Stereotactic implantation of deep brain stimulation electrodes: a review of technical systems, methods and emerging tools <s> Optical intracerebral measurements <s> Microcirculatory blood flow can be measured using a laser Doppler flowmetry (LDF) probe. However, the readings are affected by the tissue's optical properties (absorption and scattering coefficients, mu(a) and mu(s)) and probe geometry. In this study the influence of optical properties [mu(a)in(0.053,0.23) mm-1,mu(s)in(14.7,45.7) mm-1] on LDF perfusion and LDF sampling depth was evaluated for different fiber separations. In vitro measurements were made on a sophisticated tissue phantom with known optical properties that mimicked blood flow at different depths. Monte Carlo simulations were carried out to extend the geometry of the tissue phantom. A good correlation between measured and simulated data was found. The simulations showed that, for fixed flow at a discrete depth, the influence of mu(s) or mu(a) on LDF perfusion increased with an increase in flow depth and decreased with an increase in fiber separation. For a homogeneous flow distribution, however, the perfusion varied 40% due to variations in the optical properties, almost independent of the fiber separation (0.23-1.61 mm). Therefore, the effect in real tissue is likely to vary due to the unknown heterogeneous blood flow distribution. Further, the LDF sampling depth increased with a decrease in mu(s) or mu(a) and an increase in fiber separation. For fiber separation of 0.46 mm, the e-1 sampling depth ranged from 0.21 to 0.39 mm. <s> BIB001 </s> Stereotactic implantation of deep brain stimulation electrodes: a review of technical systems, methods and emerging tools <s> Optical intracerebral measurements <s> OBJECT ::: The authors have developed an intracranial near-infrared (NIR) probe that analyzes the scattering of light emitted from its tip to measure the optical properties of cerebral tissue. Despite its success in distinguishing graymatter from white matter in humans during stereotactic surgery, the limits of this instrument's resolution remain unclear. In this study, the authors determined the spatial resolution of this new probe by using a rodent model supplemented with phantom measurements and computer simulation. ::: ::: ::: METHODS ::: A phantom consisting of Intralipid and gelatin was constructed to resemble a layer of white matter overlying a layer of gray matter. Near-infrared measurements were obtained as the probe was inserted through the gray-white matter transition. A computer simulation of NIR measurements through a gray-white matter transition was also performed using Monte Carlo techniques. The NIR probe was then used to study 19 tracks from the cortical surface through the corpus callosum in an in vivo rodent preparation. The animals were killed and histological sections through the tracks were obtained. Data from the phantom models and computer simulations showed that the NIR probe samples a volume of tissue extending 1 to 1.5 mm in front of the probe tip (this distance is termed the "lookthrough" distance). Measurements obtained from an NIR probe passing through a thin layer of white matter consisted of an initial segment of increasing values, a maximum (peak) value, and a trailing segment of decreasing values. The length of the initial segment is the lookthrough distance, the position of the peak indicates the location of the superficial white matter boundary, and the length of the trailing segment is the thickness of the layer. These considerations were confirmed in experiments with rodents. All tracks passed through the corpus callosum, which was demonstrated as a broad peak on each NIR graph. The position of the dorsal boundary of the corpus callosum and its width (based on histological measurements) correlated well with the peak of the NIR curve and its trailing segment, respectively. The initial segments correlated well with estimates of the lookthrough distance. Five of the tracks transected the smaller anterior commissure (diameter 0.2 mm), producing a narrow NIR peak at the correct depth. ::: ::: ::: CONCLUSIONS ::: Data in this study confirm that the NIR probe can reliably detect and measure the thickness of layers of white matter as thin as 0.2 mm. Such resolution should be adequate to detect larger structures of interest encountered during stereotactic surgery in humans. <s> BIB002 </s> Stereotactic implantation of deep brain stimulation electrodes: a review of technical systems, methods and emerging tools <s> Optical intracerebral measurements <s> The aim of the study was to investigate if laser Doppler perfusion monitoring (LDPM) can be used in order to differentiate between gray and white matter and to what extent microvascular perfusion can <s> BIB003 </s> Stereotactic implantation of deep brain stimulation electrodes: a review of technical systems, methods and emerging tools <s> Optical intracerebral measurements <s> Diffuse reflectance spectroscopy as a method for improving intracerebral guidance during functional neurosurgery has been investigated. An optical probe was developed for measurements during stereotactic and functional neurosurgery in man. The aim of the study was to investigate the spectral differences between white and grey matter and between white matter and functional targets. Diffuse reflectance spectroscopy measurements in ten patients were recorded at incremental steps towards and in three different functional targets (STN, GPi and Zi). The recorded spectra along the trajectory were sorted into white or grey matter, based on preoperative MRI images or the recorded spectral shape and intensity. The difference between tissue types was calculated as a quotient. Significant intensity differences between white and grey matter were found to be at least 14% (p < 0.05) and 20% (p < 0.0001) for MRI and spectral-sorted data respectively. The reflectance difference between white matter and the functional targets of GPi was higher than for STN and Zi. The results indicate that diffuse reflectance spectroscopy has a potential to be developed to a