Since the initial demonstration of near-infrared spectroscopy (NIRS) for noninvasive measurements of brain perfusion and metabolism in the 1970s, and its application to functional brain studies (fNIRS) in the 1990s, the field of noninvasive optical studies of the brain has been continuously growing. Technological developments, data analysis advances, and novel areas of application keep advancing the field. In this article, we provide a view of the state of the field of cerebral NIRS, starting with a brief historical introduction and a description of the information content of the NIRS signal. We argue that NIRS and fNIRS studies should always report data of both oxy- and deoxyhemoglobin concentrations in brain tissue, as they complement each other to provide more complete functional and physiological information, and may help identify different types of confounds. One significant challenge is the assessment of absolute tissue properties, be them optical or physiological, so that relative measurements account for the vast majority of NIRS and fNIRS applications. However, even relative measurements of hemodynamics or metabolic changes face the major problem of a potential contamination from extracerebral tissue layers. Accounting for extracerebral contributions to fNIRS signals is one of the most critical barriers in the field. We present some of the approaches that were proposed to tackle this challenge in the study of cerebral hemodynamics and functional connectivity. Finally, we critically compare fNIRS and functional magnetic resonance imaging (fMRI) by relating their measurements in terms of signal and noise, and by commenting on their complementarity.

中文翻译：

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观点：利用漫射光学成像技术对人脑进行非侵入式传感的前景。

自1970年代首次展示近红外光谱（NIRS）用于无创测量脑灌注和代谢以来，到1990年代将其应用于功能性脑研究（fNIRS）以来，大脑的无创光学研究领域一直在不断发展生长。技术的发展，数据分析的进步以及新颖的应用领域不断推动着该领域的发展。在本文中，我们将从简要的历史介绍和NIRS信号信息内容的描述入手，介绍大脑NIRS领域的状况。我们认为NIRS和fNIRS研究应始终报告大脑组织中氧合和脱氧血红蛋白浓度的数据，因为它们相互补充以提供更完整的功能和生理信息，并可能有助于识别不同类型的混杂物。一项重大挑战是评估绝对组织特性，无论是光学特性还是生理特性，因此相对测量占了NIRS和fNIRS应用的绝大部分。然而，即使是相对于血液动力学或代谢变化的相对测量，也面临着来自脑外组织层的潜在污染的主要问题。大脑对fNIRS信号的贡献是该领域最关键的障碍之一。在脑血流动力学和功能连通性研究中，我们提出了一些解决这一挑战的方法。最后，我们通过将fNIRS和功能磁共振成像（fMRI）的信号和噪声测量值联系起来，并对它们的互补性进行评论，来对它们进行严格的比较。