Feature Of The Week 6/20/10: Fourier Domain Pump-Probe Optical Coherence Tomography Imaging
Feature Of The Week 6/20/10: Researchers from the Laboratory for Optical and Molecular Imaging at Texas A&M University have recently published on a fusing Optical Coherence Tomography (OCT) imaging with pump-probe spectroscopy. Shown here is a summary of some of their recent work.
Contrast in Optical Coherence Tomography (OCT) is primarily derived from local variations in the scattering coefficient, which does not vary widely among different molecular species. As a consequence, the ability to resolve specific molecular species with OCT is very limited. However, the extraction of molecular information is highly desirable because it could provide valuable insight into the biochemical makeup and physiology of the tissue. Interferometric detection requires the light wave carrying the molecular signal to be coherent with the reference arm of the interferometer, precluding the adaptation of incoherent physical processes like fluorescence or Raman scattering for molecular imaging with OCT.
In the approach shown here, molecular contrast OCT is achieved fusing OCT with pump-probe spectroscopy [1, 2] to yield Pump-Probe OCT (PPOCT). In pump-probe spectroscopy, a pulsed pump laser source interacts with the sample and is followed a short time later by a probe pulse. Coherent physical processes such as transient absorption and stimulated emission lead to a pump-dependent change in the measured probe intensity. When fused with OCT, the sample arm beam becomes the probe and any pump-induced variations in the probe intensity are reflected in the OCT A-line intensity.
Shown is a two color Fourier domain pump-probe OCT system targeted to image melanin . The base FD-OCT system utilized a tunable, femtosecond Ti:Sapphire laser operating at 830 nm and a custom spectrometer capable of line rates up to 28 kHz. Two main components were added to the base system in order to achieve the reported PPOCT system: a 415 nm pump beam and an optical delay line in the sample arm (probe) to vary the pump-probe time delay. The pump beam was amplitude modulated in order to frequency-encode the pump-probe signal. The PPOCT signal could then be extracted from an OCT M-scan using a novel algorithm.
The imaging depth and speed of the FD-PPOCT system were characterized by imaging a black, human hair embedded in raw chicken breast. Targeting the melanin in the hair, PPOCT images of the embedded hair were acquired as deep as 785 um, suggesting a maximum imaging depth of > 785 um in tissue. The imaging speed was investigated by changing the number of OCT lines (per M-scan) used for each PPOCT line. The results showed that line rates > 1 kHz were attainable while retaining an acceptable signal-to-noise ratio.
A porcine eye was imaged ex vivo to provide an initial demonstration of the PPOCT system in a biological sample. The iris contains eumelanin, one of the most common chromophores present in the human body. The iris is also one of the primary sites for the development of ocular melanoma. The resulting PPOCT image showed high melanin contrast in the iris, with minimal contrast in the lens and connective tissue. A relative melanin concentration map was derived from the PPOCT and OCT images by taking advantage of the fact that the OCT image is a direct measure of the tissue reflectivity.
A novel two color Fourier domain pump-probe OCT system has been designed and built. The system was characterized with a contrived tissue sample and used to image a porcine eye ex vivo. Molecular contrast was evident in the iris, showcasing the ability of PPOCT to isolate specific molecules in a biological sample. Improvements to the current design are being investigated, including a faster spectrometer and longer pump wavelength for possible in vivo ocular imaging of melanoma.
1. B. E. Applegate, C. Yang, and J. A. Izatt, "Theoretical comparison of the sensitivity of molecular contrast optical coherence tomography techniques", Optics Express 13, 8146-8163 (2005).
2. B. E. Applegate, and J. A. Izatt, "Molecular imaging of endogenous and exogenous molecular chromophores with ground state recovery pump-probe optical coherence tomography", Optics Express 14, 9142-9155 (2006).
3. D. Jacob, R. L. Shelton, and B. E. Applegate, "Fourier domain pump-probe optical coherence tomography imaging of Melanin", Optics Express 18, 12399-12410 (2010).