Computed tomography (CT) scanning has made a tremendous impact in medical applications and its uses have shown an exponential increase since the 1990s.1 In dental applications, the uses have mainly been in microCT, but technological innovation has remained relatively static. Recently, spectral scanning (using multiple energy bins) has been shown to be an important advancement in CT technology, particularly in material discrimination and artefact reduction.2,3 Advances in CT detectors, e.g. photon-counting detectors, which potentially improve spatial resolution compared to conventional CT detectors, are contributing to the emergence of spectral CT as a new imaging modality. With these new developments in mind, it is time to examine the use of novel spectral CT scanning in dentistry.
The MARS photon-counting spectral CT (PCSCT) scanner was designed and built by MARS Bioimaging Ltd. (MBI) (Christchurch, New Zealand). The MARS PCSCT scanner has energy discriminating capabilities and assigns incoming x-ray photons to one of eight, user-defined, energy bins (five energy bins are used for image reconstruction). Every x-ray photon is processed, therefore image quality is improved at a lower cost of radiation dose compared to conventional CT. A previous study has demonstrated that the MARS PCSCT scanner produces high-quality diagnostic images in humans without exceeding current clinical radiation dose levels (<5mGy), so it is suitable for in vivo human studies.4
Microtomography (microCT) systems have been widely used in multiple bioscience fields, as well as in dentistry. MicroCT is able to analyse various hard and soft tissue specimens with an excellent spatial resolution by generating voxels in the range of 5–50 μm.5 Therefore, microCT offers a noninvasive and precise analysis of root canal morphology. MicroCT, however, involves high radiation doses that are not compatible with in vivo human imaging.6 There are also technical limitations that limit the size of microCT systems. For these reasons, microCT is mostly used for ex vivo studies only.
Cone-beam CT (CBCT), one of the three-dimensional imaging modalities used in dentistry, has been reported to offer good accuracy with small voxel size, and reasonable costs and radiation dose compared to those known for conventional CT.7,8,9 Because of such favourable characteristics, CBCT has become the predominant imaging modality for various surgical treatments such as dental implantation.9 However, CBCT does not totally address the imaging needs in the oral and maxillofacial region. The reason for this is because orofacial structures are small, heterogeneous with different radiodensity, and in close proximity to each other. They also frequently have artificially inserted metallic objects which cause metal artefacts. Therefore, while CBCT exhibits excellent quality in outlining the bony structures in the orofacial region, its ability to visualise internal conditions of teeth and bones is not always optimal. The motivation of this paper is therefore to explore the use of PCSCT in dentistry applications as a proof-of-concept study.
The first part of this paper investigates root canal structures in teeth. The structure of root canals are complex and every detail of the root canal system needs to be considered in order to develop an appropriate plan for endodontic treatment. Sometimes, there are small structures spreading in various directions from the main canal. These structures are known as accessory canals (ACs). ACs can reach the outer surface of the root, establishing a direct relationship between the dental pulp and the periodontal space. The diameter of the ACs of primary molars has been found to range from 10 to 180 μm, with a median diameter of 67.0 μm.10 Therefore, ACs often go unnoticed.
With an optimised magnification and voxel size, the spatial resolution of the MARS PCSCT scanner is, theoretically, able to detect and visualise ACs. The first objective of this study is to qualitatively compare the ability of CBCT and PCSCT to detect and visualise ACs in teeth. MicroCT is considered the “radiological gold standard” and is therefore used as a reference. The second objective of this study is to qualitatively compare the presence of metal artefacts for CBCT, microCT and PCSCT. When x-rays pass through metal, effects such as beam hardening, photon starvation and partial volume cause artefacts.11 Metal artefacts are common in CBCT and they may interfere with the diagnostic process. PCSCT, on the other hand, has energy resolving capabilities and minimises metal artefacts in the reconstruction stage by dividing the spectrum into narrow energy bins.3 This paper focuses on the metal artefacts from two commonly used dental materials, one is a gutta percha point, and the other is a titanium implant. A gutta percha point is often used in root canal treatments. When a tooth suffers from an irreversible inflammation of the pulp, the clinician performs a root canal treatment during which the pulpal tissue is removed. The empty space is filled with a gutta percha point to prevent the ingression of bacteria. Because gutta percha points consist of metal sulfates, metal artefacts are often present when imaging the treated teeth using image modalities such as CBCT. This causes the gutta percha points to be inflated in diameter due to artefacts. With clear visualisation of the gutta percha point, without an inflated diameter, the clinician can better judge if the root filling procedure is of adequate quality or not. Titanium rods are a commonly used dental implant. They are surgically placed in the jawbone, where they serve as the roots of missing teeth. The evaluation of bone loss surrounding the dental implant is very useful. It is also useful to visualise the boundary of the implant and the jaw to see if the implant has slipped, or caused damage to the bone.
The purpose of this study is to investigate the use of the novel PCSCT scanner for dentistry applications. The performance of the MARS PCSCT scanner in (1) detecting and visualising ACs in teeth, and (2) reducing metal artefacts were qualitatively assessed and compared to CBCT and microCT. Although, the PCSCT scanner is not optimised for dentistry applications, this paper presents the first-ever explorative study of PCSCT for dentistry applications.