Dr. Akkin's research interests lie in the development and use of non-contact, non-invasive optical imaging tools to study structural and functional disorders in biological tissue with high spatiotemporal resolution.
Using phase- and polarization-sensitive interferometric techniques, Akkin Lab images tissue microstructure in real-time with a few micron spatial resolution and function with sub-nanometer scale optical path length resolution.
Because functional recovery may be possible only when the tissue structure is intact, detecting structural changes (function) during physiological activity prior to any structural loss or permanent damage is the main thrust of the work. Applications in medicine are possible, as the techniques use back-scattered light.
Brain imaging and mapping with serial optical coherence scanner
Large-scale brain imaging and mapping at microscopic resolution is feasible with intrinsic optical contrasts. Akkin Lab combined multi-contrast optical coherence tomography (OCT) and a tissue slicer to form a serial optical coherence scanner (SOCS).
It distinguishes white matter and gray matter and visualizes nerve fiber tracts that are as small as a few tens of micrometers. Axonal birefringence highlights the location and myelination of nerve fibers, while the axis orientation contrast indicates the fiber alignment in the plane. SOCS can reveal biomarkers for disease onset and progression in cerebrum and cerebellum, and support the development of therapeutics.
Depth-resolved optical imaging of neural action potentials
Akkin Lab has demonstrated non-contact depth-resolved optical imaging of neural action potentials by measuring sub-nanometer range transient structural changes. Fast signals detected by phase-sensitive optical coherence tomography (OCT) are coincident with the action potential arrival to the measurement site.
Squid giant axon preparation was used to study these changes in presence of different environmental (i.e., temperature) and physiological (i.e., ionic concentrations) conditions. Experiments with voltage-sensitive dyes allowed comparison of the phase and intensity signals at several depths. A functional OCT cross-sectional scanner was used to show that two-dimensional monitoring of small-scale neural activity would be feasible.
Integration of polarization-maintaining fiber technology into OCT
Akkin Lab has reported polarization-maintaining-fiber (PMF) based OCT systems. These systems are phase- and polarization-sensitive and are implemented in time-domain, in spectral-domain, and with swept-source technology. Conventional OCT is useful; however, often times the reflectivity images are not descriptive enough to indicate different structures in complex tissue.
Polarization-sensitive OCT (PS-OCT), on the other hand, provides additional contrasts based on birefringence, which is the optical anisotropy shared by many tissues including muscle, tendon, and nerve. The PMF-based PS-OCT devices are capable of generating reflectivity, birefringence/retardance, and axis orientation images of tissue, as well as the bidirectional blood flow, all simultaneously.
These systems combine the advantages of fiber technology with the straightforward operation and analyses of bulk setups. In this field, Akkin Lab develops these custom-made systems for disease-oriented applications.
Akkin lab has reported on several differential phase sensors based on low-coherence interferometry. The sensors are capable of measuring extremely small (Angstrom level) optical path length changes at specific depths.
The applications include imaging tissue response to electrical or photothermal stimulation, detection of neural activity, and reflection-mode measurement of Faraday rotation with a small field-depth factor.
BMEn 3211/3215 — Bioelectricity and Bioinstrumentation
BMEn 5421 — Introduction to Biomedical Optics
Liu CJ, Williams KE, Orr HT, and Akkin T; "Visualizing and mapping the cerebellum with serial optical coherence scanner," Neurophotonics, 4(1), 011006 (2017).
Liu CJ, Black AJ, Wang H, and Akkin T; "Quantifying three-dimensional optic axis using polarization-sensitive optical coherence tomography," Journal of Biomedical Optics, 21(7) 070501 (2016).
Black AJ and Akkin T; "Polarization-based balanced detection for spectral-domain optical coherence tomography," Applied Optics, 54: 7252-7257 (2015).
Yeh Y-J, Black AJ, Landowne D, and Akkin T; "Optical coherence tomography for cross-sectional imaging of neural activity," Neurophotonics, 2: 035001 (2015).
Wang H, Lenglet C, and Akkin T; "Structure tensor analysis of serial optical coherence scanner images for mapping fiber orientations and tractography in the brain," Journal of Biomedical Optics, 20: 036003 (2015).
Wang H, Zhu J, Reuter M, Vinke LN, Yendiki A, Boas DA, Fischl B, and Akkin T; "Cross-validation of serial optical coherence scanning and diffusion tensor imaging: A study on neural fiber maps in human medulla oblongata," NeuroImage, 100: 395-404 (2014).
Wang H, Zhu J, and Akkin T; "Serial optical coherence scanner for large-scale brain imaging at microscopic resolution," NeuroImage, 84: 1007-1017 (2014).