3D Doppler OCT Provides Feedback on Two-Photon Activated Photosensitizers
H. A. Collins, M. Khurana, E. H. Moriyama, A. Mariampillai, E. Dahlstedt, M. Balaz, M. K. Kuimova, M. Drobizhev, V. X. D. Yang, D. Phillips, A. Rebane, B. C. Wilson, and H. L. Anderson
Photodynamic therapy (PDT) is a therapeutic technique used to treat cancer as well as other diseases characterized by abnormal tissue growth, such as age-related skin spots. To eliminate cancerous cells, photosensitive drugs are employed. Usually benign, these drugs can be activated through exposure to specific wavelengths of light.
Three-dimensional rendered SS-OCT images of blood flow in mice whose vasculature has been exposed using a dorsal skinfold window chamber. Photosensitizers designed for two photon excitation were administered and the region denoted by the small white box in each frame was illuminated with 920 nm light (<3 mW). For the targeted blood vessel, blood flow is from left to right. Three-dimensional renderings of Doppler OCT images, in red, pretreatment (a) and post-treatment (b) demonstrate blood flow occlusion caused by two-photon excitation of the new photosensitizer. Renderings are overlaid on steromicroscope images taken pretreatment.
Traditionally, single-photon excitation has been used in PDT techniques, but recently, the advantages of two-photon excitation (TPE) -- namely its quadratic dependence on light intensity and ability to allow deeper tissue penetration -- have made it a desirable choice for targeting specific treatment areas. Since two-photon excitation only occurs at the location where the two photons illuminate the same focal volume, this technique enables high spatial targeting of the treatment area.
Recently, a group of researchers used a Thorlabs 1325 nm Swept Source OCT (SS-OCT) Imaging System to evaluate results from in vivo PDT treatment of anesthetized nude mice bearing dorsal skin windows. Using a photosensitizer that they designed for two-photon excitation, Collins and collaborators were able to induce blood vessel closure. After injecting the mice with 10 mg/kg of the photosensitizer, the volume of interest was irradiated with 920 nm light (300 fs, 90 MHz, 39 mW) for 15 minutes.
3D Doppler OCT images of the targeted vessel were acquired before and after two-photon excitation (See Fig. 1). To construct these 3D images of the blood vessels, one thousand transverse scans were used to obtain an 83 μm x 83 μm image. By looking at the Doppler frequency shift induced by moving red blood cells, researchers were able to observe flow velocities as low as 100 μm/s.
The results presented here show that SS-OCT imaging is capable of non-invasive 3D visualization of vasculature before and after two-photon PDT treatment. Such images were previously unavailable using confocal or stereomicroscopy techniques. In addition, the results demonstrate that two-photon PDT treatment is a viable choice for targeted occlusion of blood vessels.
1) H. A. Collins, M. Khurana, E. H. Moriyama, A. Mariampillai, E. Dahlstedt, M. Balaz, M. K. Kuimova, M. Drobizhev, V. X. D. Yang, D. Phillips, A. Rebane, B. C. Wilson, and H. L. Anderson, Nature Photonics. 2 (2008) 420-424.
All procedures were carried out via the approved protocol of the University Health Network, Toronto.