Ahhyun S. Nam

Complex differential variance algorithm for optical coherence tomography angiography

We describe a complex differential variance (CDV) algorithm for optical coherence tomography based angiography. The algorithm exploits both the intensity and phase changes of optical coherence tomography (OCT) signals from flowing blood to achieve high vascular contrast, and also intrinsically reject undesirable phase signals originating from small displacement axial bulk tissue motion and instrument synchronization errors. We present this algorithm within a broader discussion of the properties of OCT signal dynamics. The performance of the algorithm is compared against two other existing algorithms using both phantom measurements and in vivo data. We show that the algorithm provides better contrast for a given number of measurements and equivalent spatial averaging.

In vivo label-free measurement of lymph flow velocity and volumetric flow rates using Doppler optical coherence tomography.

Direct in vivo imaging of lymph flow is key to understanding lymphatic system function in normal and disease states. Optical microscopy techniques provide the resolution required for these measurements, but existing optical techniques for measuring lymph flow require complex protocols and provide limited temporal resolution. Here, we describe a Doppler optical coherence tomography platform that allows direct, label-free quantification of lymph velocity and volumetric flow rates. We overcome the challenge of very low scattering by employing a Doppler algorithm that operates on low signal-to-noise measurements. We show that this technique can measure lymph velocity at sufficiently high temporal resolution to resolve the dynamic pulsatile flow in collecting lymphatic vessels.

Monte Carlo modeling of angiographic optical coherence tomography.

Optical coherence tomography (OCT) provides both structural and angiographic imaging modes. Because of its unique capabilities, OCT-based angiography has been increasingly adopted into small animal and human subject imaging. To support the development of the signal and image processing algorithms on which OCT-based angiography depends, we describe here a Monte Carlo-based model of the imaging approach. The model supports arbitrary three-dimensional vascular network geometries and incorporates methods to simulate OCT signal temporal decorrelation. With this model, it will be easier to compare the performance of existing and new angiographic signal processing algorithms, and to quantify the accuracy of vascular segmentation algorithms. The quantitative analysis of key algorithms within OCT-based angiography may, in turn, simplify the selection of algorithms in instrument design and accelerate the pace of new algorithm development.

Multi-functional angiographic OFDI using frequency-multiplexed dual-beam illumination

Detection of blood flow inside the tissue sample can be achieved by measuring the local change of complex signal over time in angiographic optical coherence tomography (OCT). In conventional angiographic OCT, the transverse displacement of the imaging beam during the time interval between a pair of OCT signal measurements must be significantly reduced to minimize the noise due to the beam scanning-induced phase decorrelation at the expense of the imaging speed. Recent introduction of dual-beam scan method either using polarization encoding or two identical imaging systems in spectral-domain (SD) OCT scheme shows potential for high-sensitivity vasculature imaging without suffering from spurious phase noise caused by the beam scanning-induced spatial decorrelation. In this paper, we present multi-functional angiographic optical frequency domain imaging (OFDI) using frequency-multiplexed dual-beam illumination. This frequency multiplexing scheme, utilizing unique features of OFDI, provides spatially separated dual imaging beams occupying distinct electrical frequency bands that can be demultiplexed in the frequency domain processing. We demonstrate the 3D multi-functional imaging of the normal mouse skin in the dorsal skin fold chamber visualizing distinct layer structures from the intensity imaging, information about mechanical integrity from the polarization-sensitive imaging, and depth-resolved microvasculature from the angiographic imaging that are simultaneously acquired and automatically co-registered.

High-speed optical coherence tomography by circular interferometric ranging

Existing three-dimensional optical imaging methods excel in controlled environments, but are difficult to deploy over large, irregular and dynamic fields. This means that they can be ill-suited for use in areas such as material inspection and medicine. To better address these applications, we developed methods in optical coherence tomography to efficiently interrogate sparse scattering fields, that is, those in which most locations (voxels) do not generate meaningful signal. Frequency comb sources are used to superimpose reflected signals from equispaced locations through optical subsampling. This results in circular ranging, and reduces the number of measurements required to interrogate large volumetric fields. As a result, signal acquisition barriers that have limited speed and field in optical coherence tomography are avoided. With a new ultrafast, time-stretched frequency comb laser design operating with 7.6 MHz to 18.9 MHz repetition rates, we achieved imaging of multi-cm3 fields at up to 7.5 volumes per second.Using an ultrafast, time-stretched frequency comb laser operating with repetition rates from 7.6 MHz to 18.9 MHz, a rapid and large-volumetric-field optical coherence tomography at an imaging rate of up to 7.5 volumes per second is demonstrated.

