# Dental Applications of Optical Coherence Tomography (OCT) in Cardiology
## 2 Optical Coherence Tomography--The Methods  
In biomedical and clinical research, the Fourier-domain optical coherence tomography (FD-OCT) is the technique currently applied and has two approaches: swept source OCT (SS-OCT) and the spectrometer-based system (spectral domain OCT, SD-OCT). Both approaches, which are based on low-coherence interferometry, have been well described in numerous publications [8; 26; 31; 32; 35]. The light from the source is split into a sample and a reference arm (Michelson interferometer configuration). The back reflected light from the sample (sample arm) and the reference mirror interfere with each other, and the resulting signal (spectrum) is then recorded by the detector. In contrast, SS-OCT uses a frequency sweeping laser source combined with a photo diode; and SD-OCT applies a wideband laser source in combination with a spectrometer to split the interference signal into single wavelengths. After Fourier transform of the signal, a depth profile of backscattering along a perpendicular line to the object surface is generated (A-scan). The point-by-point scanning of the OCT beam across the sample produces 2D cross-sectional images (B-scans), and the line-by-line scanning generates a series of 2D images from which 3D image stacks can be created (Figure 3). OCT enables images to be generated from the different absorption and scattering of light of various material components in hard and soft tissues. Image contrast arises in areas with structures of different refractive index and light absorption, such as tooth-restoration interfaces, gaps, bubbles, material cracks, or porous areas in carious lesions [8; 29; 31]. The scenarios presented in this article are based on both SS- and SD-OCT. The techniques produce images non-invasively and at high-speed, allowing real-time imaging, and provide high axial and lateral resolution in the micrometer range. Dental structures and restorations can be imaged up to a depth of 2-2.5 mm thanks to the translucency and specific refractive indices of dental hard tissues (n\({}_{\text{dentin}}\): 1.48-1.80; n\({}_{\text{e}}\)manel: 1.45-1.61, own measurements) and composite restoration materials (n\({}_{\text{composite}}\): 1.42-1.63, own readings) in the range 1325 nm of near infrared light. In this wavelength range the transparency of enamel is highest [31] with high axial resolution at the same time. The refractive indices of lesions vary widely depending on the demineralization degree such as the values of natural carious enamel lesions. Usually these exceeded the values of the adjacent sound enamel up to +16% (own data). The following setups were used in our lab:  
SD-OCT: TELESTO SP II or SP 5, Thorlabs GmbH, Dachau, Germany (Figure 4a). Recording parameters in vitro, ex vivo and in vivo were: center wavelength 1310 \(\pm\) 107 nm; sensitivity \(\leq\)106 dB; axial/lateral resolution \(<\)7.5 (air)/15 \(\upmu\)m; field of view maximum 9 mm \(\times\) 9 mm \(\times\) 3.5 mm (air, pixel size 700 \(\times\) 700 \(\times\) 512 or 1024); imaging speed 76/91 kHz (in vivo) or 48-91 kHz (in vitro, ex vivo); A-Scan average 1 (in vivo) or 1-5 (in vitro, ex vivo); spot size 20 \(\upmu\)m; and power on sample 3 mW. SS-OCT: SS-OCT, OCS 1300 SS, Thorlabs Inc., Newton, NJ, USA (Figure 4b). The parameters of the OCT equipment in vitro, ex vivo and in vivo were: center wavelength 1325 \(\pm\) 100 nm; sensitivity 100 dB; axial/lateral resolution 12 (air)/25 \(\upmu\)m; field of view maximum 10 mm \(\times\) 10 mm \(\times\) 3 mm (pixel size 512 \(\times\) 512 \(\times\) 512); imaging speed 16 kHz; A-Scan average 1; and power on sample 4.5 mW.