A Homocentric Squares Filter for pulmonary nodule enhancement based on the CT images

Automatic pulmonary nodule detection in Computer Tomographic (CT) images is a challenge task for the Computer Aided Diagnosis (CAD) systems. This paper proposes a novel nodule enhancement filter (Homocentric Squares Filter) for automatic nodule detection based on CT characteristic and shape feature. First, the bright regions in the image are enhanced by calculating the CT value variation between an inner square and an outer hollow square. Then, a line-restrained function is constructed to eliminate line-like region response of the filter. Simultaneously, a uniformity function is defined to exclude the marginal region and the corner region response of the filter. Finally, the output of the proposed filter is a combination of the CT value variation function, line-restrained function and uniformity function, which can enhance those nodule-like regions and avoid the response of line-like, marginal and corner regions. Moreover, the integral image technique is applied to speed up the processing of the filter. Experimental results showed that the proposed filter outperformed the selective filter according to the nodule enhancement effect and the processing speed.