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Biomedical Optics & Medical Imaging

Contrast enhancement in optical coherence tomography

Photothermal heating helps image contrast agents at scarce concentrations over a scattering tissue background.
15 January 2013, SPIE Newsroom. DOI: 10.1117/2.1201212.004626

Molecular imaging is a powerful tool for studying disease progression and potential therapies in animals. Optical coherence tomography (OCT) is an important biomedical imaging modality, filling the niche between ultrasound and microscopy. However, OCT suffers from an inherent lack of molecular contrast (the ability to distinguish a molecule of interest from others). This is because the scattering cross-section, the source of contrast in this technique, does not vary widely between molecular species. Researchers have worked around this problem by using OCT with either endogenous or exogenous sources of contrast in a number of schemes.1, 2 But these methods require computer modeling to decouple scattering from absorption, and they depend on the elastic properties of the tissue.

An alternative technique relies on specialized optical-absorption contrast agents that enhance the visibility of the molecule of interest by emitting detectable heat when they absorb infrared light. We and others demonstrated that this photothermal detection of contrast agents can be achieved by incorporating an amplitude-modulated laser into the sample arm of a standard OCT system.3–7 This approach has the advantage of allowing for noninvasive in vivo molecular imaging in three dimensions.

In photothermal imaging, strong optical absorption by a target of interest—such as a nanoparticle—results in a change in temperature around the particle (i.e., the photothermal effect). Local photothermal heating causes index-of-refraction changes and thermoelastic expansion in the microenvironment surrounding the absorber, thus altering the local optical path length. Changes in the optical path length due to photothermal heating can be imaged directly via the phase information in an OCT image because of the linear relationship between the two parameters. This method, called photothermal OCT (PTOCT), is advantageous because it allows for sensitive detection of absorbing contrast agents over a scattering tissue background without the use of modeling and with minimal effects from tissue properties.

In our work, we demonstrated PTOCT as an imaging tool that is especially sensitive to polyethylene-glycol-coated gold nanorod contrast agents.7 We used an OCT system with an 860nm center wavelength, 51nm full-width-half-maximum super-luminescent diode for the imaging beam, with a 2048 pixel CCD acquiring lines at 10kHz. We fiber-coupled the photothermal laser into the sample arm to spatially co-locate the photothermal and OCT imaging beams (see Figure 1). PTOCT data was temporally oversampled and continuously captured while amplitude-modulating the photothermal laser at frequency fp with a mechanical chopper. To isolate the PTOCT signal at each point in depth, we performed a Fourier transform in the temporal dimension on the OCT phase signal, and defined the PTOCT signal as the amplitude of the photothermal oscillations at frequency fp.

Figure 1. Photothermal optical coherence tomography (PTOCT) instrumentation schematic. A fiber-coupled OCT system sends light from the broadband super-luminescent diode (SLD) source to the sample and reference arms (split by the 50/50 fiber splitter), and the returning light is detected by the spectrometer and CCD at 10kHz. The photothermal laser is amplitude-modulated (to create photothermal oscillations in the sample) and fiber coupled into the sample arm. Adapted with permission from the Optical Society of America.7

We quantitatively compared PTOCT data to the predictions of a numerical model (a closed-form solution of the bio-heat equation). The measured PTOCT signal from a solution of gold nanorods agreed with the temporal heating dynamics predicted by the model, as illustrated for one on-off cycle of the photothermal laser (see Figure 2). Both the model and experimental data demonstrated a linear increase in PTOCT signal strength with increased photothermal laser power, indicating that imaging sensitivity can be improved with increased laser power on the sample. However, a linear increase in the photothermal beam amplitude modulation frequency caused a logarithmic falloff in PTOCT signal strength in both the model and experimental data. The imaging speed of PTOCT is defined by the photothermal laser amplitude modulation frequency as well as the number of temporal oversamples. Therefore, imaging speed with PTOCT must be carefully selected to maximize the PTOCT signal and minimize motion artifacts in the sample. Finally, altering the signal intensity in the OCT image did not affect the amplitude of the PTOCT signal, although weaker reflections increased the noise due to decreased phase stability.

Figure 2. Experimental and modeled behavior of the PTOCT signal over one on-off cycle of the photothermal laser. For a photothermal laser amplitude modulation frequency of 200Hz, a gold nanorod solution caused ∼0.25 radian change in the PTOCT phase signal (blue solid line) during heating (first 2.5ms, laser on), followed by a gradual nonlinear cooling (last 2.5ms, laser off). The model predicts ∼2 degrees Kelvin peak increase in temperature (black dashed line), with temporal dynamics very similar to the phase changes imaged with PTOCT. Adapted with permission from the Optical Society of America.7

We performed PTOCT imaging in agarose capillary tube phantoms to demonstrate the contrast enhancement capability and spatial specificity of PTOCT. An amount of 400pM (picomolar particle concentration) of gold nanorods in agarose displayed a 15 fold increase in PTOCT signal when compared to a negative agarose control. In contrast, the mean OCT signal was not appreciably different between the gold nanorod sample and negative control. Further, we performed PTOCT imaging in vivo in the ear of a live mouse. We injected gold nanorods suspended in Matrigel (an injectable protein solution that solidifies at body temperature) into one ear, with a Matrigel control injection in the other ear. We extracted morphology, Doppler flow and PTOCT image information from the same data set, with a significant increase in PTOCT signal in the ear containing gold nanorods (see Figure 3).

Figure 3. PTOCT imaging of gold nanorods in vivo. We injected mice ears with either a negative control Matrigel suspension (top), or 400pM concentration of gold nanorods suspended in Matrigel (bottom). Morphology (gray), vessel flow (Doppler OCT, blue and red) and PTOCT (yellow) signals from the same dataset are overlaid. A significant increase in the PTOCT signal in the ear due to the presence of gold nanorods is evident in the bottom figure. Adapted with permission from the Optical Society of America.

We provided the first PTOCT images of a contrast agent in vivo. This represents an important step toward highly sensitive molecular imaging in OCT. PTOCT has potential as a powerful tool for studying in vivo disease models, especially when coupled with other functional capabilities of OCT, including tissue morphology, microvascular angiography, and flow dynamics. We are currently advancing this technology to image the kinetics of tumor-drug delivery at high resolution in three dimensions.

The authors would like to greatly acknowledge Chetan Patil for characterization assistance of PTOCT, Craig Duvall and Travis Meyer for providing gold nanorods, and Dana Brantley-Sanders for performing in vivo Matrigel injections. This study was supported in part by grant R00CA142888 from the National Institutes of Health and the National Cancer Institute.

Jason Tucker-Schwartz, Melissa Skala
Vanderbilt University
Nashville, TN

Jason Tucker-Schwartz is a PhD student in the Department of Biomedical Engineering. He received his MS from the University of Virginia in 2010.

Melissa Skala is an assistant professor of biomedical engineering. She received her PhD from Duke University in 2007 and her MS from the University of Wisconsin, Madison in 2004.

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