Concentration dependence of gold nanoshells on the enhancement of optical coherence tomography images: a quantitative study.

The effective use of gold nanoshells as a contrast agent for optical coherence tomography (OCT) may be hampered by the delivery of a wrong dose to tissue that results in unwanted signal attenuation. In this study we examine how changes in mu(s) due to concentration variations affect the OCT image and then define a dosing range that would result in appropriate scattering coefficient, mu(s), to maintain an acceptable signal attenuation level. Our results show that an increase in sample mu(s) not only enhances the OCT signal near the surface but also attenuates the signal deeper into the sample. We synthesized gold nanoshells with an 81 nm radius silica core and 23 nm shell thickness and found that a concentration range of 5.6 x 10(9)<c<2.3 x 10(10) particles/ml provided adequate signal enhancement near the surface without severely compromising the imaging depth due to signal attenuation. We also demonstrate the extraction of mu(s) from the OCT signal to estimate the gold nanoshells' concentration in vivo and verified that the estimated concentration of 6.2 x 10(9) particles/ml in a mouse tumor after intravenous delivery lies within this concentration range to effectively enhance the tumor image.