Feature Of The Week 3/3/13: Complex wavefront shaping for optimal depth-selective focusing in optical coherence tomography
We present that a novel wavefront shaping approach improves the penetration depth and sensitivity of optical coherence tomography (OCT) by systematically control multiple light scattering. As well known, the penetration depth of OCT is significantly limited due to multiple light scattering that is resulted from refractive inhomogeneity. Whereas OCT can achieve satisfactory penetration depth in ophthalmology where the tissue structures are relatively transparent (weakly turbid), the limited penetration depth of OCT prevents general applications to other highly turbid opaque tissues such as skin tissue.
Conventional approaches to attempt to solve this issue in OCT have revealed challenges in dealing with multiple light scattering. For example, the use of optical clearing agent (e.g. Glycerol) reduces light scattering by matching the refractive index of scatterers, but requires the waiting time after the addition of the agent. Other approaches such as spatial- or frequency- compounding methods increase sensitivity by averaging multiple OCT images obtained with different illumination conditions (slightly changing optical paths or using dual light sources); yet they regard multiple scattering as background noise, yielding low sensitivity in a deep position.
In this project, we propose and experimentally demonstrate a wavefront shaping approach to control multiple scattering in a sample in order to increase the penetration depth in OCT. A digital micro-mirror device is utilized to shape the incident wavefront, such that the maximal energy is focused at a specific depth in a highly scattering sample using a coherence-gated reflectance signal as feedback. 500 independent bases based on 2-D Hadamard patterns are utilized and controlled by a full phase shift method. The coherence-gated reflected OCT signal is employed for feedback; the appropriate spatial phase value that maximizes the back-scattered signal from a target depth is found for each basis. The final optimized wavefront, constructed with a combination of all optimized input bases, maximizes energy delivery at the target depth and automatically satisfies the coherence gating condition.
We used a dried leaf of cherry blossom Prunus serrulata and a phantom which consists of PDMS (polydimethylsiloxane) mixed with 10 μm dia. polystyrene beads, transparent tape, lens cleaning paper, and IR card. The experimental results clearly show that the OCT signals at specific depths can be selectively enhanced or annihilated by applying an appropriate incident wavefronts in the presence of optical inhomogeneity. The optimized wavefront significantly increases the SNR of the OCT signal at deeper depths and also extends the penetration depth.
Although the particular focus of the paper is to demonstrate that the wavefront shaping can enhance the penetration depth in the spectral domain-OCT. The present approach is sufficiently broad and general and it will directly find use in other types of OCT and several modalities of OCT, and it will be employed to in vivo applications in the near future.
For more information see recent Article. Courtesy YongKeun Park from Korea Advanced Institute of Science and Technology.