Maureen Johns

Computational and in vivo investigation of optical reflectance from human brain to assist neurosurgery.

Parkinsons disease (PD) is a chronic, progressive disease involving the globus pallidus (GP), which is a gray matter mass, surrounded by white matter deep within the brain. During a neurosurgery procedure, a thin probe is inserted into the GP to create a lesion that often relieves the cardinal symptoms of PD. The goal of this study is to develop an optical method to accurately locate the GP border. In theory, Monte Carlo simulations were performed to predict the optical reflectance from brain tissue. In experiment, a portable, real-time display spectrometer with a fiber optic reflectance probe was developed and used during human surgery. Optical reflectance values were recorded at 1 mm intervals to obtain a spatial profile of the tissue as the probe passed through regions of gray and white matter. The simulation and in vivo studies of the reflectance from the brain are in good agreement with one another. The clinical data show that the reflectance from gray matter is approximately 50% or less than that from white matter between 650 and 800 nm. A slope algorithm is developed to distinguish gray and white matter in vivo. This study provides previously unknown optical reflectance of the human brain.

Determination of Hemoglobin Oxygen Saturation from Turbid Media Using Reflectance Spectroscopy with Small Source-Detector Separations

In this study, we present an empirically modified diffusion model determining hemoglobin oxygen saturation from turbid media using steady-state, broadband (500–600 nm) reflectance with source-detector separations of a few hundred micrometers. Development of this model was conducted using Monte Carlo simulations, a gold standard modeling technique for predicting the behavior of light propagation through turbid media. Hemoglobin oxygen saturation levels of 0 and 100% at different blood concentrations were studied. Nonlinear curve fitting was used to extract the hemoglobin oxygen saturation values from the reflectance spectra, producing errors of 5% between the simulated curves and the model for both saturation cases. Further validation was performed using liquid-tissue phantoms containing intralipid and blood. Curve fitting between the in vitro data and the model produced errors of less than 2%. This validated model was then used to extract saturation values from in vivo reflectance spectra of the human index finger and human brain tissues. This empirically modified diffusion model provides the possibility of extracting local hemoglobin oxygen saturation from blood-perfused turbid media using reflectance data measured with a small source-detector separation probe.

Limited possibility for quantifying mean particle size by logarithmic light-scattering spectroscopy

Recent studies have shown that the slope of logarithmic scattering spectroscopy of a turbid medium is related to the sizes of the scattering particles within the turbid medium. Mie theory can be used to generate a logarithmic plot of the reduced-scattering coefficient versus wavelength. According to Nilsson et al. [Appl. Opt. 37, 1256 (1998)], the slope value of a linear fit of the logarithmic scattering spectroscopy between 600 and 1050 nm can be used for direct determination of particle size. We performed similar calculations using the Rayleigh-Gans approximation and obtained an analogous overall shape with additional sinusoidal features. Our calculations indicate a possible relationship between the slope and the particle size when the size is used to calculate the slope, namely, in the forward calculation. However, because of the sinusoidal pattern, the inverse calculation to obtain the particle size from the slope may be applied only for particles with a radius of <0.13 microm in combination with 650-1050-nm light. Caution should be exercised when inverse calculation is performed to determine the scattering particle sizes in the range of radii >0.13 microm, with the slope of logarithmic scattering spectroscopy within 650-1050 nm.

Calculation of hemoglobin saturation from in vivo human brain tissues using a modified diffusion theory model

During deep brain stimulation, a neurosurgical procedure to relieve tremors, a thin electrode is inserted into deep brain regions to provide stimulation. Accurate electrode placement is crucial to provide tremor suppression without damaging adjacent optical and motor regions. A portable, real-time display fiber optic reflectance probe is used to obtain reflected signals from living, human brain tissues. The optical results are compared to pre-operative MRI scans to confirm anatomical structures and verify electrode placement. In addition to reflectance, tissue oxygen saturation may assist brain tissue identification.

Guiding Pallidotomy Surgery By The Optical Reflectance Of Human Brain Tissue

Parkinsons disease (PD) is a chronic, progressive disease of the basal ganglia (gray matter). The globus pallidus is one such mass that lies deep within the brain and is surrounded by white matter. A pallidotomy surgery involves inserting a thin probe into the globus pallidus and creating a small lesion with radio frequency current. These lesions help relieve the tremors associated with PD. Precise mapping of the globus pallidus border is essential. A lesion miscalculation could cause permanent damage, such as blindness or paralysis. Currently, computed tomography (CT) and magnetic resonance imaging (MRI) are used to identify aberrant, locations prior to surgery. However, these tools are not sufficient for safe localization due to anatomic variations and their unavailability during the operation. It would be desirable to have portable, real-time display equipment available in the operating room for quick, simple and accurate lesion localization. This purpose of this research project is to develop a fiber optic, reflectance probe that can be inserted into the brain during surgery to identlfy white and gray matter, and therefore, the boundary between the two. Two kinds of optical measurements have been performed previously: (1) in vivo reflectance measurements on the human bladder and gastrointestinal tract to identlfy cancerous tissue and (2) in vitro transmittance measurements on human brain to study white and gray matter post mortem. Few studies on optical reflectance of living, human brain tissue have been reported. In our studies, we used a custom designed near infrared fiber optic probe and spectrometer to study, clinically, the reflected signals from the brain during surgery. The 1.5mm diameter probe contains one light-delivering fiber to illuminate the brain tissue and six lightcollecting fibers to gather the light reflected from the brain tissue. Our measurement procedure involved placing the sterile probe against the surface of the brain during surgery and recording the intensity of reflected light in a spectral range from 500 nm to 1000 nm. The probe was advanced into the brain using l-mm increments, and the spectnun was recorded at each position until the probe reached 15 mm below the surface of the brain. This technique allowed the optical reflectance signals from both white and gray matter to be obtained in sequential fashion. Our results show that while the in vivo reflectance from both white and gray matter have a broad spectral peak from 600 nm to 800 nm, white matter reflectance gives a more prominent peak at 650 nm than gray matter. The signal intensity within this wavelength range can be 2-3 times larger for white matter than the signal from gray matter. In addition, the reflectance spectra display a change when the probe is located at the boundary between gray and white matters. This is consistent with the results obtained ex vivo on a piece of brain tissue removed for therapeutic reasons unrelated to this study. The experimental results of reflectance from human scalp and skull appear rather scattered. Furthermore, the comparison between our measurements and Monte Carlo simulations reveals that other chromophores besides hemoglobin should be considered. Our study provides previously unreported optical reflectance properties from structures deep within the brain, and, in turn, they can be used to guide pallidotomy surgery by differentiating the boundary of gray and white matter. This research also provides the possibility to investigate absorption and scattering properties of the living brain, which offer a means to investigate blood volume and oxygenation of the brain.

Investigation of optical reflectance from human brain in vivo for guiding brain surger

Parkinsons disease (PD) is a chronic, progressive disease of the basal ganglia (gray matter). The globus pallidus is one such mass that lies deep within the brain and is surrounded by white matter. A pallidotomy surgery involves inserting a thin probe into the globus pallidus and creating a small lesion with radio frequency current. These lesions help relieve the tremors associated with PD. Precise mapping of the globus pallidus border is essential. A lesion miscalculation could cause permanent damage, such as blindness or paralysis. Currently, computed tomography (CT) and magnetic resonance imaging (MRI) are used to identify aberrant locations prior to surgery. However, these tools are not sufficient for safe localization due to anatomic variations, and they are unavailable during the operation. It would be desirable to have portable, real-time display equipment available in the operating room for quick, simple and accurate lesion localization.