Review of mesoscopic optical tomography for depth-resolved imaging of hemodynamic changes and neural activities

Abstract

Abstract. Understanding the functional wiring of neural circuits and their patterns of activation following sensory stimulations is a fundamental task in the field of neuroscience. Furthermore, charting the activity patterns is undoubtedly important to elucidate how neural networks operate in the living brain. However, optical imaging must overcome the effects of light scattering in the tissue, which limit the light penetration depth and affect both the imaging quantitation and sensitivity. Laminar optical tomography (LOT) is a three-dimensional (3-D) in-vivo optical imaging technique that can be used for functional imaging. LOT can achieve both a resolution of 100 to 200 μm and a penetration depth of 2 to 3 mm based either on absorption or fluorescence contrast, as well as large field-of-view and high acquisition speed. These advantages make LOT suitable for 3-D depth-resolved functional imaging of the neural functions in the brain and spinal cords. We review the basic principles and instrumentations of representative LOT systems, followed by recent applications of LOT on 3-D imaging of neural activities in the rat forepaw stimulation model and mouse whisker-barrel system.

High-dynamic-range fluorescence laminar optical tomography (HDR-FLOT)

“…Use of exogenous fluorescent dyes and transgenic animals can also aid in studying functional parameters, such as changes in membrane potential [voltage-sensitive dyes (VSD)] or ion concentrations (pH-, calcium-, chloride-, or potassium-sensitive dyes) [17,35]. The time-resolved acquisition protocol to record the fast neural dynamics, which requires the biological response to be repeatable for each stimulation trial [17], was investigated for FLOT [1,6,17]. The HDR-FLOT presented in this paper can be incorporated with a time-resolved acquisition protocol to provide a potentially improved mesoscopic imaging method to image neuronal activities for functional brain mapping.…”

“…For example, with more than a 20% depth increase compared to conventional FLOT in a mouse brain in vivo, we may be able to image the entire six layers in the mouse cortex with HDR-FLOT compared to four layers. Hemodynamic response usually occurs in seconds; therefore, the LOT system with ∼40 frames/s imaging speed can be used to study hemodynamic responses [8,17]. Use of exogenous fluorescent dyes and transgenic animals can also aid in studying functional parameters, such as changes in membrane potential [voltage-sensitive dyes (VSD)] or ion concentrations (pH-, calcium-, chloride-, or potassium-sensitive dyes) [17,35].…”

“…Hemodynamic response usually occurs in seconds; therefore, the LOT system with ∼40 frames/s imaging speed can be used to study hemodynamic responses [8,17]. Use of exogenous fluorescent dyes and transgenic animals can also aid in studying functional parameters, such as changes in membrane potential [voltage-sensitive dyes (VSD)] or ion concentrations (pH-, calcium-, chloride-, or potassium-sensitive dyes) [17,35]. The time-resolved acquisition protocol to record the fast neural dynamics, which requires the biological response to be repeatable for each stimulation trial [17], was investigated for FLOT [1,6,17].…”

“…Soon after, LOT was rapidly adapted for fluorescent molecular imaging, which was termed either fluorescence laminar optical tomography (FLOT) [1,[5][6][7] or mesoscopic fluorescence molecular tomography (MFMT) [10,11], and several improvements or alternative system designs were investigated [5,6,[10][11][12][13][14]. FLOT has been reported to quantify depth-resolved distribution of fluorescence-labeled tumors [13,15,16], to image fluorescence-labeled cells in tissue engineering [11,12,16], and to visualize neural activities in mouse brains in vivo [6,17]. In the FLOT system configuration, a charge-coupled device (CCD) or electronmultiplying CCD (EMCCD) can be used as the array detector since either one has a higher sampling density in comparison to a photomultiplier tube (PMT) array or an avalanche photodiode (APD) array [6,16].…”

Depth-resolved imaging of colon tumor using optical coherence tomography and fluorescence laminar optical tomography

“…LOT imaging of skin cancer based on absorption contrast has been demonstrated [46]. Fluorescence-based LOT (FLOT) has the potential to quantify depth-resolved distribution of fluorescence-labeled tumor [47][48][49][50].…”