doi: 10.1161/01.STR.0000110122.57772.C3
(Stroke. 2004;35:70.)
© 2004 American Heart Association, Inc.
Original Contributions |
Editorial Comment—Cerebral Near-Infrared Spectroscopy: How Far Away From a Routine Diagnostic Tool?
Berlin NeuroImaging Center (BNIC) and Department of Neurology, Charité, Humboldt-University, Berlin, Germany
Pathophysiological events in cerebrovascular disorders are heterogeneous, eg, the consequences of a carotid stenosis on cerebral circulation can differ considerably between patients and also the cerebral consequences of acute stroke are extremely heterogeneous, not only between different patients but also within one patient over time. Thus, there may not be the one optimal therapy for all patients, but rather the ideal therapeutic option should be tailored to the individual pathophysiological situation. Based on these considerations, a variety of methods have been developed to identify pathophysiological situations relevant for therapeutic decisions. One key issue when describing cerebral pathophysiology in the context of vascular disorders is the interplay between blood flow and oxygen consumption, and the most authoritative findings are still based on positron-emission tomography (PET) measurements. Cost, lack of general availability, poor temporal resolution, which prohibits any monitoring approach, and the exposure to radiation have motivated the search for alternatives. Beside MRI-based methods, near-infrared spectroscopy (NIRS) is a potential alternative allowing for an assessment of cerebral blood flow (CBF) and hemoglobin (Hb) oxygenation parameters of the human cerebral cortex noninvasively through skin and skull. NIRS has been termed a "promising method" for more than a quarter of a century now.1 The principle of a NIRS tool is based on a modified Beer-Lambert Law and is by far simpler than that of functional MRI or PET. Costs are low, commercial monitors are available at an expense of a head coil for an MRI scanner, and broader distribution may lower the costs even of the technically more advanced systems that allow 2-dimensional imaging and perspectively also depth resolution. This, perhaps, is the most remarkable advantage when compared with the "alpha team" of functional imaging techniques: The method is applicable in almost any environment with a regular socket, and instead of the patient being transported to a distant (and sometimes difficult to monitor) environment, the system can be brought to the patient’s bedside and is easily combined with other bedside methods, eg, transcranial Doppler sonography (TCD). Numerous articles on NIRS have been written discussing functional brain imaging in adults2 and neonates,3 and it has been used for cerebral monitoring during heart surgery.4 In addition to assessment of hemoglobin oxygenation it has been established as a technique for measuring cerebral blood flow at the bedside.5 In this issue of Stroke, Vernieri et al present a study on the combination of TCD and NIRS in the evaluation of patients with carotid stenosis.6 The combination of both bedside tools allowed for the determination of a status-specific increase in arterial blood flow velocity and oxygen saturation during a vasomotor reactivity test.6
So is NIRS ready for widespread diagnostic use? Our answer is "Yes, not today, but probably quite soon." Subsequently, we will briefly touch on (1) some technical issues yet to be overcome, (2) research applications at hand, and (3) clinical applications on the horizon.
The main methodological issue regarding the validity of all NIRS parameters mentioned above is induced by the assumptions of the path of light through tissue, and the validity of chromophore concentration measurements based on a modified Beer-Lambert law. Of particular interest is the assessment of a baseline value of cerebral hemoglobin oxygenation, which is necessary for a measurement of hemoglobin saturation: [oxy-Hb]/([oxy-Hb]+[deoxy-Hb]). While it is clear that with a certain separation (>2 cm) of the sender/receiver pair brain tissue is part of the sample volume, the amount of extracerebral contamination is difficult to assign.7 This, however, is necessary to determine "brain-specific" concentration values of oxyhemoglobin and deoxyhemoglobin or an optical contrast agent. Furthermore, to enable precise concentration measurements, the usual implicit assumption of the head as a homogeneous semi-infinite structure obviously ignores anatomy and induces a number of errors, preventing a proper quantification. However, recently models respecting the human head’s layered structure (ideally determined individually with MRI) have been successful in showing that a differentiation of different layers is well possible.8 Such an approach will be able to address the issue of depth resolution as well as of quantification. This requires NIRS technology, more demanding than most of the standard continuous-wave systems, such as time- or frequency-resolved NIRS and/or multidistance approaches. While this refinement does not interfere with the simplicity and portability of the apparatus used, thus far it is not clear how far these methodological improvements will have to go in order to provide for an absolutely reliable cerebral measurement of absolute Hb concentrations and CBF. More precisely, are current implementations such as the one used in the article by Vernieri et al6 (and supported by the promising findings herein) explored enough or will time-resolved NIRS guided by individual MRI morphology be necessary? Two types of steps need to be taken: improved NIRS technology needs to be implemented, and the new systems have to be further validated in prospective clinical studies.
With these technical improvements implemented, the usefulness of NIRS as a research tool seems secured. Research applications include studies on neurovascular coupling and9 functional imaging in situations in babies and children3 and in situations in which functional MRI or PET is not feasible (walking, standing, etc),10 but also the elucidation of pathophysiological events, eg, the search for spreading ischemia in humans.11
For clinical use, to monitor tissue at risk (perhaps after being defined by antecedent MRI) after ischemic stroke and (in combination with TCD) to assess the effect of thrombolysis—in particular, to determine the pathophysiologically relevant time-point of reperfusion—seem a likely perspective. In the preoperative assessment of patients with carotid stenosis, we envision NIRS data on CBF and Hb oxygenation, from which a clinically useful measure of the cerebral metabolic rate of oxygen can be derived.12 Time will tell whether these applications will "succeed" despite the one significant drawback of NIRS, which so far in adults can provide information only on the cerebral cortex and not on deeper structures. This drawback should not, however, significantly affect our "best bet" for a widespread clinical application. In our view it will be only a matter of time until incremental improvements of the NIRS method will help it surpass the "clinical usefulness threshold" of a rather old idea: the "cerebral oxygenation monitor" for the patient in coma (pharmacologically induced or due to brain damage) on the intensive care unit whose brain function is very difficult to assess and about whom we neurologists so often are being asked "why does she/he not wake up."
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