Method of localizing an object in a turbid medium

Abstract

The invention relates to a method of localizing an object in a turbid medium. After measurement of the intensities for a plurality of light paths between the light sources and the detectors, the measured intensities are normalized. Subsequently, an image of the interior of the turbid medium is reconstructed on the basis of the measured intensities. In order to counteract artefacts in the reconstructed image, the combination includes a weighting factor which reduces the effect exerted on the reconstructed image by the measured intensities with a high noise factor in comparison with the effect exerted thereon by the measured intensities with a low noise factor.

1 . A method of localizing an object in a turbid medium, which method includes the following steps: 
 irradiation of the turbid medium,    measurement of intensities of a part of the light transported through the turbid medium along a plurality of light paths from a source point where the light enters the turbid medium to a measuring point where the light leaves the turbid medium,    reconstruction of an image of the interior of the turbid medium from a combination of measured intensities,    characterized in that    the combination contains a weighting factor which reduces an effect exerted on the reconstructed image by the measured intensities with a high noise factor relative to the effect exerted thereon by the measured intensities with a low noise factor.    
     
     
         2 . A method as claimed in  claim 1 , characterized in that for a light source and a measuring position in which intensities is measured the method also includes the following steps: 
 determination of an estimated intensity from a transformation of a predetermined first image,    determination of a difference between the measured intensities and the estimated intensities, and    determination of a next image from the first image, a convergence factor and a back-transformation of the difference, the convergence factor being chosen to be depending on the weighting factor.    
     
     
         3 . A method as claimed in  claim 2 , in which the transformation contains a product of a sensitivity matrix and the first image.  
     
     
         4 . A method as claimed in  claim 2 , in which the back-transformation contains a product of an estimate of an inverse sensitivity matrix and the difference between the measured intensities and the calculated intensities.  
     
     
         5 . A method as claimed in  claim 1  which also includes the following steps: 
 determination of variations of the attenuation coefficients of the light paths from the measured intensities, and  
 determination of a variation of the attenuation coefficient of a volume element of the turbid medium by means of a back-transformation which contains a weighted mean value of an estimate of the inverse sensitivity matrix with the variations determined for the attenuation coefficients of the light paths, the effect of the noise factor on the reconstruction being determined by the product of the weighting factor and the elements of the estimate of the inverse sensitivity matrix.  
 
     
     
         6 . A method as claimed in  claim 1 , characterized in that the weighting factor is dependent on a reciprocal value of a noise factor.  
     
     
         7 . A method as claimed in  claim 1 , characterized in that the measured intensities are corrected by way of a normalized intensity which is determined by a combination of the measured intensity associated with a light path and a measured reference intensity associated with the light path in a reference medium.  
     
     
         8 . A method as claimed in  claim 6 , characterized in that the noise factor is determined by a combination of noise of the measurement, the measured intensity of the measurement, the noise of the reference intensity and the measured reference intensity.  
     
     
         9 . A device for the imaging of objects in a turbid medium, including 
 a light source for irradiating the turbid medium,    a photodetector for converting the light transported through the turbid medium into a photodetector signal,    means for converting the photodetector signal in measured intensities,    a control unit for reconstructing an image of the interior of the turbid medium from the measured intensities,    characterized in that    the device includes means for determining a weighing factor which reduces the effect exerted on the reconstructed image by the measured intensities with a high noise factor in comparison with the effect exerted by the measured intensities with a low noise factor.