SignificanceRaman spectroscopy is a valuable technique for tissue identification, but its conventional implementation is hindered by low efficiency due to scattering. Addressing this limitation, we are further developing the wavelength-swept Raman spectroscopy approach.AimWe aim to enhance Raman signal detection by employing a laser capable of sweeping over a wide wavelength range to sequentially excite tissue with different wavelengths, paired with a photodetector featuring a fixed narrow-bandpass filter for collecting the Raman signal at a specific wavelength.ApproachWe experimentally validate our technique using a fiber-based swept-source Raman spectroscopy setup. In addition, simulations are conducted to assess the efficacy of our approach in comparison with conventional spectrometer-based Raman spectroscopy.ResultsOur simulations reveal that the wavelength-swept configuration leads to a significantly stronger signal compared with conventional spectrometer-based Raman spectroscopy. Experimentally, our setup demonstrates an improvement of at least 200× in photon detection compared with the spectrometer-based setup. Furthermore, data acquired from different regions of a fixed monkey brain using our technique achieves 99% accuracy in classification via k-nearest neighbor analysis.ConclusionsOur study showcases the potential of wavelength-swept Raman spectroscopy for tissue identification, particularly in highly scattering media, such as the brain. The developed technique offers enhanced signal detection capabilities, paving the way for future in vivo applications in tissue characterization.
SignificanceWe assess the feasibility of using diffuse reflectance spectroscopy (DRS) and coherent anti-Stokes Raman scattering spectroscopy (CARS) as optical tools for human brain tissue identification during deep brain stimulation (DBS) lead insertion, thereby providing a promising avenue for additional real-time neurosurgical guidance.AimWe developed a system that can acquire CARS and DRS spectra during the DBS surgery procedure to identify the tissue composition along the lead trajectory.ApproachDRS and CARS spectra were acquired using a custom-built optical probe integrated in a commercial DBS lead. The lead was inserted to target three specific regions in each of the brain hemispheres of a human cadaver. Spectra were acquired during the lead insertion at constant position increments. Spectra were analyzed to classify each spectrum as being from white matter (WM) or gray matter (GM). The results were compared with tissue classification performed on histological brain sections.ResultsDRS and CARS spectra obtained using the optical probe can identify WM and GM during DBS lead insertion. The tissue composition along the trajectory toward a specific target is unique and can be differentiated by the optical probe. Moreover, the results obtained with principal component analysis suggest that DRS might be able to detect the presence of blood due to the strong optical absorption of hemoglobin.ConclusionsIt is possible to use optical measurements from the DBS lead during surgery to identify WM and GM and possibly the presence of blood in human brain tissue. The proposed optical tool could inform the surgeon during the lead placement if the lead has reached the target as planned. Our tool could eventually replace microelectrode recordings, which would streamline the process and reduce surgery time. Further developments are required to fully integrate these tools into standard clinical procedures.
Deep brain stimulation (DBS) surgery is performed on patients suffering Parkinson’s disease for whom medication is no longer effective in relieving their motor symptoms. In this surgery, a stimulating electrode is implanted in a specific structure deep within the brain, delivering electrical impulses and thus reducing the motor symptoms. The success of the surgery is highly dependent on placing the electrode accurately in the targeted structure, typically the subthalamic nucleus (STN). We developed a DBS electrode that includes optical fibers to perform coherent anti-Stokes Raman scattering (CARS) spectroscopy and diffuse reflectance spectroscopy (DRS) during the electrode insertion in the brain. We were able to identify white and grey matter using principal component analysis (PCA), showing that spectroscopic measurements could be suitable for neuronavigation.
Reconstructed depth-resolved optic axis orientation obtained by catheter based PSOCT in a tissue volume informs on the orientation of the white matter fiber bundles in the brain, owing to the birefringence of myelinated axons. The physical organization of white matter also leads to anisotropic diffusion of water molecules, which is the basis of dMRI for non-invasive imaging of the three-dimensional orientation of white matter fiber bundles. Having access to fiber orientation in both imaging modalities, we are trying to map the depth-resolved birefringence and optic axis orientation to the larger scale dMRI as well as an atlas of brain anatomy.
SignificanceTypical light sheet microscopes suffer from artifacts related to the geometry of the light sheet. One main inconvenience is the non-uniform thickness of the light sheet obtained with a Gaussian laser beam.AimWe developed a two-photon light sheet microscope that takes advantage of a thin and long Bessel-Gauss beam illumination to increase the sheet extent without compromising the resolution.ApproachWe use an axicon lens placed directly at the output of an amplified femtosecond laser to produce a long Bessel-Gauss beam on the sample. We studied the dopaminergic system and its projections in a whole cleared mouse brain.ResultsOur light sheet microscope allows an isotropic resolution of 2.4 μm in all three axes of the scanned volume while keeping a millimetric-sized field of view, and a fast acquisition rate of up to 34 mm2 / s. With slight modifications to the optical setup, the sheet extent can be increased to 6 mm.ConclusionThe proposed system’s sheet extent and resolution surpass currently available systems, enabling the fast imaging of large specimens.
The purpose is to determine whether diffuse reflectance spectroscopy (DRS) can provide optical guidance during deep brain stimulation (DBS) surgery. Experiments on monkey ex vivo brains have been performed to ensure DRS methods could differentiate white and gray matter. In this study, we use principal component analysis (PCA) to determine the composition of tissue in front of the stimulation electrode. Furthermore, our work tackles the mechanical consequences of implementing an optical probe in a DBS electrode. This multidisciplinary project shows that DRS can be used as a non-invasive, cost-effective and real-time tissue characterization.
We introduce the GRIN-axicon, a new low-cost optical component that is easy to manufacture and could replace the axicon in various setups such as a two-photon microscope. In neuroscience, the imaging of in vivo samples requires high temporal resolution in order to capture the interactions between neurons located at different depths in the tissue. To achieve this, the use of an axicon lens increases the depth of field of the microscope and reduces the number of scans to be performed. However, the axicon is difficult to manufacture and generally has defects on the tip of the cone, thus degrading the quality of the resultant Bessel-Gauss beam.
We propose a method to compute the axial distribution of non-diffracting beams. The approach stands on simple ray tracing and energy conservation principles. The proposed method is applied for four different Bessel beam generators, such as the refractive axicon, to compute the output axial intensity profile for a given input beam. We show great agreement between the results and the CODE V diffractional simulations. The method can also be used to calculate analytically the incident illumination pattern needed for a target output profile or to design an optical element surface to reach the same goal.
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