Question

Describe how we get the necessary depth and lateral information to make a B mode image using an optical coherence tomography imaging system?

Answer 1

OCT employs infrared light for the low-coherence interferometry-based, non-invasive, non-destructive technique. In this, the longer wavelength of light helps in deeper penetration than confocal microscopy or related techniques. No excision of specimen or tissue processing is needed in the process, yet results are obtained at micron scale. It can be considered that OCT imaging is analogous to ultrasound B mode imaging, with the difference that the former uses light and later sound. In OCT, cross sectional imaging is done by measuring intensity of back scattered light. OCT allows imaging at 1D depth scan called A-Scan and even 2D and 3D depth scans called B-Scan. The technique is carried out by focussing the light from OCT light source on a spot in the static specimen sample. OCT collects details from surface and sub-surface backscattered light generated during penetration of light through the transparent specimen. This along with a reference light beam is Fourier transformed to create a 1D depth profile. The OCT light probe is scanned through the surface while collecting the A-scans into datasets. A B-Scan is now produced by the 2D scan giving cross-sectional image. High frequency ultrasound is limited in depth scan compared to OCT. The ability to focus soundwaves is what governs the traverse resolution in ultrasound, but the difficulty itself is in using the sound waves. It is easy to focus light waves, as in OCT. Also, the velocity of light is much faster compared to velocity of sound. OCT technology have resolution ranging upto 15µm. use of light makes the technique sensitive to differences in refractive indices of optical scattering between different tissues.