Maryse Lapierre-Landry

Photothermal optical lock-in optical coherence tomography for in vivo imaging.

Photothermal OCT (PTOCT) provides high sensitivity to molecular targets in tissue, and occupies a spatial imaging regime that is attractive for small animal imaging. However, current implementations of PTOCT require extensive temporal sampling, resulting in slow frame rates and a large data burden that limit its in vivo utility. To address these limitations, we have implemented optical lock-in techniques for photothermal optical lock-in OCT (poli-OCT), and demonstrated the in vivo imaging capabilities of this approach. The poli-OCT signal was assessed in tissue-mimicking phantoms containing indocyanine green (ICG), an FDA approved small molecule that has not been previously imaged in vivo with PTOCT. Then, the effects of in vivo blood flow and motion artifact were assessed and attenuated, and in vivo poli-OCT was demonstrated with both ICG and gold nanorods as contrast agents. Experiments revealed that poli-OCT signals agreed with optical lock-in theory and the bio-heat equation, and the system exhibited shot noise limited performance. In phantoms containing biologically relevant concentrations of ICG (1 µg/ml), the poli-OCT signal was significantly greater than control phantoms (p<0.05), demonstrating sensitivity to small molecules. Finally, in vivo poli-OCT of ICG identified the lymphatic vessels in a mouse ear, and also identified low concentrations (200 pM) of gold nanorods in subcutaneous injections at frame rates ten times faster than previously reported. This work illustrates that future in vivo molecular imaging studies could benefit from the improved acquisition and analysis times enabled by poli-OCT.

Depth-resolved analytical model and correction algorithm for photothermal optical coherence tomography.

Photothermal OCT (PT-OCT) is an emerging molecular imaging technique that occupies a spatial imaging regime between microscopy and whole body imaging. PT-OCT would benefit from a theoretical model to optimize imaging parameters and test image processing algorithms. We propose the first analytical PT-OCT model to replicate an experimental A-scan in homogeneous and layered samples. We also propose the PT-CLEAN algorithm to reduce phase-accumulation and shadowing, two artifacts found in PT-OCT images, and demonstrate it on phantoms and in vivo mouse tumors.

In vivo photothermal optical coherence tomography of endogenous and exogenous contrast agents in the eye

Optical coherence tomography (OCT) has become a standard-of-care in retinal imaging. OCT allows non-invasive imaging of the tissue structure but lacks specificity to contrast agents that could be used for in vivo molecular imaging. Photothermal OCT (PT-OCT) is a functional OCT-based technique that has been developed to detect absorbers in a sample. We demonstrate in vivo PT-OCT in the eye for the first time on both endogenous (melanin) and exogenous (gold nanorods) absorbers. Pigmented mice and albino mice (n = 6 eyes) were used to isolate the photothermal signal from the melanin in the retina. Pigmented mice with laser-induced choroidal neovascularization lesions (n = 7 eyes) were also imaged after a systemic injection of gold nanorods to observe their passive accumulation in the retina. This experiment demonstrates the feasibility of PT-OCT to image the distribution of both endogenous and exogenous absorbers in the mouse retina.

Imaging Melanin Distribution in the Zebrafish Retina Using Photothermal Optical Coherence Tomography

Purpose To demonstrate and validate that photothermal optical coherence tomography (PT-OCT) can image melanin in the retinal pigment epithelium (RPE) and can observe light-driven melanosome translocation in the zebrafish retina. Methods A commercial spectral domain OCT system was modified to perform both OCT and PT-OCT. Four adult tyrosinase-mosaic zebrafish with varying levels of melanin expression across their retinas were imaged, and the PT-OCT signal for pigmented and nonpigmented regions were compared. Wild-type dark-adapted (n = 11 fish) and light-adapted (n = 10 fish) zebrafish were also imaged with OCT and PT-OCT. Longitudinal reflectivity and absorption profiles were generated from B-scans to compare the melanin distribution between the two groups. Results A significant increase in PT-OCT signal (P < 0.0001, Students t-test) was observed in pigmented regions of interest (ROI) compared to nonpigmented ROIs in the tyrosinase-mosaic zebrafish, which confirms the PT-OCT signal is specific to melanin in the eye. A significant increase in PT-OCT signal intensity (P < 0.0001, Students t-test) was also detected in the light-adapted wild-type zebrafish group compared to the dark-adapted group. Additionally, light-adapted zebrafish display more distinct melanin banding patterns than do dark-adapted zebrafish in PT-OCT B-scans. Conclusions PT-OCT can detect different levels of melanin absorption and characterize pigment distribution in the zebrafish retina, including intracellular changes due to light-driven melanosome translocation within the RPE. Translational Relevance PT-OCT could quantify changes in pigmentation that occur in retinal diseases. The functional information provided by PT-OCT may also enable a better understanding of the anatomical features within conventional OCT images.

