Deep Raman Imaging

Raman spectroscopy is a molecularly specific technique which allows identification of analytes due to their unique vibrational fingerprint. However, normal Raman is limited in its ability to detect analytes at depth (Raman microscopy is generally limited to depths in the order of microns), therefore the use of spatially offset Raman spectroscopy (SORS) is an emerging technique capable of providing spectral information from the analyte, even when obscuring barriers such as skin, tissue and bone are present.

SORS makes use of a spatial offset between the point of laser excitation and the point of collection which makes it possible to measure Raman scattered photons generated by analytes at depth unlike conventional backscattering Raman techniques, where excitation and collection typically take place at the same point. However, Raman scattering is intrinsically weak, particularly at the higher (NIR) excitation wavelengths required to penetrate through biological samples, since Raman scattering intensity has a 4th power dependence on excitation frequency.

Therefore, it is proposed to use surface enhanced Raman scattering (SERS), to enhance the Raman signal this leads to the new technique of surface enhanced spatially offset Raman (SESORS). SESORS has the potential to allow optical imaging at depth whilst giving a target specific response. There are currently only a few groups around the world working on this approach and it has yet to be optimised in terms of pushing the depth penetration, analysing the data to create images, using it in a targeted approach and using it for clinical samples.

This project proposes to use specifically designed functionalised metal nanoparticles as optical imaging probes to evaluate a new SORS approach known as Circular Offset Raman Scattering (CORS) which has been developed by Wasatch Photonics in collaboration with DSTL. The new approach uses a different propriety optical configuration to obtain Raman signals at depth.

In this project we will evaluate the CORS approach to fully understand and push its depth penetration abilities in different media. It also results in less signal interference from the signal from the surface of the sample versus the signal at depth. This approach will be fully characterised using functionalised nanoparticles which will be designed to display a unique Raman response at NIR excitation wavelengths that coincide with the laser excitation wavelength of the CORS system (785 nm).

The advantage of using Raman rather than fluorescence for optical imaging is the molecular specificity of the optical response and the ability to detect multiple SERS responses from multiple targets simultaneously. Our recent work has shown the ability to detect nanoparticles at depths of up to 48 mm using a handheld instrument.

We will investigate model systems e.g. tissue phantoms to optimise the nanoparticles and response to give a strong SERS response to allow them to be imaged at greater depths and also allow us to develop data analysis methodologies to create images in 2 and 3 dimensions. We will also expand on our recent work on correlating signal with nanoparticle depth in different types of tissue, muscular, lipid rich etc, effects the signal and depth prediction.

Objectives

(1) To explore the synthesis of metal nanotags with red shifted absorbances. This will involve synthesising nanoparticles with different sizes, shapes and shell structures.

(2) Evaluation of the nanotags in terms of their ability to give CORS signals at depth and how they respond to different barriers i.e. different thicknesses of clear and coloured plastics before moving to biological models.

(3) Evaluation CORS at depth in terms of ability to predict depth and location of nanotags in biological models using 3D imaging and investigation into how this is influences by different components of the model e.g. tissue type, fat layers.