Since Jobsis described the in vivo application of near-infrared spectroscopy (NIRS) to detect changes in cerebral hemoglobin oxygenation, several types of NIRS instruments have been developed.10 NIRS is widely used for the noninvasive monitoring of changes in chromophores such as oxy- and deoxy hemoglobin, lipid, and water from changes in the reflected light intensity. In the NIR region, the absorption spectra of these primary chromophores are different (Fig. 2). With determination of tissue absorption coefficients at multiple wavelengths (600–1,000 nm) and using appropriate modeling, information about absolute chromophore ­concentrations can be obtained (diffuse optical spectroscopy or DOS).

In this wavelength band, NIR light can penetrate the adult skull and propagate within the brain.1 Photons that propagate in the brain are strongly scattered elastically as they traverse several millimeters of tissue. Elastic scattering is characterized by the fact that the photon energy does not change. Since photons undergo multiple scattering events before they reach the detector, their trajectories are not straight lines; rather they resemble random walks. Multiple scattering of light which occurs as a consequence of spatial variation in refractive index influenced by cellular and extracellular optical densities, complicates quantitative measurements of light absorption in the NIR range. This complexity is a consequence of the uncertainty in optical path traveled by any given photon. Hence, a simple Beer’s law technique for deducing chromophore concentration cannot be simply applied. However, by using time-resolved or frequency-domain instruments, which effectively can be used, with appropriate selection of a model of light propagation, to deduce the average path length of detected photons, the effects of scattering can be separated from absorption. Thus absolute concentrations of tissue chromophores can subsequently be estimated in a given region of interest by application of the Beer-Lambert Law11 to the absorption coefficient. These techniques can provide quantitative measurements of optical and functional contrast between healthy and diseased tissue.20 NIRS offers an inexpensive and safe bedside monitoring tool.

Photon migration allows gathering information from relatively deep structures in highly scattering media. The light emitted from a source(s) diffuses in a random fashion through the scattering medium. However, because of statistical properties, the photons picked up by the detectors are more likely to have followed certain paths than others through the medium. If enough photons are used, the entire process of migration of photons from the sources to the detectors can be described accurately using mathematical formulations related to the diffusion process. This makes it possible to go beyond images of the first interaction of photons with the medium and produce images of structures that are up to a few centimeters deep into the medium.12

Intrinsic Signal Optical Imaging (ISOI) is widely used for studying the hemodynamic response of the brain to stimulation. It is a functional neuroimaging technique that can map a large region of the cortex with high spatiotemporal resolution. The method uses a simple configuration containing a light source such as a light-emitting diode (LED) and a CCD camera to image the optical reflectance at this wavelength. The LED light in ISOI systems uses wavelengths (for instance 630 nm) that are maximally absorbed by the deoxygenated state of hemoglobin (Fig. 5). Hence, a decrease in reflected light indicates an increased absorption by deoxyhemoglobin. Therefore, ISOI provides relative changes in the local concentration of deoxyhemoglobin as a surrogate marker for neuronal activity.

Recently, a triphasic component has been described with a second increase in local deoxyhemoglobin concentration following the BOLD signal.21 The triphasic component constitutes an initial dip, overshoot and undershoot in light reflectance. The initial dip is from increased local deoxyhemoglobin secondary to neural oxygen metabolism from activation. In contrast, the overshoot is dominated by increase in oxyhemoglobin from neurovascular coupling. This signal is analogous to the BOLD signal observed and measured with fMRI. The undershoot signal represents the effort of the brain to come back to its normal state. IOSI studies in epilepsy have shown the presence of an epileptic dip analogous to the initial dip seen with functional activation.22 While ISOI’s main role has been in functional brain imaging in animals, it has potential in intraoperative functional mapping during neurosurgical procedures.23

Optical Coherence Tomography (OCT) is a novel imaging system analogous to ultrasound except that imaging is performed with light instead of acoustic waves. OCT measures light reflected from turbid structures. OCT is based on principles of low-coherence interferometry which allows detection of back scattered near-infrared photons from selected depths and rejection of photons being scattered from other layers. It is structurally based on the Michelson’s interferometer1 (Fig. 6). As for the reflection method, it is based on the first interaction (back-scattering) of photons in tissue. Unlike the reflection measures, however, it directly visualizes the depth at which this first interaction occurs.

In combination with scanning methods, OCT provides three-dimensional information about the scattering properties of living tissue. The spatial resolution that can be reached is near-histologic (micron scale), which makes OCT suitable for in vivo optical biopsy, allowing for the visualization of tissue cross-sectional microstructure even without a contrast agent or dye. Because signals from different depths are separated by the measurement approach, OCT can detect deeper events more easily than a reflection approach. The dependence on the first interaction of photons with the medium imposes maximum penetration limits on the technology that are incompatible with using this approach to study the human brain in a noninvasive manner.

