Quantitative optical imaging of brain activity—human and animal studies

Abstract

In order to overcome the problems associated with near-infrared optical imaging (NIR-imaging) such as the lack of the quantification and poor spatial resolution, we developed a 64-channel time-resolved optical imaging system, by which we could obtain quantitative functional images of human brain activity. Reflectance tomographic images of the changes in oxy-hemoglobin [oxy-Hb], deoxy-hemoglobin [deoxy-Hb], and total-hemoglobin [t-Hb] associated with neural activation were obtained, and given as absolute concentration changes. Then, the obtained optical functional images were superimposed on 3-D images of the subject's brain reconstructed from MRI, on which fMRI images were also superimposed. Very interestingly, but curiously, we found that the activation maps of [oxy-Hb] rather than [deoxy-Hb] were very reasonable and similar to those of the fMRI. The maximum increase in [oxy-Hb] due to finger tapping was about 1 μM, whereas in several cognitive tasks such as the digit span task, the increase was much larger, at 3–8 μM. The optical imaging system employed here can be applied to the subjects of all ages and be used at the bedside as well. By simplifying and miniaturizing the imaging system, we could construct a conventional single channel oxygen monitor for clinical use, by which we could quantify the changes of [oxy-, deoxy- and total Hb] during neuronal activation in each subject and, therefore, statistical analysis became possible.