2 µm for Scanco μCT50, SCANCO Medical, Switzerland). While initial investigations using micro-CT technology were hampered by limited vertical resolution capacity of 1–2 mm (Dowker et al., 1997; Nielsen et al., 1995), improvements in the micro-CT machinery and computer software employed in reconstruction of images have allowed for significantly more accurate analysis of root canal systems (Dowker et al., 1997; Peters et al., 2000, 2001).
How Does Micro-CT Work?
Micro-CT scanners employ a micro-focus X-ray source that enables high-resolution detectors to collect magnified projection images of a small object. The first generation machines were equipped with a line detector. As the object rotated around the z-axis, differences in radiodensity were detected and a slice could then be reconstructed. With advancements along the z-axis, the acquisition of numerous two-dimensional views became possible, which were then processed by computer softwares to produce three-dimensional images (De Santis et al., 2005). The reconstructed three-dimensional images generated could then be sliced along any plane to further analyze the external and internal structures of the scanned object.
Other commercial units (e.g., SkyScan 1172, SkyScan, Belgium) use miniaturized cone beam geometry that scans the entire object in one rotation. This mode of acquiring images results in a data volume rather than individual slices. However, the data block can be resliced at selected angles and slice thicknesses. The latest generation of micro-CT units, such as newer Scanco units (SCANCO Medical), use a stacked fan beam geometry that with a special collimator are able to acquire 256 slices with one rotation. The main practical difference between fan beam and cone beam geometry is faster data acquisition (with a reliance on accurate reconstruction algorithms) with cone beam and slightly better perceived data quality (with a much longer acquisition time) with fan beam machines.
Independent of the data acquisition mode, three-dimensional reconstruction of objects with any micro-CT data sets requires segmentation. This relies on threshold values that differentiate a particular structure of interest from its surrounding material. The three-dimensional reconstruction can then be executed on these thresholds based on calculation of data slices or a data volume from the individual projections. This allows for outlines of enamel, dentine, and the root canal as well as its content to be segmented and assessed.
Applications in Endodontics
As a nondestructive imaging tool, micro-CT may be applied to assess an object many times, allowing it to remain unaltered for further experimentation and future scans. The three-dimensional images gather considerable data, allowing for both qualitative and quantitative evaluation of the sample (Rhodes et al., 1999). These characteristics make micro-CT a desirable tool for in vitro studies that evaluate root canal morphology and procedures of root canal preparation and obturation. Thus, a scanned tooth can be analyzed along its length to acquire data for calculating areas and volumes before and after endodontic procedures. The data offered by micro-CT technology can lead to clinical applications such as development of new techniques, comparative analysis of existing approaches in endodontic treatment, and enhancement of dental education in preclinical and clinical stages.
The development of the micro-CT technology has allowed for better assessment of the anatomy of the root canal system with unprecedented accuracy, which in turn has resulted in the adoption of this technology in endodontic research. Using micro-CT with a resolution of 127 µm, Nielsen et al. (1995) demonstrated accurate three-dimensional rendering of external and internal morphologies of root canals in extracted calcified human maxillary molars. They concluded that it was possible to reproduce tooth anatomy nondestructively and to assess area and volume changes after root canal instrumentation and obturation procedures (Nielsen et al., 1995). Dowker et al. (1997) improved on the work of N/>
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