Comparison of Kasai autocorrelation and maximum likelihood estimators for Doppler optical coherence tomography

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Doppler Frequency Estimators under Additive and Multiplicative Noise

In optical coherence tomography (OCT), unbiased and low variance Doppler frequency estimators are desirable for blood velocity estimation. Hardware improvements in OCT mean that ever higher acquisition rates are possible. However, it is known that the Kasai autocorrelation estimator, unexpectedly, performs worse as acquisition rates increase. Here we suggest that maximum likelihood estimators (MLEs) that utilize prior knowledge of noise statistics can perform better. We show that the additive white Gaussian noise maximum likelihood estimator (AWGN MLE) has a superior performance to the Kasai autocorrelation estimate under additive shot noise conditions. It can achieve the Cramer-Rao Lower Bound (CRLB) for moderate data lengths and signal-to-noise ratios (SNRs). However, being a parametric estimator, it has the disadvantages of sensitivity to outliers, signal contamination and deviations from noise model assumptions. We show that under multiplicative decorrelation noise conditions, the AWGN MLE performance deteriorates, while the Kasai estimator still gives reasonable estimates. Hence, we further develop a multiplicative noise MLE for use under multiplicative noise dominant conditions. According to simulations, this estimator is superior to both the AWGN MLE and the Kasai estimator under these conditions, but requires knowledge of the decorrelation statistics. It also requires more computation. For actual data, the decorrelation MLE appears to perform adequately without parameter optimization. Hence we conclude that it is preferable to use a maximum likelihood approach in OCT Doppler frequency estimation when noise statistics are known or can be accurately estimated.published_or_final_versio

Maximum likelihood estimation of blood velocity using Doppler optical coherence tomography

A recent trend in optical coherence tomography (OCT) hardware has been the move towards higher A-scan rates. However, the estimation of axial blood flow velocities is affected by the presence and type of noise, as well as the estimation method. Higher acquisition rates alone do not enable the accurate quantification of axial blood velocity. Moreover, decorrelation is an unavoidable feature of OCT signals when there is motion relative to the OCT beam. For in-vivo OCT measurements of blood flow, decorrelation noise affects Doppler frequency estimation by broadening the signal spectrum. Here we derive a maximum likelihood estimator (MLE) for Doppler frequency estimation that takes into account spectral broadening due to decorrelation. We compare this estimator with existing techniques. Both theory and experiment show that this estimator is effective, and outperforms the Kasai and additive white Gaussian noise (AWGN) ML estimators. We find that maximum likelihood estimation can be useful for estimating Doppler shifts for slow axial flow and near transverse flow. Due to the inherent linear relationship between decorrelation and Doppler shift of scatterers moving relative to an OCT beam, decorrelation itself may be a measure of flow speed.published_or_final_versio

Dual-Band Time-Multiplexing Swept-Source OCT based on Optical Parametric Amplification

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Outcome of Secondary Head & Neck Cancer in Post-irradiated Nasopharyngeal Carcinoma Patients

Conference Theme: Innovation and Integration in ORL-HN

Palladium with interstitial carbon atoms as a catalyst for ultraselective hydrogenation in the liquid phase

We report a green route to prepare Pd with interstitial carbon atoms as a new solid catalyst for fine chemical catalysis in the liquid phase. First, glucose was used as a reducing and capping agent under hydrothermal conditions for the controlled reduction of the Pd precursor and after thermal treatment, carbon atoms were found to occupy the Pd subsurface interstitial sites, as confirmed by PXRD and TPR. A simple hydrogen peroxide treatment was required to partially remove surface carbon to increase catalytic activity. Catalytic properties of the final material were investigated in the hydrogenation of 3-hexyn-1-ol and 4-octyne to the corresponding alkene products and compared with a classical Lindlar type catalyst and other commercial Pd/C. It was found that the carbonised glucose Pd nanocatalyst with subsurface carbon substantially reduced undesirable isomerisations and over-hydrogenations. This was achieved by the increase in desorption rates of alkene species in solution, an effect believed to be the result of hybridisation of the Pd d-state with the C sp-state, thereby increasing the overall cis-alkene selectivity. This journal is © The Royal Society of Chemistry 2011