Abstract
Near-infrared-spectroscopy (NIRS) has the prospect of a non-invasive diagnosis of the human brain. In this work, a time-domain NIRS apparatus has been constructed which is applicable on the adult human head. A new data analysis was developed using measured distributions of time of flight of photons (DTFs) to determine the depth and the size of an absorption change in a semi-infinite turbid medium. By using this apparatus and this new analysis, absorption changes in the brain can be distinguished from those in the skull.
For the data analysis a theoretical concept was developed, with which the relationship between a measured change in the DTF and a local absorption change in the layered medium could be formulated. This relationship is given by a set of linear equations where the coefficient matrix transforms from space (depth of the medium) to time (DTF). The matrix consists of the time-dependent mean partial photon pathlength in the different layers of the medium. These pathlength were calculated by a Monte-Carlo code which was specially developed for inhomogenous layered structures. These structures take into account the optical properties of the different compartments of the head (skalp, skull, CSF, gray and white matter). The matrix can be inverted by a truncated singular value decomposition and the absorption change can thus be quantified in certain depth.
With experiments on tissue simulating phantoms the method was examined concerning the possible spatial resolution. Simultaneous absorption changes in up to three compartments could be determined.
For the in-vivo measurements a mobile apparatus has been build suitable for bed-side monitoring. With this apparatus DTFs can be measured simultaneously at three wavenlength. The response function of the set-up (FWHM ) is about 300ps. By a new algorithm the deconvolution of the measured DTF and the response function was possible.
Measurements on human subjects were performed while the following actions induced intra- and extracerebral absorption changes: a motor stimulation, a valsalva maneuver, a venous injection of a contrast dye and a change in the concentration of inspired oxygen. For all in-vivo experiments the results one would have obtained using a measurement without depth resolution would have led to the wrong results. The method presented here is thus relevant for NIRS measurements on the adult human head.
In a second part of the work an investigation on the determination of a relevant clinical oxygen saturation value was performed. Different algorithms for the determination of this parameter were tested on simulated data. In the best case the intracerebral saturation could be determined with a maximum error of 5 % from simulated DTFs using a heuristically modified algorithm in the time domain. Measurements on a human subject showed, that this algorithm is applicable on DTFs measured in-vivo. |
