Cone–shell Raman spectroscopy (CSRS) for depth‐sensitive measurements in layered tissue

Abstract

We report the development of a depth-sensitive Raman spectroscopy system using the configuration of cone-shell excitation and cone detection. The system uses a 785 nm diode laser and three identical axicons for Raman excitation of the target sample in the form of a hollow conic section. The Raman scattered light from the sample, passed through the same (but solid) conic section, is collected for detection. Apart from its ability of probing larger depths (~ few mm), an important attraction of the system is that the probing depths can be varied by simply varying the separation between axicons in the excitation arm. Furthermore, no adjustment is required in the sample arm, which is a significant advantage for noncontact, depth-sensitive measurement. Evaluation of the performance of the developed setup on nonbiological phantom and biological tissue sample demonstrated its ability to recover Raman spectra of layers located at depths of ~2-3 mm beneath the surface.

Inverse spatially‐offset Raman spectroscopy using optical fibers: An axicon lens‐free approach

“…The tibia had a highly scattering bone tissue beneath a 0.8‐mm‐thick muscle tissue. While muscle is believed to have a plenty of protein and lipid molecules, phosphate (PO 4 3− ) is known to be the main constituent of bone . As expected when the angle of incidence is changed from 0° to 6° the intensities of Raman peaks at ~1658 cm −1 (amide‐I, C=O stretching), 1445 cm −1 (C‐H deformation) and at 1250 and 1315 cm −1 (amide‐III bands, C‐N stretching and N‐H in plane bending) are found to decrease and that for ~958 cm −1 (due to PO 4 3− ) is seen to substantially increase.…”

“…The above results clearly demonstrate the viability of the proposed scheme for implementing an inverse SORS configuration. The present scheme executes SORS measurements of layered turbid media by using a multi‐mode optical fiber and performs the same function that usually is done by an axicon lens . However, the use of a fiber over the axicon lens has several advantages.…”

“…It is important to mention here that the much higher spatial offsets may not always be practically useful as the SNR of the Raman signal significantly deteriorates with increasing spatial offsets between illumination and collection. For example, in an axicon lens based configuration, the offsets >8 mm have rarely been used . Another point important to mention here is that though the proposed scheme is not completely free of moving parts and requires changing the angle between the incident laser beam and the optical fiber for the generation of illumination rings of varying radii, it does not require any adjustment in the sample arm for reaching various depths inside a sample.…”

“…In recent years, although an array of techniques based on the SDS principle such as defocused SORS , surface enhanced SORS , defocused μ‐SORS , full μ‐SORS , off‐confocal Raman spectroscopy , frequency‐offset Raman spectroscopy and more recently cone‐shell Raman spectroscopy has been proposed for depth‐sensitive Raman measurements in layered turbid media, inverse SORS appears to be a more convenient way of implementing the SDS principle, particularly in probing ~1 to 5 mm depths beneath the tissue surface, which is why the technique has made a remarkable impact in several biomedical applications . However, an important issue related to the implementation of the technique is that its classical configuration which uses an axicon lens puts limit on making a compact I‐SORS probe for carrying out depth‐sensitive Raman measurements in situ from a layered turbid sample.…”

Imaging behind opaque obstacle: a potential method for guided in vitro needle placement

“…Axicon lenses in recent years, specifically in the area of microscopy have been principally utilized to image deeper into inhomogeneous scattering media [9,10] as well as to attain high resolution imaging [11,12]. In a notable study, using axicon lens phenomenon of light beam reconstruction after propagating through a plane of beads or a suspension of small beads was demonstrated [13].…”