Magnetic and Plasmonic Contrast Agents in Optical Coherence Tomography

Abstract

Optical coherence tomography (OCT) has gained widespread application for many biomedical applications, yet the traditional array of contrast agents used in incoherent imaging modalities do not provide contrast in OCT. Owing to the high biocompatibility of iron oxides and noble metals, magnetic and plasmonic nanoparticles, respectively, have been developed as OCT contrast agents to enable a range of biological and pre-clinical studies. Here we provide a review of these developments within the past decade, including an overview of the physical contrast mechanisms and classes of OCT system hardware addons needed for magnetic and plasmonic nanoparticle contrast. A comparison of the wide variety of nanoparticle systems is also presented, where the figures of merit depend strongly upon the choice of biological application.

Keywords: Contrast agents; magnetomotive; optical coherence tomography; plasmonic nanoparticles; superparamagnetic iron oxides.

Fig. 1

Concept diagram of sinusoidal MMOCT. Tissue containing MNPs (black dots) is placed in the fringe field (blue lines) of a solenoid, while the OCT imaging beam scans through its bore. A square-root sinusoidal voltage applied to the solenoid produces a pure sinusoidal force on MNPs directed along the field gradient. OCT detects the resulting axially-directed motion (z) of light-scattering structures within the tissue that are mechanically coupled to the MNP motion.

Fig. 2

General trends in the optical properties of gold nanospheres and prolate ellipsoids as a function of size and aspect ratio, respectively. The various contrast methods (SOCT, PTOCT, and DOCT) each have different figures of merit that dictate different particle sizes and shapes.

Fig. 3

he schematic and output of a dual-coil magnetomotive OCT setup employing magnetically labeled microspheres (MSs) in a flow phantom containing phosphate-buffered saline (PBS). (Reprinted with permission from [33]).

Fig. 4

Sample B-mode images of ex vivo porcine arteries following exposure in a flow chamber to SPIO-RL platelets. (a) OCT image of control artery (b) OCT image of damaged artery (c) magnetomotive OCT image of control artery (d) magnetomotive OCT image of damaged artery. Note that each artery is longitudinally cut with the luminal wall facing upward. Scale bar: 0.5 mm (Reprinted with permission from [30]).

Fig. 5

Spectroscopic OCT images of TiO₂ gelatin phantom, with right side doped with 35 nm nanocages. (a) Standard OCT showing decreasing signal in location with absorbing nanocages. (b) Spectroscopic OCT showing changes in scattered spectra. (c) Hue-Saturation-Value (HSV) image of the same cross section with hue representing centroid wavelength, and saturation/value representing OCT intensity. (d) Plot of intensity versus depth versus wavelength. (Reprinted with permission from [47]).

Fig. 6

Matrigel containing gold nanorods injected into an in vivo mouse ear showing blood flow via speckle variance (red) and nanorods via PTOCT (green). (a) Blood flow without nanorods and with modulating laser off. (b) Blood flow without nanorods and with modulating laser on. (c) Blood flow with nanorods and the modulating laser off. (d) Blood flow with nanorods and the modulating laser on. (Reprinted with permission from [61]).

Fig. 7

MAPS signature in a cell culture containing mammary epithelial cells where DOCT was used to enhance contrast of tissue containing gold nanorods against cells void of nanorods. (Reprinted with permission from [42]).