Shadi Masoumi

Annually, around 23 million people worldwide suffer from brain disorders, with 14 million requiring surgery for issues such as brain tumors, traumatic brain injuries, and neurodegenerative diseases such as Parkinson’s disease (PD).¹ Our focus is on deep brain stimulation (DBS), a stereotactic neurosurgical procedure that involves implanting electrodes in specific brain regions to regulate abnormal neural activity.² This technique, introduced by Benabid,³ is primarily used to treat movement disorders such as PD, dystonia, and essential tremor. MRI is an essential part of neurosurgical planning, where a pre-operative MRI (pre-op MRI) of the patient’s head is used to identify critical targets. DBS targets vary according to the disorder being treated: for instance, the subthalamic nucleus (STN) and internal pallidum (GPi) are commonly targeted for PD, the ventral intermediate thalamic nucleus (VIM) for essential tremor, and the caudate nucleus (CD) and the internal capsule (IC) for severe forms of obsessive-compulsive disorder (OCD).^{4 – 7} Once the target of interest is identified in MR images, its stereotactic coordinates are determined relative to anatomical landmarks such as the anterior and posterior commissures and to the stereotaxic frame placed on the patient’s head. After identifying the target, final adjustments and confirmation of the target may be performed in the operating room through microelectrode recording (MER) and macrostimulation when available.⁵

Deep Brain Stimulation (DBS) is an effective neurosurgical intervention for movement disorders. Comparing PS-OCT and MRI in a cadaver macaque head revealed matching neuroanatomical structures, suggesting that intrasurgical optical imaging may improve DBS targeting precision.

Imaging depth-resolved birefringence and optic axis orientation with polarization sensitive optical coherence tomography (PS-OCT) unveils details of tissue structure and organization that can be of high pathophysiologic, mechanistic, and diagnostic value. For catheter-based PS-OCT, the dynamic rotation of the fiber optic probe, in addition to the polarization effects of the system components, complicates the reliable and robust reconstruction of the sample’s optic axis orientation. Addressing this issue, we present a new method for the reconstruction of absolute depth-resolved optic axis orientation in catheter-based PS-OCT by using the intrinsic retardance of the protecting catheter sheath as a stable guide star signal. Throughout the paper, we rigorously inspect the retardance and optic axis orientation of the sheath and validate our method by imaging a birefringent phantom with known optic axis orientation. Reconstructing the optic axis orientation of the phantom, placed at different locations around the catheter, we measured an average absolute deviation (AAD) for the mean optic axis orientation over cross-sectional images of 3.28°, even with significant bending stress on the catheter. This corresponds to an almost three-fold improvement compared to our earlier method (optic axis AAD of 9.41°). We finally highlight the capability of our reconstruction with stereotactic catheter-based PS-OCT of a fresh sheep brain.

Conventional dual-input state PS-OCT incorrectly assumes that the two probing input states provide equally reliable measurements. In this work, we overcome this assumption by adapting a maximum-likelihood framework which combines all input state and spectral bin measurements to find the most likely sample Jones matrix. This processing method (MLDIPS) shows a significantly reduced retardance noise floor as well as improved qualitative characterization of white matter versus grey matter in porcine brain tissue, displaying better contrast to conventional dual-input processing.

Parkinson’s disease (PD) is the second most common neurodegenerative disorder after Alzheimer’s disease.¹ The primary clinical signs of PD are motor symptoms that include resting tremors, muscular rigidity, and slowness of movements.² The initial treatment of PD usually starts by administering the dopamine precursor l-dopa. However, over time, this pharmacological treatment becomes less effective, and most patients will eventually suffer from l-dopa-induced dyskinesia.^3,4 For these patients, deep brain stimulation (DBS) surgery is often the recommended treatment option.⁵

We present improved reconstruction of depth-resolved optic axis orientation in catheter-based polarization-sensitive optical coherence tomography, by using the intrinsic retardance of the catheter-sheath as reference, and image white matter fiber bundles in sheep brain.

Engineers and scientists operating in fields such as neurophotonics, remote sensing, medicine, or even industrial manufacturing are often tasked with building or modifying imaging systems for customized applications. They must often resort to expert advice from optical designers to obtain clear and quantitative answers to their specific optics problems. Optical designers, with their knowledge and specialized software tools such as Zemax, Oslo, or CODE V, can indeed provide these answers. However, third-party expert advice can be prohibitively expensive, difficult to interpret without basic knowledge, and can lead to sub-optimal solutions if the requirements are miscommunicated to the external consultant. For the general engineer, scientist, and especially for trainees, learning these professional software tools is often not an option because of the abundance of features targeted to advanced optical designers that end up confusing trainees and non-experts alike. Yet, there should be a solution, between simplified analytical solutions and expert tools that can help non-experts tackle moderately complicated problems by themselves to obtain quantitative answers. It is therefore the purpose of the present tutorial to provide both the knowledge and the tools for this audience.

Recently, compression optical clearing (OC) was applied to detect dermal carotenoid using reflection spectroscopy. To enhance the precision and accuracy of reflection spectroscopy to better detect the spectral absorption of beta-carotene inside biological phantom, here, we simultaneously use compression and immersion OC using dimethyl sulfoxide. In addition, we analytically extract the absorption coefficient of beta-carotene using diffuse reflectance spectroscopy (as an analytical OC). Our results show that the presented analytical OC can be applied alone as a noninvasive method to measure cutaneous chromophores at deep tissues. Finally, we also improve the ability of the analytical clearing method mediated with experimental OC. Our result demonstrates that the combination of analytical and experimental clearing methods enhance the ability of diffuse reflection spectroscopy for extracting the absorption coefficient of beta-carotene as one of the chromospheres inside biological phantom.