1. A method for extracting image information from images of a target object in an optical coherence tomography (OCT) device obtained via scanning a reference path length relative to a signal path length of a signal path in the interferometer of the OCT device where the target object is located at one end of the signal path, comprising: obtaining different OCT images of the target object under different reference path lengths; processing the different OCT images to obtain a derivative with respect to the signal path length of image information of the different images; and processing the derivative to extract improved image information of the target object to reduce an effect of a signal decay due to scattering of light in the target object, wherein obtaining different OCT images of the target object under different reference path lengths comprises: obtaining different OCT images of the target object at different locations shifted in position represented by x and y coordinates along a direction substantially perpendicular to a direction of incidence of light into the target object represented by z coordinate; and obtaining different OCT images of the target object at different reference path lengths at each of the different locations to obtain 3-dimensional image information of the target object at the different locations and at different depths with different z coordinate below a surface of the target object, and wherein processing the different OCT images to obtain a derivative with respect to the signal path length of image information of the different images comprises: averaging a first number of consecutive B-scan images among the different OCT images to obtain an averaged B-scan image; obtaining a normalized OCT image intensity of a position in the averaged B-scan image by dividing a signal intensity of the position in the averaged B-scan image by a signal intensity at a surface of the target object; calculating a gradient value of the normalized OCT image intensity in the signal path length direction by subtracting a normalized OCT image intensity in a next depth to the normalized OCT image intensity; and obtaining the derivative with respect to the signal path length by dividing the gradient value of the normalized OCT image intensity by double actual distance of one pixel.