As a result, the basic measurement reported by optical coherence tomography systems is reflectivity versus depth1. While optical coherence tomography is a light-based imaging modality, the design of contrast agents compatible with it is complicated by the requirement that the minimal coherence of the light source be maintained during and after its interaction with the contrast agent. The degree to which light is absorbed relative to reflected is related to the overall size of the nanorod.
As previously mentioned, optical coherence tomography systems natively only provide reflectivity versus depth information, which can be a limitation in heterogeneous tissue. Another OCT adjunct is spectroscopic OCT, which includes split-spectrum and spectral-fractionation optical coherence tomography (SF-OCT). These technologies examine the entire spectrum of light emitted and detected by an optical coherence tomography system to measure changes in tissue reflectivity over narrower bands of this spectrum30.
However, with homegrown optical coherence tomography systems approaching submicron axial resolutions, this trade-off could be advantageous in many circumstances.
Gold Nanorod Detoxification, Functionalization, and OCT Imaging
This SPR peak is used for all GNR in the studies reported in this chapter because we theorized that this peak would maximally interact with the 800-900nm spectrum of the OCT system we use for imaging. A microscope coverslip coupled to the eye with a 2.5% hydroxypropylmethylcellulose solution was used to aid in the surgery. Wild-type (C57BL/6) mice aged 8–10 weeks were treated in the LCNV model as previously described.
However, previous studies published from similar injections have modeled the formation of an amorphous opacity in the vitreous. The presence of discrete scatterers (arrows) in the tissue is evident near the inner retina extending into the vitreous. It was also unclear where this GNR could localize in the retina for our imaging purposes.
To address these concerns, we chose to image our GNR in the laser-induced choroidal neovascularization, or LCNV, model41. Mice were treated in the LCNV model and on day 5 after lasering, they were injected in the tail vein with PBS, untargeted or targeted GNR. We gave GNR 8 h to accumulate in the lesions and then isolated the choroids.
This is a representative image of a mouse retina treated in the LCNV model taken five days after lasering. There is still some anti-rat IgG staining in the lesion of the mouse injected with untargeted GNR (7e), and there is virtually none in the lesion of the PBS-injected mouse (7d). All three lesions show DAPI staining, but the strongest anti-rat IgG counterstaining is present in the lesion of the mouse injected with targeted GNR.
This is a representative image of a day 5 LCNV lesion in the mouse retina, imaged 8 hours after intravenous PBS injection. This is a representative image of an LCNV lesion in the mouse retina obtained by OCT five days after lasering and 8 hours after the intravenous injection of targeted GNR.
In Vivo Imaging of GNR Using Photothermal Optical Coherence Tomography
In this chapter, we demonstrate the ability of PT-OCT technology to achieve in vivo retinal imaging of endogenous (RPE melanin) and exogenous (GNR) contrast agents in the LCNV model. Furthermore, we demonstrate the utility of PT-OCT detection of molecularly targeted GNR to generate high-resolution contrast-enhanced images of physiological changes in the retina. We then use PT-OCT to image treated mice in the LCNV model after intravenously injecting them with ICAM2-targeted GNRs to assess our ability to detect these GNRs in vivo above the background melanin signal.
By comparing the lesion-associated photothermal signal of these cohorts, we can assess our ability to detect targeted exogenous contrast agents in the mouse retina using PT-OCT. The other question we investigate in this chapter is whether PT-OCT imaging of GNR allows the detection of clinically relevant physiological changes in the mouse retina in vivo. In assessing our ability to detect anti-VEGF-induced changes in LCNV lesions in vivo, we assess the utility of PT-OCT imaging of GNR to detect clinically relevant physiological changes in the retina, which is one of our most important goals with this research. .
There is a significant increase in PT-OCT signal in the wild-type mice, with almost no signal present in the albino mice. Following our successful detection of endogenous photothermal contrast in the mouse retina, we assess our ability to detect and evaluate exogenous contrast agents in vivo. Note the increased concentration of photothermal signal associated with passive accumulation of GNR in the lesions, and the greater increase associated with the injection of targeted GNR.
