According to a recent study published in the Journal of Biomedical Optics, a group of researchers proposed using a single objective lens to produce a line-field confocal optical coherence tomography system based on a Mirau interferometer. A single objective lens allows dynamic adjustment of the camera frequency during scanning.
The beam-splitter reflectivity in the Mirau interferometer can be tuned to increase detection sensitivity. The apparatus includes a galvanometer scanner that could scan the light line laterally. It is possible to acquire a stack of B-scans that together produce a 3D image. Line-field confocal optical coherence tomography device improves detection sensitivity and can acquire images more quickly without lowering their resolution.
Line-Field Confocal Optical Coherence Tomography (LC-OCT)
A recently developed high-resolution imaging technique called line-field confocal optical coherence tomography (LC-OCT) combines reflectance confocal optical microscopy with line illumination and detection with low-coherence optical interferometry. The LC-OCT technology's predecessor is the TD-OCT with line detection and illumination rather than point scanning.
A vertical segment image (a B-scan) is produced in LC-OCT by scanning in the depth direction by obtaining numerous A-scans concurrently. As a result, a B-scan can be obtained while retaining a similar acquisition time for the entire image with a scanning rate slower than point-scanning TD-OCT. A high numerical aperture (NA) microscope lens can be focused for imaging with a great lateral resolution by scanning at a frequency of 10 Hz.
Line-Field Confocal Optical Coherence Tomography (LC-OCT) in Dermatology
Line-field confocal optical coherence tomography has mostly been used in dermatology and dermo-cosmetology as it can produce in vivo three-dimensional (3D) images of the skin with cellular resolution. The described LC-OCT systems that capture three-dimensional images are developed around a Linnik interferometer with two identical microscope lenses.
A broadband super-continuum laser is used for high axial resolution imaging as the light source with ultrashort temporal coherence. LC-OCT enables an isotropic image resolution of 1 μm. The linear sensor of the LC-OCT camera, which is optically coupled to the illumination line, serves as a confocal slit by filtering most of the parasitic out-of-focus light. As a result, the total signal-to-noise ratio (SNR) is strong enough to provide real-time in vivo imaging in highly scattering tissues like human skin at depths of up to 400 μm.
Limitations of LC-OCT
The LC-OCT systems reported so far are capable of taking three-dimensional (3D) images and are based on a Linnik interferometer with two identical microscope objectives. The devices could not be made very small and light in this configuration. The inertia caused by the bulk being moved only allows a 10 Hz B-scan acquisition rate. Recently, a Mirau interferometer-based LC-OCT device with the advantages of being smaller and lighter is proposed.
Development of Mirau Interferometer-based LC-OCT
Xue et al. developed an LC-OCT system based on a Mirau interferometer that can acquire 3D pictures in addition to B-scans. The 3D image can be post-processed to provide cross-sectional images with any orientation, including horizontal section views. The LC-OCT apparatus is detailed, together with the specially created Mirau interferometer. Performance is described in terms of acquisition speed and spatial resolution. To demonstrate how the LC-OCT gadget can provide 3D images of the skin in vivo, the researchers exhibited the images of skin tissues.
In this research, the researchers have demonstrated an enhanced LC-OCT system based on a Mirau interferometer that can capture and display B-scans at 17 frames per second. 3D images with a quasi-isotropic resolution of 1.5 μm can be produced by stacking B-scans at various positions. The system can show 3D images of human skin at a cellular level. At 17 fps, the B-scan capture rate breaks all previous records for LC-OCT.
The reported Mirau-based LC-OCT device acquired B-scans roughly twice as quickly as the traditional LC-OCT devices based on a Linnik interferometer. A faster operation speed was attained by dynamically adjusting the camera frequency during the depth scan to account for the trajectory's nonlinearity and using a quicker camera and a more potent light source. A greater speed was made possible by reducing the mass needed to oscillate. According to reports, the fastest frequency-domain OCT devices can acquire A-scans up to 300 kHz and even up to 20 GHz by obtaining 4 A-scans simultaneously. They can produce B-scans made of 2048 A-scans at a speed of up to 104 frames per second for comparison with the LC-OCT gadget produced in this research.
Xue, W., Ogien, J., Bulkin, P., Coutrot, A.-L., & Dubois, A. (2022). Mirau-based line-field confocal optical coherence tomography for three-dimensional high-resolution skin imaging. Journal of Biomedical Optics, 27(8), 086002. https://www.spiedigitallibrary.org/journals/journal-of-biomedical-optics/volume-27/issue-8/086002/Mirau-based-line-field-confocal-optical-coherence-tomography-for-three/10.1117/1.JBO.27.8.086002.full