Question

Describe the setup of an ODT imaging system. Why do we use near infrared light for these systems ?

Answer 1

ODT or Optical Coherence Doppler Tomography is a new technique for imaging tissue microcirculation with ultra high velocity sensitivity. It is a non invasive microvascular imaging .

It is based on 200KHz VCSEL, swept source and GPU accelerated GUI to implement real time 3D flow image reconstruction and display. An FBG filter in the input arm is used to reduce phase errors induced by sweep trigger signalling.

The laser is connected to a fiber Bragg gating [FBG] . It illuminates a custom fiber optic interferometer consisting of a circulator [CIR], A 50/50 fiber coupler that split light into the reference and sample arms.Light exiting the sample arm is steered transversely by a Galvo mirror and then focussed into the sample using a 5 * NIR scan lense , yielding a focal spot size i.e. lateral resolution of 14.3 um in air in a biological tissue.

In the reference arm , a titled neutral density filter is placed in front of a gold coated mirror mounted on a translational stage to match the light density and pathlength in sampling arm.

Reflected light beams from both the sample and the reference arms are recombined in the detection arms where the interference fringes are detected using a balance photo detector with a bandwith of DC~350 MHz . It is then digitized using a 12 bit, 1.8GS/s data acquisition card.

The k-clock is generated by a delayed Mach-Zhender interferometer [ MZI] module and connected to the data acquisition card as sampling clock to allow accurate depth profile decoding.

Red colour of the white light has the longest wavelength. Infra red light has wavelength even longer than that of the red light of the visible spectrum. This helps in a sharper imaging of the biological tissue and a clearer picture .This is why near infra red light is preferably used for these systems.