Complex differential variance angiography with noise-bias correction for optical coherence tomography of the retina

Complex differential variance (CDV) provides phase-sensitive angiographic imaging for optical coherence tomography (OCT) with immunity to phase-instabilities of the imaging system and small-scale axial bulk motion. However, like all angiographic methods, measurement noise can result in erroneous indications of blood flow that confuse the interpretation of angiographic images. In this paper, a modified CDV algorithm that corrects for this noise-bias is presented. This is achieved by normalizing the CDV signal by analytically derived upper and lower limits. The noise-bias corrected CDV algorithm was implemented into an experimental 1 μm wavelength OCT system for retinal imaging that used an eye tracking scanner laser ophthalmoscope at 815 nm for compensation of lateral eye motions. The noise-bias correction improved the CDV imaging of the blood flow in tissue layers with a low signal-to-noise ratio and suppressed false indications of blood flow outside the tissue. In addition, the CDV signal normalization suppressed noise induced by galvanometer scanning errors and small-scale lateral motion. High quality cross-section and motion-corrected en face angiograms of the retina and choroid are presented.

High-speed subsampled optical coherence tomography imaging with frequency comb lasers

We demonstrate how frequency comb lasers can be used to induce optical-domain compression in optical ranging. In the context of coherent tomography, this compression enables ultra-high speed volumetric microscopy. We describe this concept and a novel high-speed laser based on stretched-pulse mode locking (SPML).

Assessment of vascularization and myelination following peripheral nerve repair using angiographic and polarization sensitive optical coherence tomography (Conference Presentation)

A severe traumatic injury to a peripheral nerve often requires surgical graft repair. However, functional recovery after these surgical repairs is often unsatisfactory. To improve interventional procedures, it is important to understand the regeneration of the nerve grafts. The rodent sciatic nerve is commonly used to investigate these parameters. However, the ability to longitudinally assess the reinnervation of injured nerves are limited, and to our knowledge, no methods currently exist to investigate the timing of the revascularization in functional recovery. In this work, we describe the development and use of angiographic and polarization-sensitive (PS) optical coherence tomography (OCT) to visualize the vascularization, demyelination and remyelination of peripheral nerve healing after crush and transection injuries, and across a variety of graft repair methods. A microscope was customized to provide 3.6 cm fields of view along the nerve axis with a capability to track the nerve height to maintain the nerve within the focal plane. Motion artifact rejection was implemented in the angiography algorithm to reduce degradation by bulk respiratory motion in the hindlimb site. Vectorial birefringence imaging methods were developed to significantly enhance the accuracy of myelination measurements and to discriminate birefringent contributions from the myelin and epineurium. These results demonstrate that the OCT platform has the potential to reveal new insights in preclinical studies and may ultimately provide a means for clinical intra-surgical assessment of peripheral nerve function.

Angiographic imaging using an 18.9 MHz swept-wavelength laser that is phase-locked to the data acquisition clock and resonant scanners(Conference Presentation)

In this study, we present an angiographic system comprised from a novel 18.9 MHz swept wavelength source integrated with a MEMs-based 23.7 kHz fast-axis scanner. The system provides rapid acquisition of frames and volumes on which a range of Doppler and intensity-based angiographic analyses can be performed. Interestingly, the source and data acquisition computer can be directly phase-locked to provide an intrinsically phase stable imaging system supporting Doppler measurements without the need for individual A-line triggers or post-processing phase calibration algorithms. The system is integrated with a 1.8 Gigasample (GS) per second acquisition card supporting continuous acquisition to computer RAM for 10 seconds. Using this system, we demonstrate phase-stable acquisitions across volumes acquired at 60 Hz frequency. We also highlight the ability to perform c-mode angiography providing volume perfusion measurements with 30 Hz temporal resolution. Ultimately, the speed and phase-stability of this laser and MEMs scanner platform can be leveraged to accelerate OCT-based angiography and both phase-sensitive and phase-insensitive extraction of blood flow velocity.

Label-free in-vivo measurement of lymph flow velocity using Doppler optical coherence tomography(Conference Presentation)

Alterations in lymphatic network function contribute to the lymphedema development, cancer progression and impairment in regional immune function. However, there are limited tools available to directly measure lymphatic vessel function and transport in vivo. Existing approaches such as fluorescence recovery after photo-bleaching (FRAP) require injection of exogenous labels which intrinsically alter the physiology of the local lymphatic network. A label-free approach to imaging lymph flow in vivo would provide direct and unaltered measurements of lymphatic vessel transport and could catalyze research in lymphatic biology. Here, we demonstrate and validate the use of Doppler optical coherence tomography (DOCT) to measure lymph flow in vivo at speeds as low as 50µm/s. Compared to blood, lymph is relatively acellular (under normal conditions), but contains similar soluble components to blood plasma. We demonstrate that the small but detectable scattering signal from lymph can be used to extract fluid velocity using a dedicated algorithm optimized for Doppler analysis in low signal-to-noise settings (0 to 6 dB typical). We demonstrate the accuracy of this technique by comparing DOCT to FRAP measurements, using an intralipid lymph proxy in microfluidic devices and in vivo in the mouse ear. Finally, we demonstrate the label free measurement of lymph speed in the hind-limb of mice with a temporal resolution of 0.25s that agree well with prior literature reports. We anticipate that DOCT can become a powerful new tool in preclinical lymphatic biology research—including the relationship between lymphatic function and metastasis formation—with the potential to later expand also to clinical settings.