Feature of the Week 04/02/2016: Contrast-Enhanced Optical Coherence Tomography with Picomolar Sensitivity for Functional in vivo Imaging (with Audio)
Optical Coherence Tomography (OCT) enables real-time imaging of living tissues with cellular resolution over large 3D fields of view. However, functional and molecular capabilities for OCT remain elusive due to the difficulties of distinguishing exogenous contrast agents from intrinsic tissue scattering and absorption. Previous reports have detailed the use of magnetic probes, absorbent dyes, and gold nanoparticles to produce OCT contrast enhancement through various mechanisms. In the current study, optimized large gold nanorods (LGNRs) and a customized aberration-free spectral detection algorithm were developed to demonstrate an improved platform for molecular imaging with OCT. LGNRs, which produce 30-fold greater backscattering than conventional GNRs, were used as novel spectral domain (SD) OCT agents with near infrared plasmon peaks. LGNRs were biofunctionalized using sequential modification with poly(sodium 4-styrenesulfonate) (PSS, MW 70 kDa) and poly(ethylene glycol) (PEG, MW 5 kDa) reagents. Surface chemistry methods were also developed to produce LGNRs coated with any antibody of choice for molecular specificity. While typical OCT images convey structural details of the sample, the contrast enhancement approach developed herein provides detailed angiography and spectral information from the sample. By implementing spectral identification methods, LGNRs were detected with 50 pM sensitivity in whole blood and < 250 pM sensitivity (< 40 LGNRs per imaging voxel) in the circulation of live mice. Furthermore, LGNRs exhibited > 100-fold increased spectral signal than GNRs previously used as OCT agents. A SD-OCT system operating at 30 kHz (A-scan rate) with a broadband superluminescent diode source (SLD, lamda = 800-1000 nm) was used to acquire contrast-enhanced images in several live mouse models. First, LGNRs were administered intravenously to 6-8 week old nude (nu-/nu-) mice bearing U87MG (glioblastoma) tumors. The spectral detection method achieved a 100% imaging depth improvement for the noninvasive detection of angiogenic microvasculature with 8 um axial and lateral resolution. Separately, LGNRs prepared with discrete near-infrared resonances (815nm and 925nm) were subcutaneously injected into ear pinnae of healthy nude mice to image fluid drainage in lymphatic capillaries. The algorithm distinguished LGNRs of each resonance, which enabled the study of spatial patterns of drainage, including unidirectional flow. Lymphatic valves in closed and open conformations were also identified with 8 um axial and 4 um lateral resolution. The reported LGNR molecular targeting methods, contrast sensitivity, spectral multiplexing, and high resolution comprise a promising approach for future OCT molecular imaging studies, which we call MOZART.
 A.G. Podoleanu, Brit. J. Rad. 78(935) (2005)
 R. John et al, PNAS 107(18) (2010)
 W. Kim, B.E. Applegate, Opt. Lett. 40(7) (2015)
 J.M. Tucker-Schwartz et al, Biomed. Opt. Exp. 6(6) (2015)
 E.D. SoRelle et al, Langmuir 31(45) (2015)
For more information see recent Article. Courtesy Adam de la Zerda from Stanford.