History compounding: a novel speckle reduction technique for OCT guided cochleostomy

Optical coherence tomography (OCT) is a promising candidate for monitoring the bottom of the drilled channel during cochleostomy to prevent injury to the critical structure under the bone tissue. While the thickness of the overlaying bone tissue is changed during the drilling process, the wave front of the backscattered light is also altered, resulting in changing speckle patterns of the observed structures in the sequential historical scans. By averaging the different patterns in these scans, named history compounding, the speckles can be reduced and the detection of critical structure becomes much easier. Before averaging, the refractive index of bone tissue n_b has to be compensated so that the speckles of the same structure in different historical scans can be aligned together. An accurate method for measuring the refractive index nb using OCT is presented. Experiments were conducted to evaluate history compounding and the new technique is proved to be an effective, flexible and intuitive speckle reduction technique for OCT guided cochleostomy as well as hard tissue ablation of other kind.