13 Jun 2008
The first system to combine Raman spectroscopy and optical coherence tomography could improve the diagnosis of skin cancers.
An imaging system that combines Raman spectroscopy (RS) and optical coherence tomography (OCT) has been unveiled by a group of researchers directed by Anita Mahadevan-Jansen (Vanderbilt University, US) and Ton van Leeuwen (University of Twente, NL). The set-up allows each technique to compensate for the limitations of the other and could aid the optical detection and diagnosis of epithelial cancers, such as skin cancer (Optics Letters 33 1135).
"This is the first time that stokes RS and OCT have been combined into the same instrument," researcher Chetan Patil from Vanderbilt University told optics.org. "We have integrated the sampling optics of the two techniques to create an instrument whose performance in each modality is independent and not compromized."
The strength of RS lies in its ability to detect biochemical features of disease with unparalleled specificity. Unfortunately, due to the weak nature of Raman scattering, acquisition times are on the order of seconds and visual guidance is the primary cue for selecting measurement sites.
OCT on the other hand can detect features of disease with high spatial sensitivity and screen large areas of tissue at high speed. However, the primary source of contrast in OCT is tissue backscatter, which does not provide much specificity. The clinical application of OCT could therefore benefit from a technique that provides biochemical characterization of the structural irregularities it detects.
"A combined RS-OCT device is a multimodal tool for thorough tissue analysis," explained Patil. "It is able to image and screen large areas of tissue for the microstructural features of disease with OCT, and then selectively acquire spectra from these areas with high biochemical specificity using RS."
The OCT system uses a semiconductor optical amplifier source with a centre wavelength of 1310 nm, while the Raman source is a 785 nm diode laser. A custom dichroic mirror reflects the 785 nm and broadband 1310 nm light, while transmitting 800–950 nm signals with a relatively flat passband.
"The transition between RS and OCT is instantaneous," commented Patil. "The Raman and OCT lasers both illuminate the sample concurrently. When operated in OCT mode, the scanning mirrors in the sample arm are active and generate 2D OCT images. When a feature of interest is targeted by OCT, the scanning mirrors are stopped, and Raman acquisition starts. After Raman acquisition is complete, the mirrors are restarted, and live OCT imaging starts again."
Having successfully performed proof-of-principle tests on ex vivo samples, Patil and his colleagues are eager to perform clinical trials. "The system performance was comparable to that of similarly designed independent devices," he said. "The next step is to miniaturize the sample arm optics for use in a clinical setting."
The first target for the team is skin cancer. "Ultimately, we believe a combined approach can be beneficial in any tissue where structural irregularities make it difficult to characterize features using strictly their morphological appearance," concluded Patil.
Jacqueline Hewett is editor of Optics & Laser Europe magazine.