Photothermal optical coherence tomography of indocyanine green in the eye (Conference Presentation)

Optical coherence tomography (OCT) is a standard for retinal imaging and has been integrated to surgical microscopes to evaluate tissue-instrument interactions during macular surgery. One common procedure during such surgery, membrane peeling, is done under a white light microscope. Indocyanine green (ICG) can be used to specifically dye the inner limiting membrane (ILM) and facilitates this surgery. However, there is no equivalent contrast mechanism to specifically target the ILM on OCT images. We propose to use photothermal OCT (PT-OCT) to detect ICG in the OCT field-of-view, which would increase contrast between the ILM and other structures of the retina. As preliminary data for this project, we have collected PT-OCT images of different ICG phantoms over a wide range of laser powers and ICG concentrations, including concentrations lower than the clinical standard. We have also detected a PT-OCT signal from ICG on a mouse tail with low photothermal laser powers (0.56 mW) to evaluate the feasibility of this technique for in vivo ocular imaging. Finally, we have collected PT-OCT images of a fixed monkey retina after the ILM was dyed with ICG, and obtained a PT-OCT signal from the ICG and the melanin present in the retinal pigment epithelium and the choroid. Those preliminary results indicate that ICG can be detected with PT-OCT at low concentrations and low laser powers. PT-OCT has never been demonstrated in the human eye and has only been recently demonstrated in the mouse eye. This experiment establishes feasibility for PT-OCT in clinical applications.

In vivo photothermal optical coherence tomography of gold nanorods in the mouse eye (Conference Presentation)

Optical coherence tomography (OCT) has become standard in retinal imaging at the pre-clinical and clinical level by allowing non-invasive, three-dimensional imaging of the tissue structure. However, OCT lacks specificity to contrast agents that could be used for in vivo molecular imaging. We have performed in vivo photothermal optical coherence tomography (PT-OCT) of targeted gold nanorods in the mouse retina after the mice were injected systemically with the contrast agent. To our knowledge, we are the first to perform PT-OCT in the eye and image targeted gold nanorods with this technology. As a model of age-related wet macular degeneration, lesions were induced by laser photocoagulation in each mouse retina (n=12 eyes). Untargeted and targeted (anti-mouse CD102 antibody, labeling neovasculature) gold nanorods (peak absorption λ=750nm) were injected intravenously by tail-vein injection five days after lesion induction, and imaged the same day with PT-OCT. Our instrument is a spectral domain OCT system (λ=860nm) with a Titanium:Sapphire laser (λ=750nm) added to the beam path using a 50:50 coupler to heat the gold nanorods. We acquired PT-OCT volumes of one lesion per mouse eye. There was a significant increase in photothermal intensity per unit area of the lesion in the targeted gold nanorods group versus the saline control group and the untargeted gold nanorods group. This experiment demonstrates the feasibility of PT-OCT to image the distribution of molecular contrast agents in the mouse retina, including in highly scattering lesions. In the future we will use this method to identify new biomarkers linked with retinal disease.

In vivo photothermal optical coherence tomography in the mouse eye(Conference Presentation)

OCT has become a standard in retina imaging at the pre-clinical and clinical level by allowing non-invasive, three-dimensional imaging of the tissue structure. However, OCT lacks specificity to contrast agents that could be used for in vivo molecular imaging. We have performed in vivo photothermal optical coherence tomography (PTOCT) of gold nanorods in the mouse retina after the mice were injected intravenously with the contrast agent. To our knowledge, we are the first team to perform PTOCT in the eye. Four lesions were induced by laser photocoagulation in each mouse retina (n=6 mice) and gold nanorods (untargeted and targeted with anti-mouse CD102 antibody, which labels neovasculature, peak absorption λ=750nm) were injected intravenously by tail-vein injection five days later in four mice (two mice are controls). The mice were imaged with PTOCT the same day. Our instrument is a spectral domain OCT system (λ=860nm) with a Titanium:Sapphire laser (λ=750nm) added to the beam path using a 50:50 splitter to target the gold nanorods. We acquired PTOCT B-scans over one lesion per mouse eye. There was a significant increase in photothermal intensity at the center of the lesion in the gold nanorod group versus the control group. This experiment demonstrates the feasibility of PTOCT to image the distribution of contrast agents in the mouse retina. In the future we will use this method to optimize drug delivery to the retina in pre-clinical models.

An analytical model of photothermal optical coherence tomography to predict signal intensity with dept

We present the first analytical model of photothermal optical coherence tomography to replicate an experimental A scan. We also present the PT-CLEAN algorithm to remove two imaging artefacts: phase accumulation and shadowing.