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| Year : 1999 | Volume
: 47
| Issue : 2 | Page : 83-4 |
Near infrared spectroscopy : an emerging non-invasive optical imaging technique.
Tandon PN
How to cite this article:
Tandon P N. Near infrared spectroscopy : an emerging non-invasive optical imaging technique. Neurol India 1999;47:83 |
As early as l949, it was reported that the activity of the nerve cells was associated with changes in their optical properties.[1] When photons impinge on biological materials, their transmission depends on a combination of reflectance, scattering and absorption effects. Absorption occurs at specific wave lengths, determined by the molecular properties of the materials in the light path. The relatively good transparency of biological materials in the near-infrared (NIR) region of the spectrum, permits sufficient photon transmission through organs in situ for monitoring cellular events. Thus, in the 700-1300 nm range of NIR, a significant amount of radiation can be effectively transmitted through biological material over long distances.[2] It has been known for many years that some intrinsic changes in the optical properties of the tissue are dependent on electrical or metabolic activity.[3],[4] Changes in optical properties of brain cells have been reported in cell cultures, brain slices, as well as in intact cortical tissue.[5] Grinvald et al[6] used optical signals to map functional architecture of cortex after exposure of cortical tissue in rats, cats and monkeys. They concluded that optical imaging of cortical activity offered several advantages over conventional electorphysiological and anatomical techniques. It can map relatively large area, obtain successive maps to different stimuli in the same cortical area and follow variations overtime.
Based on assessment of absorption and scattering, three types of activity-related signals have been recorded non-invasively : (i) changes in haemoglobin oxygenation, (ii) changes in cytochrome-c-oxidase (co), and (iii) optical signals presumably related to changes in light scattering reflecting either membrane potential (fast signals) or cell swelling (slow signal). Villringer and Chance,[7] claimed that the advantages of the optical methods include biochemical specificity, a temporal resolution in the millisecond range, the potential of measuring intracellular and intravascular events simultaneously and the portability of the devices enabling bed side examination.
Following earlier in-vitro studies and work in experimental animals, such studies were initially carried out in new born infants.[8],[9] However, recently it has been shown that it is possible to assess brain activity through the intact skull in adult human subjects.[7],[10],[11],[12],[13],[14] While in infants it is possible to use transmission spectroscopy, for the larger adult head only reflectance spectroscopy (scattered light sampled by an ipsilateral receiving probe) is possible.
NIR spectroscopes are now available commercially. Basically these consist of pulsed laser diodes generating near infrared light of different wave lengths (between 7-1300 nm wave lengths) carried to the patient via a fibre optic cable (optode). Scattered light is collected by a second optode and detected by a photomultiplier tube. The data is displayed graphically or numerically as changes in chromophore concentration (micro mole per cm). It can also calculate the absolute changes in chromophore concentration (micro mole per liter). Most studies till recently were performed with single-site NIRS systems but the multi-optode arrays have already been used for multisite mapping of brain activity.[14]
NIR-spectroscopy and imaging, compared to other functional neuroimaging methods such as positron emission tomography (PET) and functional magnetic resonance imaging (FMRI) lacks spatial resolution and depth penetration. Comparison between NIR spectroscopy and PET data shows the best correlation in the outer 1 cm of brain tissue. However, intracerebral haematomas in the depth of the brain could be detected. It is, thus restricted to study of the cortical grey matter. The possibility of contributions from extra cerebral tissues e.g. scalp and skull contaminating the observation cannot be ruled out. However, Kirkpatrick et al[13] have demonstrated lack of any significant contribution from extracranial tissues. NIR method, on the other hand, has some specific advantages. It offers biochemical specificity by directly measuring concentration of molecules like oxy-haemoglobin, deoxy-haemoglobin and Co redox state. The data obtained provides information not only about vascular response, consequent upon neuronal activation but also intracellular events (Co redox state light scattering). The temporal resolution of NIR methods is in millisecond range, not possible with other imaging techniques. Unlike PET and MRI, the equipment is portable and flexible permitting its use at the bed side of the patient. The patient can be examined repeatedly or even continuously for long periods. The advanced versions of the equipment are going to be cheaper and more cost effective.
The technique has already been used successfully in clinical practice. Initially most of the studies in adults were carried out for functional localization of sensory, motor, visual, auditory and even cognitive tasks including speech.[15],[16] As mentioned earlier it has already been used for measuring cerebral blood flow and volume in critically ill new born infants.[8],[9],[17] Haglund et al[18] utilized high resolution optical imaging, (which is not the same as NIR spectroscopy technique) to study the effect of biopolar cortical stimulation in five patients undergoing surgery for intractable epilepsy. They correlated the optical changes with the duration of stimulation-evoked epileptiform after discharges and cognitively evoked functional activity. They concluded, "The adaptation of high-resolution optical imaging for use on human cortex provides a new technique for investigation of the organization of the sensory and motor cortical, language, and other cognitive processes".
Kirkpatrick et al[13] used NIR spectroscopy to monitor changes in cerebral oxygenation state in 13 patients during carotid endarterectomy. NIR spectroscopy permitted real time measurements, of cerebral perfusion through intact skull, during clamping of the carotid arteries in neck and after their release. Though they found the method more sensitive for detecting cerebral hypoxia than scalp electroencephalography, they recommended formal quantitative future validation of the method. Gopinath et al[12] utilized NIR spectroscopy for early detection of delayed traumatic intracranial haematoma in a series of 167 patients. The difference in absorption of light (delta OD) at 760 nm between the normal and haematoma side was measured serially for 3 days after injury. In 24 of the 27 patients who developed delayed haematoma, a significant increase (>0.3) in the delta OD occurred prior to an increase in intracranial pressure, a change in neurological examination or a change in CT scan.
Considering the vast potentials of non-invasive optical spectroscopy and imaging both as a clinical diagnostic method and a research tool it is surprising that it has not attracted the attention of neurologists and neurosurgeons more widely. On the basis of the available information the author is convinced that it would soon become a routine bed side investigation specially to monitor cerebral blood flow dynamics in patients with stroke and head injury.
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