CHORONARY IMAGE ENHANCEMENT USING DECIMATION-FREE DIRECTIONAL FILTER BANKS

CHORONARY IMAGE ENHANCEMENT USING DECIMATION-FREE

DIRECTIONAL FILTER BANKS

• The detection and enhancement of Coronary arterial trees (CAT’s)in an angiogram image is an important pre-processing task.
• In this paper, they have proposed a decimation-free directional filter bank (DFB) structure.It provides output in the form of directional images as opposed to directional sub-bands provided in previous DFBs.
• Angiograms acquired are low in contrast.Therefore we have to prepare angiogram image before it can given as input to the proposed DFB structure.
• The preparation steps involve removing non-uniform illumination from the image.Then proposed DFB structure outputs directional images.The final enhanced result is constructed on a block-by-block basis by comparing energy of all the directional images and picking one that provides maximum energy. The enhancement that results in the final image is due to the fact that we can separate omni-directional background noise from CAT structure which is
  pre-dominantly a directional feature.

• An angiogram image enhancement algorithm receives an input angiogram image, applies a set of intermediate steps on the input image and finally outputs the enhanced image.

1.   Non-uniform Illumination Correction
   An input angiogram image has a varying illumination pattern that needs to be removed. Although, there are many spatial domain techniques available to get rid of non-uniform illumination structure,we opted for homomorphic filtering to extract non-uniform illumination of the test image
2.   Normalization 
    is a pixel-wise operation. The main purpose of normalization is to get an output image with desirable mean and variance, which facilitates the subsequent processing.
3.    Creation of Directional Images

3.1.      Design of Directional Filters 

• The directional analysis employed in this paper decomposes the spectral region of a given image into wedge-shaped passband regions.
• It is easily shown that these wedge-shaped regions correspond to directional components of an image. 
• The filters related towedge-shaped regions are commonly referred to as fan filters.
• However, in this structure,decimators at each stage are taken out and filters are designedby linear transformation in the frequency domain to get fan filters.
• Furthermore, to avoid ringing artifact in the output, ideal fan filters are avoided by employing non-ideal hour-glass filters using an FIR

                 3.2 Directional Images

• The first step employed in directional images creation is to remove the spatial varying mean term by filtering with a highpass filter.
•  In this paper, rectangularly separable highpass filter was used.

4.   Reconstruction of Enhanced Image