The poor spatial resolution and reproducibility of the images are disadvantages of near infrared topography. The authors
proposed the combination of the double-density probe arrangement and the image reconstruction algorithm using a
spatial sensitivity profile to improve the spatial resolution and the reproducibility. However, the proposed method was
evaluated only by the simplified adult head model. It is uncertain whether the proposed method is effective to the actual
head that has complicated structure. In this study, the proposed method is evaluated by the virtual head phantom the 3Dstructure
of which is based upon an MRI scan of an adult head. The absorption change the size of which is almost
equivalent to the width of the brain gyri was measured by the conventional method and the proposed method to evaluate
the spatial resolution of the topographic images obtained by each method. The positions of the probe arrangements are
slightly changed and the topographic images of the same brain activation measured by two probe positions are compared
to evaluate the reproducibility of the NIR topography. The results indicate that the combination of the double-density
probe arrangement and the image reconstruction algorithm using the spatial sensitivity profile can improve both the
spatial resolution and the reproducibility of the topographic image of brain activation in the virtual head phantom.
However, the uneven thickness of the superficial tissues affects the accuracy of the position of activation in the images.
The spatial resolution of current near-infrared topography is not enough for clinical application. In this study, the image reconstruction algorithm using prior knowledge about spatial sensitivity profile in the tissue and constraint of spatial frequency in image was proposed and was evaluated by simulation. The spatial resolution of topographic image obtained from the image reconstruction method is better than that obtained from the conventional mapping-interpolation method. The most appropriate cut-off frequency for the constraint for the image reconstruction method depends on the arrangement of fibres.
Multi-channel optical imaging system can obtain a topographical distribution of the activated region in the brain cortex by a simple mapping algorithm. Near-infrared light is strongly scattered in the head and the volume of tissue that contributes to the change in the optical signal detected with source-detector pair on the head surface is broadly distributed in the brain. This scattering effect results in poor resolution and contrast in the topographic image of the brain activity. We report theoretical investigations on the spatial resolution of the topographic imaging of the brain activity. The head model for the theoretical study consists of five layers that imitate the scalp, skull, subarachnoid space, gray matter and white matter. The light propagation in the head model is predicted by Monte Carlo simulation to obtain the spatial sensitivity profile for a source-detector pair. The source-detector pairs are one dimensionally arranged on the surface of the model and the distance between the adjoining source-detector pairs are varied from 4 mm to 32 mm. The change in detected intensity caused by the absorption change is obtained by Monte Carlo simulation. The position of absorption change is reconstructed by the conventional mapping algorithm and the reconstruction algorithm using the spatial sensitivity profiles. We discuss the effective interval between the source-detector pairs and the choice of reconstruction algorithms to improve the topographic images of brain activity.
A near infrared topographic system is an effective instrument for obtaining an image of brain activation. In the conventional mapping method, the signals detected with the source-detector pairs are simply mapped and interpolated to obtain the topographic image.
It is likely that an image reconstruction algorithm using a spatial
sensitivity profile will improve the spatial resolution of the topographic image. In this study, a one-dimensional distribution of the absorption change in the head model is calculated from the signals detected with various intervals of source-detector pairs by the conventional mapping method and an image reconstruction algorithm using the spatial sensitivity profile to evaluate the limit of spatial resolution of topographic imaging. Small intervals of the source-detector pairs improve the position of the absorption change in the topographic image calculated by both the conventional mapping method and the reconstruction algorithm. The size of the absorption change calculated from the intensity detected with a small interval of the source-detector pairs is sufficiently improved by the image reconstruction algorithm using the spatial sensitivity profile.