# Dental Applications of Optical Coherence Tomography (OCT) in Cardiology
## 2 Optical Coherence Tomography--The Methods
In vitro/ex vivo studies: OCT signals were verified within a predefined region of interest (ROI) by comparison with corresponding X-ray microtomography images (\(\upmu\)CT, Skyscan 1172-100-50, Bruker MicroCT, Kontich, Belgium); and after sectioning specimens with images from the reference methods transverse microradiography (TMR, PW 3830/40, Philips, Amsterdam, The Netherlands); scanning electron microscopy (SEM, 5 kV, Phenom, Phenom World, Eindhoven, The Netherlands); spinning disc confocal laser microscopy (SDCM, Smartproof 5, Carl Zeiss Microscopy, Oberkochen Germany); and different modes of light microscopy. Figure 3: SD-OCT imaging (2D, 3D) of an extracted human premolar restored with composite (C). FOV: field of view, E: enamel, D: dentin, EDJ: enamel-dentin junction. In vitro/ex vivo studies were performed on extracted human teeth. The teeth were sound or had caries lesions, were unrestored or restored with composite. The focus on the specimen was set on a ROI and the surface was dried using a cotton pellet to leave it moist with no visible water droplets (controlled hydrated condition). 2D cross-sectional images of the ROI and 3D image stacks were recorded and images processed (Image) from version 1.45S; [PERSON], NIH, Bethesda, MD, USA). In vivo studies: Because of the geometric dimensions of the probes (diameter 34 mm, working distance about 42 mm), the imaging was limited to vestibular smooth surfaces of incisors, canines and premolars, after restorations and tooth surfaces were cleaned. The handheld and mechanically stabilized scanning probe was positioned at a rough right-angle to the restoration surface (Figure 4c). The restoration area was brought into focus and within 6 s, 2D cross-sectional images and 3D image stacks were generated and processed (Image) from version 1.45S). The studies were conducted in accordance with the Declaration of Helsinki, and the protocols were approved by the Ethics Committee of the University of Leipzig. The extracted teeth were used with patients' approvals (informed consents, protocol no. 299-10-04102010), and all subjects gave their informed consent for inclusion before they participated in the study (protocols no. 087/2003, 131-11-18042011, 196-14-14042014, 294-15-13072015). With OCT, in vivo imaging is a challenge if dental hard tissues (enamel, dentin), which induce more scattering and containing less water, are to be displayed together with soft tissues (gingiva, pulp), which contain more water. The optical properties of these tissues, such as light scattering and absorption, are very diverse and also vary with imaging wavelength. One limitation of OCT is the shallow penetration depth, caused by signal attenuation due to scattering and absorption. Depth penetration can be enhanced by optimizing the center wavelength. In particular, in the presence of structures such as carious lesions which induce increased scattering, multiple scattering can further reduce imaging quality at deeper penetration depths. As well as the technical parameters of the OCT equipment, the center wavelength of the light source and the optical bandwidth are key determinants for axial resolution. For soft and hard tissues, there are characteristic dependences of wavelength of OCT signal attenuation [36]. Some authors suggested that an imaging window at 1700 nm offers an advantage over shorter wavelengths by increasing penetration depth as well as enhancing the image contrast at deeper penetration depths by reducing multiple scattering [37]. For biological tissues, it is expected an increase in OCT imaging depth at 1600 nm compared to 1300 nm on samples with high scattering power and low water content (enamel) [38]. Otherwise at longer wavelengths, absorption increases with greater water content, which has a counterbalancing effect [37]. Also, as in the case of enamel, transparency is higher at 1300 nm than at 1600 nm. In general, however, axial resolution decreases with increasing imaging wavelength.