Magnetic contrast agents for optical coherence tomography

The magneto-mechanical effect is exploited as a means of producing background-free contrast in optical coherence tomography (OCT). Contrast agents consisting of iron-oxide particles and protein microspheres encapsulating colloidal iron-oxide have a sufficiently high magnetic susceptibility to be detected by modulation of a magnetic field gradient using a small solenoid coil. The externally-applied magnetic field mechanically rotates or translates these highly scattering contrast agents within the sample at the modulation frequency, which is subsequently detected as amplitude modulation of the OCT signal. Pairs of sequential axial scans (A-lines) are acquired with the magnetic field on and off, allowing one to build up a pair of images corresponding to the "on" and "off" states of the magnetic field. These image pairs are differenced to look for magnetic-specific effects, allowing one to distinguish the magnetic contrast agents from non-magnetic structures within the sample with a signal-to-background ratio of ~23dB. This technique has the potential to be very powerful when coupled with targeting for in vivo molecular imaging. To evaluate this potential we demonstrate in vitro imaging of magnetically-labeled macrophage cells embedded in a 3D tissue phantom, in vitro tissue doped with contrast agents, and in vivo imaging of Xenopus laevis (African frog) tadpoles.