suitable complement to other intracerebral guidance methods. <s> BIB004 </s> Stereotactic implantation of deep brain stimulation electrodes: a review of technical systems, methods and emerging tools <s> Optical intracerebral measurements <s> AIM ::: The aim of this study is to assess reflected light intensity combined with impedance as a navigation aid during stereotactic neurosurgery. ::: ::: ::: METHODS ::: During creation of 21 trajectories for stereotactic implantation of deep brain stimulation electrodes in the globus pallidus internus or subthalamus (zona incerta or subthalamic nucleus), impedance at 512 kHz and reflected light intensity at 780 nm were measured continuously and simultaneously with a radio frequency electrode containing optical fibres. The signals were compared with the anatomy, determined from pre- and post-operative MRI and CT. The measurements were performed within minutes, and signal analysis was done post-operatively. ::: ::: ::: RESULTS ::: Reflected light intensity was low from the cortex, lateral ventricle, caudate nucleus and putamen; intermediate from the globus pallidus and thalamus; while it was high from the subcortical white matter, internal capsule and subthalamus. The electrical impedance was less consistent, but generally low in the cortex, intermediate in the subcortical white matter, putamen, globus pallidus and thalamus, and high in the internal capsule and subthalamus. ::: ::: ::: CONCLUSION ::: Reflected light intensity and electrical impedance give complementary information about passed tissue, and the combination seems promising as a navigation aid during stereotactic neurosurgery. <s> BIB005 </s> Stereotactic implantation of deep brain stimulation electrodes: a review of technical systems, methods and emerging tools <s> Optical intracerebral measurements <s> An electrode with adjacent optical fibers for measurements during navigation and radio frequency lesioning in the brain is modeled for Monte Carlo simulations of light transport in brain tissue. Relative reflected light intensity at 780 nm, I780, from this electrode and probes with identical fiber configuration are simulated using the intensity from native white matter as reference. Models are made of homogeneous native and coagulated gray, thalamus, and white matter as well as blood. Dual layer models, including models with a layer of cerebrospinal fluid between the fibers and the brain tissue, are also made. Simulated I780 was 0.16 for gray matter, 0.67 for coagulate gray matter, 0.36 for thalamus, 0.39 for coagulated thalamus, unity for white matter, 0.70 for coagulated white matter, and 0.24 for blood. Thalamic matter is also found to reflect more light than gray matter and less than white matter in clinical studies. In conclusion, the reflected light intensity can be used to differentiate between gray and white matter during navigation. Furthermore, coagulation of light gray tissue, such as the thalamus, might be difficult to detect using I780, but coagulation in darker gray tissue should result in a rapid increase of I780. <s> BIB006
Optical intracerebral measurement is a technique for realtime presentation of grey-white tissue boundaries during stereotactic procedures. A thin probe with dimensions adapted to the stereotactic system and with optical fibres aligned along the interior side of the probe towards the tip is connected to the optical system. The probe is used to create tracts for the DBS electrodes along the precalculated trajectories; at the same time as the reflected light intensity, i.e. tissue type is recorded, processed and displayed in real time. In this manner, reflectance spectroscopy measurements in the near infrared region have been performed by several investigators in order to discriminate between white and grey brain matter during both experimental and clinical stereotactic neurosurgery  BIB002 . Up to date, the spectral measurements during clinical implantations of DBS electrodes have been used by Giller et al.  in more than 200 implantation procedures and also been compared with microelectrode recording. The concept developed by Wårdell and colleagues, which is based on both laser Doppler perfusion monitoring (LDPM) BIB003 and diffuse reflection spectroscopy BIB004 , have been clinically evaluated on more than 50 DBS lead implantations and compared to impedance recordings BIB005 . This group extracts curve data from specific wavelength intervals, e.g. around 573 nm (blood content) and 780 nm (greywhite boundaries), the latter wavelength also applicable in the LDPM system, whereas Giller and co-workers are using the tilt of the curve within an interval in the near infrared region. Despite slightly different signal processing, both research groups present similar optical signatures along trajectories towards the STN and the thalamus. In general, all time-curves start with a low intensity value in cortex and increases to a maximum intensity value when passing the subcortical white matter. The last section differs depending on the target area aimed at. For the GPi a characteristic ''double peak'', representing light intensity changes during insertion through the white lamina surrounding the Putamen, GPe and GPi has been identified BIB005 (Fig. 4) . This is possible due to the fact that white matter is more opaque than grey matter, and thus the white matter lamina in grey matter is easier to detect than grey matter nuclei in white matter. In general, the light interaction with tissue, and thus the measurement depth, is affected by several aspects such as probe design, light source and the tissue's scattering and absorption characteristics. Experimental investigations and Monte Carlo simulations show that the optical sampling depth in brain tissue is less than 1 mm BIB006 BIB001  . Qian et al.  studied the ''look ahead distance'' in brain matter by means of Monte Carlo simulation, and found that in the near infrared region and for small fibre separation, grey matter is expected to have a slightly increased ''look ahead distance'' than white matter. This was confirmed by Johansson et al. BIB006 .