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Non-Parametric Image Subtraction

The non-parametric image subtraction algorithm described in TINA Memo no 2002-004 is implemented within the Imcalc tool as the dscat button. The algorithm subtracts two images by constructing a joint histogram, normalising it to produce a probability histogram and then, for each pair of corresponding pixels from the original image pair, performing an integration along the column of the histogram defined by the pixel grey-level value from the top-of-stack image, summing all values smaller than that at the position defined by the pixel pair. This is directly equivalent to constructing a confidence interval to generate the probability that the grey-level pairing of the pixels was not generated by the same distribution as the bulk of the images. The algorithm is therefore ideal for detecting small changes in image pairs, such as enhancement of MS lesions between pre- and post-GdDTPA contrast agent injection.

The major advantage of the technique over simple image subtraction is that the resulting difference image can be interpreted directly as a probability image, with a uniform distribution (i.e. the probabilities are ``honest" in the sense that they reflect frequencies of occurrence). In addition, the difference image is not contaminated with consistent differences between the original image pair e.g. overall illumination differences.

The usage of the algorithm is as follows:


next up previous contents
Next: Edge Geom Tool Up: Imcalc Tool Previous: Standard Usage   Contents
root 2017-11-24