OCT has the potential to provide new imaging, visualization, and quantification capabilities for a wide range of investigations in neuroscience and neuroimaging. Research has demonstrated its potential in the areas of developmental neurobiology, retinal imaging, cellular imaging, brain tumor imaging, and image-guided surgery for the repair of peripheral nerves.20 By incorporating OCT technology into an endoscope Bohringer et al. were able to visualize tissue lining the walls of the third ventricle such as peri-aqueductal tissue, floor of the third ventricle, as well as the rostral wall of the basilar artery in a cadaveric study.24 The same group also analyzed human brain tumor biopsies as well as an orthotopic mouse glioma model using time domain and spectral domain OCT and found good correlation of OCT findings of normal brain, tumor infiltrative brain and tumor with histology.25 OCT has been used to visualize retinal hamartomas in patients with tuberous sclerosis,26 to measure the retinal nerve fiber layer thickness in patients with mild cognitive impairment and Alzheimer’s disease and multiple sclerosis.27,28 Cadaveric studies have shown the potential of OCT for providing navigational guidance of deep brain stimulation probes.29 Recently, the authors of this chapter have used a catheter based OCT system for in vivo cerebrovascular imaging in patients [unpublished].

Lasers have been utilized in neurosurgical operations for frameless stereotactic registration. Marmulla et al. described the use of frameless stereotactic laser registration of the auricles for lateral skull base surgery.91 Schicho et al. compared laser surface scanning to fiducial marker-based registration in frameless stereotaxy with comparable results in four cadaveric heads.92 Sinha et al. used laser ranges scanning for intraoperative cortical surface characterization in eight patients with brain tumors and mesial temporal epilepsy.93

In 1986 Kelly et al. described the results of CO₂ laser integration into a computer assisted volumetric stereotactic system for brain tumor resection.94 It involved reconstruction of tumor volume from CT and MRI imaging data followed by a computer-monitored stereotactically directed laser to vaporize intracranial tumor. The position of the laser with respect to reformatted planar slices through the tumor was monitored on a computer terminal. In a separate report the same year they reported on 83 computer assisted stereotactic laser procedures in 78 tumor patients.95 Although the authors mention that the stereotactic laser technique was particularly useful in aggressive resection of deep seated tumors within eloquent brain regions, no direct comparison was performed to standard microsurgical techniques. In 1989 Kelly et al. reported on excision of 12 colloid cysts of the third ventricle using stereotactic laser application with excellent results.96

Neuroablative lasers have been utilized in functional neurosurgery particularly in dorsal root entry zone (DREZ) lesions and commissural myelotomies.97–99 DREZ lesions are primarily used for intractable pain following spinal cord injury, brachial plexus avulsions, post herpetic neuralgia, and phantom limb pain.100

Lasers have been used adjunctively in the management of epileptic syndromes. Kelly et al. reported in 18 patients with medically intractable epilepsy that underwent stereotactic amygdalohippocampectomy using a CO₂ laser.101 Later they reported on stereotactic laser treatment of epileptogenic lesions associated with tuberous sclerosis.102 While effective, it is unclear whether laser assisted techniques have advantages over conventional microsurgery in the treatment of epileptic lesions.

Endoscopic applications of lasers in neurosurgery have been mainly in the management of hydrocephalus. Bucholz and Pittman used a Nd:YAG laser to endoscopically coagulate the choroid plexus in an infant with hydrocephalus.103 Vandertop et al. performed reported endoscopic third ventriculostomy with a Nd. YAG laser and a new-generation laser diode in 33 patients and reported no morbidity.104 Powers used an Argon laser to perform endoscopic fenestration in two infants with compartmentalization of the lateral ventricles secondary to ventriculitis.105 Zammorano et al. integrated image-guided stereotaxis with rigid-flexible endoscopy and the Nd-YAG laser (endoscopic laser stereotaxis) and found this system useful in cystic and intraventricular lesions.106 Otsuki et al. used a stereotactic guiding tubes and endoscopes to deliver laser irradiation to deep seated brain tumors.107

Due to the limitations of microsurgical nerve repair such as scar formation, fascicle mismatching, neuroma formation, inflammatory response to suture material, researchers have been interested in the use of lasers for nerve repair.108 This application is termed “laser neurorrhaphy.”

Laser neurorrhaphy was first reported in 1984 by Almquist et al. with the use of argon laser to repair peripheral nerves in rats and monkeys.109 Fischer et al. reported using CO₂ lasers for sciatic nerve repair in rats.110 Seifert and Stoke demonstrated feasibility of microsurgical CO₂ laser assisted repair of the oculomotor nerve in cats.111 Bailes et al. compared CO₂ laser with 9-0 nylon microsurgical neurorrhaphy in a primate model of sural to peroneal nerve graft and found comparable nerve conduction velocities (NCV) and histologic findings at 5, 8,10 and 12 months.112 Huang et al. compared microsuture technique to CO₂ laser repair of sciatic nerves in rats and found the EMG and NCV studies to be comparable113 although Maragh et al. compared laser neurorrhaphy to standard microsurgical nerve repair in rat sciatic nerves and found that nerve conduction velocities were better in the latter group.114 Campion et al. reported improved neuromuscular function of argon laser neurorrhaphy compared to microsurgical epineural surgical repair in rabbits.115 Korff et al. found similar amounts of regeneration in severed rat sciatic nerves repaired with suture or laser, at 2 months.116 Menovsky and Beek compared CO₂ laser assisted nerve repair with fibrin glue or absorbable sutures in transected rat sciatic nerves and found no significant differences in motor function at 16 weeks and no histological differences.117