The photothermal signal density in the untargeted GNR cohort (nm/pixel, n=14) is 1.49 times greater than in the PBS cohort, a statistically significant difference. PT-OCT imaging of targeted GNR accumulation after anti-VEGF injection in an LCNV model. To achieve this goal using PT-OCT and GNR technology, we treated wild-type mice in an LCNV model.
Finally, we assessed our ability to use exogenous contrast agents to detect physiological changes in the retina. There are a number of challenges associated with PT-OCT imaging of the retina in vivo, including the presence of endogenous contrast from melanin in the RPE layer.
In Vitro Imaging of Intracellular GNR Using Spectral Fractionation Optical Coherence Tomography
First, we tested whether or not we could detect GNR in water or tissue phantoms with SF-OCT. Then we tested our ability to use SF-OCT to image intracellular localized GNR, also in tissue phantoms. Initial SF-OCT imaging of GNR occurred using standard CTAB-coated “bare” GNR suspended in tissue phantoms.
These findings demonstrate that exposed GNR can be detected using SF-OCT in water and in tissue phantoms. The OCT image is in grayscale with the SF-OCT SLoW ratio colored in blue or red for +1 and -1 dB ratio measurements, respectively. GNRs are clearly visible in red in water and intralipid due to their enhancement of the long part of the SLoW ratio.
SF-OCT images of (a) 1% gelatin, (b) aRPE-19 cells fixed and suspended in 1% gelatin, and (c) aRPE-19 cells previously incubated with Tat-GNR fixed and suspended in gelatin 1%. This short chapter focuses on some of the exciting results obtained from SF-OCT imaging of GNR. The SPR of the GNR was chosen to be offset from the center of the OCT systems in order to maximally influence the SLoW ratio.
We demonstrate that GNR can be detected using SF-OCT in a tissue phantom, which is a valuable proof of principle. Considering the negative results from unlabeled cells, this result thus suggests that our GNR construct was able to enter cells at concentrations sufficient for detection and that we could use GNRs with SF-OCT to generate intracellular contrast in tissue phantoms. This has been demonstrated for the first time and indicates promising future implementations of GNR and SF-OCT technology.
Summary and Conclusions
So we turned to intravenous injections of GNR to try to study it in the retina. However, the clinical utility of improved melanin quantification in the retina remains to be proven. This very strong endogenous contrast agent represented a significant source of background noise for subsequent studies imaging injected GNR in the LCNV model.
The photothermal signal from melanin is currently an obstacle to the clinical translation of GNR imaging using PT-OCT. Despite this difficulty, we were able to achieve positive and novel results from our PT-OCT imaging studies of intravenously delivered GNR. We found that both non-targeted GNR and ICAM2-targeted GNR can be detected in LCNV lesions using PT-OCT in vivo.
A relevant piece of evidence in Chapter 2 is the immunohistochemical study of flat-mounted retinas treated in the LCNV model and injected with targeted vs non-targeted nanorods. Although there are issues with the quality of this image, these different results may suggest that the process of flat mounting itself may remove some of the unbound GNR from the lesions, allowing the apparent targeting effect of IHC to be consistent with the lack of one of PT-OCT imaging. In addition to using PT-OCT as an adjunct to imaging GNR, we also performed some experiments with the Huang lab using SF-OCT to detect GNR, although these experiments were performed in tissue phantoms30.
In addition, PT-OCT takes much longer to complete a scan of a given area than SF-OCT, and this can be as long as 40 minutes for scanning a complete LCNV lesion. Also, at least in our studies, SF-OCT of GNR has not yet been demonstrated in vivo in the eye, while PT-OCT has. Finally, PT-OCT can image a potentially useful endogenous contrast source without the need for additional exogenous contrast agents, improving the translatability of this technology.
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