Abstract
We introduce the approach of variable time encoding for multichannel optical coherence tomography (OCT). High-speed fiber optical switches are applied for sequential sample arm switching to enable quasisimultaneous image acquisition from three different orientation angles. In comparison with previous multichannel OCT (using simultaneous sample illumination), time-encoded multichannel OCT has no need for division of illumination power among the respective channels to satisfy laser safety requirements. Especially for ophthalmic applications-in particular retinal imaging, which the presented prototype was developed for-this advantage strongly influences image quality through an enhanced sensitivity. Nevertheless, time encoding comes at the cost of a decrease in imaging speed due to sequential channel illumination. For the typical multichannel OCT modality Doppler OCT, this results in a reduction of the maximum unambiguously determinable Doppler velocity. However, we demonstrate that this drawback can be overcome by adaptation of the illumination channel switching scheme. Thus, a re-extension of the maximum unambiguously determinable Doppler frequency to the full A-scan rate of the tunable light source is presented. The performance of the technique is demonstrated by flow phantom experiments and measurements of retinal blood flow in the eyes of healthy human volunteers.
Highlights
Tackling the quantification of flow in living tissue, optical coherence tomography’s (OCT) functional extension Doppler OCT (DOCT) is in principle limited to the determination of the axial velocity component.[1]
We introduce a concept of multichannel OCT, which combines the advantages of both our previously published multichannel instruments: (i) active recoupling of all three illumination channels,[9] (ii) full illumination power for each channel[11]
The enhanced signal quality due to increased illumination power especially benefits DOCT investigations because of decreased phase noise. This advantage comes at the cost of a reduced imaging speed and a reduced dynamic range for DOCT velocity measurements
Summary
Tackling the quantification of flow in living tissue (primarily blood flow), optical coherence tomography’s (OCT) functional extension Doppler OCT (DOCT) is in principle limited to the determination of the axial velocity component.[1]. Some approaches used additional structural data to externally retrieve information about flow orientation.[2,3,4] Others integrated an additional illumination channel to obtain simultaneous axial velocity component measurements from two different observation angles including advanced alignment strategies.[5,6] Again others combined both the additional structural data and the dual-channel approach.[7,8] multichannel OCT only enabled first absolute velocity measurements without any prior knowledge of flow orientation by increasing the number of individual linearly independent sampling directions to three.[9]
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